Ocular surface changes with short-term contact lens wear · conjunctiva and tear film surface...

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Ocular surface changes with short-term contact lens wear Garima Tyagi B.S. (Optom), PGDHRM A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy Professor Michael Collins (Principal supervisor) Dr Scott Read (Associate supervisor) Mr Brett Davis (Associate supervisor) Institute of Health and Biomedical Innovation School of Optometry Queensland University of Technology Brisbane, Australia 2011

Transcript of Ocular surface changes with short-term contact lens wear · conjunctiva and tear film surface...

Page 1: Ocular surface changes with short-term contact lens wear · conjunctiva and tear film surface quality. The experimental paradigm used in these studies was a repeated measures, cross-over

Ocular surface changes with short-term contact lens wear

Garima Tyagi B.S. (Optom), PGDHRM

A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy

Professor Michael Collins (Principal supervisor)

Dr Scott Read (Associate supervisor)

Mr Brett Davis (Associate supervisor)

Institute of Health and Biomedical Innovation

School of Optometry

Queensland University of Technology

Brisbane, Australia

2011

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Keywords

Contact lens

Cornea

Corneal topography

Corneal thickness

Corneal swelling

Pentacam

Medmont

Videokeratoscope

Eyelids

Blepharoptosis

Lid-wiper epitheliopathy

Tarsal conjunctiva

Tear film

Tear film surface quality

Non-invasive

High-speed videokeratoscopy

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Abstract Contact lenses are a common method for the correction of refractive errors of

the eye. While there have been significant advancements in contact lens

designs and materials over the past few decades, the lenses still represent a

foreign object in the ocular environment and may lead to physiological as well

as mechanical effects on the eye. When contact lenses are placed in the eye,

the ocular anatomical structures behind and in front of the lenses are directly

affected. This thesis presents a series of experiments that investigate the

mechanical and physiological effects of the short-term use of contact lenses on

anterior and posterior corneal topography, corneal thickness, the eyelids, tarsal

conjunctiva and tear film surface quality.

The experimental paradigm used in these studies was a repeated

measures, cross-over study design where subjects wore various types of

contact lenses on different days and the lenses were varied in one or more key

parameters (e.g. material or design). Both, old and newer lens materials were

investigated, soft and rigid lenses were used, high and low oxygen permeability

materials were tested, toric and spherical lens designs were examined, high

and low powers and small and large diameter lenses were used in the studies.

To establish the natural variability in the ocular measurements used in the

studies, each experiment also contained at least one “baseline” day where an

identical measurement protocol was followed, with no contact lenses worn. In

this way, changes associated with contact lens wear were considered in

relation to those changes that occurred naturally during the 8 hour period of the

experiment.

In the first study, the regional distribution and magnitude of change in

corneal thickness and topography was investigated in the anterior and posterior

cornea after short-term use of soft contact lenses in 12 young adults using the

Pentacam. Four different types of contact lenses (Silicone hydrogel/

Spherical/–3D, Silicone Hydrogel/Spherical/–7D, Silicone Hydrogel/Toric/–3D

and HEMA/Toric/–3D) of different materials, designs and powers were worn for

8 hours each, on 4 different days. The natural diurnal changes in corneal

thickness and curvature were measured on two separate days before any

contact lens wear. Significant diurnal changes in corneal thickness and

curvature within the duration of the study were observed and these were taken

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into consideration for calculating the contact lens induced corneal changes.

Corneal thickness changed significantly with lens wear and the greatest corneal

swelling was seen with the hydrogel (HEMA) toric lens with a noticeable

regional swelling of the cornea beneath the stabilization zones, the thickest

regions of the lenses. The anterior corneal surface generally showed a slight

flattening with lens wear. All contact lenses resulted in central posterior corneal

steepening, which correlated with the relative degree of corneal swelling. The

corneal swelling induced by the silicone hydrogel contact lenses was typically

less than the natural diurnal thinning of the cornea over this same period (i.e.

net thinning). This highlights why it is important to consider the natural diurnal

variations in corneal thickness observed from morning to afternoon to

accurately interpret contact lens induced corneal swelling.

In the second experiment, the relative influence of lenses of different

rigidity (polymethyl methacrylate – PMMA, rigid gas permeable – RGP and

silicone hydrogel – SiHy) and diameters (9.5, 10.5 and 14.0) on corneal

thickness, topography, refractive power and wavefront error were investigated.

Four different types of contact lenses (PMMA/9.5, RGP/9.5, RGP/10.5,

SiHy/14.0), were worn by 14 young healthy adults for a period of 8 hours on 4

different days. There was a clear association between fluorescein fitting pattern

characteristics (i.e. regions of minimum clearance in the fluorescein pattern)

and the resulting corneal shape changes. PMMA lenses resulted in significant

corneal swelling (more in the centre than periphery) along with anterior corneal

steepening and posterior flattening. RGP lenses, on the other hand, caused

less corneal swelling (more in the periphery than centre) along with opposite

effects on corneal curvature, anterior corneal flattening and posterior

steepening. RGP lenses also resulted in a clinically and statistically significant

decrease in corneal refractive power (ranging from 0.99 to 0.01 D), large

enough to affect vision and require adjustment in the lens power. Wavefront

analysis also showed a significant increase in higher order aberrations after

PMMA lens wear, which may partly explain previous reports of “spectacle blur”

following PMMA lens wear.

We further explored corneal curvature, thickness and refractive changes

with back surface toric and spherical RGP lenses in a group of 6 subjects with

toric corneas. The lenses were worn for 8 hours and measurements were taken

before and after lens wear, as in previous experiments. Both lens types caused

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anterior corneal flattening and a decrease in corneal refractive power but the

changes were greater with the spherical lens. The spherical lens also caused a

significant decrease in WTR astigmatism (WRT astigmatism defined as major

axis within 30 degrees of horizontal). Both the lenses caused slight posterior

corneal steepening and corneal swelling, with a greater effect in the periphery

compared to the central cornea.

Eyelid position, lid-wiper and tarsal conjunctival staining were also

measured in Experiment 2 after short-term use of the rigid and SiHy contact

lenses. Digital photos of the external eyes were captured for lid position

analysis. The lid-wiper region of the marginal conjunctiva was stained using

fluorescein and lissamine green dyes and digital photos were graded by an

independent masked observer. A grading scale was developed in order to

describe the tarsal conjunctival staining. A significant decrease in the palpebral

aperture height (blepharoptosis) was found after wearing of PMMA/9.5 and

RGP/10.5 lenses. All three rigid contact lenses caused a significant increase in

lid-wiper and tarsal staining after 8 hours of lens wear. There was also a

significant diurnal increase in tarsal staining, even without contact lens wear.

These findings highlight the need for better contact lens edge design to

minimise the interactions between the lid and contact lens edge during blinking

and more lubricious contact lens surfaces to reduce ocular surface micro-

trauma due to friction and for.

Tear film surface quality (TFSQ) was measured using a high-speed

videokeratoscopy technique in Experiment 2. TFSQ was worse with all the

lenses compared to baseline (PMMA/9.5, RGP/9.5, RGP/10.5, and SiHy/14) in

the afternoon (after 8 hours) during normal and suppressed blinking conditions.

The reduction in TFSQ was similar with all the contact lenses used, irrespective

of their material and diameter. An unusual pattern of change in TFSQ in

suppressed blinking conditions was also found. The TFSQ with contact lens

was found to decrease until a certain time after which it improved to a value

even better than the bare eye. This is likely to be due to the tear film drying

completely over the surface of the contact lenses. The findings of this study

also show that there is still a scope for improvement in contact lens materials in

terms of better wettability and hydrophilicity in order to improve TFSQ and

patient comfort.

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These experiments showed that a variety of changes can occur in the

anterior eye as a result of the short-term use of a range of commonly used

contact lens types. The greatest corneal changes occurred with lenses

manufactured from older HEMA and PMMA lens materials, whereas modern

SiHy and rigid gas permeable materials caused more subtle changes in corneal

shape and thickness. All lenses caused signs of micro-trauma to the eyelid

wiper and palpebral conjunctiva, although rigid lenses appeared to cause more

significant changes. Tear film surface quality was also significantly reduced

with all types of contact lenses. These short-term changes in the anterior eye

are potential markers for further long term changes and the relative differences

between lens types that we have identified provide an indication of areas of

contact lens design and manufacture that warrant further development.

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Table of contents

Keywords .................................................................................................. i Abstract .................................................................................................. iii Table of contents ................................................................................... vii List of Figures ......................................................................................... xi List of Tables ........................................................................................ xix

Abbreviations ....................................................................................... xxiii Statement of original authorship .......................................................... xxv Acknowledgements ............................................................................ xxvii Chapter 1 ................................................................................................ 1 Literature Review ................................................................................... 1

1.1 Eyelid: structure and functions .................................................. 1

1.2 Tarsal conjunctiva ..................................................................... 2 1.3 Tear film: structure and functions .............................................. 3

1.4 Cornea ...................................................................................... 3 1.4.1 Corneal anatomy ................................................................ 3 1.4.2 Corneal physiology............................................................. 4

1.5 Shape of the cornea .................................................................. 5

1.5.1 Current methods of measuring corneal topography ........... 7 1.5.2 Corneal topographic reference points .............................. 11

1.5.3 Classification of corneal topography ................................ 12 1.5.4 Corneal topography in normal population ........................ 15 1.5.5 Variations in corneal topography ...................................... 18

1.6 Corneal thickness .................................................................... 20 1.7 Contact lenses ........................................................................ 21

1.7.1 Properties of contact lens materials ................................. 22 1.8 Contact lenses and the eyelids ............................................... 25

1.9 Contact lenses and the tarsal conjunctiva ............................... 26 1.10 Contact lenses and the tear film .............................................. 26

1.11 Contact lenses and anterior corneal topography ..................... 28 1.11.1 PMMA contact lens wear and corneal topography ........... 28

1.11.2 RGP contact lens wear and corneal topography .............. 29 1.11.3 Soft hydrogel contact lenses and corneal topography...... 30 1.11.4 Silicone hydrogel contact lenses and corneal topography 31

1.11.5 Extended wear contact lenses and corneal topography ... 31 1.11.6 Toric soft contact lenses and corneal topography ............ 32

1.11.7 Time of recovery of corneal changes caused by contact lenses …………………………………………………………………33

1.12 Contact lenses and posterior corneal topography ................... 33 1.13 Contact lenses and corneal thickness ..................................... 34

1.13.1 Effect of contact lens wear on central corneal thickness .. 34 1.13.2 Effect of contact lens wear on peripheral corneal thickness 35

1.13.3 Mechanism of corneal thinning ........................................ 36 1.14 Contact lenses and orthokeratology ........................................ 36 1.15 Rationale ................................................................................. 37

Chapter 2 .............................................................................................. 41 Corneal changes following short-term soft contact lens wear........ 41

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2.1 Introduction ............................................................................. 41

2.2 Methodology ........................................................................... 43 2.2.1 Subjects ........................................................................... 43 2.2.2 Instrumentation ................................................................ 43

2.2.3 Contact lenses ................................................................. 44 2.2.4 Measurements and Protocol ............................................ 46

2.3 Data analysis .......................................................................... 48 2.3.1 Curvature and thickness difference maps ....................... 48 2.3.2 Regional analysis ............................................................ 49

2.3.3 Corneal best fit sphero-cylindrical power ......................... 49 2.3.4 Statistical analysis ........................................................... 50 2.3.5 Contact lens centration and rotation ................................ 50 2.3.6 Baseline day diurnal changes .......................................... 51

2.4 Results .................................................................................... 54

2.4.1 Diurnal Changes .............................................................. 54 2.4.2 Corneal thickness ............................................................ 54 2.4.3 Anterior corneal curvature ............................................... 56

2.4.4 Posterior corneal curvature ............................................. 57 2.4.5 Association between changes in thickness and curvature58 2.4.6 Corneal best fit sphero-cylindrical power ......................... 59

2.4.7 Contact lens centration and rotation ................................ 62 2.5 Discussion .............................................................................. 63 2.6 Conclusion .............................................................................. 67

Chapter 3 ............................................................................................. 69 Corneal changes following short-term rigid contact lens wear ...... 69

3.1 Introduction ............................................................................. 69 3.2 Methodology ........................................................................... 71

3.2.1 Subjects ........................................................................... 72 3.2.2 Contact Lenses................................................................ 72

3.2.3 Measurements and Instruments ...................................... 73 3.3 Data Analysis .......................................................................... 77

3.3.1 Corneal topography and thickness data .......................... 77 3.3.2 Pentacam data: Corneal curvature and thickness ........... 77

3.3.3 Medmont data: Correlation between the rigid lens fluorescein pattern and corneal topography changes .................... 78 3.3.4 Medmont data: Corneal refractive power ......................... 82 3.3.5 COAS data: Ocular wavefront error ................................. 82 3.3.6 Lens movement videos: Position of contact lens with respect to limbus centre ................................................................. 82

3.4 Results .................................................................................... 83

3.4.1 Anterior corneal axial curvature ....................................... 83 3.4.2 Posterior corneal axial curvature ..................................... 84 3.4.3 Corneal thickness ............................................................ 85 3.4.4 Correlation between corneal curvature and thickness ..... 87 3.4.5 Correlation between rigid lens fluorescein pattern and corneal topography changes .......................................................... 89 3.4.6 Refractive power .............................................................. 89 3.4.7 Ocular wavefront error ..................................................... 90 3.4.8 Position of contact lens .................................................... 91

3.5 Discussion .............................................................................. 92

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3.6 Conclusion .............................................................................. 96

Chapter 4 .............................................................................................. 99 Corneal changes with spherical versus back surface toric rigid contact lens wear ................................................................................ 99

4.1 Introduction ............................................................................. 99 4.2 Methodology .......................................................................... 100

4.2.1 Subjects ......................................................................... 100 4.2.2 Contact lenses ............................................................... 101 4.2.3 Measurements and Instruments ..................................... 103

4.3 Data Analysis ........................................................................ 104 4.3.1 Corneal topography and thickness data ......................... 104 4.3.2 Pentacam data: Corneal curvature and thickness .......... 104 4.3.3 Medmont data: Corneal refractive power ....................... 105 4.3.4 COAS data: Ocular wavefront error ............................... 105

4.3.5 Lens movement videos: Position of contact lens on cornea (with respect to limbus centre) ...................................................... 106

4.4 Results .................................................................................. 106

4.4.1 Anterior corneal axial curvature ..................................... 106 4.4.2 Posterior corneal axial curvature .................................... 107 4.4.3 Corneal thickness........................................................... 108

4.4.4 Refractive power ............................................................ 109 4.4.5 Ocular wavefront error ................................................... 109 4.4.6 Position of contact lenses .............................................. 110

4.5 Discussion ............................................................................. 111 4.6 Conclusion ............................................................................ 113

Chapter 5 ............................................................................................ 115 Eyelid changes following short-term rigid and soft contact lens wear .................................................................................................... 115

5.1 Introduction ........................................................................... 115

5.2 Methodology .......................................................................... 117 5.2.1 Subjects ......................................................................... 118 5.2.2 Contact lenses ............................................................... 118 5.2.3 Measurements and Instruments ..................................... 118

5.3 Data Analysis ........................................................................ 121 5.3.1 Eyelid position (Blepharoptosis) ..................................... 121 5.3.2 Tarsal staining ................................................................ 122 5.3.3 Lid-wiper epitheliopathy ................................................. 123

5.4 Results .................................................................................. 125

5.4.1 Eyelid position (Blepharoptosis) ..................................... 125 5.4.2 Tarsal conjunctival staining ............................................ 126

5.4.3 Lid-wiper epitheliopathy ................................................. 127 5.4.4 Association between blepharoptosis and tarsal conjunctival staining ……………………………………………………………….128

5.5 Discussion ............................................................................. 128 5.6 Conclusion ............................................................................ 134

Chapter 6 ............................................................................................ 135 Tear film surface quality with rigid and soft contact lenses .......... 135

6.1 Introduction ........................................................................... 135 6.2 Methodology .......................................................................... 137

6.2.1 Subjects ......................................................................... 137

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6.2.2 Instrument ..................................................................... 138

6.2.3 Contact lenses ............................................................... 138 6.2.4 Protocol ......................................................................... 139

6.3 Data Analysis ........................................................................ 140

6.4 Results .................................................................................. 143 6.4.1 TFSQ in natural blinking conditions ............................... 143 6.4.2 Blink frequency in natural blinking conditions ................ 144 6.4.3 TFSQ in suppressed blinking conditions ....................... 145 6.4.4 Trend of TFSQ with time in suppressed blinking conditions ……………………………………………………………….147 6.4.5 Association between TFSQ value and blink rate ........... 150 6.4.6 Association between TFSQ value and tarsal conjunctival and lid-wiper staining ................................................................... 150

6.5 Discussion ............................................................................ 151

6.6 Conclusion ............................................................................ 153 Chapter 7 ........................................................................................... 155 Conclusions ...................................................................................... 155

7.1 Changes in ocular structures posterior to the contact lens ... 156 7.1.1 Corneal thickness changes and contact lenses ............. 156 7.1.2 Anterior corneal curvature changes and contact lenses 158

7.1.3 Posterior corneal curvature changes and contact lenses ………………………………………………………………..161 7.1.4 Wavefront aberrations and rigid contact lenses ............. 161

7.2 Changes in ocular structures anterior to the contact lens ..... 162 7.2.1 Lid related changes and contact lenses ........................ 162

7.2.2 Tear film surface quality (TFSQ) and contact lenses ..... 165 7.3 Conclusion and clinical implications...................................... 166

References......................................................................................... 169 Appendices........................................................................................ 199

Appendix A: Ethics and Consent form ............................................. 199 Appendix B: Conference abstracts arising from this thesis ............. 199 Appendix C: Publications arising from this thesis ............................ 199

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List of Figures Figure 1-1: Structure of the eyelid. Redrawn from Bergmanson (2009). ........... 2

Figure 1-2: Layers of the cornea. Thicknesses as described by Gipson (1994). 4

Figure 1-3: (a) Medmont E300 videokeratoscope (b) Reflection of Placido disc

image from cornea (c) Medmont Placido disc (d) Subject‟s eye in position for

measurement. .................................................................................................. 9

Figure 1-4 (a) Oculus Pentacam system (b) Rotating Scheimpflug camera

system (c) Anterior segment image with the Pentacam .................................. 10

Figure 1-5: Various corneal topographic reference points. Note the

misalignment of the videokeratoscope axis from line of sight. Adapted from

Mandell (1996). .............................................................................................. 12

Figure 1-6: Anatomical classification of the corneal surface. Adapted from

Mountford et al. (2004). .................................................................................. 13

Figure 1-7: Explanation of axial and tangential curvatures at a point on the

cornea. Adapted from Mejía-Barbosa and Malacara-Hernández (2001). ........ 14

Figure 1-8: Illustration of the types of topography maps for a representative

subject, as captured by the Medmont E300 videokeratoscope: (a) Axial power

(b) Tangential power (c) Refractive power and (d) Elevation .......................... 14

Figure 1-9: Pre- and post-lens tear films and a contact lens on the cornea. Pre-

and post-lens thickness values by King-Smith et al. (2004). ........................... 27

Figure 2-1: The powers, designs and materials of the contact lenses used. The

comparisons to investigate the effect of lens characteristics on corneal

thickness and curvature are represented by curved arrows. The materials were

silicone hydrogel (SiHy) and hydroxyethyl methacrylate (HEMA). .................. 45

Figure 2-2: Contact lens thickness profiles for the SiHy/Sph/–3 (a), SiHy/Sph/–7

(b) and SiHy and HEMA/Toric/–3 (c). The color scale represents lens thickness

in mm. The thickness profile for the SiHy and HEMA toric contact lens is

identical (c). .................................................................................................... 46

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Figure 2-3: Axial curvature difference maps for a subject after 60 minutes of

downgaze task in (a) baseline (no contact lens wear) and (b) with a soft contact

lens in eye ...................................................................................................... 48

Figure 2-4: Cornea divided into central (4 mm diameter) and peripheral (4 mm

annulus) regions. ............................................................................................ 49

Figure 2-5: Digital image of a soft contact lens (SiHy/Toric/–3) on a subject‟s

eye. The lens centration for this subject was recorded as 0 (optimal), with less

than 0.5 mm decentration. The lens rotation for this lens was calculated using

the Imetrics software to be 16 degrees nasal. ................................................ 51

Figure 2-6: Diurnal variation in corneal pachymetry analysis. This figure shows

thickness difference maps for subject 2, SiHy/Toric/–3 lens. .......................... 53

Figure 2-7: Group mean changes in corneal thickness (mm) relative to baseline

days for the four different types of contact lenses. The lenses included different

combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)],

design [spherical (Sph), toric] and power (–3.00, –7.00 D). ............................ 55

Figure 2-8: Group mean changes in anterior axial curvature (mm) relative to

baseline days for the four different types of contact lenses. The lenses included

different combinations of lens material [hydrogel (HEMA) and silicone hydrogel

(SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D). ............... 57

Figure 2-9: Group mean changes in posterior axial curvature (mm) relative to

baseline days for the four different types of contact lenses. The lenses included

different combinations of lens material [hydrogel (HEMA) and silicone hydrogel

(SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D). ............... 58

Figure 2-10: (a) Correlation between changes in posterior (central) corneal

curvature with (a) central corneal thickness (b) peripheral corneal thickness. P-

values in are shown in red. ............................................................................. 59

Figure 2-11: Changes in best fit sphere (M), with-the-rule and against-the-rule

astigmatism (J0), oblique astigmatism (J45) and sphero-cylinder RMSE (from

baseline) for the anterior cornea. Significant change indicated by * p<0.05 and

# p<0.01. Error bar represents one standard error of the mean. Negative

change in M represents a decrease in corneal axial power (hypermetropic

shift). Negative change in J0 represents a decrease in WTR astigmatism.

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Positive change in J0 represents an increase in WTR astigmatism. Positive J45

represents a negative cylinder axis closer to 45° and negative J45 represents a

negative cylinder axis closer to 135°. ............................................................. 60

Figure 3-1: Photo of the set up with digital camera to record movement of the

contact lens. Illumination of the eye is provided by a fluorescent ring light,

mounted behind a diffuser. ............................................................................. 75

Figure 3-2: Sequence of measurements taken in the morning before and

following insertion of contact lens in eye ........................................................ 76

Figure 3-3: Sequence of measurements taken in the afternoon after 8 hours of

lens wear. ....................................................................................................... 77

Figure 3-4: Steps involved in correlating rigid lens fluorescein pattern and

corneal topographic changes. (b) White cross showing LC (c) small white cross

showing LC and bigger white cross showing VK centre. HVID: horizontal visible

iris diameter, VK: videokeratoscope centre, LC: limbus centre. ...................... 81

Figure 3-5: Image showing the position of a rigid contact lens on cornea (light

blue ring), limbus (yellow ring) and upper (red arc) and lower eyelid (blue arc).

....................................................................................................................... 83

Figure 3-6: Group mean changes in anterior axial corneal curvature (mm)

relative to baseline day for the four different types of contact lenses. The lenses

included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5

and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change

represents flattening and negative change represents steepening. ................ 84

Figure 3-7: Group mean changes in posterior axial corneal curvature (mm)

relative to baseline day for the four different types of contact lenses. The lenses

included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5

and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change

represents flattening and negative change represents steepening. ................ 85

Figure 3-8: Group mean changes in corneal thickness (mm) relative to baseline

day for the four different types of contact lenses. The lenses included different

materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm).

Details of the lenses are shown in Table 3-1. Positive change represents

swelling and negative change represents thinning. ........................................ 86

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Figure 3-9: Correlation between changes in central corneal thickness and

central (a) and peripheral (b) back curvature. Correlation between changes in

peripheral corneal thickness and central (c) and peripheral (d) back curvature.

....................................................................................................................... 88

Figure 3-10: Correlation between distance of points of minimum clearance

(between cornea and contact lenses, in fluorescein pattern) and points of

maximum corneal flattening, from the videokeratoscope (VK) centre. Data is

shown for inferior (V2) points along vertical meridian and nasal (H1) points

along the horizontal meridian. ........................................................................ 89

Figure 3-11: Schematic demonstration of anterior and posterior curvatures and

thickness of the cornea, before and after PMMA and RGP contact lens wear for

8 hours based on the experimental data. The solid lines represent the baseline

anterior and posterior surfaces of cornea. The dotted line represents the

anterior and posterior surfaces of the cornea after contact lens wear for 8

hours. (a) PMMA contact lens showing greater central corneal swelling

compared to peripheral resulting in anterior corneal steepening and posterior

corneal flattening. (b) RGP contact lens showing greater peripheral corneal

swelling resulting in anterior corneal flattening and posterior corneal

steepening. Note that the diagram is not to scale. .......................................... 93

Figure 4-1: Axial corneal curvature maps of all subjects showing pattern of

corneal astigmatism and difference in curvature of the two principal meridians.

Note that all subjects had central astigmatism except for subject 04 who

showed limbus-to-limbus astigmatism. ......................................................... 101

Figure 4-2: Fluorescein patterns with a spherical (a) and back surface toric (b)

lens (same eye) on a subject (04) with high astigmatism (∆K = 3.3 D), limbus-

to-limbus. In the lower panels a spherical (c) and back surface toric (d) lens

(same eye) on a subject (06) with a lower amount of corneal astigmatism (∆K =

1.4 D), central. Note axis markings/scribe marks of the toric lens on the flatter

corneal meridian in both subjects (panels b and d). ...................................... 103

Figure 4-3: Group mean changes in anterior axial corneal curvature (mm)

relative to baseline day for the spherical and back toric RGP lenses. Details of

the lenses are shown in Table 4-1. Positive change represents flattening and

negative change represents steepening. ...................................................... 106

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Figure 4-4: Mean axial radii of curvature (mm) in the vertical meridians for

baseline, spherical lens and back surface toric lens. VK: Videokeratoscope. 107

Figure 4-5: Group mean changes in posterior axial corneal curvature (mm)

relative to baseline day for the spherical and back surface toric RGP lenses.

Positive change represents flattening and negative change represents

steepening. .................................................................................................. 108

Figure 4-6: Group mean change in corneal thickness (mm) relative to baseline

day for the spherical and back toric RGP lenses. Details of the lenses are

shown in Table 4-1. Positive change represents swelling and negative change

represents thinning. ...................................................................................... 108

Figure 5-1: (a) Set up of digital camera to take the photo of external eyes (b)

Ruler next to the eye to allow calibration. ..................................................... 119

Figure 5-2: (a) Position of eyelids on the baseline day afternoon (no contact

lens in eyes) (b) Position of eyelids with contact lens in left eye in the afternoon

after 8 hours of lens wear. Palpebral aperture (PA) height is shown in mm.

Yellow rings indicate the limbus outline, upper eyelid margin is shown in red

and lower eyelid margin is shown with blue. ................................................. 120

Figure 5-3: External photo of the eye showing the palpebral aperture (PA)

height with respect to limbus centre. Markings in yellow indicate the limbus

margins, Markings in red indicate the position of upper lid and markings in blue

indicate the position of the lower lid. ............................................................. 122

Figure 5-4: Digital images showing upper tarsal conjunctival staining with

fluorescein on a grade of 0 (None) to 4 (Severe). ......................................... 123

Figure 5-5: Examples showing grading of lid-wiper epitheliopathy from three

representative subjects. ............................................................................... 124

Figure 5-6: Changes in height of palpebral aperture (mm) of the right and left

eye relative to baseline afternoon. Negative values mean that palpebral

aperture height is less compared to baseline afternoon. Each error bar

represents one standard error of the mean. # represents statistically significant

p-values (<0.03), * represents p-value approaching significance (p=0.06). Right

eye is control eye (no lens) and left eye is the contact lens wearing eye. ..... 125

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Figure 5-7: Mean lid-wiper epitheliopathy grades with different types of contact

lenses and on the baseline (BL) day (no contact lens), in the morning and

afternoon. p-values indicated for change in lid-wiper compared to baseline. #

indicates p-value <0.05. Each error bar indicates one standard deviation. .. 127

Figure 5-8: Lens edge profiles of the RGP/9.5 and soft lenses used in the

study. PMMA and RGP/10.5 lenses had similar edge profiles to the RGP/9.5

lens. ............................................................................................................. 133

Figure 6-1: Steps involved in estimation of TFSQ value on the corneal or

contact lens surface. ROI: region of interest. AOA: area of analysis ............. 142

Figure 6-2: Image frames from high speed videokeratoscopy with an RGP lens

on the cornea. Reflections of the Placido disc pattern (a) Immediately after blink

(TFSQ value = 0.85) and (b) few seconds after blink showing tear break up

(TFSQ value = 0.68). Yellow lines enclose the area of analysis. .................. 142

Figure 6-3: Mean TFSQ values in 30 seconds with the four contact lenses and

on baseline day (no contact lens), in the morning and afternoon, in natural

blinking conditions. The TFSQ is calculated on a scale of 0 to 1 where 0 is very

poor and 1 is very good quality. * indicates significant difference compared to

baseline. Error bars represent standard error of the mean. ......................... 143

Figure 6-4: Mean blink frequencies (number of blinks per minute) in natural

blinking conditions with and without contact lenses. Error bars represent

standard error of the mean. .......................................................................... 145

Figure 6-5: The group mean TFSQ values in suppressed blinking conditions

with time for the 6 seconds after a blink, for the baseline day and with the four

different contact lenses in the morning (a) and afternoon (b). ....................... 146

Figure 6-6: The four different types of representative patterns of TFSQ with

time over 30 seconds. .................................................................................. 148

Figure 6-7: TFSQ over time for a representative subject, showing an increase

during the first second post-blink (build-up time) and then a constant reduction

in TFSQ over time till the end of the measurement. Corresponding Placido disc

maps can be seen at the beginning (clear rings), middle (breaks in the ring

pattern) and end (severe distortion of the ring pattern) of the measurement.

Yellow lines enclose the area of analysis. .................................................... 149

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Figure 6-8: TFSQ over time for a representative subject, showing an increase

during first second post-blink, then a reduction is seen with time till a certain

point after which it shows an improvement and reaches a value more than the

baseline. Corresponding Placido disc maps can be seen at the beginning (clear

rings), middle (few breaks in the ring pattern) and end (very clear and regular

ring pattern) of the measurement. This later period seems to correspond to

complete drying of the lens surface which now acts like a mirror to produce a

high TFSQ value. Yellow lines enclose the area of analysis. ........................ 149

Figure 6-9: Correlation between mean TFSQ values and blink rates (number of

blinks per minute) in the morning and afternoon, for all the lenses combined.

(Afternoon measurements only) ................................................................... 150

Figure 6-10: Correlation between mean TFSQ values and (a) Tarsal staining

(b) Lid-wiper staining for all the lenses combined for morning and afternoon.151

Figure 7-1: Schematic representation of ocular structures and parameters

affected by short-term use of contact lenses presented in this thesis. .......... 155

Figure 7-2: Changes in ocular structures and parameters (posterior to contact

lenses) affected by short-term use of contact lenses, in comparison to baseline

day changes. ................................................................................................ 156

Figure 7-3: Schematic diagram showing difference between central and uniform

anterior swelling. .......................................................................................... 160

Figure 7-4: Changes in ocular structures and parameters (anterior to contact

lenses) affected by short-term use of contact lenses, in comparison to baseline

day changes. PA: Palpebral aperture. .......................................................... 162

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List of Tables Table 1-1: Mean anterior corneal radius of curvature R and anterior corneal

asphericity Q as reported by various authors. ................................................ 16

Table 1-2: Distribution of qualitative topographic patterns as reported in various

studies with videokeratoscopes ...................................................................... 16

Table 1-3: Mean posterior corneal radius of curvature R (mm) and asphericity

Q as reported by various authors. V and H represent vertical and horizontal

meridians, respectively. .................................................................................. 18

Table 2-1: Details of the lenses used in the study. ......................................... 45

Table 2-2: Methods to study the diurnal changes in corneal curvature and

thickness. ....................................................................................................... 52

Table 2-3: Relationship studied to check for diurnal changes. ........................ 52

Table 2-4: Mean corneal thickness changes relative to baseline days, with the

four contact lenses in central and peripheral corneal regions. Values where

pair-wise comparison revealed a significant change from baseline are

highlighted with asterisks (p-value ≤ 0.001 is ***). Positive change represents

swelling and a negative change represents thinning. ..................................... 56

Table 2-5: Mean changes in anterior and posterior axial corneal curvatures

relative to baseline days, with the four contact lenses in the central and

peripheral corneal regions. Values where pair-wise comparison revealed a

significant change from baseline are highlighted with asterisks (p-value ≤ 0.05

is *, ≤ 0.01 is ** and ≤ 0.001 is ***). ................................................................ 56

Table 2-6: Mean lens centrations calculated using custom-written software and

digital images of lenses on the corneas .......................................................... 62

Table 3-1: Details of the four lenses used in the study ................................... 73

Table 3-2: Mean changes in anterior and posterior axial corneal curvatures

relative to baseline days with the four contact lenses in the central and

peripheral regions. ......................................................................................... 85

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Table 3-3: Mean corneal thickness changes relative to baseline days with the

four contact lenses in central and peripheral corneal regions. ........................ 86

Table 3-4: Correlation between corneal thickness with anterior and posterior

curvatures for the four different types of contact lenses. ................................. 88

Table 3-5: Mean changes in best fit sphere(M), with/against the rule

astigmatism (J0) and oblique astigmatism (J45) in Dioptres, relative to baseline

day with the four contact lenses for the 4 and 6 mm corneal diameter............ 90

Table 3-6: Mean changes in HO RMS, 2nd, 3rd and 4th order RMS, relative to

baseline day with the four contact lenses for 4 mm (n=14) and 5.5 mm (n=10)

pupil diameters. „n‟ is the number of subjects included in the analysis. ........... 91

Table 3-7: Mean distances of contact lens centre to limbus centre (mm) and

ranges (mm) in the horizontal and vertical directions for the three types of rigid

contact lenses. ............................................................................................... 92

Table 4-1: Details of the lenses used in the study. ....................................... 102

Table 4-2: Mean changes in anterior and posterior axial corneal curvatures

relative to baseline days with the two lens types in the central and peripheral

regions. ........................................................................................................ 107

Table 4-3: Mean corneal thickness changes relative to baseline days with the

two contact lens types in central and peripheral corneal regions. ................. 109

Table 4-4: Mean changes in best fit sphere (M), with/against the rule

astigmatism (J0) and oblique astigmatism (J45) in dioptres, relative to baseline

day with the two lens types for the 4 and 6 mm corneal diameters. .............. 110

Table 4-5: Mean changes in HO RMS, 3rd and 4th order RMS, relative to

baseline day with the two lens types for 4 and 5.5 mm pupil diameters. ....... 110

Table 4-6: Mean distance of contact lens centre to limbus centre (mm) and

range in the horizontal and vertical directions for the two lens types. ........... 111

Table 5-1: Grades of horizontal length and sagittal width staining of lid-wiper.

Grading was done using both fluorescein and lissamine green. ................... 124

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Table 5-2 Lid-wiper epitheliopathy classification system for final score as

described by Korb et al. (2005). ................................................................... 124

Table 5-3: Changes in upper tarsal conjunctival staining (relative to baseline) in

morning and afternoon. Positive values mean that tarsal staining has increased

compared to baseline. Note: The increase in staining in the mornings following

approximately 45 minutes of lens wear. ....................................................... 126

Table 5-4: Changes in upper tarsal conjunctival staining in the afternoon

(relative to morning). Positive values mean that tarsal staining has increased

compared to morning. .................................................................................. 126

Table 5-5: Changes in lid-wiper staining grade in the afternoon (relative to

morning). Positive values indicate increased staining compared to morning. 128

Table 6-1: Dry eye screening tests, screening criterion and mean scores of the

study subjects. Subjects who failed 2 or more dry eye tests were not included in

the study. ..................................................................................................... 138

Table 6-2: Description of the lenses used in the study. ................................. 140

Table 6-3: Mean change in TFSQ values relative to baseline, with the four

contact lenses in the morning and afternoon, in natural blinking conditions over

a period of 30 seconds. Negative values of TFSQ indicate that TFSQ is worse

with contact lenses. ...................................................................................... 144

Table 6-4: Mean change in TFSQ values in the afternoon relative to morning,

with the four contact lenses in natural blinking conditions. Negative values of

TFSQ indicate that TFSQ is worse in the afternoon. .................................... 144

Table 6-5: Mean changes in TFSQ values in suppressed blinking conditions

relative to baseline. ...................................................................................... 146

Table 6-6: Analysis of recordings showing an increase in TFSQ value with time.

..................................................................................................................... 148

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Abbreviations

PMMA – Polymethyl methacrylate

RGP – Rigid gas permeable

HEMA – Hydroxyethyl methacrylate

SiHy – Silicone hydrogel

TFSQ – Tear film surface quality

PLTF– Pre-lens tear film

PCTF – Pre-corneal tear film

Dk – Oxygen permeability

Dk/t – Oxygen transmissibility

BOZR – Back optic zone radius

FOZD – Front optic zone diameter

BOZD – Back optic zone diameter

WTR – With-the-rule

ATR – Against-the-rule

M – Best fit sphere

J0 – With/against the rule astigmatism

J45 – Oblique astigmatism

RMS – Root mean square

HORMS – Higher order root mean square

Ortho-k – Orthokeratology

LC – Limbus centre

PA – Palpebral aperture

VK – Videokeratoscope

TBUT – Tear break-up time

NITBUT – Non-invasive tear break-up time

ROI – Region of interest

AOA – Area of analysis

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Statement of original authorship

The work contained in this thesis has not been previously submitted to

meet requirements of a degree or diploma or for any other higher

education institution. To the best of my knowledge and belief, the thesis

contains no material previously published or written by another person

except where due reference is made.

Signature:

Date:

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Acknowledgements

It gives me great joy and satisfaction to submit this work as a doctorate thesis

and I would like to thank the people who have been instrumental in making this

dream possible.

This work has been possible due the support, guidance, motivation and

constant encouragement provided by my supervisor, Professor Michael Collins.

I have been very fortunate to have him as my supervisor and I sincerely thank

him for being the best supervisor.

I am grateful to Dr Scott Read, my associate supervisor, for his time,

help and involvement which have been invaluable to me. I am also very

thankful to my associate supervisor Mr Brett Davis, for his especially useful

ideas and friendly discussions.

I would like to acknowledge the financial support during my PhD

candidature: International Postgraduate Research Scholarship, Queensland

University of Technology and Australian Government, Cornea and Contact

Lens Society of Australia awards for two years and the Faculty of Health

research scholarship, Queensland University of Technology.

I acknowledge Dr Robert Iskander for the analysis software used in my

studies. Thanks to Ms Shila Roshani for help with analysis of digital images and

Dr David Alonso-Caneiro for help with analysis of videokeratoscopy recordings.

Special thanks to each and every participant of my studies for their help

and patience and for completing the studies with a smile. Thanks to everyone

in our lab for their company and for making the last three years special and

memorable for me.

Finally, I wish to thank my husband, Ankit, for the encouragement and

understanding. A big thank you to everyone in my family, who mean a lot to

me, for always being there for me. This thesis is dedicated to my parents for

their unconditional love, support and encouragement.

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- 1 -

Chapter 1

Literature Review Contact lenses are commonly used for the correction of refractive errors of the

eye. These lenses are placed on the anterior surface of the eye and are

therefore in direct contact with the ocular structures in front of and behind the

lenses. In the following Sections these ocular structures and the effects of

contact lenses on these structures, are discussed.

1.1 Eyelid: structure and functions

The eyelids protect the eyes from injury and excessive light. The upper and

lower lids join at the nasal and temporal canthi and are separated by an

elliptical opening called the palpebral aperture. They are also responsible for

the even distribution of tears over the ocular surface during blinking.

The eyelid margins play an important role both in the formation and

distribution of the tear film across the ocular surface (Lemp et al. 1970; Guillon

and Guillon 1994). They contain the openings of meibomian glands (Figure 1-

1), and secrete the outermost lipid layer of the tear film, preventing the inner

aqueous layer of tears from evaporating quickly (Craig and Tomlinson 1997).

Additionally, the portion of the palpebral conjunctiva adjacent to the lid margin,

referred to as the “lid-wiper”, sweeps and spreads the tears over the ocular

surface (Korb et al. 2002; Ruskell and Bergmanson 2007). Disruption of this

portion of the conjunctiva observed by staining with fluorescein or rose bengal

dye is termed “lid-wiper epitheliopathy” (Korb et al. 2002; Korb et al. 2005).

During a blink, the upper eyelid does most of the movement whereas

the lower eyelid moves minimally (McCulley 1988). Eyelid closure during a blink

occurs as a result of relaxation of the levator palpebral superioris muscle

(innervated by the oculomotor nerve III) (McCulley 1988), followed by

contraction of the palpebral portion of orbicularis oculi (innervated by the facial

nerve VII) (Figure 1-1). The opening of the lids occurs as a result of the

contraction of levator palpebral superioris muscle, and the Müller‟s muscle

(innervated by the sympathetic nerves) assists in widening of palpebral fissure

during fear or excitement (Forester et al. 2008). The average spontaneous blink

rate is approximately 12-15 blinks per minute (Carney and Hill 1982; Moses

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2

1987) and a normal blink takes about 250 ms to complete (Hung et al. 1977;

Doane 1980).

Figure 1-1: Structure of the eyelid. Redrawn from Bergmanson (2009).

1.2 Tarsal conjunctiva

The conjunctiva is a thin translucent mucous membrane which attaches the

eyeball to the tarsal surface of the lids and also forms the superior and inferior

fornices or conjunctival sacs (McCulley 1988). The conjunctival sacs behave as

tear reservoirs. The conjunctiva is responsible for producing the mucous

component of the tear film from its goblet cells, which makes the otherwise

hydrophobic corneal surface hydrophilic for the formation of the tear film

(McCulley 1988). The conjunctiva is also immunologically active and has a

variety of defence mechanisms against infections (Knop and Knop 2005).

The conjunctiva is divided into 3 parts: tarsal or palpebral, fornicial and

bulbar. The fornicial conjunctiva forms the conjunctival sac and the bulbar

conjunctiva lines the anterior portion of the eye ball from limbus to fornix

(Ruskell and Bergmanson 2007; Forester et al. 2008). The tarsal conjunctiva

lines the tarsal surface of the lids and is firmly attached to the tarsal plate. The

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3

vessels of the tarsal conjunctiva are thought to provide an oxygen supply to the

cornea when the lids are closed (Brandell et al. 1988; Friend and Hassell

1994).

1.3 Tear film: structure and functions

The pre-ocular tear film covering the cornea and conjunctiva, is typically

described as a three layered structure with an estimated thickness of 3 microns

(King-Smith et al. 2004). The superficial lipid layer secreted by the meibomian

glands is important to prevent evaporation of the underlying aqueous layer

(Mishima and Maurice 1961) and contamination of tear film by skin lipids

(McDonald 1968). The middle aqueous layer, secreted by the main lacrimal

gland and the accessory glands of Krause and Wolfring (Figure 1-1) contains

dissolved ions and proteins. The inner mucous layer of the tear film secreted by

the goblet cells of the conjunctiva and crypts of Henle, helps in lubrication and

protects the epithelial surfaces. Thus the tear film plays a vital role in

maintaining a healthy and functional ocular surface and visual system.

A healthy tear film regulates the small irregularities in the corneal

surface to provide a smooth and regular optical surface, moistens the ocular

surface and minimizes friction for movement of the eyelids and therefore

prevents the desiccation of ocular surface cells. It flushes the cellular debris

and foreign matter towards the caruncle for removal. It also contains

antibacterial components and antibodies (e.g. lysozyme, secretory IgA) and

acts as the first line of defense against the ocular infections (McClellan et al.

1973; Farris 1985).

1.4 Cornea

This section discusses the anatomy and physiology of the cornea.

1.4.1 Corneal anatomy

The cornea is an avascular, transparent, richly innervated surface tissue of the

eye which encloses (together with sclera) the delicate internal ocular structures.

The surface area of the cornea is estimated to be 1.1 cm2, which is about 7% of

the surface area of the globe (Maurice 1984).

The cornea consists of five distinct layers: the epithelium, Bowman‟s

layer, stroma, Descemet‟s membrane and endothelium (Figure 1-2). The

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4

epithelium represents about 10% of the corneal thickness and forms a cellular

barrier to minimize fluid loss and entry of substances and pathogens into the

eye (Klyce and Beuerman 1998). Bowman‟s layer, also known as the anterior

limiting membrane, is composed of randomly oriented groups of fine collagen

fibrils that merge into the more organized anterior stroma. The stroma

represents 90% of the corneal thickness. It mainly consists of collagen fibrils

embedded in a matrix of proteoglycan which are more regularly arranged in

posterior stroma than in the anterior stroma (Klyce and Beuerman 1998).

Descemet‟s membrane is the basement membrane of the corneal endothelium

and is secreted by the endothelium. The endothelium is a single layer of

squamous cells with a density range of 1,400 to 3,400 cells/mm2 in adults

(Sturrock et al. 1978; Klyce and Beuerman 1998). It plays an important role in

maintaining corneal deturgescence that is the relative state of dehydration

required for maintaining corneal transparency (Edelhauser et al. 1994).

Figure 1-2: Layers of the cornea. Thicknesses as described by Gipson (1994).

1.4.2 Corneal physiology

The cornea maintains its transparency, cell reproduction, temperature, and

transport of tissue materials through constant metabolic activities. It requires a

constant supply of oxygen, glucose and amino acids for these functions. When

the eyes are open, oxygen is mainly supplied to the cornea by the pre-corneal

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5

tear film from the atmosphere by the process of diffusion. In the closed eye

condition, the oxygen level in the tears is in balance with the palpebral

vasculature (Brandell et al. 1988; Friend and Hassell 1994). Glucose, amino

acids and other nutrients are provided by the aqueous humour (Friend and

Hassell 1994).

Normally, the corneal uptake of glucose from the aqueous humor is

balanced by the loss of corneal lactate (O'Neal and Bonanno 1994). However,

during some forms of corneal metabolic stress or when the oxygen supply to

the cornea is reduced, for example during sleep, the rate of lactate production

increases due to the cornea being more reliant upon anaerobic metabolic

processes. This may result in epithelial edema which can alter the cellular

refractive index and produce Sattler‟s veil phenomenon which is often reported

as increased glare and halos surrounding bright lights (Dallos 1946). Stromal

edema that occurs during hypoxia is also a result of excess osmotic solute load

caused by lactate accumulation. Finally, increased metabolic lactate production

can cause localized stromal acidosis which can alter endothelial morphology

and function. All of these changes can compromise the optical properties of the

cornea (Kwok 1994).

1.5 Shape of the cornea

The cornea is the most powerful optical surface of the eye, and provides two-

thirds of the total refractive power of an unaccommodated eye (approximately

42 D), which is mostly due to the change in refractive index at the air-tear

interface present over the cornea (Guillon and Guillon 1994). Therefore, even

subtle changes in corneal shape can considerably affect vision. Measurement

of corneal shape forms an important clinical technique for procedures such as

contact lens fitting and refractive surgery. In addition, evaluating the sequential

changes in corneal topography with time has an important role in monitoring

corneal pathologies, contact lens-induced changes (Wilson et al. 1990), corneal

refractive treatments (e.g. orthokeratology) (Lui and Edwards 2000), diagnosis

and management of keratoconus (Schwiegerling and Greivenkamp 1996) and

the management of surgically induced astigmatism (Holck et al. 1998).

Given its importance and accessibility, corneal shape and other

parameters have been extensively investigated. The average corneal diameter

horizontally is 12.6 mm and vertically is about 11.7 mm (Klyce and Beuerman

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6

1998). The radius of curvature of the anterior surface of the central cornea is

about 7.8 mm whereas for the posterior surface it is around 6.5 mm (Atchison

and Smith 2000). Due to this difference in the curvatures of the two surfaces,

the cornea shows a variation in thickness from centre to periphery. The central

corneal thickness is approximately 520 µm which increases to about 650 µm or

more in the periphery (Klyce and Beuerman 1998). The mean refractive index

of the cornea is 1.376 (Gullstrand 1909; Atchison and Smith 2000).

The anterior corneal surface often exhibits toricity, which may result in

overall ocular astigmatism. The anterior corneal surface is generally steeper

along the vertical meridian than along the horizontal meridian in young eyes

(Hayashi et al. 1995; Goto et al. 2001). This is often referred to as “with-the-rule

(WTR)” astigmatism. With age, the cornea changes its shape such that the

horizontal meridian becomes steeper. This is referred to as “against-the-rule

(ATR)” astigmatism (Atchison and Smith 2000). Grosvenor (1978) suggested

that the band-like pressure on the cornea due to tightness of the lids (especially

upper lid) in young adults may result in corneal steepening along the vertical

meridian, causing WTR astigmatism and as the lid tension decreases with age

there is a tendency towards ATR astigmatism.

The corneal surface is also aspheric, that is, it flattens progressively

from centre to periphery. The surface can be mathematically expressed as a

conicoid using the equation: h2 + (1 + Q) Z2 – 2 ZR = 0, where the Z axis is the

optical axis, R is the apical radius, Q is the asphericity constant and h2 = X2 +

Y2, where X and Y are horizontal and vertical Cartesian coordinates. Q

determines the way the shape of a surface will change away from the apex and

is a sphere when Q = 0, oblate (aspheric surface that steepens away from the

apex) when Q > 0 is and, prolate (aspheric surface that flattens away from the

apex such as cornea) when 1 < Q < 0 (Kiely et al. 1982). The asphericity

constant Q varies from 0.30 to 0.18 (Atchison and Smith 2000). An aspheric

corneal surface helps in reducing the spherical aberration of the eye (Kiely et

al. 1982).

Another common method to describe corneal shape mathematically, is

to use Zernike polynomials (Schwiegerling et al. 1995). An advantage of using

these functions is that they are orthogonal over a unit circle, that is, the

coefficients are independent of each other and are not affected by addition or

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deletion of terms used for describing the surface. Zernike polynomials are also

commonly used to describe corneal wavefront aberrations.

1.5.1 Current methods of measuring corneal topography

This section briefly describes the various techniques that have been used to

measure corneal shape.

1.5.1.1 Keratometry

Keratometry is one of the oldest and most common methods for measuring

corneal curvature. A keratometer measures the anterior curvature of the central

cornea. The optical principle of keratometry involves a relationship between the

object size and the image size reflected from the cornea (Purkinje image I)

which acts as a convex mirror (Wilson and Klyce 1991). The radius of curvature

of the cornea is then calculated based upon the object size and the distance

between the image and the object. It uses a standard keratometric refractive

index of 1.3375 to convert these values to dioptric power.

The keratometer assumes the cornea to be sphero-cylindrical and

measures anterior corneal radius of curvature at two locations 2.5 - 4.0 mm

apart in the central cornea (depending on the corneal power) along each of the

two orthogonal principal meridians with maximum and minimum power

(Mandell 1988; Klyce and Wilson 1989). It provides a good estimate of central

curvature for a normal cornea, which is nearly spherical. However, it is not

accurate for measuring aspheric or irregular surfaces and provides no

information about corneal shape in the periphery. This is a major drawback of

keratometry which has led to development of more advanced instruments

providing detailed information of peripheral corneal topography.

1.5.1.2 Photokeratoscopy and videokeratoscopy

Photokeratoscopes are instruments to assess the curvature and topography of

the anterior surface of the cornea. They work on a principle similar to the

keratometer in which the cornea behaves as a convex mirror (Wilson and Klyce

1991). However, unlike keratometers, photokeratoscopes normally consist of

an alternating pattern of black and white illuminated concentric rings (called a

Placido Disc pattern), which is reflected from the cornea (Figure 1-3b), and is

imaged by the camera. The image of these concentric rings reflected from the

cornea is then compared with the rings of the instruments‟ Placido disc pattern

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to determine the slope of corneal surface at various locations across the cornea

(Schwiegerling et al. 1995; Seitz et al. 1997). The optical power of the cornea is

computed from the derivatives of the corneal surface slopes (Schwiegerling et

al. 1995). This instrument gives the corneal topography of both the central and

peripheral cornea unlike a keratometer.

Videokeratoscopes are an advanced version of photokeratoscopes and

are equipped with a video camera and image processing computer software to

record and analyze the keratoscope images and display the topographic data

on the monitor (Morrow and Stein 1992). These instruments provide corneal

surface information from a large number of corneal locations using a Placido

disc pattern. The number of data points for determining corneal power can be

as much as 15120 with 32 rings in the Placido disc pattern (Medmont E300

corneal topographer, Figure 1-3) and 300 data points in each semi meridians.

The exact number of corneal data points depends on the target/instrument

design and some features of the individual eye being measured. The target can

be large or small. Larger targets reduce the chances of alignment errors as

they allow larger working distances. Smaller targets (Figure 1-3c) allow a larger

corneal coverage by minimizing the corneal area obscured by eyelashes, nose

and eye brow (Courville et al. 2004). The videokeratoscopes made by different

manufacturers may also differ in the focusing and alignment method and

computer features.

The Placido disc based videokeratoscopes have been shown to be

accurate and repeatable (Cho et al. 2002) with the accuracy reported to vary

from 0.1 D to 0.25 D in terms of corneal power (or 0.018 mm to 0.045 mm of

axial radius of curvature). However, inaccuracies in Placido disc based

instruments may arise due to focusing and centration errors (Seitz et al. 1997)

and corneas with sudden change in slopes (Belin and Ratliff 1996). Modern

videokeratoscopes are equipped with various mechanisms to minimize these

errors, for example range finding devices that are used determine the distance

from corneal apex to the instrument‟s camera and captures the image

automatically when the eye is in good focus and well aligned (Mattioli and

Tripoli 1997).

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Figure 1-3: (a) Medmont E300 videokeratoscope (b) Reflection of Placido disc image from cornea (c) Medmont Placido disc (d) Subject’s eye in position for measurement.

1.5.1.3 Corneal profile topography

Corneal profile photography or an optical slit-scanning mechanism is used by

the Orbscan topographer. This technique allows acquisition of topographic

measurements of the anterior and posterior corneal surfaces, as well as the

anterior lens surface. The Orbscan II is a more recent computerized

topographer which uses the slit-scanning technology in combination with a

Placido disc to measure the corneal curvature (Cairns and McGhee 2005). The

instrument scans across the anterior corneal surface, obtaining 40 slit images

(20 from the right and 20 from the left) in a sequence in 2.1 seconds from an

angle 45 degrees to the instrument axis. It also records eye movements and

reflection data from a Placido disc device at the same time. The data are then

combined to form a three-dimensional anterior and posterior corneal surface.

The instrument provides information in the form of curvature topography maps.

The repeatability of anterior elevation as measured by Orbscan II is reported to

be approximately 2 µm, which is said to decrease towards the periphery of the

cornea (Cairns and McGhee 2005).

1.5.1.4 Scheimpflug imaging

Instruments that use Scheimpflug photography have recently been introduced

for imaging the anterior eye. The Pentacam HR system (Oculus Inc, Wetzlar

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Germany) (Figure 1-4) is an instrument based on this principle that allows

measurement of the anterior and posterior corneal topography, corneal

thickness and anterior chamber depth. It consists of a 180-degree rotating

Scheimpflug camera and a monochromatic, slit light source (blue LED at 475

nm) that rotate together around the optical axis of the eye. It works as a

topographer in that its software creates three-dimensional models of the cornea

based upon cross-sectional images. This instrument has been shown to have

good repeatability for central and peripheral corneal thickness (Barkana et al.

2005; Lackner et al. 2005; O'Donnell and Maldonado-Codina 2005; Amano et

al. 2006; Uçakhan et al. 2006; Khoramnia et al. 2007; Shankar and Pesudovs

2008), anterior and posterior corneal curvature (Chen and Lam 2007; Shankar

et al. 2008) and anterior chamber depth measurements (Lackner et al. 2005;

Meinhardt et al. 2006; Savant et al. 2008; Shankar et al. 2008).

Figure 1-4 (a) Oculus Pentacam system (b) Rotating Scheimpflug camera system (c) Anterior segment image with the Pentacam

1.5.1.5 Other methods of measuring corneal shape

Corneal shape has been evaluated using many other techniques such as raster

photogrammetry (for example – PAR corneal topography system, PAR vision

systems, USA) in which a grid pattern is projected on the cornea and

distortions in the pattern based on the grid projection and camera angles are

analysed to determine corneal elevation data (Belin et al. 1995). Although

these instruments have certain advantages such as they are not dependent on

reflection from cornea (therefore can be used for scarred or irregular corneas)

and can measure larger area of the cornea, they are not as accurate as Placido

disc videokeratoscopes (Tang et al. 2000).

Another method that has been used to measure corneal shape is laser

holographic interferometry (e.g. CLAS 1000, Kerametrics Corp., USA) in which

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the optical path difference between the laser beam illuminated and reflected

from the cornea is used to calculate corneal elevation (Kasprzak et al. 1995;

Naufal et al. 1997). These instruments have not been adopted for widespread

clinical use because it is hard to describe a variety of corneal shapes using a

single interference reference (Courville et al. 2004).

1.5.2 Corneal topographic reference points

Corneal topography can be referenced to a number of points on the cornea

such as the corneal geometric centre and the corneal sighting centre. The ideal

reference point depends on the purpose of corneal topography. If corneal

topography measurement was done for contact lens fitting then corneal

geometric centre is appropriate. Corneal sighting centre (Figure 1-5), which is

where the line joining the object and centre of entrance pupil (line of sight)

intersects the cornea (Mandell 1994), is a more appropriate reference point

when one is interested in the optical effects of the corneal shape. The corneal

sighting centre, on average, has been found to be located 0.21 ± 0.16 mm

nasally relative to the corneal geometric centre (Pande and Hillman 1993).

However, the videokeratoscopic images are usually referenced to a point

where the instrument axis intersects the cornea called the “vertex normal”

(Figure 1-5) which is different from both corneal sighting and corneal geometric

centre. The vertex normal is, on average, 0.38 ± 0.1 mm away from the corneal

geometric centre (Mandell et al. 1995). The relative distances between these

reference points are usually small and therefore will typically have little effect

on corneal topography measurements (Mandell et al. 1995). However, large

misalignments between vertex centre and line of sight may result in

considerable errors when comparing the corneal topographic measurements

with other measures of ocular optics which are referenced to the line of sight,

such as total wavefront aberration measurements (Salmon and Thibos 2002).

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Figure 1-5: Various corneal topographic reference points. Note the misalignment of the videokeratoscope axis from line of sight. Adapted from Mandell (1996).

1.5.3 Classification of corneal topography

The corneal surface is most commonly described as a sphero-cylinder as

measured by a keratometer, but this description is valid only for the central

cornea and provides no information about the peripheral cornea. With the

development of videokeratoscopes, profile topographers and Scheimpflug

topographers, detailed measurement of the corneal surface is possible.

Corneal topography can be described qualitatively as well as quantitatively.

This section discusses different ways of describing the corneal topography.

1.5.3.1 Qualitative descriptors of corneal topography

Modern videokeratoscopes measure corneal shape at a large number of data

points. Therefore, the videokeratoscopes generate various maps to summarize

information such as axial curvature or power, tangential curvature or power,

elevation maps and refractive power maps. The maps can be further classified

into regions (Figure 1-6) such as central, paracentral, peripheral and limbal

zones (Mountford et al. 2004).

A detailed description of the various corneal topographic maps (Figure 1-8) is

as follows:

Elevation map (Corneal height map)

The videokeratoscope uses corneal slope data to derive the height or sag of

the cornea relative to a reference plane (Schwiegerling et al. 1995). Therefore,

this method obscures any localized changes in the corneal height due to the

marked spherical shape of the cornea (Young and Siegel 1995; Schwiegerling

and Greivenkamp 1997). However, the localized changes in height can be

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more easily visualized by subtracting the corneal best fit sphere from the

corneal height data (Young and Siegel 1995; Schwiegerling and Greivenkamp

1997).

Figure 1-6: Anatomical classification of the corneal surface. Adapted from Mountford et al. (2004).

Axial and tangential curvatures

Corneal surface slope relative to the vertex normal can be expressed using an

axial radius or a tangential radius (Figure 1-7). Axial or sagittal radius is defined

as the perpendicular distance from a point on the cornea to the vertex normal

of the instrument. These maps are a reasonable representation of corneal

topography for normal corneas, but they do not highlight localized corneal

changes similar to those seen in conditions such as keratoconus (Bafna et al.

1998; Elsheikh et al. 2007).

The tangential or instantaneous radius of curvature is defined as the

mathematical radius of curvature for the surface. It is only dependent on the

local curvature and its radius is independent of any axis. Therefore, it is more

sensitive to show localized changes in radius such as those in keratoconus. A

tangential map shows the position of the corneal apex more accurately than an

axial map.

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Refractive power map

The refractive power maps are generated from the focal length of the cornea

using Snell‟s law applied to paraxial rays intersecting the cornea (Atchison and

Smith 2000). These maps represent the effect of the shape of the cornea on

the optics of the cornea (Bafna et al. 1998).

Figure 1-7: Explanation of axial and tangential curvatures at a point on the cornea. Adapted from Mejía-Barbosa and Malacara-Hernández (2001).

Figure 1-8: Illustration of the types of topography maps for a representative subject, as captured by the Medmont E300 videokeratoscope: (a) Axial power (b) Tangential power (c) Refractive power and (d) Elevation

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1.5.3.2 Quantitative descriptors of corneal regularity

Some authors have attempted to represent the regularity of the central corneal

shape using topographic indices (Wilson and Klyce 1991; Smolek et al. 1998;

Chastang et al. 1999). Two such indices are surface regularity index (SRI) and

surface asymmetry index (SAI). SRI is a representation of localized variation in

corneal power as measured over 10 central Placido disc rings of the

videokeratoscope. The lower the value of SRI the smoother is the surface. SAI

is a measure of the central corneal surface power asymmetry. SAI is a centrally

weighted summation of differences in corneal power at points 180° apart over

128 equally spaced meridians (Wilson and Klyce 1991). Again, the lower the

value of SAI, the more symmetric is the surface.

1.5.4 Corneal topography in normal population

Advances in corneal topographers have allowed detailed topography

measurement of not only anterior cornea but also the posterior cornea. This

section discusses the topography of the anterior and posterior cornea as seen

in the normal population.

1.5.4.1 Anterior corneal topography

Knowledge of corneal topography is important for contact lens fitting (Szczotka

1997; Reddy et al. 2000; Szczotka et al. 2002), conventional and customized

laser refractive surgery (Alessio et al. 2000; Knorz and Jendritza 2000;

Ambrosio et al. 2003; Kanjani et al. 2004; Kymionis et al. 2004; Varssano et al.

2004), diagnosis of corneal ecstatic disorders such as keratoconus

(Schwiegerling and Greivenkamp 1996), and to understand the effects of these

abnormalities on vision (Dingeldein and Klyce 1989). Generally, the cornea

flattens in the periphery or is prolate in shape as described in Section 1.5.

However, the topography of the normal cornea exhibits variations across the

normal population. Corneal asphericity also shows variation with the nasal and

superior cornea typically more prolate than the inferior and temporal cornea

(Clark 1974).

A common way of describing the corneal shape is in terms of the best

fitting conic section with radius of curvature R and asphericity Q, as described

earlier in Section 1.5. The mean corneal anterior radius of curvature varies from

7.67 mm to 7.86 mm among various studies (Kiely et al. 1982; Guillon et al.

1986; Eghbali et al. 1995; Atchison 2006; Read et al. 2006; Atchison et al.

2008). Read et al. (2006), using a videokeratoscope, measured corneal

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topography in 92 subjects and found mean corneal radius of curvature of 7.77 ±

0.2 mm and corneal asphericity of 0.19 ± 0.1 when a conic fit was applied to

data from the cornea‟s central 6 mm. Table 1-1 shows the mean values of

anterior corneal radius and asphericity as reported by various studies.

Table 1-1: Mean anterior corneal radius of curvature R and anterior corneal asphericity Q as reported by various authors.

Author (year) Method N Age

range (years)

Conic fit diameter

(mm)

Mean R (mm)

Mean Q

Kiely et al. (1982) Photokeratoscope 88 16 - 80 6 7.72 ± 0.3 0.26 ± 0.2

Guillon et al. (1986) Photokeratoscope and keratometer

110 17 - 60 9 # 7.78 ± 0.3 0.15 ± 0.2

Eghbali et al. (1995) Videokeratoscope 41 23 - 61 6 7.67 ± 0.2 0.18 ± 0.2

Douthwaite et al. (1999)

Videokeratoscope 98 20 - 59 6 7.86 ± 0.2 0.21 ± 0.1

Franklin et al. (2006) Videokeratoscope 15 19 - 36 7 7.72 ± 0.2 0.24 ± 0.01

Read et al. (2006) Videokeratoscope 92 18 - 35 6 7.77 ± 0.2 0.19 ± 0.1

Atchison et al. (2008) Videokeratoscope 106 18 - 69 6 7.75 ± 0.24

0.13 ± 0.14

# The topographer used by Guillon et al. (1986) was capable of measuring up to 9 mm, however the actual conic fit corneal diameter was smaller but not defined. N is the number of subjects.

Some authors have also qualitatively described corneal shape based on

the patterns seen in color coded axial topographic maps generated by the

videokeratoscope. The results of these studies are shown in Table 1-2.

Table 1-2: Distribution of qualitative topographic patterns as reported in various studies with videokeratoscopes

Topographic pattern Bogan et al.

(1990), N = 216 Rabinowitz et al. (1996), N = 195

Kanpolat et al. (1997), N = 114

Round 22.6% 25.1% 14%

Oval 20.8% 37% 11%

Symmetric bowtie 17.5% 21.8% 29%

Asymmetric bowtie 32.1% 10.2% 33%

Irregular 7.1% 5.9% 12%

To make the results consistent between studies, the distribution for superior steep and inferior steep patterns were combined with oval pattern, symmetric bowtie-skewed radial axis pattern was combined with symmetric bowtie pattern and asymmetric bowtie-superior steep, asymmetric bowtie-inferior steep and asymmetric bowtie-skewed radial axis patterns were combined with asymmetric bowtie pattern. N is the number of subjects.

1.5.4.2 Posterior corneal topography

The posterior corneal surface is optically less powerful than the anterior surface

due to the small difference in the corneal and aqueous humour‟s refractive

indices. It exhibits toricity (Dunne et al. 1991) and is thought to change in shape

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before the anterior corneal surface in disorders such as keratoconus (Mannis et

al. 1992; Tomidokoro et al. 2000) and therefore its topography can aid early

detection. Dunne et al. (1992) reported that the posterior corneal surface is

more toric than the anterior corneal surface. Accurate in-vivo measurement of

the posterior corneal shape is complicated due to the magnification and

distortion produced by the anterior cornea. Various methods have been

employed to measure posterior corneal shape such as Purkinje imaging

(Royston et al. 1990; Dunne et al. 1992; Garner et al. 1997; Lam and

Douthwaite 1997), combination of videokeratoscopy and pachymetry (Patel et

al. 1993; Edmund 1994; Lam and Douthwaite 1997), combination of slit

scanning and Placido disc (Liu et al. 1999; Módis et al. 2004; Patel et al. 2008),

and Scheimpflug photography (Dubbelman et al. 2002; Dubbelman et al. 2006;

Read et al. 2007; Uçakhan et al. 2008).

The Purkinje imaging technique is applicable only to perfectly spherical

surfaces, so was incapable of measuring corneal asphericity, which was then

estimated by combining the videokeratoscopic and pachymetry data (Patel et

al. 1993; Edmund 1994; Lam and Douthwaite 1997). The method of combining

videokeratoscopic and pachymetry data is susceptible to errors due to

misalignment and is rather time consuming.

Scheimpflug imaging on the other hand, is a newer technique that has

been applied to the measurement of posterior corneal topography (Shankar et

al. 2008). It eliminates any alignment errors as the anterior corneal curvature,

corneal thickness and posterior corneal curvature measurements are taken

simultaneously, accelerating the measurement procedure (Brown 1973).

Standard Scheimpflug imaging however, requires correction of the distortion

due to the geometry of the system and the refraction of anterior corneal

surface, which can otherwise lead to erroneous measurements of posterior

corneal shape (Dubbelman and Van der Heijde 2001; Dubbelman et al. 2005).

Posterior corneal curvature measurements reported by various authors are

summarized in Table 1-3.

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Table 1-3: Mean posterior corneal radius of curvature R (mm) and asphericity Q as reported by various authors. V and H represent vertical and horizontal meridians, respectively.

Author (year) Method N Age (yrs)

(years)

Mean R (mm) Mean Q

Royston et al. (1990) Purkinje imaging 5 - V = 6.42 -

Dunne et al. (1991) Purkinje imaging - V = 6.4 -

Patel et al. (1993) Videokeratoscopy and pachymetry

20 19 - 23 V = 5.80 ± 0.42 H = 5.82 ± 0.40

V = 0.36 ± 0.37

H = 0.48 ± 0.30

Garner et al. (1997) Purkinje imaging 120 16 - 17 V = 6.42 -

Edmund (1994) Videokeratoscopy and pachymetry

- V = 6.71 ± 0.23 V = 0.35

Lam and Douthwaite (1997)

Videokeratoscopy and pachymetry

60 20 # 6.51 ± 0.40 0.66 ± 0.38

Dubbelman et al. (2002)

Scheimpflug imaging 83 16 - 62 6.40 ± 0.28 0.52 ± 0.27

# median, N is the number of subjects.

1.5.5 Variations in corneal topography

Corneal topography has been shown to vary with age and gender and also

show some diurnal variations. It can also be affected by eyelid forces, refractive

error and ocular rubbing. This section discusses how the corneal topography is

affected by some of the above factors.

1.5.5.1 Diurnal variations in corneal topography and thickness

The shape and thickness of the cornea show slight diurnal variation. The

knowledge of these variations in corneal parameters is of importance for any

clinical or research related purposes requiring accurate measurements of these

corneal parameters. The diurnal changes in anterior corneal shape have been

documented by many authors with some differences amongst the studies.

Typically, the anterior cornea is flattest immediately after waking and then

gradually steepens as the day progresses (Reynolds and Poynter 1970;

Rengstorff 1972; Kiely et al. 1982; Read et al. 2005). The rate of corneal

steepening after waking is higher during the first half of the day compared to

the second half (Kiely et al. 1982; Read et al. 2005). Also, the increase in

corneal curvature along the horizontal meridian is more than the increase in

vertical meridian (Reynolds and Poynter 1970; Kiely et al. 1982). On the other

hand, the posterior cornea is found to be steepest immediately after waking,

followed by a gradual flattening during the day. Posterior corneal shape also

shows a slight increase in astigmatism in the morning with steepening of the

vertical meridian (Read and Collins 2009).

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1.5.5.2 Eyelid forces and corneal topography

Eyelid pressure due to reading (Buehren et al. 2003) and other near work

(Collins et al. 2006; Vasudevan et al. 2007) has been reported to cause a

range of corneal topographic (Buehren et al. 2001; Buehren et al. 2003; Collins

et al. 2005; Read et al. 2005; Collins et al. 2006; Collins et al. 2006; Shaw et al.

2009) and astigmatic changes (Read et al. 2007). Read et al. (2007) reported

that their subjects with larger palpebral apertures also had slightly flatter

corneas. They also reported that the angles of the upper and lower lid were

associated with corneal oblique astigmatism (J45).

Monocular diplopia after reading has also been recorded in a few

studies due to changes in corneal topography because of lid positions while

reading (Mandell 1966; Knoll 1975; Ford et al. 1997). Certain eyelid

abnormalities are also associated with changes in corneal topography, for

example ptosis (Holck et al. 1998; Brown et al. 1999; Ugurbas and Zilelioglu

1999), chalazia (Asseman et al. 1965; Rathschuler 1970; Nisted and Hofstetter

1974; Santa Cruz et al. 1997) and lagophthalmos (Goldhahn et al. 1999).

Narrowing and retraction of palpebral fissure have been shown to alter corneal

astigmatism (Wilson et al. 1982; Grey and Yap 1986; Lieberman and Grierson

2000). The narrower palpebral fissure during reading has been found to result

in corneal topographic changes causing slightly increased corneal power

(Shaw et al. 2008) and increased higher order aberrations (Buehren et al.

2005).

Authors studying the corneal topographic changes due to lid forces

have reported wave-like corneal distortions, especially in the areas of lid

contact on the cornea (Buehren et al. 2001; Buehren et al. 2003; Collins et al.

2005; Read et al. 2005; Collins et al. 2006; Collins et al. 2006; Shaw et al.

2009). The topographic changes are greater with longer durations (Collins et al.

2005; Shaw et al. 2009) and for larger angles of down gaze (Collins et al.

2006). Also, tasks involving eye movements affect greater change in corneal

topography than those requiring static fixation (Collins et al. 2006).

1.5.5.3 Refractive errors and corneal topography

Since the cornea is responsible for two-thirds of ocular refractive power, it is

logical to think that it may also have a contribution to the eye‟s refractive status.

Many studies have reported that the cornea becomes steeper with increasing

myopia (Stenstrom 1948; Sheridan and Douthwaite 1989; Goh and Lam 1994;

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Grosvenor and Scott 1994; Carney et al. 1997; Goss et al. 1997; Budak et al.

1999; Atchison 2006) and flattens with hypermetropia (Sheridan and

Douthwaite 1989; Mainstone et al. 1998; Strang et al. 1998; Llorente et al.

2004)

Most studies have reported no change in corneal asphericity with the

degree of myopia (Sheridan and Douthwaite 1989; Pärssinen 1991; Carkeet et

al. 2002; Atchison 2006) or hypermetropia (Sheridan and Douthwaite 1989;

Mainstone et al. 1998; Budak et al. 1999; Carkeet et al. 2002). However,

Carney et al. (1997) and Horner et al. (2000) found an increase in corneal

asphericity with myopia, whereas Davis et al. (2005) noticed a decrease in

corneal asphericity with myopia. Llorente et al. (2004) found that

hypermetropes had more positive corneal asphericity than myopes.

In spite of the inconsistencies among studies reporting changes in

corneal topography with myopia, it appears that refractive error does influence

the shape of the cornea (i.e. myopes have steeper corneas whereas

hypermetropes have flatter corneas than emmetropes). Corneal asphericity

does not seem to be affected by refractive status of the eye. However, it is still

unclear whether the change in corneal radius is the cause or consequence of

altered refractive status of the eye.

1.6 Corneal thickness

Corneal thickness provides an index of corneal hydration and indicates the

metabolic status of the cornea (Hedbys and Mishima 1966). Changes in

thickness indicate the physiological status of the cornea during hypoxia, trauma

and disease (Klyce 1981; Johnson et al. 1985; Mandell et al. 1989; Huff 1991).

There have been many studies reporting the diurnal changes in corneal

thickness. The central corneal thickness is maximum immediately after waking

and gradually decreases throughout the day (Mertz 1980; Kiely et al. 1982;

Feng et al. 2001; du Toit et al. 2003; Read et al. 2005; Read and Collins 2009).

The overnight increase in corneal thickness is about 5.5% (Harper et al. 1996).

The time to reach minimum corneal thickness varies from 5 to 10 hours after

waking (Kiely et al. 1982; Harper et al. 1996) although the largest changes are

noted in the first 1 to 2 hours after waking. The overnight increase in corneal

thickness results because the closure of the lids blocks the atmospheric oxygen

supply to the cornea, thereby inducing anaerobic metabolism and subsequent

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accumulation of lactate. This triggers osmotic influx of water into the cornea,

and increasing its thickness (Efron and Carney 1979; Klyce 1981; Holden et al.

1983). There is a strong correlation in diurnal changes seen in the anterior

(Kiely et al. 1982; Read and Collins 2009) and posterior (Read and Collins

2009) corneal curvatures and corneal thickness, suggesting that a substantial

proportion of the diurnal change in corneal curvature can be explained by the

diurnal change in corneal thickness.

1.7 Contact lenses

Contact lenses are optical devices placed on the cornea, most commonly used

to correct refractive errors such as myopia, hypermetropia, astigmatism and

presbyopia; but they can also be used for cosmetic and therapeutic purposes

(Lazarus 2007). Contact lenses are currently estimated to be used by 125

million people worldwide (Barr 2004). They are the second most common

option to correct refractive errors after spectacle lenses.

Contact lenses can be broadly classified into three main types: hard or

rigid, soft hydrogel and silicone hydrogel contact lenses depending on the

rigidity of the material from which they are made. Today the term “rigid lens”

refers to rigid gas permeable (RGP) lenses, unlike a couple of decades ago

when it was used for lenses made of polymethyl methacrylate (PMMA). PMMA

contact lenses were used in the past but their use is now limited to trial lenses.

These lenses are not gas permeable and therefore, interfere with the normal

metabolic activity of the cornea and can result in hypoxia and corneal edema.

RGP lenses were developed in the 1970s as an alternative to PMMA

contact lenses (DeRubeis and Shily 1985). These lenses permit increased

oxygen to the cornea compared to the PMMA contact lenses by direct

transmission through the lens material and by tear exchange due to lens

movement. RGP contact lenses are more flexible compared to the PMMA

lenses which aids in the tear exchange behind these lenses. Though soft

contact lenses are used more commonly today, RGPs are still widely used for

irregular (Griffiths et al. 1998) and post-surgical corneas (Beekhuis et al. 1991;

Steele and Davidson 2007) when soft contact lenses are not able to sufficiently

improve vision quality, for orthokeratology (Swarbrick 2006), and for general

refractive power correction (Johnson and Schnider 1991; Fonn et al. 1995).

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Soft hydrogel contact lenses are the most popular type of contact

lenses. Due to the lack of rigidity they conform to the shape of cornea providing

an increased level of comfort compared to RGP lenses. One of the earliest soft

contact lens materials available is hydroxyethyl methacrylate (HEMA). These

are stable, hydrophilic materials with good wettability but low oxygen

permeability. Soft hydrogel contact lenses are available in materials of varying

oxygen permeability. These lenses are more comfortable and require very little

adaptation compared to RGP lenses.

Silicone hydrogel (SiHy) contact lenses introduced in the late 1990‟s are

the latest development in soft contact lens materials (Tighe 2002). These

lenses have high oxygen permeability and are more comfortable compared to

the rigid lenses. Being hydrophobic by nature, SiHy lenses (first and second

generation) are treated by application of hydrophilic coatings or the addition of

certain wetting agents to make them hydrophilic. These lenses have very high

oxygen transmissibility and are better suited for extended and continuous wear.

Some of the third generation SiHy lenses do not require any hydrophilic

coatings or internal wetting agent.

1.7.1 Properties of contact lens materials

It is important to have an understanding of contact lens material properties

before we can investigate how they affect the corneal topography and

thickness. This section discusses some of the important contact lens material

properties.

In an open eye without a contact lens, the cornea receives most of its

oxygen from the atmosphere through the tear film, in addition to contributions

from the aqueous humour in the anterior chamber and limbal blood vessels

(Benjamin 1994). However, each of these routes caters to different parts of the

cornea (i.e. posterior corneal layers receive their oxygen through diffusion from

aqueous humour, about 1 mm of peripheral cornea receives it from limbal

vessels and the rest of the anterior cornea receives it from the tear film). When

the eye is closed the atmospheric oxygen is partly replaced by oxygen diffusion

from the palpebral conjunctival vessels (Benjamin 1994).

When a contact lens is placed on the cornea, it acts as a barrier

between the cornea and atmosphere and impedes oxygen supply to the cornea

resulting in hypoxic changes in the cornea (Smelser 1952). Therefore, to

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maintain normal corneal physiology, contact lenses need to be designed in

such a way that they allow constant exchange of lacrimal fluid between the lens

and the cornea, and also allow transmission of oxygen through the lens. The

ability of a contact lens to transmit oxygen to the cornea is described using two

parameters: oxygen permeability and oxygen transmissibility.

1.7.1.1 Oxygen permeability

Oxygen permeability (Dk) is the oxygen transmitted through a unit area of

contact lens material of unit thickness under unit pressure difference (Benjamin

1994). It is the product of the diffusion constant (D) and solubility (k) of oxygen

in the lens (Hwang et al. 1971; Brennan et al. 1987; Brennan et al. 1987;

Weissman and Fatt 1991; Tighe 2004). Dk is measured in the units of (cm/s)

(ml O2/ [ml x mmHg]). Dk is a physical property of contact lens material and is

not affected by contact lens design.

1.7.1.2 Oxygen transmissibility

Oxygen transmissibility (Dk/t) is oxygen permeability per unit thickness (in cm)

of a contact lens under specific conditions (Benjamin 1994). The units of Dk/t

are (cm2/s) (ml O2/ [ml x mmHg]). Dk/t is a physical property of the material and

the thickness and design of the lens. Dk/t is dependent on thickness of the

contact lens, and so varies with the power and design of the lens. Therefore,

Dk/t is more representative of the on-eye performance of the contact lens than

Dk alone.

1.7.1.3 Water content

Water content is a material property. The oxygen permeability of a contact lens

material is influenced by the water content of the material for the hydrogel

lenses. The logarithm of oxygen permeability shows a linear relationship with

the water content of the material (Sarver et al. 1981; Fatt and Chaston 1982).

In hydrogel materials, most of the oxygen diffuses through the water within the

hydrogel material and not through its polymer structure (Benjamin 1994).

However, this linear relationship does not present the true picture of the on-eye

performance of the lens because with increase in water content of the lens

material, there is an associated increase in lens thickness required to fabricate

a lens of a given optical power. The increase in thickness in turn results in a

decrease in Dk/t. For a given lens material, a critical minimum thickness is

required for durability and stability of lens parameters. The critical thickness is

about 0.03 mm for a 38% water hydroxyethyl methacrylate (HEMA) lens, while

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for a 70% water content material it is about 0.12 mm (Brennan and Carney

1987).

Oxygen transmissibility of rigid and silicone hydrogel lenses are not

limited by the water content of the material. The oxygen permeable component

in these materials is usually either silicone (siloxane) and/or a fluoro-polymer.

Although, the incorporation of these components in the rigid lens material is

limited by their inherent flexibility and non-wettability (Benjamin and Bourassa

1989), the rigid and silicone hydrogel lenses provide more oxygen to cornea

than hydrogel lenses. The Dk of rigid lens materials varies from 9 to 63 units

whereas that of silicone elastomer is as high as 104 units. The reason behind

the high Dk of silicone-based materials is that oxygen is more soluble in

silicone than it is in water. Therefore, unlike hydrogels, in silicone-based

contact lenses, oxygen is transmitted through the silicone polymer and the

water content of the material has little effect on the Dk of the material (Tighe

2004). The silicone contact lenses are usually coated to improve their

wettability, but this has negligible effect on their oxygen transmissibility (Refojo

et al. 1982).

1.7.1.4 Modulus of elasticity

This is a mechanical property of the contact lens material. Young‟s modulus of

a contact lens material is a measure of its ability to retain its shape against

forces such as the wrapping associated with the difference in contact lens base

curve and corneal curvature and lid pressure. A hard (or rigid contact lens)

material has high modulus (PMMA ≈ 2000 MPa, RGP (Boston XO) = 1500

MPa) whereas a soft material has lower modulus (Silicone elastomer = 8 MPa,

pHEMA = 0.5 MPa) and therefore, conforms to the shape of the cornea

(Stevenson 1994). In hydrogel lenses, the modulus is largely dependent on

their water content such that the modulus of a hydrogel lens (38% water

content is about 0.64 MPa whereas with 73% water content it is about 0.35

MPa) so that modulus exhibits an inverse relationship with the water content of

hydrogel lenses (Stevenson 1994).

1.7.1.5 Lubricity/coefficient of friction

Friction or coefficient of friction is a measure of the „lubricity‟ of a material, a

term first used by Isaac Newton (Jacobson 1991). Tribology is the study of

friction and lubrication on living tissues (Cher 2003). Lubricity of a contact lens

material is the amount of friction experienced by the eyelid during a blink when

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moving across the lens surface (Cher 2003; Tighe 2004). Plasma treatment is

used for the RGP (Yin et al. 2008; Yin et al. 2009) and silicone hydrogel contact

lenses (Valint Jr et al. 2001; Valint Jr et al. 2001) to increase its wettability or

hydrophilicity thus reducing the friction and improving comfort. The silicone-

based contact lenses which are hydrophobic, are treated to make them

hydrophilic by one of two processes. This is done by either surface treatment

with plasma gas to form an ultra-thin coat or by plasma oxidation to convert the

silicone to silicate. Another way to increase wettability and reduce friction is to

add a wetting agent to the material, for example polyvinylpyrrolidone (PVP)

(Carney et al. 2008).

1.8 Contact lenses and the eyelids

The eyelids constantly interact with the contact lens edges and surface during

each blink during the waking hours. If the contact lens surface is not smooth or

the edges are rough, it can lead to trauma of the eyelid surface or structures. A

range of changes in the eyelids and related structures have been linked with

contact lens wear.

The lid-wiper is a part of the marginal conjunctiva of the upper eyelid

that spreads the tears across the ocular surface or contact lens surface during

blinking (Korb et al. 2002). The lid-wiper consists of conjunctival tissue as well

as stratified squamous epithelium (similar to the cornea) and has thus been

shown to stain with both fluorescein as well as rose bengal/lissamine green

dyes. Lid-wiper epitheliopathy is often seen in contact lens wearers with dry

eye symptoms and in some asymptomatic soft contact lens wearers. It is

identified by an increase in fluorescein and rose bengal staining or lissamine

green staining (Korb et al. 2002; Yeniad et al. 2010). The influence of shorter

periods of contact lens wear upon the presence and severity of lid-wiper

staining and eyelid position is not known.

Blepharoptosis or ptosis is drooping of the upper eyelids which can be

noticed in early adaptive period of rigid lenses and has been documented after

long-term use of PMMA and RGP contact lens wear (Fonn and Holden 1988;

Van den Bosch and Lemij 1992; Fonn et al. 1995). Ptosis in the early adaptive

period is due to lid edema because of mechanical trauma due to lens edge and

usually dissipates as the eye gets adapted to the lens but the changes due to

long-term lens wear are usually persistent (Phillips 2007).

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1.9 Contact lenses and the tarsal conjunctiva

The tarsal conjunctiva is in close contact with the contact lens surface during

lens wear, and therefore can be affected directly by constant friction during

blinks. Redness and roughness of the tarsal conjunctiva secondary to use of

contact lenses have been widely reported in the past (Efron et al. 2001;

Skotnitsky et al. 2002; Maldonado-Codina et al. 2004). Differences in severity

of tarsal conjunctival redness was noted in soft and hard contact lens wearers,

with soft lenses showing greater redness (Korb et al. 1981; Korb et al. 1983).

No differences in palpebral roughness or redness between long-term high-Dk

silicone hydrogel contact lens wearers and non-lens wearers have been

reported (Covey et al. 2001).

Long term contact lens has also been reported to lead to a condition

called “contact lens papillary conjunctivitis (CLPC)” which manifests as

inflammatory changes in the tarsal conjunctiva (Allansmith et al. 1977;

Allansmith et al. 1978; Korb et al. 1980; Efron 1999). Silicone hydrogel lenses

are commonly associated with CLPC (Skotnitsky et al. 2002). However, CLPC

can occur with both soft and hard contact lenses especially in long-term contact

lens wearers (Allansmith et al. 1977; Maldonado-Codina et al. 2004).

1.10 Contact lenses and the tear film

As described in Section 1.3, the tear film plays a vital role in maintaining a

healthy, normal, functioning cornea. Dry eye symptoms are reported in about

50% of contact lens wearers (Doughty et al. 1997; Begley et al. 2000; Nichols

et al. 2002) and contact lens-related dry eye is frequently seen in the clinical

setting (Lemp 1995; Begley et al. 2000; Begley et al. 2001). When a contact

lens is placed on the cornea it divides the tear film into pre- and post-lens tear

films (Figure 1-9). The pre-lens tear film is thinner than the pre-corneal tear film

(Wang et al. 2003; Nichols and King-Smith 2004).

Normally, the oily lipid layer is spread over the aqueous layer by the

blinking action of the eyelids. But in the presence of a contact lens the lipid

layer rests on a considerably thinner aqueous layer and there is also a

disturbance in the smooth ocular surface over which the lids must move to

spread the tears (Guillon 1986). Contact lenses also result in changes in the

composition of the tear film due to altered rate of tear flow (Tomlinson 1992).

The tear film osmolarity is initially reduced (due to reflex tearing) followed by an

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increase in long term contact lens wearers either due to increased evaporation

as a consequence of disturbed lipid layer (Tomlinson and Cedarstaff 1982;

Iskeleli et al. 2002), or due to a decreased tear production as a consequence of

reduction in corneal sensitivity (Gilbard et al. 1986). Changes in mucin

production has also been reported in contact lens users (Greiner and

Allansmith 1981).

The lipid layer is an effective evaporation barrier and is reported to be

altered over the hydrogel contact lens (Guillon 1986; Young and Efron 1991)

and absent in the case of rigid lenses (Guillon 1986). Thinning of the lipid layer

is associated with an instability of the tear film in contact lens wear as indicated

by reduction in tear break-up time (TBUT) (Young and Efron 1991; Guillon and

Guillon 1994) and development of corneal staining (Guillon et al. 1990). The

average thinning rate of the pre-lens tear film has been shown to be less

compared to the pre-corneal tear film (Nichols et al. 2005).

Figure 1-9: Pre- and post-lens tear films and a contact lens on the cornea. Pre- and post-lens thickness values by King-Smith et al. (2004).

Studies have reported tear film surface quality using invasive and non-

invasive techniques. Bhatia et al. (1993) found mean pre-corneal fluorescein

TBUT in RGP contact lens wearers reduced significantly if the lenses were

worn for more than 8 hours per day. The pre-lens non-invasive tear break-up

time (NIBUT) in studies has been shown to be as low as 2-3 seconds on RGP

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contact lenses and 5-6 seconds in soft contact lenses (Guillon and Guillon

1993; Morris et al. 1998). The mean NIBUT of the pre-lens tear film of hydrogel

contact lens wearers using a custom instrument was found to be 6.1 seconds

(Faber et al. 1991). Pre-lens NIBUT was found to be 13.7 seconds after 6

hours of hydrogel contact lens use compared to 21.3 seconds at baseline in

tolerant contact lens users (Glasson et al. 2006). Whereas the pre-corneal

NIBUT in symptomatic contact lens wearers is reported to be as low as 3-10

seconds (Guillon and Guillon 1993). Tear film surface quality (TFSQ) has also

been described using high speed dynamic videokeratoscopy. A significant

reduction in TFSQ has been shown with hydrogel and silicone hydrogel contact

lenses compared to bare eye (Kopf et al. 2008; Alonso-Caneiro et al. 2009).

1.11 Contact lenses and anterior corneal topography

Contact lens wear is known to alter anterior corneal topography. The

magnitude of contact lens-induced topographic changes has been found to be

largely dependent on the material, design and duration of wear of the contact

lens. Corneal topography changes have been recorded with PMMA, RGP as

well as soft contact lenses (Miller 1968; Phillips 1990; Wilson et al. 1990; Ruiz-

Montenegro et al. 1993).

Many researchers have studied the effect of different types of contact

lenses on corneal topography and these studies are summarized below. The

studies are classified on the basis of instrument used (i.e. keratometer,

photokeratoscope and videokeratoscope).

1.11.1 PMMA contact lens wear and corneal topography

PMMA contact lens wear can lead to changes in corneal curvature due to the

mechanical forces associated with the wear of a hard lens and due to edema

caused by decreased oxygen supply to the cornea (DeRubeis and Shily 1985).

Studies using a keratometer

Central corneal steepening in 53% of his subjects was recorded by Miller

(1968) with the use of PMMA contact lenses and he correlated the steepening

with increased central corneal thickening due to corneal edema after 1 to 3

months of lens wear. On the other hand, studies measuring central corneal

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curvature after long-term use of PMMA lenses have found corneal flattening

(Rengstorff 1969; Levenson 1983), corneal steepening followed by flattening

(Hovding 1983), deformation (Bonnet and el-Hage 1968) or irregularity

(Levenson 1983).

Studies using a photokeratoscope

An increase in corneal asymmetry and irregularity in PMMA contact lens

wearers was reported by Ruiz-Montenegro et al. (1993) using a topographic

modelling system. They also noticed relative flattening of corneal curvature

under the contact lens and a relative steepening in other parts of the cornea.

These topographic changes correlated with the resting position of the contact

lenses (Wilson et al. 1990). Wilson et al. (1990) suggested that the cornea

begins to return to its original shape after cessation of lens wear.

To summarize the above studies, the central corneal curvature changes

after the use of PMMA contact lenses, showing corneal steepening with short-

term use of these lenses and flattening with long-term use of more than a year.

Local corneal edema in the central cornea could be the cause of corneal

steepening occurring in the early stages of the lens use.

1.11.2 RGP contact lens wear and corneal topography

Studies using a keratometer

Previous studies have found central corneal steepening with long term (3

months to few years) RGP contact lens wear (Bailey and Carney 1970;

Rengstroff 1973). Others have reported central corneal distortion after long

term use of many years (Calossi et al. 1996) or little changes after short-term

wear (DeRubeis and Shily 1985).

Studies using a photokeratoscope or videokeratoscope

An increase in corneal asymmetry and irregularity (Ruiz-Montenegro et al.

1993), altered corneal topographic pattern (Wilson et al. 1990) and a reversal

of normal topographic pattern from prolate to oblate (Maeda et al. 1994) have

been reported with the use of RGP contact lenses. On the other hand, no

significant corneal curvature changes after 21 days of high Dk RGP contact

lens wear using a videokeratoscope has also been observed (Yebra-Pimentel

et al. 2001). A relative flattening of the cornea under a decentred contact lens

and a relative steepening of the rest of the cornea has also been reported with

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RGP contact lens wear (Ruiz-Montenegro et al. 1993; Maeda et al. 1994;

Calossi et al. 1996).

In summary, most of the investigations on RGP contact lenses users

seem to suggest central corneal steepening occurs with these lenses. An

important observation of relative flattening of corneal curvature under a

decentred contact lens and a relative steepening of the rest of cornea have

been made in both PMMA and RGP contact lenses wearers, using the

photokeratoscope.

1.11.3 Soft hydrogel contact lenses and corneal topography

There have been many studies reporting the effect of soft hydrogel contact lens

on corneal topography but there is some inconsistency in the reports.

Studies using a keratometer

Most investigators who have investigated corneal curvature changes with the

use of soft contact lenses for a few months to many years have found central

corneal steepening (Harris et al. 1975; Montés-Micó et al. 2002; Schornack

2003). Montés-Micó et al. (2002) found a mean corneal steepening of up to

0.07 mm after 1 day of lens wear. On the contrary, Hovding (1983) reported

corneal flattening with the use of these lenses for a period of 1 year. Some

authors have noticed initial central flattening followed by steepening (Grosvenor

1975; Barnett and Rengstorff 1977; Hovding 1983) whereas others found no

change (Carney 1972; Sanaty and Temel 1996) after a period of a few months

to years of lens wear.

Studies using a photokeratoscope or videokeratoscope

Carney (1972) and Bailey et al. (1972) have reported corneal thickness

changes but no change in corneal curvature with short and long term use of

soft contact lenses. Mid-peripheral corneal steepening of about 0.5 D (Collins

and Bruce 1993) and steepening of most regions from the central to about 6.6

mm diameter (Yeniad et al. 2003) has been observed in soft contact lens

wearers after the lenses were worn for a period of 3 months to 12 months,

respectively. Ruiz-Montenegro et al. (1993) have reported corneal topography

alterations (change in asymmetry and irregularity) whereas Tomlinson (1976)

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observed inconsistent changes (steepening, flattening or no change) in central

and peripheral corneal topography in a small group of subjects.

In summary, most studies indicate corneal steepening after short and

long term use of soft contact lenses but reports suggesting corneal flattening

after long-term wear are also in the literature. The differences in the results of

changes in corneal curvature with contact lenses are likely to be due to

differences in lens designs, materials and duration of lens wear or time of

measurements. The mechanism of the contact lens induced corneal curvature

changes is unclear. The physical molding of the cornea by the back surface of

the contact lens and contact lens induced corneal edema have most commonly

been suggested as the causes of these changes (Carney 1975; Hovding 1983;

Ruiz-Montenegro et al. 1993).

1.11.4 Silicone hydrogel contact lenses and corneal topography

No studies describing corneal topography changes after short-term (hours or

days) use of silicone hydrogel lenses in open eye conditions are available.

Alba-Bueno et al. (2009) reported no significant changes in corneal topography

in a group of subjects using two different types of silicone hydrogel contact

lenses (first and second generation) worn for a period of 3 months on a daily

wear basis.

Corneal flattening during the first 3 months (which returned to baseline

at the end of 12 months) was observed in subjects fitted with silicone hydrogel

contact lenses on a continuous wear basis (Gonzalez-Meijome et al. 2003).

Santodomingo-Rubido et al. (2005) reported no change in corneal topography

after 6 months use of silicone hydrogel lenses in daily or continuous wear

basis.

1.11.5 Extended wear contact lenses and corneal topography

Extended wear of contact lenses can be defined as “the wearing of lenses

without removal during eye closure, for periods ranging from occasional

overnight wear to 30 days or more of continuous wear” (Phillips and Speedwell

1997). During the 1980‟s continuous use of contact lenses was considered

unsafe and thus this modality was not popular. But it was reintroduced in the

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late 1990‟s after the introduction of improved contact lens materials (Tighe

2002).

Studies using a keratometer

Central corneal steepening has been reported by many authors with the use of

hydrogel contact lenses on an extended wear basis (Binder 1979; Montés-Micó

and Ferrer-Blasco 2002). Iskeleli (1996) reported central corneal flattening with

the use of RGP contact lenses on an extended wear basis for a period of 6

months. Jalbert et al. (2004) found central corneal steepening in low Dk

hydrogel lens wearers and flattening in high Dk silicone hydrogel contact

lenses over a period of 12 months.

Studies using a videokeratoscope

Gonzalez-Meijome (2003) found initial corneal flattening returning to baseline in

subjects using silicone hydrogel lenses for 12 months. Significant corneal

curvature changes (Masnick 1971) and an altered corneal topographic pattern

(Ruiz-Montenegro et al. 1993) was reported in two different groups of subjects

using soft hydrogel contact lenses.

In summary, not many reports are available on corneal topography with

extended wear of contact lenses. Central corneal steepening with the use of

hydrogel contact lenses and central flattening with RGP contact lenses have

been shown. Studies using silicone hydrogel lenses suggested overall corneal

flattening. Jalbert et al. (2004) proposed redistribution of corneal tissue due to

pressure exerted by silicone hydrogel lenses as the cause of corneal flattening

with these lenses. The studies of contact lens induced corneal changes in

open-eye cannot be directly compared to those in closed-eye conditions due to

differences in hypoxia and eyelid pressure under closed-eye conditions.

1.11.6 Toric soft contact lenses and corneal topography

Corneal topography changes with toric soft contact lenses have not been

reported by many authors and no controlled studies are available in the

literature. Some case reports of patients presenting with corneal changes after

use of toric lenses have been presented. Hagan (1998) reported inferior

corneal steepening after soft toric lens wear using a videokeratoscope and

Schornack (2003) reported infero-nasal steepening using a slit-scanning

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topographer in toric soft contact lens wearing subjects. These reports of inferior

corneal steepening could be due to mechanical pressure by the thickened

region of the toric lenses, but the reports were based on just a few subjects.

1.11.7 Time of recovery of corneal changes caused by contact lenses

The time taken by the cornea to return to normal or baseline topographic

values has been investigated by many researchers. This has an important

application in contact lens patients opting for refractive surgery. Corneal and

refractive stability is vital before any such procedure can be performed.

Wang et al. (2002) found that the mean corneal topography recovery

rates in long term contact lens wearers (mean duration of 21.2 years) differed

for different types of contact lenses (soft daily wear 2.5 weeks, soft extended-

wear contact lens 11.6 weeks, soft toric 5.5 weeks and RGP lens 8.8 weeks).

Wilson et al. (1990) suggested that the time to reach corneal stability was up to

6 weeks for soft, 21 weeks for RGP and 6 months for PMMA lenses. Machat

(1996) reported the recovery time for soft daily wear (2-7 days), soft extended

wear (1-2 weeks), and PMMA (4 to 6 weeks) contact lens wearers. Budak et al.

(1999) found no differences in corneal topographic patterns between non-

contact and contact lens wearers after discontinuing soft contact lens wear for

2 weeks and RGP contact lens wear for 5 weeks. Ng et al. (2007) studied time

to stability in full-time soft contact lens wearers and found mean time for

stability of all subjects was about 16 days for keratometry, 28 days for

topography and 35 days for pachymetry after discontinuing the lens wear.

In summary it can be concluded that the time to stability of corneal

topography is proportional to the duration of lens wear. It also depends on the

type of the lens (corneal topography takes longer to recover from PMMA and

RGP contact lens wear compared to soft contact lenses wear), design of the

lens (corneas with toric contact lenses take longer to recover compared to

spherical contact lenses), modality of lens wear (eyes with extended wear

modality take longer to recover compared to daily wear).

1.12 Contact lenses and posterior corneal

topography

The posterior cornea is not affected mechanically by contact lenses. However,

posterior corneal curvature changes have been associated with corneal edema

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(Kikkawa and Hirayama 1970; Lee and Wilson 1981; Erickson et al. 1999).

Previous studies have recorded posterior corneal flattening along with central

corneal swelling associated with low Dk soft (Martin et al. 2009) and low Dk soft

and PMMA lens wear (Moezzi et al. 2004). These studies were conducted in

closed eye or extended wear conditions. The posterior corneal curvature

changes related to daily wear (open eyes) of various contact lenses have not

been studied.

1.13 Contact lenses and corneal thickness

Corneal (central and peripheral) thickness is an important indicator of the

metabolic changes occurring in the cornea with the use of contact lenses. As

mentioned earlier, contact lenses can affect the corneal oxygen uptake, thereby

altering the cornea‟s metabolism. One of the most important indicators of

corneal hypoxia is a change in corneal thickness (Section 1.4.2). The effect of

contact lens wear on corneal thickness has been investigated in many studies

and is discussed in the sections below. Central and peripheral corneal

thickness changes are discussed in separate sections.

1.13.1 Effect of contact lens wear on central corneal thickness

A review of the literature reveals a number of studies documenting the effect of

short and long term use of contact lenses. Contact lens wear of up to 6 hours

results in an increase in central corneal thickness. Fonn et al. (1984) reported

corneal thickening of 5.5% and 2.2% with PMMA and RGP lenses,

respectively. They also observed that thinner RGP lenses cause less swelling

than thicker lenses, and flatter fitting lenses caused less swelling than steeper

RGP lenses. Carney (1974) found an increase in thickness of about 30 to 37

µm subsequent to PMMA contact lens wear for 3 months. Harris et al. (1981)

reported a 4% increase in corneal thickness after 6 hours of hydrogel soft lens

wear under closed eye conditions.

Previous studies reporting the longer term effects of more than 2 years

of contact lens wear have generally found central corneal thinning ranging from

13 to 37 µm with RGP lenses and 3 to 22 µm with soft lenses (Myrowitz et al.

2002; Braun and Anderson Penno 2003; Iskeleli et al. 2006). Liu and

Pflugfelder (2000) found a reduction in corneal thickness of 40 µm in subjects

who had worn soft and RGP contact lenses for more than 5 years. Braun and

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Anderson Penno (2003) and Iskeleli et al. (2006) found a similar magnitude

(difference of 2 µm) of corneal thinning in subjects who had worn RGP and soft

contact lens wearers for 2 to 5 years. Myrowitz et al. (2002), on the other hand,

reported corneal thinning of 3.2 µm with soft and 37 µm with RGP in contact

lens wearers of about 16 years.

In summary, corneal thickness changes appear to relate to the duration

of lens wear. Generally, short-term contact lens wear seems to increase the

central corneal thickness whereas long-term contact lens wear leads to

decrease in central corneal thickness. With long-term contact lens wear, central

corneal thinning is greater with RGP contact lenses compared to soft contact

lenses.

1.13.2 Effect of contact lens wear on peripheral corneal thickness

Carney (1974) reported peripheral corneal swelling of up to 20 µm at the two

peripheral corneal points after 3 months of PMMA contact lens wear. Liu and

Pflugfelder (2000) found corneal thinning of 40 µm in long term (more than 5

years) contact lens wearers at 8 peripheral corneal locations. Yeniad et al.

(2003) studied the corneal thickness at the same 8 peripheral corneal locations

and found that both RGP and soft contact lens wear caused significant corneal

thinning after 6 and 12 months.

Wang et al. (2003) found 3.8% and 3.0% increase in peripheral corneal

thickness with soft (HEMA) and PMMA contact lenses respectively after 3

hours of wear under closed eye conditions. Martin et al. (2008) measured

thickness at 4 peripheral corneal locations and found that high Dk lenses

induced significantly less corneal swelling than low Dk lenses after 1 week of

extended wear.

To conclude, as in the central cornea, short-term lens wear leads to

peripheral corneal swelling whereas long-term lens wear leads to peripheral

corneal thinning. Corneal thickness changes associated with contact lens wear

depend on many factors, including the type and material of lens (such as

PMMA, RGP or soft contact lens), the duration of lens wear (such as short-term

- few hours and long term - many years), the modality of lens wear (such as

daily wear and extended wear) and also on the thickness and fit of the lenses.

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1.13.3 Mechanism of corneal thinning

Long-term contact lens wear has been shown to cause corneal thinning. One of

the mechanisms proposed is a loss of keratocyte cells found in the stroma (Liu

and Pflugfelder 2000). Keratocytes are responsible for the synthesis of

collagen, glycoproteins and proteoglycans which form the mass of the corneal

stroma (Holden et al. 1985). Apoptosis of the keratocytes occurs due to

secretion of mediators such as interleukin-1 from the damaged corneal

epithelial cells (Bonanno and Polse 1985). Loss of stromal tissue could also be

caused by accumulation of lactic acid in the cornea secondary to chronic

hypoxia, resulting in loss of mucopolysaccharide ground substance (Braun and

Anderson Penno 2003).

1.14 Contact lenses and orthokeratology

Orthokeratology is a clinical procedure to temporarily reduce or eliminate

refractive error by using specially designed contact lenses to alter corneal

topography. Orthokeratology gained acceptance and popularity in the mid-

1990s with the advent of the computerized corneal topography, development of

high Dk RGP materials and reverse-geometry lens designs. The typical design

of a reverse-geometry contact lens for myopia correction consists of a central

BOZR that is flatter than the central corneal curvature which flattens the central

corneal curvature by pressure. This technique is most popular for correction of

low to moderate levels of myopia of up to 4 D but has also been attempted for

correction of hyperopia (Gifford and Swarbrick 2008; Gifford and Swarbrick

2009) and with-the-rule astigmatism (Mountford and Pesudovs 2002).

Changes in anterior corneal curvature with reverse-geometry ortho-k

contact lenses have been well studied (Swarbrick et al. 1998; Owens et al.

2004). Significant central corneal flattening most prominent in the central 5-6

mm diameter with ortho-k lenses within one night of wear has been reported

using videokeratoscopy (Swarbrick et al. 1998; Nichols et al. 2000; Swarbrick

and Alharbi 2001; Soni et al. 2003; Owens et al. 2004). These changes have

been shown to occur within minutes after open eye orthokeratology (Sridharan

and Swarbrick 2003; Jayakumar and Swarbrick 2005). Researchers have

presented varying results on changes in posterior corneal curvature with ortho-

k. Tsukiyama et al. (2008) showed evidence that no changes occur in posterior

cornea using a Pentacam and that ortho-k changes are mainly anterior

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whereas posterior corneal flattening in both central and peripheral regions has

been reported by others (Owens et al. 2004).

Corneal curvature and thickness changes have been suggested to

occur due to a redistribution of corneal tissue rather than overall bending of the

cornea (Swarbrick et al. 1998; Choo et al. 2004; Tsukiyama et al. 2008). This

results in central corneal thinning, with the reported thinning from 12 to 24 µm

(Swarbrick et al. 1998; Nichols et al. 2000; Swarbrick and Alharbi 2001; Soni et

al. 2003; Owens et al. 2004) along with mid-peripheral corneal thickening

(Swarbrick et al. 1998; Wang et al. 2003). Most of the corneal thickness change

occurs after one day of ortho-k contact lens wear (Nichols et al. 2000; Soni et

al. 2003). Further evidence for this is provided by authors who have reported

histological changes in animal corneas (Matsubara et al. 2004; Cheah et al.

2008) showing central epithelial thinning with mid-peripheral thickening. These

findings were supported by Choo et al. (2008) who showed that changes in the

cornea are dependent on the design of the lens back surface and duration of

the contact lens wear.

The corneal changes that occur with orthokeratology show a quick

recovery to the baseline values. The recovery is as rapid as the development of

the anterior corneal curvature changes (Gonzalez-Meijome et al. 2008). Haque

et al. (2004) observed a recovery of corneal curvature changes within 3 days of

lens discontinuation whereas Soni et al. (2004) reported full recovery after 1

week of lens discontinuation. The corneal thickness returns to baseline values

even faster. The thickness recovers almost fully just after one night of lens

discontinuation (Soni et al. 2004).

1.15 Rationale

The influence of soft, hard and rigid contact lenses on corneal topography has

typically been limited to the measurement of central anterior corneal curvature

with keratometers. However, the advent of new technologies such as

computer-based videokeratoscopes and Scheimpflug imaging allow

assessment of corneal topography of large corneal areas with many thousands

of data points. Additionally, the magnitude and nature of changes in the

posterior cornea after daily wear (open eyes) of various contact lenses is not

known.

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Similarly, the effects of contact lenses on corneal thickness have been

measured either at the centre of the cornea or at a few locations in the

peripheral cornea along the horizontal meridian. Regional changes in corneal

periphery with soft toric lenses (which have their thickest portions in the

periphery) have never been investigated. Again the use of Scheimpflug imaging

technology allows the thickness of the cornea to be measured across most of

the cornea and also provides information about the shape of the posterior

cornea, a surface which has received little research attention. In the following

series of experiments we plan to use Scheimpflug imaging and

videokeratoscopy to accurately assess the changes in corneal topography and

thickness associated with the short-term wear of contact lenses, considering

both the central and peripheral corneal surface.

Many of the previous studies of the effects of contact lens wear on the

cornea were conducted with only a few subjects and often the lens type,

design, material or duration of lens wear were not strictly controlled. Therefore,

one aim of this project was to provide better control of wearing time and lens

parameters, so as to allow more meaningful comparisons of the influence of a

range of contact lens variables (materials, designs, powers and diameters) on

corneal curvature and thickness. This was achieved by using a randomised

controlled cross-over study design. A range of older (HEMA, PMMA) and newer

(SiHy, Boston XO) lens materials were selected in order to compare the best

available lens materials with high oxygen permeability against some older

materials with poor oxygen permeability. The experiments were conducted over

a period of 8 hours in order to study a range of lenses on each subject, with

care taken to ensure a sufficient wash-out period after each lens wearing day,

before the next lens was worn. In addition to corneal changes, contact lenses

can also affect the eyelids and tarsal conjunctival surface which are in constant

friction with the lens edge and surface. This micro-trauma has been shown to

result in ptosis, lid-wiper epitheliopathy or contact lens papillary conjunctivitis

with long-term lens wear. However, there is a lack of understanding of the

effect of short-term wear of contact lenses of different types on eyelid

structures and this will be investigated by grading the staining of tarsal and lid-

wiper conjunctiva and evaluating the lid position.

The tear film forms an important optical surface of the eye and also

lubricates the ocular surface. Contact lenses are known to disrupt and cause a

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variety of changes to the tear film. New method has been developed to quantify

TFSQ using non-invasive technique which is based on properties of Placido

disc images. This technique has been used to quantity TFSQ with and without

soft contact lenses. But the TFSQ with RGP contact lenses using a non-

invasive technique is not known.

We hypothesized that there are no significant ocular surface changes

with short-term contact lens wear. The following series of experiments therefore

aimed to study the short-term (8 hours) influence of contact lens wear on the

surface of the eye and the eyelids. By carefully controlling the contact lens

variables and the lens wearing times and accounting for natural diurnal

variations, we could directly compare the changes that these contact lens

parameters caused to the anterior eye.

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Chapter 2

Corneal changes following short-term soft contact lens wear

2.1 Introduction

Contact lens wear is known to alter corneal shape and thickness. The anterior

corneal topography can be altered by changes in the mechanical forces (e.g.

orthokeratology) and/or normal metabolism (e.g. induced swelling). Changes in

anterior corneal topography are reported with different types of contact lenses

including poly methyl methacrylate (PMMA) (Hartstein 1965; Miller 1968;

Wilson et al. 1990), rigid gas permeable (RGP) (Bailey and Carney 1970; Ruiz-

Montenegro et al. 1993; Schwallie et al. 1995; Yebra-Pimentel et al. 2001), soft

hydrogel (Carney 1975; Grosvenor 1975; Harris et al. 1975; Collins and Bruce

1993; Montés-Micó et al. 2002; Arranz et al. 2003; Schornack 2003) and

silicone hydrogel (Gonzalez-Meijome et al. 2003) contact lenses. Signs of

corneal topography changes may include central corneal flattening or

steepening, changes in regular or irregular astigmatism, changes in the axis of

astigmatism and loss of radial symmetry (Wilson et al. 1990; Ruiz-Montenegro

et al. 1993) or changes in optical higher order aberrations (Lu et al. 2003).

While these contact lens-induced topography changes are well reported with a

range of different lens materials and designs, no controlled studies

investigating the effect of soft toric contact lens designs on corneal shape and

thickness are available.

Additionally, little is known about the effects of contact lens wear on

posterior corneal shape, with some evidence of posterior corneal flattening

found to be associated with one week of extended wear (Martin et al. 2009)

and 3 hours of closed eye soft and PMMA lens wear (Moezzi et al. 2004). The

magnitude and nature of posterior corneal change associated with daily wear of

various contact lenses is not known. Whilst the posterior cornea makes a

smaller contribution to the eyes total refractive power than the anterior surface,

knowledge of how contact lens wear influences its shape is important for any

research and clinical application requiring accurate and precise posterior

corneal measures, particularly for monitoring corneal changes over time.

Knowledge of the impact of contact lens wear on the posterior cornea may also

be important given recent suggestions that posterior corneal measures may be

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useful in the early detection of corneal ectatic disorders (Rao et al. 2002;

Bessho et al. 2006).

Corneal thickness is an important indicator of the metabolic status of the

cornea (Smelser and Ozanics 1952; Hedbys and Mishima 1966) and hypoxic

stress induced by the contact lenses determines the changes in corneal

thickness with contact lens wear (Bergmanson and Chu 1982). Corneal

swelling has been reported after short periods of wear of soft contact lenses in

open (Bailey and Carney 1973) and closed eye conditions (Harris et al. 1975;

Harris et al. 1981) in the central cornea as well as in closed eye conditions in

the peripheral cornea (Wang et al. 2003). The results of majority of these

studies are based on corneal swelling measurement of a single point in the

centre (Harris et al. 1977) or single points in the periphery (Kaluzny et al. 2003)

to give the peripheral swelling. This does not give an accurate measure of

regional corneal swelling. An important aim of this study was to investigate

regional swelling in corneal periphery since soft toric lenses have their thickest

portions in the periphery of these lenses.

The diurnal changes in corneal thickness are well studied. These

changes are small but significant with the largest changes being obvious in the

morning on awakening (du Toit et al. 2003; Read and Collins 2009) with

significant corneal swelling along with slight flattening of anterior corneal

curvature and slight steepening of the posterior corneal surface (Read and

Collins 2009). Slight thinning of the cornea throughout the day (morning to

afternoon) has also been reported (Feng et al. 2001; Read and Collins 2009).

These natural corneal diurnal variations can potentially confound studies

investigating contact lens induced corneal changes (for both daily and

extended wear conditions) if they are not taken into consideration.

Thus the main aims of this study were to investigate the short-term

effect of different types of contact lenses on the regional distribution and

magnitude of change in corneal thickness and topography in the anterior and

posterior cornea. We also measured the natural diurnal changes in corneal

thickness and curvature on separate days to account for these variations and

provide an accurate measure of the corneal changes related to the contact

lenses.

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2.2 Methodology

2.2.1 Subjects

Twelve healthy young adult (mean age = 26.8 ± 2.9 years; range 21 – 32

years) subjects (5 females and 7 males) with mean spherical equivalent

refractive error of –1.6 ± 2.6 D (range: –6.25 D to 0.75 D) were recruited for the

study. All subjects had a refractive or corneal astigmatism of ≤ 1.50 D and a

best corrected visual acuity of 6/6 or better. Normal tear film, anterior segment

and central corneal thickness [in the range of 475 to 596 microns, based on the

normative values reported by Doughty and Zaman (2000)] were ensured by a

series of preliminary tests. None of the subjects were regularly using any ocular

or systemic medication and none reported any history of ocular injury, infection

or surgery.

One of the subjects was a regular soft contact lens wearer but had

discontinued lens wear at least one month before the start of the study. None of

the subjects were RGP contact lens users. It was calculated based upon pilot

studies conducted before the start of this study, that the sample size of 12

subjects would give 80% power to detect 2.7 microns change in corneal

thickness and 0.01 mm change in anterior corneal curvature at the 0.05 level of

significance. The study was approved by Queensland University of Technology

(QUT) Human Research Ethics Committee and followed the tenets of

Declaration of Helsinki. All subjects signed a written informed consent before

the start of the study (Appendix A).

2.2.2 Instrumentation

The Pentacam HR system (Oculus, Wetzlar, Germany) which uses a rotating

Scheimpflug camera (a digital camera with a slit illumination system) to assess

the anterior segment of the eye was used to measure regional corneal

thickness and anterior and posterior corneal topography. The instrument has a

central fixation target and a monochromatic (blue light emitting diode at 475

nm) slit of light which illuminates the anterior eye of the subject. The “25 picture

3D scan” mode which gives 25 cross-sectional images of the anterior eye was

used for the measurements. It automatically captures images when correct

alignment is attained and provides a measurement of reliability as a „quality

specification‟ (QS) for each 3D scan which checks for alignment, eye

movements and any missing or invalid data. Any unreliable measurements

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were repeated and a total of 5 reliable corneal scans were captured during

each measurement session. Studies have reported the Pentacam to be highly

repeatable for measurements of central and peripheral corneal thickness

(Shankar et al. 2008; Miranda et al. 2009). The Pentacam also shows excellent

repeatability and reasonable accuracy for measuring the anterior corneal axial

curvature (Read et al. 2009), as well as reasonable repeatability for posterior

corneal curvature, especially when the average of a few readings are taken

(Chen and Lam 2007).

2.2.3 Contact lenses

The lenses were empirically ordered according to manufacturer‟s

recommendations (Gelflex Laboratories, Perth, Western Australia) but any

unacceptable fits were reordered to obtain optimal movement and centration.

The lenses used were specifically ordered for the study and were of 4 different

types and were chosen to compare the effects of different lens materials

(hydrogel and silicone hydrogel), designs (spherical and toric) and powers (–

3.00 and –7.00 D) on corneal thickness and curvature (Figure 2-1 and Table 2-

1). For example SiHy/Sph/–3 (lens 1) and SiHy/Sph/–7 (lens 2) are of same

design with different powers whereas SiHy/Sph/–3 (lens 1) and SiHy/Toric/–3

(lens 3) are of different designs (sphere vs. toric) but with the same power. The

toric lenses had no cylindrical power but a spherical power in the optic zone

and a toric stabilized design. The thickest portion of the toric lenses was 0.35

mm which was at the location of the stabilization zones (i.e. at 4 and 8 o‟clock

positions or about 30 degrees below horizontal) (Figure 2-2c). This allowed the

effect of the lens stabilizing design to be studied in the comparison between

SiHy/Sph/–3 (lens 1) and SiHy/Toric/–3 (lens 3), with all other parameters

remaining the same. SiHy/Toric/–3 (lens 3) and HEMA/Toric/–3 (lens 4) had the

same design and power but different materials [i.e. silicone hydrogel (SiHy) and

hydroxyethyl methacrylate (HEMA)] (Figure 2-1).

The lenses used in the study were selected based on a number of pilot

studies in which a variety of contact lenses were worn for varying durations. All

the contact lenses were tested for base curve, power and lens diameter by the

manufacturer. The lenses passed the acceptance tolerances of the

manufacturer, which were centre thickness ± 0.01 mm (when measured dry),

base curve: ± 0.20 mm, lens diameter: ± 0.20 mm, and lens power: ± 0.50 D.

The back vertex power, total diameter and BOZR were also rechecked and

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found to conform to the manufacturers stated tolerances. The contact lens

parameters are described in Table 2-1.

Figure 2-1: The powers, designs and materials of the contact lenses used. The comparisons to investigate the effect of lens characteristics on corneal thickness and curvature are represented by curved arrows. The materials were silicone hydrogel (SiHy) and hydroxyethyl methacrylate (HEMA).

Table 2-1: Details of the lenses used in the study.

Parameter Lens 1 Lens 2 Lens 3 Lens 4

Acronym SiHy/Sph/–3 SiHy/Sph/–7 SiHy/Toric/–3 HEMA/Toric/–3

Design Spherical Spherical Toric Toric

Material SiHy SiHy SiHy HEMA

Power (Dioptres)

–3.00

(±0.50) *

–7.00

(±0.50) *

–3.00

(±0.50) *

–3.00

(±0.50) *

BOZR (mm) 7.7 – 8.9

(± 0.20) *

7.7 – 8.9

(± 0.20) *

7.7 – 8.9 (± 0.20) *

7.8 – 9.0 (± 0.20) *

Total diameter (mm)

14.8 (± 0.20) *

14.8 (± 0.20) *

14.8 (± 0.20) *

14.0 (± 0.20) *

FOZD (mm) 8.00 8.00 8.00 8.00

BOZD (mm) 13.8 13.8 13.8 13.0

Water content 54% 54% 54% 38%

Dk 53 53 53 8-10

Modulus of elasticity (MPa)

0.35 0.35 0.35 0.50

Manufacturing method

Lathe Lathe Lathe Lathe

Surface treatment

Plasma Plasma Plasma None

Centre thickness (mm)

0.11 (± 0.01) *

0.11 (± 0.01) *

0.11 (± 0.01) *

0.11 (± 0.01) *

Edge thickness (mm)

0.10 0.10 0.08 0.09

Manufacturer lens parameter tolerances are given in brackets *. Lens centre thickness was checked by the manufacturer in its dry state. SiHy: silicone hydrogel, HEMA: hydroxyethyl methacrylate (hydrogel), BOZR: back optic zone radius, mm: millimetre, FOZR: front optic zone radius, BOZR: back optic zone radius, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity. The lenses supplied by the manufacturer were checked for back vertex power, lens diameter, BOZR and edge and surface quality after use in the study.

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The contact lens thickness profile was calculated for the soft contact

lenses using the design parameters provided by the manufacturer (Figure 2-2).

The edge of the front optic of higher power –7.00 D spherical lens (Figure 2-2

b) is thicker compared to the –3.00 D lens (Figure 2-2 a). The thickness profile

of the two toric lenses (Figure 2-2 c) shows the thickest portion of the lens at

the location of the stabilization zones i.e. at 4 and 8 O clock or about 30

degrees below horizontal.

Figure 2-2: Contact lens thickness profiles for the SiHy/Sph/–3 (a), SiHy/Sph/–7 (b) and SiHy and HEMA/Toric/–3 (c). The color scale represents lens thickness in mm. The thickness profile for the SiHy and HEMA toric contact lens is identical (c).

2.2.4 Measurements and Protocol

Subjects wore a different type of contact lens in the left eye on four separate

days, for 8 hours each day. All 4 types of contact lenses were worn by each

study participant and the order of wear of the contact lens type was

randomized. New lenses were used for each subject and for each trial. The

lens wearing eye (left eye) was occluded during the 8 hours of lens wear with a

frosted spectacle lens, to avoid any visual discomfort due to aniseikonia. Eight

hours lens wear duration was chosen because it was practical (most students

and staff spent about 8 hours at work/university), we found statistically

significant corneal changes within 8 hours of lens wear in our pilot experiments,

and this short duration allowed us to study a variety of lenses on each of the

subjects along with a sufficient wash-out period between the wearing of

different lenses.

Measurements of the cornea were taken twice daily. Morning

measurements were taken between 8 and 11 am and at least 2 hours after

waking. This was done to avoid the influence of overnight corneal swelling on

the morning measurements. It has been observed that the cornea is the

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thickest immediately after waking in the morning and typically returns to normal

thickness within 2 hours (Kiely et al. 1982; Harper et al. 1996; Read and Collins

2009). A second set of measurements were taken in the afternoon (between 4

and 7 pm). Pentacam measurements were completed within 5 - 10 minutes of

lens removal in the afternoon to avoid any substantial recovery of corneal

changes caused by contact lens wear.

In order to record the individual‟s natural diurnal and between day

variations in corneal thickness and topography two days of baseline

measurements were taken on the left eye without any contact lenses being

worn, both in the morning between 8 and 11 am) and repeated in the afternoon

(between 4 and 7 pm ) after 8 hours.

A two day wash-out period was used after each lens wearing day to

allow for the full recovery of corneal curvature and thickness changes due to

contact lens wear. This wash-out period was chosen based on pilot studies

performed using soft and rigid contact lenses worn for a period of 8 hours. We

found that regression of corneal changes was rapid, with the majority of

changes recovered by the first day after lens wear, while on the second follow-

up day the corneal curvature and thickness had returned to baseline values

(within the limits of measurement error). As a further precaution, for the first two

subjects in the study, measurements were taken on two consecutive days

following each lens wearing day to monitor the regression of the contact lens

induced curvature and thickness changes. We again found that the regression

of corneal changes was rapid and by the second follow-up day the corneal

curvature and thickness returned to baseline values. Based on these findings, a

two day recovery period was scheduled after each day of lens wear and before

the wearing of the next lens, to ensure no persistence of lens related ocular

changes into subsequent lens wearing days.

Given that the subjects were typically at work or university during the

day, there could be some topography changes associated with reading and

downward gaze (Buehren et al. 2003; Collins et al. 2006; Vasudevan et al.

2007). However, a pilot study using contact lenses revealed that these changes

in corneal curvature caused by the eyelids were relatively small in comparison

with the effects of the contact lenses on corneal shape and thickness (See

Figure 2.3). Subjects were also instructed to avoid any significant reading work

(or any other activity involving down gaze), at least one hour before the

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measurement sessions, in order to avoid substantial corneal topography

changes due to reading and down gaze. A questionnaire was completed at the

end of the day to record the type of visual tasks performed during the 8 hours

of lens wear and the subjects were mostly found to be involved in computer

work. The use of computers typically involves about 10 degrees down gaze and

should cause minimal corneal changes compared with reading which involves

20 degrees or more down gaze and leads to eyelid locations closer to the

corneal centre (Shaw et al. 2008; Shaw et al. 2009).

In order to determine the centration of lenses and rotation of the toric

lens in relation to the cornea, digital images of the eye with contact lens in vivo

were captured in the morning on the lens wearing days with a high resolution

digital camera attached to a slit lamp and sufficient time (10 to 20 minutes) was

given for the contact lens to settle in the eye. The subject was positioned on the

head rest of the slit lamp with eyes in primary position and an external white

ring light was used for illumination.

Figure 2-3: Axial curvature difference maps for a subject after 60 minutes of downgaze task in (a) baseline (no contact lens wear) and (b) with a soft contact lens in eye

2.3 Data analysis

2.3.1 Curvature and thickness difference maps

Corneal thickness and axial curvature maps, in the form of a square grid with a

point spacing of 0.1 mm, were exported from the Pentacam. An average of the

5 maps (taken during each measurement session) was calculated using

custom software developed in the Contact Lens and Visual Optics Laboratory,

QUT. Thickness and curvature difference maps were generated to compare the

baseline maps to post-lens removal maps from all the 12 subjects, for each of

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the 4 lens types. „Thickness difference maps‟ were generated by subtracting

the „average baseline thickness‟ map from the „average thickness map after 8

hours of lens wear‟. Similarly, „curvature difference maps‟ were generated by

subtracting the „average baseline curvature map‟ from the „average curvature

map after 8 hours of lens wear‟.

2.3.2 Regional analysis

To study the regional changes in corneal thickness and curvature after contact

lens wear, the data from all the subjects was averaged. The average corneal

thickness and curvature was then calculated for each subject within two corneal

regions (i.e. central and peripheral) as shown in the Figure 2-4. A diameter of 8

mm was selected for this analysis, since data was available for all the subjects

out to this diameter and there were no gaps (e.g. upper lid interference) in the

data.

Figure 2-4: Cornea divided into central (4 mm diameter) and peripheral (4 mm annulus) regions.

2.3.3 Corneal best fit sphero-cylindrical power

Corneal axial power data was used to calculate the best fit corneal sphero-

cylinder in the form of power vectors M (best fit sphere), J0 (with and against-

the-rule astigmatism) and J45 (oblique astigmatism) using the method

described by (Thibos et al. 1997) for the central (0-4 mm) and peripheral (4-8

mm) corneal annulus regions. The root mean square error (RMSE) between

the axial corneal power and best fit corneal sphero-cylinder were also

calculated for each map. The RMSE from the best fit corneal sphero-cylinder

represents the higher order aberrations of the corneal surface height. This is

similar to the optical aberrations of the anterior corneal surface, but represents

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the surface height deviations not the optical path deviations. The best fit

corneal sphero-cylinder was calculated both for anterior and posterior axial

corneal power maps.

2.3.4 Statistical analysis

The data was found to be normally distributed. A repeated measures analysis

of variance (ANOVA) was used to calculate statistical significance of changes

in corneal curvature and thickness due to contact lens wear, with lens type,

region and segment as within-subject factors. Degrees of freedom were

adjusted using the Greenhouse-Geisser correction to prevent any type 1 errors,

where violation of the sphericity assumption occurred. Bonferroni adjusted pair-

wise comparisons were carried out for individual comparisons.

2.3.5 Contact lens centration and rotation

Soft contact lens fit was assessed in terms of corneal coverage, lens centration

(in the horizontal and vertical meridian), lens movement (with blink) and lens

rotation (only for the toric lenses). The grading scale described by Guillon

(1994) was used to assess the lens fit (including corneal coverage, lens

centration and lens movement).

Digital images of contact lens on the eye were also analysed to

calculate the centration of the contact lens in relation to the corneal limbus,

using custom written software (Iskander et al. 2004). A common scale for each

of the digital images was calculated using the Medmont E300

videokeratoscope images to calculate the horizontal visible iris diameter (HVID)

for each subject‟s eye. The distance between the contact lens centre and

corneal geometric centre (centroid of the limbus) was used to calculate the lens

centration on the eye. Lens rotation for the toric contact lenses was calculated

in degrees using the software with average rotation values from two digital

images for each lens for every subject. Figure 2-5 shows a digital image of one

of the subject‟s eye with a toric lens showing the toric lens markings.

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Figure 2-5: Digital image of a soft contact lens (SiHy/Toric/–3) on a subject’s eye. The lens centration for this subject was recorded as 0 (optimal), with less than 0.5 mm decentration. The lens rotation for this lens was calculated using the Imetrics software to be 16 degrees nasal.

2.3.6 Baseline day diurnal changes

To study the diurnal changes in corneal thickness occurring during the 8 hours

of the contact lens wearing period, measurements at the beginning and end of

the 8 hours period on the two baseline days (without contact lenses wear) were

examined. The diurnal difference was analysed by applying repeated measures

ANOVA on the baseline day data.

There were two possible analysis methods that could be used to assess

the changes in corneal thickness and topography associated with contact lens

wear for 8 hours. In the first method, curvature difference maps are generated

by subtracting the curvature map before lens wear (morning) from the curvature

map after lens wear (afternoon). In the second method, curvature difference

maps are generated by subtracting the curvature map of the baseline day

(afternoon) from curvature map after lens wear (also afternoon) (Table 2-2).

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Table 2-2: Methods to study the diurnal changes in corneal curvature and thickness.

Method 1: Difference between afternoon & morning (lens wear day)

Curvature difference map = Curvature map after lens wear (pm) – Curvature map before lens wear (am)

Similarly,

Thickness difference map = Thickness map after lens wear (pm) – Thickness map before lens wear (am)

Method 2: Difference between afternoon (lens wear) and afternoon (baseline)

Curvature difference map = Curvature map after lens wear (pm) – Curvature map of baseline day (pm)

Similarly,

Thickness difference map = Thickness map after lens wear (pm) – Thickness map of baseline day (pm)

It was concluded that method 2 (afternoon of lens wear minus afternoon

of baseline) was a more accurate representation of the corneal changes

associated with the contact lenses because it takes into consideration the

diurnal changes occurring in the curvature as well as thickness of the cornea.

In order to confirm that the diurnal change was appropriately compensated in

the analysis, we studied the following relationship (Table 2-3). This relationship

is also illustrated in the Figure 2-6.

Table 2-3: Relationship studied to check for diurnal changes.

[Thickness map after lens wear (pm) – Thickness map before lens wear (am)]

should equal

[Thickness map after lens wear (pm) – Thickness map of baseline day (pm)] +

[Thickness map baseline day (pm) – (am)]

It is clear from the results in Figure 2-6 that some diurnal variation does

occur in corneal thickness and curvature and that the method we used was

providing an accurate representation of the corneal changes associated with

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contact lens wear, without the influence of diurnal changes in the cornea. Thus

all difference maps in this study were calculated using the second method,

where thickness difference map was generated by subtracting the thickness

map of baseline day (afternoon) from thickness map after lens wear

(afternoon).

Figure 2-6: Diurnal variation in corneal pachymetry analysis. This figure shows thickness difference maps for subject 2, SiHy/Toric/–3 lens.

(a) Thickness difference map = Thickness map in afternoon – Thickness map in morning

(b) Thickness difference map – Thickness map in afternoon – Thickness map in afternoon of baseline day

(c) Normal diurnal change in thickness or Thickness difference map = Thickness map in afternoon – morning of baseline day

(d) Thickness difference map from (b) + Thickness difference map from (c) = Thickness difference map from (a)

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2.4 Results

2.4.1 Diurnal Changes

There was a significant difference between the morning and afternoon

measurements on the baseline days (no contact lens wear). The group mean

diurnal change in corneal thickness showed a significant thinning of 7.9 ± 1.1

microns (p=0.001) in the central corneal region and 9.3 ± 1.7 microns (p=0.001)

in the peripheral corneal annular region. The anterior corneal curvature

exhibited a slight steepening of 0.01 ± 0.02 mm (p=0.49) centrally, which was

not significant and significant steepening of 0.02 ± 0.02 mm (p=0.005)

peripherally. The posterior corneal curvature exhibited a slight flattening of 0.01

± 0.02 mm (p=0.31) centrally, which was not significant and significant

flattening of 0.01 ± 0.01 mm (p=0.007) peripherally. Given the magnitude of

these diurnal changes occurring during the day with no lens wear, especially in

corneal thickness, the corneal changes associated with contact lens wear have

been analysed by taking this diurnal thinning into consideration.

To study the variability in morning measurements (prior to lens wear) on

the six study days (2 baseline days + 4 contact lens wear days), the curvature

and pachymetry measurements on the 6 days for each subject were compared.

A repeated measures ANOVA was applied with „day‟ as the within subject

factor. No significant differences (all p>0.05) were found between the days

suggesting that the morning measurements were not significantly different

across the study days, for all subjects.

2.4.2 Corneal thickness

The type of contact lens had a significant effect on corneal thickness change

after 8 hours of lens wear (p<0.001). There was also a significant interaction

between the type of lens thickness change in corneal annular regions

(p<0.001). Group mean changes in corneal thickness relative to the baseline

days, with the four contact lenses are shown in Table 2-4 and Figure 2-7. The

HEMA/Toric/–3 contact lens caused the greatest level of corneal thickening in

the central (20.3 ± 10.0 microns or 3.5 ± 1.7%, p<0.001) as well as peripheral

cornea (24.1 ± 9.1 microns or 3.7 ± 1.4%, p<0.001) after 8 hours of lens wear.

In post-hoc testing, this change in corneal thickness with the HEMA/Toric/–3

lens was significantly greater than all the other lenses (all p<0.001) used in the

study. The average thickness difference map (lens wear pm minus baseline

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pm) for the HEMA/Toric/–3 shows an obvious regional thickening in the nasal

and temporal edges of the cornea, in the areas beneath the stabilizing zones of

the lens (Figure 2-7 d). However, these regional changes were not obvious in

the average thickness difference map for the other toric lens, the SiHy/Toric/–3

lens (Figure 2-7 c).

The three silicone hydrogel lenses produced similar patterns of change

in corneal thickness (Figure 2-7 a, b, c) The SiHy/Toric/–3, SiHy/Sph/–7 and

SiHy/Sph/–3 lenses on average caused slight central corneal thinning and

minor peripheral corneal thickening, but these changes were not statistically

significant compared with the baseline days (p>0.05).

Figure 2-7: Group mean changes in corneal thickness (mm) relative to baseline days for the four different types of contact lenses. The lenses included different combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D).

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Table 2-4: Mean corneal thickness changes relative to baseline days, with the four contact lenses in central and peripheral corneal regions. Values where pair-wise comparison revealed a significant change from baseline are highlighted with asterisks (p-value ≤ 0.001 is ***). Positive change represents swelling and a negative change represents thinning.

Central Peripheral

Mean change

Lens type µm ± SD % µm ± SD %

SiHy/Sph/–3 (Lens 1) –1.4 ± 6.6 -0.3 2.3 ± 7.0 0.4

SiHy/Sph/–7 (Lens 2) –0.3 ± 6.2 -0.1 3.9 ± 6.2 0.6

SiHy/Toric/–3 (Lens 3) –0.6 ± 5.2 -0.1 4.5 ± 5.5 0.7

HEMA/Toric/–3(Lens 4) 20.3 ± 10.5 *** 3.5 24.1 ± 9.6 *** 3.7

Table 2-5: Mean changes in anterior and posterior axial corneal curvatures relative to baseline days, with the four contact lenses in the central and peripheral corneal regions. Values where pair-wise comparison revealed a significant change from baseline are highlighted with asterisks (p-value ≤ 0.05 is *, ≤ 0.01 is ** and ≤ 0.001 is ***).

Anterior axial curvature Posterior axial curvature

Central Peripheral Central Peripheral

Lens type Mean change ± SD (mm) Mean change ± SD (mm)

SiHy/Sph/–3 (Lens 1) 0.01 ± 0.03 0.03 ± 0.02 * –0.02 ± 0.03 –0.02 ± 0.01 **

SiHy/Sph/–7 (Lens 2) 0.03 ± 0.02 * 0.02 ± 0.02 * –0.03 ± 0.03 –0.02 ± 0.01 **

SiHy/Toric/–3 (Lens 3) 0.01 ± 0.03 0.03 ± 0.02 ** –0.03 ± 0.03 –0.03 ± 0.02 **

HEMA/Toric/–3 (Lens 4) 0.02 ± 0.03 0.02 ± 0.02 –0.07 ± 0.04 *** –0.02 ± 0.02

Positive change represents flattening and a negative change represents steepening.

2.4.3 Anterior corneal curvature

The type of lens had a significant effect on the change in anterior corneal

curvature following lens wear (p<0.001). Table 2-5 and Figure 2-8 show the

group mean change in anterior axial curvature relative to the baseline days,

with the four contact lenses. The anterior corneal surface generally showed

slight flattening after 8 hours of contact lens wear (Table 2-5, Figure 2-8),

except for SiHy/Sph/–3 which caused some localized areas of steepening

(Figure 2-8 a). The three silicone hydrogel contact lenses (i.e. SiHy/Sph/–3,

SiHy/Sph/–7 and SiHy/Toric/–3) caused slight flattening of 0.03 ± 0.02, 0.02 ±

0.02, 0.03 ± 0.02 mm (respectively) in the peripheral annular region of the

anterior cornea which was statistically significant (all p<0.05) compared with

the baseline days.

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Figure 2-8: Group mean changes in anterior axial curvature (mm) relative to baseline days for the four different types of contact lenses. The lenses included different combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D).

2.4.4 Posterior corneal curvature

The type of lens had a significant effect on posterior corneal curvature change

(p<0.001). There was also a significant interaction between the type of lens and

corneal annular region (p<0.001). Group mean changes in posterior axial

curvature, relative to the baseline days, with the four contact lenses are shown

in Table 2-5 and Figure 2-9.

The wear of all contact lenses resulted in posterior corneal steepening

compared with the baseline days, which was more prominent in the inferio-

nasal cornea (Figure 2-9) for the three silicone hydrogel lens types (all p≤0.01

for the peripheral annulus), and was greater in the central region after wear of

the HEMA/Toric/–3 lenses (p≤0.001) (Figure 2-9 d).

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Significant corneal steepening occurred in the posterior corneal

periphery for the 3 silicone hydrogel lenses SiHy/Sph/–3 (0.02 ± 0.01 mm),

SiHy/Sph/–7 (0.02 ± 0.02 mm) and SiHy/Toric/–3 (0.04 ± 0.02 mm) (Figure 2-

9). The HEMA/Toric/–3 lens, on the other hand showed the greatest steepening

in the posterior central cornea of –0.07 ± 0.04 mm (p<0.01) (Figure 2-9).

Figure 2-9: Group mean changes in posterior axial curvature (mm) relative to baseline days for the four different types of contact lenses. The lenses included different combinations of lens material [hydrogel (HEMA) and silicone hydrogel (SiHy)], design [spherical (Sph), toric] and power (–3.00, –7.00 D).

2.4.5 Association between changes in thickness and curvature

To study the association between changes in anterior and posterior corneal

curvatures and corneal thickness, we calculated the linear regression and

significance of the association between each of the study variables. There was

a negative correlation between the change in posterior central corneal

curvature and change in central corneal thickness (R2 = 0.302, p=0.06, F =

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4.468, B = -2.39) which approached significance (Figure 2-10 a). The change

in posterior central corneal curvature and change in peripheral corneal

thickness was significantly negatively correlated (R2 = 0.446, p=0.02, F = 7.44,

B = - 2.86) (Figure 2-10 b). The change in anterior corneal curvature and

posterior peripheral curvature did not show any significant correlation with

change in corneal thickness (all p>0.05).

Figure 2-10: (a) Correlation between changes in posterior (central) corneal curvature with (a) central corneal thickness (b) peripheral corneal thickness. P-values in are shown in red.

2.4.6 Corneal best fit sphero-cylindrical power

Changes in best fit sphero-cylinder for the anterior and posterior corneal axial

powers were analysed for the central and peripheral annular regions (i.e. 0 – 4

mm diameter and 4 – 8 mm diameter). The difference in refractive index at the

anterior (air to cornea) and posterior (cornea to aqueous) corneal surfaces

means that a flattening of the anterior corneal radius leads to a decrease in

anterior corneal power, whereas a steepening of the posterior corneal radius

also leads to a decrease of posterior corneal power. The decrease in axial

power (M) of the posterior cornea was smaller in magnitude than the anterior

cornea due to the refractive index difference at this surface, even though the

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change in radius was greater in the posterior surface (Figures 2-11 & 2-12

respectively).

The change in anterior corneal best-fit sphere (M) was significantly

affected by the type of lens and there was a significant interaction with corneal

annular region (repeated measures ANOVA, both p<0.001). The change in

anterior corneal best fit sphere (M), with/against the rule astigmatism (J0),

oblique astigmatism (J45), and sphero-cyl RMS error is shown in Figure 2-11.

There were significant hyperopic shifts (decrease in refractive power of the

cornea) in best fit sphere (M) for SiHy/Sph/–7 lens in the central cornea (0-4

mm) and for SiHy/Sph/–3, SiHy/Sph/–7, SiHy/Toric/–3 and HEMA/Toric/–3

lenses in the peripheral cornea (4-8 mm). The changes in J45 and sphero-cyl

RMS were not significant for any of the lenses.

Figure 2-11: Changes in best fit sphere (M), with-the-rule and against-the-rule astigmatism (J0), oblique astigmatism (J45) and sphero-cylinder RMSE (from baseline) for the anterior cornea. Significant change indicated by * p<0.05 and # p<0.01. Error bar represents one standard error of the mean. Negative change in M represents a decrease in corneal axial power (hypermetropic shift). Negative change in J0 represents a decrease in WTR astigmatism. Positive change in J0 represents an increase in WTR astigmatism. Positive J45 represents a negative cylinder axis closer to 45° and negative J45 represents a negative cylinder axis closer to 135°.

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The posterior corneal best-fit sphere (M) was also significantly affected

by the type of lens (repeated measures ANOVA, p<0.001). There was also a

significant interaction (p<0.001) between the type of contact lens and corneal

annular region. Changes in posterior corneal best fit sphere (M), with/against

the rule astigmatism (J0), oblique astigmatism (J45), and sphero-cyl RMS error

are shown in Figure 2-12. There was a significant hyperopic shift in best fit

sphere (M) for the HEMA/Toric/–3 lens in the central posterior cornea (0-4 mm)

and for the SiHy lenses (i.e. SiHy/Sph/–3, SiHy/Sph/–7 and SiHy/Toric/–3) in

the peripheral posterior cornea (4-8 mm). There was a significantly greater

hyperopic shift in M in the central corneal region with the HEMA/Toric/–3 lens

(–0.07 ± 0.03 D) compared to the SiHy/Sph/–3 (–0.02 ± 0.02 D) and

SiHy/Sph/–7 (–0.03 ± 0.03 D).

Posterior corneal J0, J45 and sphero-cyl RMS error did not show any

significant changes from baseline with any of the lenses. But there was a

significantly greater increase in sphero-cyl RMS error in the peripheral region

with the HEMA/Toric/–3 lens (–0.03 ± 0.02 D) compared to the SiHy/Sph/–3 (–

0.02 ± 0.02 D) and SiHy/Toric/–3 lenses (–0.02 ± 0.02 D).

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Figure 2-12: Changes in best fit sphere (M), with-the-rule and against-the-rule astigmatism (J0), oblique astigmatism (J45) and sphero-cylinder RMSE (from baseline) for the posterior cornea. Significant change indicated by * p<0.05 and # p<0.01. Each error bar represents one standard error of the mean. Negative change in M represents a decrease in corneal axial power (hypermetropic shift). Negative change in J0 represents a decrease in WTR astigmatism. Positive change in J0 represents an increase in WTR astigmatism. Positive J45 represents a negative cylinder axis closer to 45° and negative J45 represents a negative cylinder axis closer to 135°.

2.4.7 Contact lens centration and rotation

Mean lens centrations for all subjects in the horizontal and vertical direction are

shown in Table 2-6. The mean lens rotations (in degrees) for the two toric

lenses used in this study were 8.0 ± 13.8 degrees nasal for SiHy/Toric/–3 and

5.0 ± 10.1 degrees nasal for the HEMA/Toric/–3 lenses but the difference

between the two lenses was not statistically different (p>0.05).

Table 2-6: Mean lens centrations calculated using custom-written software and digital images of lenses on the corneas

Lens Horizontal centration (x) - mm

Direction Vertical centration (y) - mm

Direction

SiHy/Sph/–3 –0.02 ± 0.16 Nasal –0.02 ± 0.33 Inferior

SiHy/Sph/–7 –0.05 ± 0.1 Nasal –0.08 ± 0.25 Inferior

SiHy/Toric/–3 –0.02 ± 0.16 Nasal +0.05 ± 0.29 Superior

HEMA/Toric/–3 +0.03 ± 0.19 Temporal +0.01 ± 0.23 Superior

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2.5 Discussion

This is the first systematic investigation of the effect of soft toric contact lens

design on corneal thickness and topography. Previous anecdotal reports

discuss cases of patients presenting with blurred vision and inferior corneal

steepening after long term soft hydrogel toric contact lens wear (Hagan et al.

1998; Schornack 2003). In this study, the effect of the thicker lens stabilization

zones on corneal swelling can be clearly observed in the thickness difference

map, where maximum edema can be observed in the corneal periphery at the

locations corresponding to the thickest regions of the soft toric contact lenses

(approximately 4 and 8 o‟clock positions). This suggests that these regions of

the cornea would be most likely to suffer from the negative consequences of

chronic hypoxia. The magnitude of corneal thickness and curvature changes

that we have observed following a short period of soft toric lens wear are

unlikely to influence clinical measures of vision or refraction, however it is likely

that corneal changes associated with longer term toric lens wear may be larger

(Hagan et al. 1998; Schornack 2003). Future research involving controlled

clinical studies of longer term soft toric lens wear is required to improve our

understanding of the nature and magnitude of the longer term corneal effects.

The HEMA/Toric/–3 contact lens caused significant corneal thickening

in the central as well as peripheral cornea (3.5% centrally and 3.7%

peripherally). This corneal swelling was significantly greater than those

observed with all other lenses including the other toric lens (SiHy/Toric/–3). The

difference in corneal swelling between the HEMA/Toric/–3 and SiHy/Toric/–3

lenses is most likely due to the difference in oxygen permeability of the two lens

materials (SiHy, Dk = 53 and HEMA, Dk = 8 to 10), since the lenses were

identical in design. Previous studies have shown a smaller increase in central

corneal thickness of 0.8% (Polse et al. 1976) and 0.5% (Harris et al. 1977) after

8 hours wear but with spherical ultra-thin hydrogel contact lenses. The

increased average thickness of the toric design of HEMA/Toric/–3 lens has

presumably led to a slightly greater amount of corneal swelling in this study

(3.5% centrally and 3.7% peripherally).

We found the magnitude of corneal curvature change associated with

contact lens wear to be greater in the central posterior cornea. This is

consistent with various reports that have shown that the anterior corneal

curvature shows little changes in response to corneal hypoxia, contact lens

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induced or otherwise, and it is the posterior cornea that shows the greatest

changes in curvature in response to corneal edema (Kikkawa and Hirayama

1970; Lee and Wilson 1981; Erickson et al. 1999). This has been attributed to

the differences in the structure (Kikkawa and Hirayama 1970; Komai and Ushiki

1991; Muller et al. 2001; Bergmanson et al. 2005) and composition of the

stroma (Bettelheim and Plessy 1975; Castoro et al. 1988). Reports indicate that

the posterior stroma is capable of swelling more than the anterior stroma at a

given swelling pressure (Kikkawa and Hirayama 1970; Lee and Wilson 1981;

Erickson et al. 1999). The swelling of the posterior cornea was also shown to

be significantly greater in the central region compared to the peripheral region

in rabbit, cat and bovine corneas (Kikkawa and Hirayama 1970). These

differences in the physiological properties of the cornea relate to stromal

structure and composition. The anterior lamellae in the stroma are reported to

be tightly interwoven compared to the posterior stroma in human and animal

corneas (Kikkawa and Hirayama 1970; Komai and Ushiki 1991; Muller et al.

2001). The density of the lamellae in the anterior portion of the stroma is about

50% greater than the posterior stroma in human eye bank corneas

(Bergmanson et al. 2005). There are also differences in the anterior and

posterior stroma in terms of composition of the proteoglycan, causing the

differences in swelling properties of the stroma. For example, keratin sulphate

(a more hydrophilic proteoglycan) is commonly present in the posterior stroma

whereas dermatan sulphate (a much less hydrophilic proteoglycan) is

commonly present in the anterior stroma in bovine corneas (Bettelheim and

Plessy 1975; Castoro et al. 1988).

We also studied the correlation between the corneal curvature and

thickness changes and found that the posterior (central) corneal curvature

showed a negative correlation with the central and peripheral corneal

thicknesses. The change in anterior corneal curvature did not show any

significant correlation with change in corneal thickness in this study, in

agreement with previous studies that have reported substantial changes in

corneal thickness without any change in anterior corneal topography with the

use of contact lenses (Bailey and Carney 1972; Carney 1972; Carney 1975).

The difference in power between the two spherical lenses (SiHy/Sph/–3 and

SiHy/Sph/–7), resulted in greater front optic zone edge thickness in the – 7.00

D lens and hence lower regional and average Dk/t. This difference in thickness

profile did not lead to any significant difference in the amount of change in

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anterior corneal curvature or corneal thickness with these lenses. The mean

peripheral thickening with SiHy/Sph/–7 lens (3.9 ± 2.0 microns) was slightly

greater than the SiHy/Sph/–3 lens (2.3 ± 7.0 microns), but this difference was

not statistically significant. The changes in anterior and posterior corneal

curvatures were also minimal with both of these lenses. This suggests that at

least for short-term silicone hydrogel lens wear, these differences in lens

thickness do not lead to significantly different corneal changes.

To gain better understanding of why the posterior corneal steepening

was greatest in the inferior-nasal corneal region with all the lens types, we

estimated the mean distance of the centre of the topography map from the

geometric centre of the cornea (limbus centroid). A mean offset of 0.36 ± 0.19

mm temporally and 0.05 ± 0.09 mm superiorly was calculated using

videokeratoscope maps of all subjects. It has been shown that the centre of the

topography map from the Medmont E300 videokeratoscope coincides closely

with the centre of the topography map from the Pentacam system (Read et al.

2009). Thus the nasal and inferior corneal steepening can partly be explained

by the offset of the topography map in the same direction.

There were some variations in the amount of swelling induced by the

same lens in different subjects. For example, the average corneal swelling with

HEMA/Toric/–3 lens ranged from 5 to 40 microns (0.8 to 6.5%). This is

consistent with reports in which the amount of corneal edema in response to

hypoxia has been shown to vary from 3.6 to 12.2% between subjects (Sarver et

al. 1983; Efron 1986). Bonanno et al. (2003) demonstrated that inter-subject

variability in corneal swelling is affected by corneal metabolic activity and that

there is an association between corneal swelling and endothelial function.

Therefore the amount of oxygen required to maintain normal corneal

metabolism and avoid edema varies from subject to subject and these inter-

subject differences in corneal physiology are the likely reason for the variability

we observed in corneal edema.

This is the first study to investigate the influence of „open eye‟ daily

contact lens wear upon posterior corneal shape. Whilst statistically significant

changes were observed with a number of lenses, the majority of changes were

small (–0.03 mm and smaller) and unlikely to be of clinical significance. The

largest magnitude of change was observed with the HEMA/Toric/–3 lens, with a

central steepening of the posterior cornea observed (average change of –0.07

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mm). The larger magnitude of posterior corneal change observed with this lens

is likely due to the greater amount of corneal swelling also observed with this

lens. In contrast to our findings, previous studies investigating posterior corneal

change associated with extended wear of soft spherical lenses have noted a

significant flattening of the posterior cornea (Martin et al. 2009). The central

steepening of the posterior cornea that we have observed with the hydrogel

toric lens appears to be related to the regional pattern of swelling with this lens,

where the thickness changes within the central corneal region (4 mm) have led

to central corneal steepening (Figure 2.7d). This difference in corneal swelling

within the central 4 mm zone of the cornea is likely to be due to the reduced

oxygen transmissibility of the thicker peripheral stabilization zones in this lens

causing more peripheral corneal regions to have a higher degree of swelling

than the centre.

We were careful in this study to measure the natural diurnal variation in

corneal thickness and curvature in each of the subjects and then use these

data to measure the true change in the cornea associated with contact lens

wear (factoring out the diurnal changes). This was proven to be important in

arriving at reliable results, since the magnitude of natural diurnal changes in

corneal thickness from morning to afternoon were typically much larger (8 to 9

microns of thinning) than the changes associated with the contact lenses (0.4

to 4.5 microns of thickening), with the exception of the HEMA toric lenses (20 to

24 microns of thickening). The magnitude of corneal thinning that we observed

in the subjects from morning to afternoon without contact lens wear was similar

to that reported by Read et al. (2009).

We found slightly greater swelling in the corneal periphery (3.7 ± 1.7%)

compared to the centre (3.5 ± 1.7%) with HEMA/Toric/–3 lens. This observation

is similar to that of Kaluzny et al. (2003) who also found greater corneal

swelling in the corneal periphery (3.26%) than in the centre (1.54%) after 2

weeks of soft spherical contact lenses used on a daily wear basis. Martin at al.

(2008) found no difference between central and peripheral corneal swelling with

low Dk soft contact lens after 1 week of extended wear. The measurements in

Martin at al. (2008) study were done between 4 and 8 pm to ensure that the

corneal edema induced by overnight eye closure had resolved and that

changes were mostly due to contact lens wear. However many earlier studies

performed under closed eye conditions have reported greater swelling in the

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centre of the cornea compared to the peripheral cornea (Bonanno and Polse

1985; Holden et al. 1985; Herse et al. 1993; Moezzi et al. 2004).

In our study, the greater corneal swelling in the peripheral cornea could

be attributed to thicker peripheral thickness of the negative power lenses and

thicker stabilization zones in the corneal periphery of the toric lenses. It is well

known that decrease in Dk/t (with increase in contact lens thickness) leads to a

greater swelling in the peripheral cornea (Bonanno and Polse 1985; Bonanno

et al. 1986). The greater central corneal swelling observed after eye closure

could be because of lack of atmospheric oxygen, which affects the central

cornea more, while the peripheral cornea is still supplied by the limbal

vasculature. Other explanations for greater central corneal swelling compared

to the periphery, reported in studies under closed eye conditions, is reduced

tear mixing, thereby averaging the oxygen tension under the lens (Bonanno

and Polse 1985; Bonanno et al. 1986) and the physical clamping by the limbus

which limits swelling of the peripheral cornea so that further hypoxia causes

increase only in the centre of the cornea (Maurice and Giardini 1951).

In contrast, Martin et al. (2009) reported a significant amount of central

corneal swelling (2.41%) accompanied by posterior corneal flattening with the

low Dk lenses after 1 week of extended wear. The anterior corneal curvature

steepened slightly but not significantly. In another study Moezzi et al. (2004)

also found posterior corneal flattening but no changes in anterior corneal

curvature in response to corneal swelling (more in the centre than the

periphery) with low Dk soft contact lenses, after 3 hours under closed eyelid

conditions. In the above studies the central corneal thickening may have

caused a backward movement of the central posterior surface resulting in

corneal flattening.

2.6 Conclusion

To conclude, there was an obvious regional corneal swelling apparent

after wearing the low Dk soft toric lenses, due to the location of the thicker

stabilization zones of the toric lenses. The corneal swelling and curvature

changes seen in this study after 8 hours of lens wear are comparable to those

seen after overnight sleep and are not likely to affect the wearer‟s fit, comfort or

vision. The natural diurnal variations in corneal thickness that we measured

from mid-morning to afternoon, were typically larger than the changes caused

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by the silicone hydrogel contact lenses and this factor should be considered in

short-term studies of contact lens induced corneal swelling.

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Chapter 3

Corneal changes following short-term rigid contact lens wear

3.1 Introduction

In Chapter 2, corneal thickness and curvature changes were investigated with a

variety of soft contact lens materials (silicone hydrogel and hydrogel), designs

(spherical and toric) and powers (–3.00 and –7.00 D). The largest changes

were observed following wear of the hydrogel toric lens which caused

significant corneal swelling and these changes correlated with the corneal

curvature changes. We also found significant diurnal changes in these corneal

parameters over the 8 hour duration of the study.

Rigid gas permeable (RGP) lenses, although currently less popular than

soft contact lenses as a refractive correction option, may still offer some

advantages for the wearer. Apart from a superior quality of vision (Johnson and

Schnider 1991; Fonn et al. 1995), these lenses have been reported to cause

fewer complications compared to any other available contact lens type or

modality. RGP lenses have been shown to have the lowest incidence of

microbial keratitis (Stapleton et al. 2008), severe and non-severe keratitis in

daily and extended wear (Morgan et al. 2005), and corneal infiltrative events

(Efron et al. 2005) compared to other types of contact lenses. Dart et al. (2008)

found that these lenses reduce the risk of microbial keratitis by 84% compared

to programmed replacement soft lenses. The incidence of toxic or allergic

reactions to lens care solutions is also reported to be lower with RGP lenses

(Stapleton et al. 1992).

RGP lenses are an important option for conditions such as keratoconus

(Mandell 1997; Griffiths et al. 1998), post laser refractive surgery (Steele and

Davidson 2007), post keratoplasty (Beekhuis et al. 1991) and post trauma

(Kanpolat and Ciftci 1995) where the cornea is irregular and vision is not

satisfactorily corrected with spherical or toric soft lenses. RGP lenses provide

better visual performance compared to soft lenses when fitted to irregular

corneas because the anterior surface of these lenses in-eye is not affected by

the corneal shape. The rigidity of these lenses forms a post-lens tear lens in

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between the cornea and the contact lens which neutralizes the optical

aberrations of the anterior cornea. The refractive index of the tear lens (n =

1.336) being very similar to that of the cornea (n = 1.376), helps in neutralising

the majority of aberrations of the anterior corneal surface. Thus, these lenses

help in providing better visual acuity compared to soft lenses.

Reports of long term (1 month to few years) corneal topographic

changes induced by daily wear of RGP contact lenses are inconsistent, with

some studies reporting corneal steepening (Rengstroff 1973; Sanaty and

Temel 1996), some reporting flattening under decentred lens (Maeda et al.

1994; Calossi et al. 1996), and others reporting no significant changes

(DeRubeis and Shily 1985; Yebra-Pimentel et al. 2001). Yeniad et al. (2003)

noted corneal flattening in the first month of wear, with steepening at 6 months

in a group of subjects using RGP contact lenses on a daily wear basis. There

are no controlled studies in the literature reporting the effect of rigid contact

lenses on corneal curvature after short-term use. These changes are important

to study as RGP lenses can be worn for short-term occasional wear by people

with irregular corneas, who can mostly manage with glasses, but require better

vision for certain activities such as sports. A better understanding of the short-

term variations in the cornea with RGP lenses may also provide insights into

the potential longer term effects of these lenses.

During RGP contact lens fitting, the assessment of the post-lens tear

film using sodium fluorescein (NaFl) is an important component of determining

lens fitting characteristics. It is also a basis for achieving the desired curvature

and refractive results in myopic and hyperopic orthokeratology (Soni et al.

2003; Lu et al. 2007). Some studies in the past have found correlation between

corneal topography and the resting position of the contact lens on the cornea

(Wilson et al. 1990; Ruiz-Montenegro et al. 1993) but no statistical analysis was

performed in these studies. Since the fluorescein pattern is an integral part of

assessing the fit of standard and specialized RGP contact lenses, studying the

association between fluorescein fitting characteristics (i.e. regions of minimum

clearance in the fluorescein pattern) and the resulting corneal shape changes

may provide important insights into the underlying causes of the corneal

changes associated with short-term wear of RGP contact lenses.

The reports of corneal thickness changes due to short-term RGP

contact lens wear are also inconsistent. Fonn et al. (1984) noticed an increase

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in corneal thickness of 1.2 to 4.4% after 6 hours wear of RGP contact lenses of

different centre thickness and fits. Sarver et al. (1977) did not find any

significant changes in mean corneal thickness with RGP lenses after 8 hours of

use. Yeniad et al. (2003) reported an increase in corneal thickness in the first

month, but thinning was seen after 6 months of RGP contact lens use on a

daily wear basis. There are no studies available that have systematically looked

at the influence of factors such as contact lens material (Dk) and diameter on

corneal thickness after short-term use of RGP contact lenses.

Therefore, the aim of this controlled cross-over study is to investigate

the changes in corneal thickness and anterior and posterior corneal topography

with the wear of different types of rigid contact lenses for eight hours. We also

studied the relative influence of different contact lens materials (PMMA, RGP-

Boston XO) and diameters (9.5, 10.5) on the measurements of corneal

topography and thickness, as an extension of Experiment 1. A commercially

available silicone hydrogel lens was also included as a control, to provide a link

to Experiment 1 and to compare the results with a lens used commonly in

current clinical practice.

3.2 Methodology

This study was approved by the QUT university human research ethics

committee (see Appendix A) and followed the tenets of declaration of Helsinki.

All subjects were asked to read the study information sheet and were given an

opportunity to ask any questions before signing an informed consent. A

required sample size of 12 subjects was calculated based upon pilot studies, to

provide 80% power to detect 0.01 mm change in anterior corneal curvature and

2.7 microns change in corneal thickness at the 0.05 level of significance.

The protocol followed was similar to that in Experiment 1 (Chapter 2).

This study was conducted over a period of 5 days (one baseline and 4 lens

wearing days). On each day, measurements were taken in the morning and

then again in the afternoon 8 hours later. On day one, baseline measurements

were taken without any contact lens in the eye, in the morning (0 hours) and

repeated in the afternoon after 8 hours. On days 2, 3, 4 and 5 of the study the

subjects wore different types of contact lenses in the left eye only, with

measurements collected in the morning before the lens was inserted and again

after 8 hours of wear. A 2-3 day recovery period was observed after each lens

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wear day before commencing wear of another lens, based on pilot studies

(described in Methods section of Chapter 2). Since for most people the right

eye is the dominant eye (Eyre and Schmeeckle 1933), it was decided that the

contact lens will cause less visual disturbance for the majority of subjects if

used in the left eye. Lens wear typically commenced between 8 and 11 am and

at least 2 hours after waking, to limit the potential influence of the corneal

changes that are typically evident immediately after sleep (Read and Collins

2009). The lenses were removed in the afternoon between 4 and 7 pm, after 8

hours of lens wear.

3.2.1 Subjects

The study included 14 young, healthy adult subjects aged between 20 to 33

years (mean age 27.8 ± 4.0 years) with visual acuity of 6/6 or better and

corneal astigmatism of ≤ 1.5 D, as determined by the Medmont E300

videokeratoscope (Medmont Pty. Ltd., Victoria, Australia). Five of the subjects

were females. The mean spherical equivalent refractive error was –0.6 ± 1.3 D.

Prior to commencement of the study, all subjects were screened for any tear

film abnormalities and anterior segment pathology using slit-lamp

biomicroscopy. None of the subjects had a history of corneal injury, infection or

surgery. Two of the subjects were habitual soft contact lens wearers but they

were asked to discontinue lens wear one month prior to the start of the study,

to allow any effects of soft lens wear to largely resolve. None of the subjects

were previous rigid contact lens wearers.

3.2.2 Contact Lenses

A contact lens trial fitting was performed for each subject with the rigid lenses

before ordering, to determine the optimum back optic zone radius (BOZR) for

each lens in the left eye only. Similar fitting characteristics of central alignment,

midperipheral touch and moderate edge lift was achieved with all the rigid

lenses for all subjects. Three different types of custom made rigid contact

lenses and one soft contact lens were ordered of each subject. The rigid lenses

were 9.5 or 10.5 mm in diameter with a spherical BOZR and an aspheric

periphery, and made from either PMMA or Boston XO material. The soft

contact lens was a Bausch & Lomb PureVision (Balafilcon A) silicone hydrogel

lens with 14.0 mm diameter. Other details of the lenses are shown in Table 3-1.

The type of lens to be worn on each study day was randomised.

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Table 3-1: Details of the four lenses used in the study

Lens PMMA/9.5 RGP/9.5 RGP/10.5 SiHy/14.0

Design (BOZ) Spherical Spherical Spherical Spherical

Design (BPZ) Aspheric Aspheric Aspheric B&L PureVision

Material PMMA RGP (Boston

XO) RGP (Boston

XO) Silicone Hydrogel

Power (Dioptre) –0.50 –0.50 –0.50 –0.50

Mean BOZR (mm)

7.77 ± 0.32 7.77 ± 0.32 7.85 ± 0.30 8.6

Total diameter (mm)

9.5 9.5 10.5 14.0

BOZD (mm) 8.1 8.1 8.8 8.9

Water content (%)

0 0 0 36

Dk 0.1 100 100 99

Modulus (MPa) ≈ 2000 1500 1500 1.1

Manufacturing method

Lathe Lathe Lathe Cast moulding

Surface treatment

None None None Performa

Centre thickness (mm)

0.18 ± 0.02 0.19 ± 0.01 0.20 ± 0.01 0.12 *

Mean Dk/t 0.06 52.6 50 82.5

Asterisk (*) based on assumption from other lens design. PMMA: polymethyl methacrylate, RGP: rigid gas permeable, SiHy: silicone hydrogel, BOZ: back optic zone, BPZ: back peripheral zone, B&L: Bausch and Lomb, BOZR: back optic zone radius, BOZD: back optic zone diameter, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity, mm: millimetres. Unit of Dk/t = (cm/sec) (mLO2/mL X mmHg). The lenses supplied by the manufacturer were checked for back vertex power, lens diameter, BOZR and edge and surface quality before use in the study.

3.2.3 Measurements and Instruments

A range of ocular measurements were collected at each measurement session

in order to quantify corneal shape and thickness, ocular optics and contact lens

fitting and centration characteristics over the course of the study.

Anterior and posterior corneal topography and regional corneal

thickness were measured using the Pentacam HR system (Oculus, Wetzlar,

Germany) which uses a rotating Scheimpflug camera (a digital camera with a

slit illumination system) to evaluate the anterior segment of the eye. A total of 5

measurements were completed using the “25 picture 3D scan” mode, which

gives 25 cross-sectional images of the anterior eye.

Anterior corneal topography was also measured using the Medmont

E300 videokeratoscope (Medmont Pty. Ltd., Victoria, Australia) which is based

on the Placido disc principle. A total of 4 Medmont images, with a quality score

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of 95 or greater were taken at each measurement session and saved for

analysis.

Ocular monochromatic aberrations were measured using the Complete

Ophthalmic Analysis System (COAS, Wavefront Sciences Ltd, USA).

Measurements were performed without the use of any eye drops in natural

pupil conditions and in dim room illumination. A total of 4 measurements were

taken during each measurement session, with 20 frames per measurement.

Subjects were instructed to keep away from any significant reading

work or any other activities involving long hours of downward gaze (Shaw et al.

2008), before taking the measurements. A questionnaire was completed by

each subject to monitor the visual tasks performed during the period of lens

wear. Subjects were engaged in similar tasks (e.g. computer work) during the

study period each day. The morning and afternoon measurements on contact

lens wearing days were conducted at around same time of day as on day 1

(baseline day) to allow comparison without confounding effects due to diurnal

variations, as discussed in Chapter 2.

Digital photos of the rigid contact lens on eye were taken to record the

fluorescein pattern using a Canon Digital Rebel EOS 300 D 6.3 mega pixels

Digital SLR (Canon Inc Tokyo, Japan) camera attached to a slit lamp. The slit

aperture was maximized and the slit lamp magnification was kept constant at

10X for all images. The same colour balance setting of the camera was used

for all the images in auto-mode. The images were taken at approximately the

same time of day every day, in the same room with the same temperature (24.9

± 1.0 ºC), humidity (58.0 ± 5.2%), and ambient lighting conditions (slit lamp

illumination: approximately 990 lux, plane of subject‟s eye). Fluorescein strip

(Fluorets, fluorescein sodium sterile ophthalmic strips) moistened with a drop of

unpreserved sterile unit dose saline was lightly touched on the upper bulbar

conjunctiva. The photos were taken in white light and with cobalt blue light (and

a Wratten filter # 12), 4-5 sec after fluorescein instillation.

A 30-second video recording was also taken for each subject with each

lens in order to analyse the most frequent position of the lens on the cornea (in-

between blinks). A Casio Digital SLR EX-F1 (Casio computer Co., Tokyo,

Japan) camera in auto mode, attached to a custom made adjustable camera

mount and illumination system was used for this (Figure 3-1). The illumination

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was kept at approximately 390 lux at the plane of subject‟s eye using an

external fluorescent ring light mounted behind a diffuser. The standard movie

mode with frame rate of 30 frames per second was used which gave images

with an aspect ratio of 4:3 and resolution of 640 x 480 pixels. The videos were

recorded in same room with approximately same humidity (59.6 ± 7.5%) and

temperature (24.8 ± 1.08 ºC), in dark room conditions at approximately the

same time of day every day (morning and afternoon). The subject was

positioned in the head rest with eyes in primary position and was instructed to

fixate on the middle of camera lens. The subject was instructed to make gentle,

complete blinks during the recording, and two video recordings were captured

for 30 seconds each. Measurements were performed in the following order at

each session: Medmont, Pentacam, COAS and then digital photography and

video recording.

Figure 3-1: Photo of the set up with digital camera to record movement of the contact lens. Illumination of the eye is provided by a fluorescent ring light, mounted behind a diffuser.

Figures 3-2 and 3-3 describe the sequence of measurements taken in

the morning and afternoon on contact lens wearing days. On the baseline days

when no contact lenses were worn, the same measurements and in same

order were taken. The measurements for Experiments described in chapters 5

and 6 were also collected together with measurements for this experiment

using the same set of contact lenses.

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Figure 3-2: Sequence of measurements taken in the morning before and following insertion of contact lens in eye

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Figure 3-3: Sequence of measurements taken in the afternoon after 8 hours of lens wear.

3.3 Data Analysis

3.3.1 Corneal topography and thickness data

Pentacam corneal thickness and axial (anterior and posterior) curvature data

and Medmont anterior tangential curvature and corneal height data from each

measurement session were exported from the two instruments. An average of

the 4 maps (Medmont) and 5 maps (Pentacam), taken for each subject during

each measurement session, was calculated using custom-written software

developed at the Contact Lens and Visual Optics Laboratory, QUT.

3.3.2 Pentacam data: Corneal curvature and thickness

The thickness and curvature difference maps from Pentacam data were

generated to compare the baseline maps to post-lens wear maps. „Thickness

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difference maps‟ and „curvature difference maps‟ were generated as described

in Chapter 2 (Section 2.3.1). Group average difference and significance maps

were also generated by averaging the data from all the subjects for each of the

4 lens types. From the results of Experiment 1, it was concluded some small

but significant diurnal variations in corneal thickness and curvature occur within

the 8 hours duration of the study. Thus all difference maps in this study were

generated by subtracting the thickness map of the baseline day (afternoon)

from the thickness map after lens wear (afternoon).

The Pentacam data from all the subjects were averaged using a

custom-written software (Topoview, developed at Contact Lens and Visual

Optics Laboratory) in order to study the regional changes in corneal thickness

and curvature following lens wear. The average corneal thickness and

curvature was calculated for each subject within two corneal regions i.e. central

(0 - 4 mm) and peripheral (4 - 8 mm) as illustrated in Figure 2-4 (Chapter 2).

To study the statistical significance of corneal changes due to contact

lens wear, a repeated measures analysis of variance (ANOVA) was used with

lens type and region as within-subject factors. Degrees of freedom were

adjusted using Greenhouse-Geisser correction to prevent any type 1 errors,

where violation of the sphericity assumption occurred. Bonferroni adjusted pair-

wise comparisons were carried out for individual comparisons. Pearson‟s

correlation was calculated to study the association between the changes in

corneal thickness and anterior and posterior curvature changes in the central

and peripheral corneal regions, using SPSS statistical software. The correlation

was calculated for the mean of the results from all four lenses and then for the

results of each of the lenses individually.

3.3.3 Medmont data: Correlation between the rigid lens fluorescein pattern and corneal topography changes

Tangential curvature difference maps were calculated for each subject with the

smaller diameter rigid lenses (PMMA/9.5, RGP/9.5) as described above using

the Medmont data, and the location of the points of maximum corneal flattening

were determined along the vertical and horizontal meridians along the map

centre. The tangential curvature map was used for this because it describes

localised changes in corneal topography. Additionally each digital slit lamp

image of the fluorescein fitting pattern was analysed to determine the region of

minimal clearance along the vertical and horizontal meridians. Pearson‟s

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correlation was then used to investigate the association between the spatial

location of the points of minimum clearance in the fluorescein image and the

points of corneal flattening in the topographic map.

Figure 3-4 shows the steps involved in calculating the points of

minimum clearance (in the fluorescein images) and points of maximum

flattening (topographic maps). First, the horizontal visible iris diameter (HVID)

and the distance from the videokeratoscope (VK) centre to limbus centre (LC)

was calculated for each subject using the Placido disc image from the

Medmont E300, using custom written software (Iskander et al. 2004). The

location of the limbus and its centroid was then located in the fluorescein

image, and the image was scaled using each subject‟s HVID measure from the

Placido disc image. The coordinates of the location of the VK centre in the

fluorescein image was then determined using the VK centre to LC offset from

the Placido disc image. The fluorescein pattern image was then analysed

using a Matlab-based algorithm which quantifies fluorescence along a

meridian. Fluorescence was estimated for the vertical and horizontal corneal

meridians along the coordinates of the VK centre, and the point of least

fluorescence was taken as the point of minimum clearance. The locations of

four points of minimum clearance were therefore estimated: superior and

inferior points of clearance along the vertical meridian and nasal and temporal

points of clearance along the horizontal meridian. The points of maximum

flattening in the midperipheral corneal region of the tangential curvature

topographic map, along the vertical and horizontal corneal meridians (centred

on VK axis), were also calculated using custom-written software (Topoview,

developed at Contact Lens and Visual Optics Laboratory). Pearson correlation

and significance was then calculated for these points.

Due to the constraints of size of the tangential curvature map from the

VK, the points of flattening (topographic map) corresponding to the

midperipheral points of minimum clearance in the fluorescein map were not

available in the superior and temporal regions for a substantial number of

subjects. Hence, these points (superior and temporal) were not included in the

analysis. Sufficient data was available for the inferior (19 lenses) and nasal (26

lenses) points, thus data for only these points (inferior and nasal) of minimum

clearance and flattening data for the smaller diameter lenses (PMMA/9.5 and

RGP/9.5) were included in the analysis. The data from the large diameter lens

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(RGP/10.5) were also excluded due to the same reasons (limited number of

data points of flattening in the topographic map).

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Figure 3-4: Steps involved in correlating rigid lens fluorescein pattern and corneal topographic changes. (b) White cross showing LC (c) small white cross showing LC and bigger white cross showing VK centre. HVID: horizontal visible iris diameter, VK: videokeratoscope centre, LC: limbus centre.

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3.3.4 Medmont data: Corneal refractive power

Corneal refractive power was estimated based upon each subject‟s average

corneal height maps, from the Medmont VK, assuming a corneal refractive

index of 1.376. Least squares fitting of a sphero-cylindrical surface to the

refractive power map (Maloney et al. 1993) was performed using custom-

written software. The surface was referenced to the VK axis. The refractive

power change was described and analysed in terms of power vectors (Thibos

et al. 1997): best fit sphere (M), with/against-the-rule astigmatism (J0) and

oblique astigmatism (J45) for 4 and 6 mm corneal diameters. The refractive

power changes were tested for statistical significance using repeated measures

ANOVA with lens type and corneal diameter as within-subject factors.

3.3.5 COAS data: Ocular wavefront error

Zernike coefficients up to the 8th radial order were exported from the COAS

aberrometer and then averaged for each subject using a custom-written

software (developed at Contact Lens and Visual Optics Laboratory) for 4 mm

(photopic) and 5.5 mm (scotopic) pupil size. These coefficients were further

analysed to calculate higher-order root mean square (HO RMS), 2nd, 3rd and 4th

order RMS for the baseline and contact lens wearing days. Repeated

measures ANOVA with lens type as within-subject factor was performed to

calculate the statistical significance of the changes.

3.3.6 Lens movement videos: Position of contact lens with respect to limbus centre

The videos were exported as image frames for further analysis. A 30 seconds

video gave 30 frames per second, i.e. approximately 900 image frames. In

order to calculate the most frequent position of contact lens (in between blinks)

in relation to the limbus centre, 3 image frames were analysed after 3 different

blinks. The image frames 1 second after each blink were selected for analysis

to give the contact lens time to settle.

The image analysis was performed by the same independent masked

observer using custom-written software (Topoview, developed at Contact Lens

and Visual Optics Laboratory). It involves the operator manually locating the

position of the coordinates of the limbus (8 points), contact lens (8 points), the

upper lid margin (8 points) and the lower lid margin (8 points) Figure 3-5. The

software then determines the best fitting ellipse to the limbus and contact lens

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co-ordinates and the best fitting quadratic function to the upper and lower

eyelid coordinates. The position of contact lens centre with respect to the LC

(x-horizontal and y-vertical) was then calculated for the 3 images after 3

different blinks. A mean, SD and range of the horizontal and vertical

coordinates of the lens position were then calculated for the 3 images with each

lens for all the subjects.

Figure 3-5: Image showing the position of a rigid contact lens on cornea (light blue ring), limbus (yellow ring) and upper (red arc) and lower eyelid (blue arc).

3.4 Results

3.4.1 Anterior corneal axial curvature

Figure 3-6 and Table 3-2 show the group mean change in anterior axial corneal

curvature (relative to the baseline day) for the four types of lenses. The type of

lens had a significant effect on the changes in anterior axial corneal curvature

(p<0.001, repeated measures ANOVA). Overall, the PMMA/9.5 lens showed

primarily steepening (significant in the centre) whereas RGP/9.5, RGP/10.5

lenses caused flattening in both the central and peripheral anterior corneal

surface (Table 3-2). RGP/10.5 lens led to significant flattening in the central

(0.05 ± 0.04 mm, p=0.007) and peripheral (0.03 ± 0.02 mm, p<0.001) corneal

regions. Significant flattening in the peripheral corneal region was observed

following wear of the RGP/9.5 lens (Table 3-2). Only small magnitude changes

in corneal curvature were observed with the SiHy/14.0 lens, and these changes

did not reach statistical significance.

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Figure 3-6: Group mean changes in anterior axial corneal curvature (mm) relative to baseline day for the four different types of contact lenses. The lenses included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change represents flattening and negative change represents steepening.

3.4.2 Posterior corneal axial curvature

The type of lens also had a significant effect on the changes in posterior

corneal axial curvature (p<0.001, repeated measures ANOVA). Figure 3-7 and

Table 3-2 show the group mean changes in posterior corneal curvature

(compared to the baseline day) for the four lenses. PMMA/9.5 lens showed

flattening in both the central (0.09 ± 0.05 mm, p<0.001) and peripheral (0.04 ±

0.03 mm, p=0.006) cornea, whereas the RGP/9.5, RGP/10.5 and SiHy/14.0

lenses showed no significant changes.

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Figure 3-7: Group mean changes in posterior axial corneal curvature (mm) relative to baseline day for the four different types of contact lenses. The lenses included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change represents flattening and negative change represents steepening.

Table 3-2: Mean changes in anterior and posterior axial corneal curvatures relative to baseline days with the four contact lenses in the central and peripheral regions.

Anterior axial curvature Posterior axial curvature

Lens Central Mean

change ± SD

(mm)

Peripheral mean

change ± SD

(mm)

Central Mean

change ± SD

(mm)

Peripheral mean

change ± SD

(mm)

PMMA/9.5 –0.05 ± 0.05

(p=0.03 )

–0.01 ± 0.02

(p=1.0)

0.09 ± 0.05

(p<0.001)

0.04 ± 0.03

(p=0.006)

RGP/9.5 0.01 ± 0.04

(p=1.0)

0.03 ± 0.02

(p<0.001)

–0.01 ± 0.03

(p=1.0)

–0.01 ± 0.02

(p=0.59)

RGP/10.5 0.05 ± 0.04

(p=0.007)

0.03 ± 0.02

(p<0.001)

–0.02 ± 0.03

(p=0.41)

–0.02 ± 0.02

(p=0.10)

SiHy/14.0 0.004 ± 0.03

(p=1.0)

0.004 ± 0.01

(p=1.0)

–0.003 ± 0.04

(p=1.0)

–0.002 ± 0.02

(p=1.0)

Positive change represents flattening and negative change represents steepening.

3.4.3 Corneal thickness

Corneal thickness was significantly affected by the type of lens and corneal

region (both p<0.001, repeated measures ANOVA).The group mean changes

in corneal thickness (relative to the baseline day) with the four lenses is shown

in Figure 3-8 and Table 3-3. PMMA/9.5 lens showed the greatest level of

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corneal swelling in both the central (27.92 ± 15.49 µm, p<0.001) and peripheral

(17.78 ± 12.11 µm, p=0.001) corneal regions. RGP/9.5 and RGP/10.5 lenses

showed lesser amounts of corneal swelling whereas the SiHy/14.0 lens showed

smaller changes again. The corneal swelling seen with PMMA/9.5 lens was

significantly greater than the swelling seen with the RGP/9.5, RGP/10.5 and

SiHy/14.0 lenses in the central region and the SiHy/14.0 lens in the peripheral

annular region. There were no significant differences in corneal swelling

between the RGP and SiHy lenses.

Table 3-3: Mean corneal thickness changes relative to baseline days with the four contact lenses in central and peripheral corneal regions.

Lens Mean change in central corneal thickness (relative to baseline)

Mean change in peripheral corneal thickness (relative to baseline)

(µm) ± SD (%) (µm) ± SD (%)

PMMA/9.5 27.92 ± 15.49 (p<0.001) 4.77 17.78 ± 12.11 (p=0.001) 2.71

RGP/9.5 2.30 ± 12.46 (p=1.0) 0.41 6.26 ± 13.30 (p=1.0) 0.97

RGP/10.5 4.47 ± 12.56 (p=1.0) 0.80 9.42 ± 12.15 (p=0.12) 1.46

SiHy/14.0 –1.88 ± 13.07 (p=1.0) –0.34 –1.03 ± 14.21 (p=1.0) –0.16

Positive change represents swelling and negative change represents thinning.

Figure 3-8: Group mean changes in corneal thickness (mm) relative to baseline day for the four different types of contact lenses. The lenses included different materials (PMMA, RGP and SiHy) and diameters (9.5, 10.5 and 14.0 mm). Details of the lenses are shown in Table 3-1. Positive change represents swelling and negative change represents thinning.

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3.4.4 Correlation between corneal curvature and thickness

A linear regression was performed on changes in corneal curvature and

thickness with all the lenses. Significant negative correlation between the

changes in central corneal thickness (swelling) and central (R2 = 0.63, p=0.001,

F = 20.03, B = –2.183) and peripheral (R2 = 0.57, p=0.002, F = 16.22, B = –

1.386) posterior corneal curvatures change (Figures 3-9 a & b) were found.

Similarly, there was a significant negative correlation between changes in

peripheral corneal thickness and central (R2 = 0.74, p<0.001, F = 34.54, B = –

2.328) and peripheral (R2 = 0.69, p<0.001, F = 26.78, B = –1.487) posterior

corneal curvatures change (Figure 3-9 c & d). The change in anterior corneal

curvature did not show a significant correlation with the change in corneal

thickness (all p>0.05).

Relationships between corneal thickness and front and back curvatures

for the individual lenses are shown in Table 3-4. Generally, RGP/9.5, RGP/10.5

and SiHy/14.0 lenses showed a highly significant negative correlation (all

p≤0.003) between corneal thickness and posterior corneal curvature whereas

PMMA/9.5 lens did not show any significant correlations between these

parameters.

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Figure 3-9: Correlation between changes in central corneal thickness and central (a) and peripheral (b) back curvature. Correlation between changes in peripheral corneal thickness and central (c) and peripheral (d) back curvature.

Table 3-4: Correlation between corneal thickness with anterior and posterior curvatures for the four different types of contact lenses.

Correlations Lens

PMMA/9.5

Lens

RGP/9.5

Lens

RGP/10.5

Lens

SiHy/14.0

Central thickness &

posterior central curvature NS

R2 = 0.832

p<0.001

R2 = 0.610

p=0.001

R2= 0.641

p<0.001

Central thickness &

posterior peripheral curvature NS

R2 = 0.834

p<0.001

R2 = 0.534

p=0.003

R2 =0.776

p<0.001

Peripheral thickness &

posterior central curvature NS

R2 = 0.828

p<0.001

R2 = 0.671

p<0.001

R2 = 0.841

p<0.001

Peripheral thickness &

posterior peripheral curvature NS

R2 = 0.857

p<0.001

R2 = 0.64

p=0.001

R2 = 0.821

p<0.001

Central thickness &

anterior central curvature NS NS

R2 = 0.303

p=0.04 NS

Central thickness &

anterior peripheral curvature

R2=0.354

p=0.03 NS

R2 = 0.462

p=0.007 NS

Peripheral thickness &

anterior central curvature NS NS NS

R2 = 0.313

p=0.04

Peripheral thickness &

anterior peripheral curvature NS NS

R2 = 0.386

p=0.02 NS

NS: not statistically significant, all correlations are in negative direction

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3.4.5 Correlation between rigid lens fluorescein pattern and corneal topography changes

To study the association between the fluorescein pattern‟s regions of minimal

clearance and changes in corneal curvature (in the topographic map), a

Pearson‟s correlation was calculated for the spatial location of these regions.

There was a significant positive correlation between the location of points of

minimum clearance (in fluorescein pattern) and maximum corneal flattening (in

the topographic maps) for inferior (R2 = 0.599, p<0.001) points along vertical

meridian and nasal (R2 = 0.528, p<0.001) points along the horizontal meridian

(Figure 3-10).

Figure 3-10: Correlation between distance of points of minimum clearance (between cornea and contact lenses, in fluorescein pattern) and points of maximum corneal flattening, from the videokeratoscope (VK) centre. Data is shown for inferior (V2) points along vertical meridian and nasal (H1) points along the horizontal meridian.

3.4.6 Refractive power

The change in anterior corneal best fit sphere (M) was significantly affected by

lens type and the size of corneal diameter analysed (both p<0.001, repeated

measures ANOVA). The group mean changes in M relative to the baseline day

for the four types of lenses, for 4 and 6 mm corneal diameter are shown in

Table 3-5. PMMA/9.5 lens (0.11 ± 0.07 D, p=1.00) showed an increase in

corneal power (which was not significant) whereas the RGP/9.5 (–0.34 ± 0.05

D, p<0.001), RGP/10.5 (–0.44 ± 0.07 D, p<0.001) and SiHy/14.0 (–0.11 ± 0.03

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D, p=0.01) lenses showed a significant decrease in refractive power for a 6 mm

corneal diameter.

With/against-the-rule astigmatism (J0) generally showed a decrease

with all the lenses, but the change only approached significance with RGP/9.5

lens (–0.08 ± 0.03 D, p=0.06). Oblique astigmatism (J45) showed an increase

with lenses PMMA/9.5, RGP/9.5 and RGP/10.5 whereas there was a decrease

with SiHy/14.0 lens. The increase in oblique astigmatism (J45) was significant

only with the RGP/9.5 lens (0.06 ± 0.01 D, p=0.01).

Table 3-5: Mean changes in best fit sphere(M), with/against the rule astigmatism (J0) and oblique astigmatism (J45) in Dioptres, relative to baseline day with the four contact lenses for the 4 and 6 mm corneal diameter.

Lens

Mean change in

M ± SD

(Dioptres)

p-value

Mean change in

J0 ± SD

(Dioptres)

p-value

Mean change in

J45 ± SD

(Dioptres)

p-value

4 mm corneal diameter

PMMA/9.5 0.19 ± 0.32 0.47 –0.04 ± 0.18 1.0 0.02 ± 0.12 1.0

RGP/9.5 –0.31 ± 0.22 0.002 –0.09 ± 0.13 0.28 0.01 ± 0.08 1.0

RGP/10.5 –0.49 ± 0.32 0.001 –0.06 ± 0.16 1.0 0.02 ± 0.07 1.0

SiHy/14.0 –0.13 ± 0.15 0.09 –0.02 ± 0.07 1.0 –0.01 ± 0.06 1.0

6 mm corneal diameter

PMMA/9.5 0.11 ± 0.24 1.0 –0.02 ± 0.10 1.0 0.02 ± 0.06 1.0

RGP/9.5 –0.34 ± 0.17 < 0.001 –0.08 ± 0.10 0.06 0.06 ± 0.06 0.01

RGP/10.5 –0.44 ± 0.26 < 0.001 –0.05 ± 0.13 1.0 0.04 ± 0.07 0.54

SiHy/14.0 –0.11 ± 0.10 0.01 –0.01 ± 0.05 1.0 –0.01 ± 0.04 1.0

Positive change represents increase and negative change represents decrease in corneal refractive power. Negative change in M represents decrease in corneal refractive power (hypermetropic shift). Positive change in M represents increase in corneal refractive power (myopic shift). Negative change in J0 represents decrease in WTR astigmatism. Positive J45 represents negative cylinder axis closer to 45° and negative J45 represents negative cylinder axis closer to 135°.

3.4.7 Ocular wavefront error

HO RMS, 2nd, 3rd and 4th order RMS wavefront errors were calculated for all 14

subjects for 4 mm pupil and for 10 subjects for 5.5 mm pupil (as the pupil size

for other subjects was less than 5.5 mm). The type of lens had a significant

effect on HO RMS, 2nd, 3rd and 4th order RMS wavefront errors for both 4 (all

p≤0.002) and 5.5 (all p≤0.02) mm pupil diameters. The group mean changes in

HO RMS, 2nd, 3rd and 4th order RMS relative to the baseline day for the four

types of lenses, for 4 and 5.5 mm pupil diameters are shown in Table 3-6.

Overall, PMMA/9.5, RGP/9.5 and RGP/10.5 lenses showed an increase in HO

RMS, 3rd and 4th order RMS.

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PMMA/9.5 lens showed a significant increase in HO RMS (0.09 ± 0.07

µm, p=0.005), 2nd (0.22 ± 0.19 µm, p=0.009), 3rd (0.08 ± 0.07 µ, p=0.007) and

4th (0.03 ± 0.04 µm, p=0.05) order RMS and RGP/9.5 lens showed significant

increase in 4th (0.02 ± 0.02 µm, p=0.01) order RMS, for the 4 mm pupil

diameter (Table 3-6). There was also a significant increase in HO RMS (0.29 ±

0.10 µm, p=0.001), 3rd (0.09 ± 0.06 µm, p=0.01) and 4th (0.06 ± 0.05 µm,

p=0.05) order RMS with PMMA/9.5 lens for 5.5 mm pupil diameter (Table 3-6).

The soft contact lens caused no significant changes in any of the terms.

Table 3-6: Mean changes in HO RMS, 2nd, 3rd and 4th order RMS, relative to baseline day with the four contact lenses for 4 mm (n=14) and 5.5 mm (n=10) pupil diameters. ‘n’ is the number of subjects included in the analysis.

Lens Mean HO RMS

change ± SD

(µm)

Mean 2nd

order

RMS change ± SD

(µm)

Mean 3nd

order

RMS change ± SD

(µm)

Mean 4nd

order

RMS change ± SD

(µm)

4 mm pupil

PMMA/9.5 0.09 ± 0.07 (p=0.005)

0.22 ± 0.19 (p=0.009)

0.08 ± 0.07 (p=0.007)

0.03 ± 0.04 (p=0.05)

RGP/9.5 0.13 ± 0.13

(p=0.38) –0.02 ± 0.15

(p=1.00) 0.03 ± 0.08

(p=1.00) 0.02 ± 0.02

(p=0.01)

RGP/10.5 0.01 ± 0.02

(p=0.80) –0.10 ± 0.15

(p=0.27) 0.01 ± 0.03

(p=1.00) 0.01 ± 0.02

(p=1.00)

SiHy/14.0 –0.001 ± 0.01

(p=1.00) –0.03 ± 0.10

(p=1.00) 0.002 ± 0.01

(p=1.00) –0.004 ± 0.01

(p=1.00)

5.5 mm pupil

PMMA/9.5 0.29 ± 0.10 (p=0.001)

0.26 ± 0.39 (p=0.62)

0.09 ± 0.06 (p=0.01)

0.06 ± 0.05 (p=0.05)

RGP/9.5 0.21 ± 0.09

(p=1.00) –0.01 ± 0.31

(p=1.00) 0.12 ± 0.09

(p=1.00) 0.03 ± 0.04

(p=1.00)

RGP/10.5 0.17 ± 0.07

(p=1.00) –0.10 ± 0.26

(p=1.00) –0.02 ± 0.06

(p=1.00) 0.01 ± 0.05

(p=1.00)

SiHy/14.0 0.10 ± 0.08

(p=1.00) –0.09 ± 0.24

(p=1.00) –.02 ± 0.05

(p=1.00) –0.01 ± 0.03

(p=1.00)

Positive change represents increase and negative change represents decrease.

3.4.8 Position of contact lens

The mean distance of rigid contact lens centre from the limbus centre one

second after 3 different blinks is shown Table 3-7. This also represents the

most frequent on-eye resting position of the contact lens in between the blinks.

The decentration in the vertical direction was greater than in the horizontal

direction for all the lenses and PMMA/9.5 lens showed maximum decentration.

The relatively large standard deviations associated with each of the mean lens

decentration and relatively large mean range indicates a high degree of

variability in lens position both within and between subjects.

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Table 3-7: Mean distances of contact lens centre to limbus centre (mm) and ranges (mm) in the horizontal and vertical directions for the three types of rigid contact lenses.

Lens Mean horizontal distance ± SD

Range Mean vertical distance ± SD

Range

PMMA/9.5 0.24 ± 0.23 –0.33 to 0.56 1.05 ± 0.65 0.02 to 2.44

RGP/9.5 0.28 ± 0.32 –0.34 to 0.91 0.78 ± 0.58 –0.51 to 2.04

RGP/10.5 0.10 ± 0.28 –0.50 to 0.73 0.43 ± 0.42 –0.38 to 1.38

Positive sign represents temporal direction and negative sign represents nasal (horizontally) and positive sign represents superior direction and negative sign represents inferior (vertically).

3.5 Discussion

We investigated the effect of short-term (8 hours) wear of 3 different types of

contact lenses (RGP, PMMA and SiHy) of different Dk and diameters on

corneal thickness and curvature. Corneal swelling with the wear of PMMA

contact lenses has been widely documented in the literature (Carney 1974;

Fonn et al. 1984; Wang et al. 2003; Moezzi et al. 2004), and our results are

consistent with these findings. We found significantly more corneal swelling

with the PMMA contact lens compared to the RGP lenses as reported earlier by

Fonn et al. (1984) which was of greater magnitude in the central compared to

peripheral corneal region. The corneal swelling with the RGP lenses did not

reach statistical significance, however on average a greater magnitude of

swelling was observed in peripheral corneal regions compared to central

regions (both p<0.01) with both of the RGP lenses (small and larger diameter)

worn. PMMA and RGP lenses were also observed to have opposite effects on

corneal curvature. Overall, PMMA lenses caused anterior corneal steepening

(significant centrally) and significant posterior corneal flattening whereas RGP

lenses resulted in anterior corneal flattening and posterior steepening (although

not significant posteriorly) (Table 3-2). Silicone hydrogel lens wear had little

effect on corneal thickness or curvature (Table 3-2 and 3-3). The changes we

observed in corneal curvatures were well correlated with corneal thickness

changes (Figure 3-9, Table 3-4). The changes in thickness and curvatures with

the different rigid contact lenses are illustrated schematically in Figure 3-11.

With PMMA lenses the anterior steepening and posterior flattening was

associated with central corneal thickening (Figure 3-11 a). For RGP lenses,

anterior flattening and posterior steepening was accompanied by peripheral

corneal thickening (Figure 3-11 b).

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Figure 3-11: Schematic demonstration of anterior and posterior curvatures and thickness of the cornea, before and after PMMA and RGP contact lens wear for 8 hours based on the experimental data. The solid lines represent the baseline anterior and posterior surfaces of cornea. The dotted line represents the anterior and posterior surfaces of the cornea after contact lens wear for 8 hours. (a) PMMA contact lens showing greater central corneal swelling compared to peripheral resulting in anterior corneal steepening and posterior corneal flattening. (b) RGP contact lens showing greater peripheral corneal swelling resulting in anterior corneal flattening and posterior corneal steepening. Note that the diagram is not to scale.

The corneal swelling due to the PMMA lenses was of much higher

magnitude than that observed for the RGP lenses, and was confined largely to

the central cornea resulting in overall anterior corneal steepening and posterior

flattening. The difference in the effects of PMMA and RGP on corneal thickness

was expected due to the difference in Dk of the two lens materials. The fitting

characteristics of the RGP and PMMA lens were similar for these subjects, so

the curvature changes due to mechanical forces should also be similar. For the

RGP lenses, the changes in curvature are likely to be at least in part driven by

mechanical forces on anterior cornea resulting in slight flattening. More

prominent corneal flattening due to mechanical forces of RGP lenses, that

involves the migration of epithelial cells is observed with orthokeratology (Choo

et al. 2008). For PMMA lenses, any changes in curvature due to mechanical

forces (which might have been similar to those due to RGP lenses), were

probably masked by the thickness and curvature changes because of hypoxia.

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We did notice small changes in the posterior corneal curvature

(steepening) with RGP lenses, presumably related to slight peripheral corneal

swelling, but they were not significant. The larger diameter RGP lens showed

slightly more pronounced changes for thickness and central anterior curvatures

compared to the smaller diameter. The changes in curvatures and thickness

following SiHy lens wear were very small and not statistically significant. The

minimal corneal changes noted with the SiHy lens most likely relate to the lens

being thinner and having a substantially lower modulus of elasticity, which

would be expected to result in less metabolic and mechanical related corneal

changes.

The mechanical forces across the cornea due to a contact lens are

distributed based on areas of touch and clearance. For rigid lenses these areas

are identified using the corneal fluorescein pattern during the contact lens

fitting. Fluorescein patterns are used to examine whether the lens will fit loose

and ride low on the cornea or fit tight and prevent any tear exchange. Usually,

rigid lens fitting procedure aims for an optimal fit which allows enough tear

exchange and also ensures lens centration on the cornea.

The profile of the tear layer thickness should also influence the changes

in corneal curvature due to mechanical forces. For example, the areas of

minimal clearance would be expected to induce flattening, whereas areas of

increased clearance may result in steepening. Although, a rigid lens moves

with every blink, it will most likely result in corneal curvature changes based

primarily on its final resting position after the completion of the blink (the inter-

blink location). We found that regions of minimal fluorescein clearance

correlated spatially to the areas of anterior corneal flattening. We could find no

previous reports which quantitatively analyse the fluorescein pattern for rigid

contact lens fitting. Information provided by this analysis could be useful in

predicting the changes in corneal curvature that the rigid contact lens will

produce.

We investigated the change in the best fit sphero-cylinder of the anterior

corneal surface with short-term wear of different types of contact lenses. The

results were similar for both 4 mm and 6 mm corneal diameters. The PMMA

lens showed a small (but statistically insignificant) increase in best fit sphere

(M). However there was a clinically and statistically significant decrease (mean

approximately 0.50 D) in best fit sphere (M) with the RGP lenses. This

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significant refractive power change may be due to mechanical forces on the

anterior surface of the RGP lens (which was likely to be masked in case of

PMMA lens due to significant corneal swelling). This significant refractive power

change could also be a short-term consequence of peripheral corneal swelling

but if it were to persist it would require alteration in the lens power to

compensate. There are no reports of this phenomenon in the clinical literature,

which suggests that it may be a transient occurrence. Further research

investigating corneal changes with longer periods of RGP lens wear are

required to clarify the time course of these corneal refractive power changes.

For the 6 mm corneal diameter, the RGP/9.5 lens also induced a clinically

insignificant change in with/against-the-rule (J0) and oblique (J45) astigmatism.

The large diameter RGP contact lens caused greater central anterior

corneal flattening compared to the small diameter RGP lens. This could be

because of greater interaction of larger diameter lens with the upper lid

compared to smaller diameter lens, leading to differences in pressure

distribution by the two lenses. The corneal swelling caused by the large

diameter lens was also slightly more compared to small diameter lens, both in

central and peripheral cornea but these changes were not significant.

The soft SiHy contact lens resulted in a clinically insignificant change in

corneal best fit sphere (M). These results support the view that the soft contact

lenses predominantly conform to the shape of the cornea and therefore

produce little refractive power changes due to lens pressure, whereas rigid

contact lenses can alter the shape of the cornea and can produce considerable

changes in corneal refractive power.

There are many studies reporting the effects of contact lenses on-eye

on the ocular wavefront aberrations (Hong and Himebaugh 2001; Dorronsoro et

al. 2003; Lu et al. 2003). The changes in aberrations associated with contact

lens wear depend on both the optical properties of the lens and the nature of

the tear optics (Hong and Himebaugh 2001). Therefore, RGP lens wear has

been shown to result in a reduction in total ocular aberrations whereas soft

contact lens wear has usually been found to result in increases in aberrations

(Griffiths et al. 1998; Lu et al. 2003). To our knowledge there are no

experimental studies reporting residual effects of contact lens wear on ocular

aberrations measured after removal of the lens. We investigated the effect of

short-term wear of various contact lenses on wavefront aberrations of the eye

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for 4 mm and 5.5 mm pupil size. We found a significant increase in HO RMS,

2nd, 3rd and 4th order RMS wavefront error after PMMA contact lens wear for

a 4 mm pupil diameter. For a 5.5 mm pupil, there was also a significant

increase in 3rd, 4th and HO RMS but not in 2nd order RMS. This suggests that

the anterior corneal steepening and posterior flattening associated with corneal

thickening as seen with the PMMA lens, not only affects the lower order

wavefront aberrations but also results in increased higher order terms. The

increase in most aberration terms including the non-symmetric aberrations

probably depends on the final on-eye resting position of the lens which induced

curvature changes and corneal thickening asymmetrically relative to the pupil

centre (Table 3-6). It is evident from Figures 3-6 to 3-8 that the corneal

changes associated with the PMMA lens are often decentred away from the

topographic map centre. We found the mean decentration of PMMA lens centre

with respect to the limbus centre to be 0.24 ± 0.23 mm temporally and 1.05 ±

0.65 mm superiorly. These changes may result in significant reduction in ocular

image quality after lens removal (during lens wear the post-lens tear layer

would neutralize most of the higher order aberrations) and may be at least in

part be responsible for some reports of “spectacle blur” associated with PMMA

lens wear (Levenson 1983; Wilson et al. 1990).

3.6 Conclusion

To conclude, PMMA contact lens wear resulted in corneal swelling (more in the

centre compared to periphery) consistent with previous reports, and RGP

lenses caused more corneal swelling in the periphery compared to the centre.

The difference in the pattern of regional corneal swelling seen with PMMA and

RGP lenses, due to hypoxia induced by the PMMA lens material, led to

opposite effects on corneal curvature for these lenses. Overall, PMMA lenses

caused anterior corneal steepening and posterior corneal flattening whereas

RGP lenses resulted in anterior corneal flattening and posterior steepening. We

also found a significant correlation between the locations of minimum clearance

in the fluorescein pattern and the resultant corneal flattening in the topographic

maps, with the rigid lenses. This highlights the importance of lens fitting

characteristics and the resultant corneal curvature changes associated with

lens wear. This may also aid in anticipating the changes in corneal curvature

due to lens wear which can be up to 0.50 D, enough to affect vision and require

an adjustment in the lens power. Evidence of “spectacle blur” following PMMA

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lens wear has also been shown in the form of an increase in higher order

aberrations.

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Chapter 4

Corneal changes with spherical versus back surface toric rigid contact lens wear

4.1 Introduction

Astigmatism is a commonly occurring refractive error and is found in about 13%

of the ametropic population (Porter et al. 2001). The origin of astigmatism can

be corneal or lenticular, with either one or both (anterior and posterior) corneal

and/or lenticular surfaces having different powers along the two principal

meridians. In a study by Fledelius and Stubgaard (1986) it was shown that 46%

of the population exhibits corneal astigmatism of >0.5 D but only 4.7% have

astigmatism of > 1.5 D, which was later supported by McKendrick and Brennan

(1996).

A spherical back surface RGP lens may correct up to 2 to 2.5 D corneal

astigmatism (through neutralisation of astigmatism by the tear lens) whereas

higher levels of astigmatism require that the back surface of the lens is toric in

order to provide a stable fit. However, stability of the fit also depends on a

range of factors including lens centration, lid forces and aspects of the lens

material and design characteristics (e.g. thickness, specific gravity of the

material).

In Experiments 1 (Chapter 2) and 2 (Chapter 3) we investigated

changes in corneal curvature and thickness with short-term use of soft and

RGP contact lenses respectively, in subjects with only small amounts of

astigmatism. These contact lenses caused changes in corneal thickness and

curvature, although the results varied with different materials and lens designs.

Back surface toric RGP lenses are usually fitted on or near alignment (Lindsay

2007) to the two principal meridians of astigmatic corneas, in order to ensure

rotational stability. We hypothesise that the regional mechanical pressure on

the cornea and tear exchange due to back surface toric RGP lenses will vary

from that of spherical RGP lenses and that this will result in different corneal

thickness and curvature changes with the two lens designs. In this chapter we

aim to investigate corneal curvature, thickness and refractive changes after

short-term use of back surface toric RGP lenses compared with spherical RGP

lenses on subjects with astigmatic corneas.

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4.2 Methodology

The study was conducted over a period of 3 days and the protocol was similar

to that in Chapters 2 and 3. Baseline measurements were taken without any

contact lens on day one, in the morning and the afternoon after 8 hours. On

days 2 and 3 of the study the subjects wore two different types of contact

lenses in the left eye only, and measurements were taken in the morning

before the lens was inserted and repeated after 8 hours of wear. Lens wear

typically started between 8 and 11 am and at least 2 hours after waking, to limit

the potential influence of the corneal changes that are typically evident

immediately after sleep (Read and Collins 2009). The lenses were removed in

the afternoon between 4 and 7 pm, after 8 hours of lens wear. The type of lens

to be worn on each study day was randomized and a 2-3 day recovery period

was allowed after each lens wear day before commencing wear of the second

lens.

All subjects were asked to read the study information sheet before

signing an informed consent. The study followed the tenets of the declaration of

Helsinki and was approved by the QUT university human research ethics

committee (see Appendix A).

4.2.1 Subjects

The study included 6 young, healthy adult subjects, aged between 19 to 31

years (mean age ± SD = 24.8 ± 4.1 years) with best-corrected visual acuity of

at least 6/6 and a difference in central corneal curvature of at least 0.25 mm

(1.4 D) between the principal meridians. Subjects were selected to have with-

the-rule astigmatism (i.e. major axis within 30 degrees of horizontal). The mean

central corneal curvatures (simulated K reading) were 7.9 ± 0.3 mm (42.8 ± 1.6

D) in the flatter meridian and 7.6 ± 0.3 mm (44.7 ± 2.1 D) in the steeper

meridian, as determined by the Medmont E300 videokeratoscope. One of the 6

subjects showed limbus-to-limbus astigmatism and the rest showed central

corneal astigmatism (Figure 4-1). The mean spherical refractive error was –3.0

± 1.9 D and mean refractive astigmatism was –0.7 ± 0.8 D.

All subjects were screened for any anterior segment pathology or tear

film abnormalities using slit-lamp biomicroscopy. No history of corneal injury,

infection or surgery was reported by any of the subjects. One of the subjects

was an habitual soft lens wearer and three others were occasional soft contact

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lens wearers. These subjects discontinued lens wear one month prior to the

commencement of the study to allow any effects of soft lens wear to largely

resolve (Wilson et al. 1990; Wang et al. 2002). None of the subjects were

previous rigid contact lens wearers.

Figure 4-1: Axial corneal curvature maps of all subjects showing pattern of corneal astigmatism and difference in curvature of the two principal meridians. Note that all subjects had central astigmatism except for subject 04 who showed limbus-to-limbus astigmatism.

4.2.2 Contact lenses

Two different types of custom made rigid contact lenses were ordered for the

left eye of each subject. The rigid lenses were both 9.5 mm in diameter and

one had a spherical BOZR and the second had a toric back surface. A contact

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lens fitting trial with fluorescein assessment was performed for each subject in

order to determine the optimum back optic zone radius (BOZR) for the

spherical lens. The back surface toric lens was ordered based on the corneal

curvature derived from videokeratoscopy. Examples of the fluorescein pattern

fitting with a spherical and back surface toric RGP lens on subjects with high

and low corneal astigmatism are shown in Figure 4-2. Details of the spherical

and toric lenses are shown in Table 4-1.

Table 4-1: Details of the lenses used in the study.

Lens Sph Toric

Design (BOZ) Spherical Toric

Design (BPZ) Aspheric Aspheric

Material RGP (Boston XO) RGP (Boston XO)

Power (Dioptre) –0.5 –0.5

Mean BOZR (mm) 7.8 ± 0.3 K1 = 7.6 ± 0.3, K2 = 7.9 ± 0.3

Total diameter (mm) 9.5 9.5

BOZD (mm) 8.1 8.1

Water content (%) 0 0

Dk 100 100

Modulus (MPa) 1500 1500

Manufacturing method Lathe Lathe

Surface treatment Plasma Plasma

Centre thickness (mm) 0.18 0.19

RGP: rigid gas permeable, BOZ: back optic zone, BPZ: back peripheral zone, BOZR: back optic zone radius, BOZD: back optic zone diameter, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity, mm: millimetres. The lenses supplied by the manufacturer were checked for back vertex power, lens diameter, BOZR (spherical lens) and edge and surface quality before use in the study. K1 = steeper corneal curvature, K2 = flatter corneal curvature.

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Figure 4-2: Fluorescein patterns with a spherical (a) and back surface toric (b) lens (same eye) on a subject (04) with high astigmatism (∆K = 3.3 D), limbus-to-limbus. In the lower panels a spherical (c) and back surface toric (d) lens (same eye) on a subject (06) with a lower amount of corneal astigmatism (∆K = 1.4 D), central. Note axis markings/scribe marks of the toric lens on the flatter corneal meridian in both subjects (panels b and d).

4.2.3 Measurements and Instruments

Corneal topography and thickness, ocular wavefront aberrations and contact

lens fitting and centration characteristics were assessed at each measurement

session. The protocol followed was similar to that in Chapters 2 and 3.

Anterior and posterior corneal topography and regional corneal

thickness were measured using the Pentacam HR system (Oculus, Wetzlar,

Germany). A total of 5 measurements were completed using the “25 picture 3D

scan” mode, which gives 25 cross-sectional images of the anterior eye. Corneal

refractive power was measured using the Medmont E300 videokeratoscope

(Medmont Pty. Ltd., Victoria, Australia). A total of 4 videokeratoscope images,

with a quality score of 95 or greater were taken at each measurement session

and saved for analysis. Ocular monochromatic aberrations were measured

using the Complete Ophthalmic Analysis System (COAS, Wavefront Sciences

Ltd, USA). Measurements were performed without the use of any eye drops

under natural pupil conditions with dim room illumination. A total of 4

measurements were taken during each measurement session, with 20 frames

per measurement.

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Digital photos of the rigid contact lens on eye were taken to record the

fluorescein pattern using a digital camera. The protocol was similar to that used

in Chapter 3 (Section 3.2.3). In order to analyse the most frequent position of

the contact lens on the cornea (in between blinks) a 30 second video was also

recorded the details of which are described in Section 3.2.3.

4.3 Data Analysis

The analysis carried out on the data was similar to that in Chapter 3, Section

3.3.

4.3.1 Corneal topography and thickness data

Corneal thickness and anterior and posterior axial curvature data from the

Pentacam and Medmont corneal height data from each measurement session

were exported. An average of the 4 maps (Medmont) and 5 maps (Pentacam),

taken for each subject during each measurement session, was calculated using

custom-written software (Topoview, developed at Contact Lens and Visual

Optics Laboratory).

4.3.2 Pentacam data: Corneal curvature and thickness

All difference maps in this study were generated by subtracting the curvature or

thickness map of the baseline day (afternoon) from the curvature or thickness

map after lens wear (afternoon). This was considering small but significant

diurnal variations in corneal thickness and curvature that occurred within the 8

hours duration of the study, as seen in Chapter 2. The thickness and curvature

difference maps were generated using Pentacam data to compare the baseline

to post-lens wear maps. Thus, „Thickness difference maps‟ were generated by

subtracting the average baseline thickness map from the average thickness

maps after 8 hours of lens wear. Similarly, „curvature difference maps‟ were

generated by subtracting the average baseline curvature map from the average

curvature map after 8 hours of lens wear. Group average difference maps were

generated by averaging the data from all subjects for each of the two lens

types.

The average Pentacam data for each subject were further analysed

using the custom-written Topoview software to investigate the regional changes

in corneal thickness and curvature after contact lens wear. The average

corneal thickness and curvature was calculated for each subject within two

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corneal regions, the central area (0 – 4 mm) and peripheral area (4 – 8 mm) as

shown in Figure 2-4. Complete data for each subject were available for 8 mm

corneal diameter, thus analyses were performed in this region. The mean radii

of curvature along the vertical and horizontal meridians with the spherical and

back surface toric lens and on the baseline day were calculated using the

custom-written Topoview software.

Statistical significance of changes in corneal curvature and thickness

due to contact lens wear was tested using repeated measures analysis of

variance (ANOVA), using SPSS 17.0. Lens type and region were considered as

within-subject factors. Bonferroni adjusted pair-wise comparisons were

conducted for individual comparisons. To avoid any type 1 errors, degrees of

freedom were adjusted using Greenhouse-Geisser correction, where violation

of the sphericity assumption occurred.

4.3.3 Medmont data: Corneal refractive power

Average corneal height maps for each subject from the Medmont

videokeratoscope were used to calculate corneal refractive power, assuming a

corneal refractive index of 1.376. Custom-written software (Topoview) was

used to perform a least squares fitting of a sphero-cylindrical surface to the

refractive power map as described by Maloney et al. (1993). The

videokeratoscope axis was used as a reference for the surface fit. The

refractive power sphero-cylinder was converted into power vectors (Thibos et

al. 1997): best fit sphere (M), with/against-the-rule astigmatism (J0) and oblique

astigmatism (J45) for 4 and 6 mm corneal diameters. The refractive power

changes were tested for statistical significance using repeated measures

ANOVA with lens type and corneal diameter as within-subject factors.

4.3.4 COAS data: Ocular wavefront error

Zernike coefficients (measured using the COAS aberrometer) were averaged

for each subject using custom-written software (WFM, developed at Contact

Lens and Visual Optics Laboratory) for 4 mm (photopic) and 5.5 mm (scotopic)

pupil sizes up to the 8th radial order. Higher-order root mean square (HO RMS)

wavefront error, 2nd, 3rd and 4th order RMS for the baseline and contact lens

wearing days were further calculated. A repeated measures ANOVA with lens

type as within-subject factor was performed to test the statistical significance of

these changes.

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4.3.5 Lens movement videos: Position of contact lens on cornea (with respect to limbus centre)

Each 30 second video was exported into image frames (with 30 frames per

second i.e. a total of approximately 900 image frames). Three image frames

were analysed after 3 different blinks to calculate the most frequent position of

contact lens in between blinks. The contact lens was allowed to settle after the

blink by selecting the image frame 1 second after the completion of each blink

for the analysis. The mean positions of contact lens centre with respect to the

limbus centre (x- horizontal and y- vertical) were calculated for the 3 images

using a custom written software, details of which have been described earlier

(Chapter 3, Section 3.3.6).

4.4 Results

4.4.1 Anterior corneal axial curvature

The group mean changes in anterior axial corneal curvature relative to the

baseline day for the two lenses are shown in Table 4-2 and Figure 4-3. The

type of lens had a significant effect on the changes in anterior axial corneal

curvature (p<0.05, repeated measures ANOVA). Generally, both lenses caused

a small magnitude of flattening in both central and peripheral corneal regions,

but the changes were significant only in the peripheral cornea (p<0.05, pairwise

comparison). The rate of flattening in the inferior cornea from centre to

periphery appeared higher with the toric lens than with the spherical lens, but

this was not statistically significant (p>0.05) (Figure 4-4).

Figure 4-3: Group mean changes in anterior axial corneal curvature (mm) relative to baseline day for the spherical and back toric RGP lenses. Details of the lenses are shown in Table 4-1. Positive change represents flattening and negative change represents steepening.

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Table 4-2: Mean changes in anterior and posterior axial corneal curvatures relative to baseline days with the two lens types in the central and peripheral regions.

Anterior axial curvature Posterior axial curvature

Lens Central mean

change ± SD

(mm)

Peripheral mean

change ± SD

(mm)

Central mean

change ± SD

(mm)

Peripheral mean

change ± SD

(mm)

Sphere 0.04 ± 0.03

(p=0.11 )

0.03 ± 0.02

(p=0.05) –0.02 ± 0.03

(p=0.32 )

–0.02 ± 0.01

(p=0.08)

Back

toric 0.02 ± 0.02

(p=0.54 )

0.03 ± 0.02

(p=0.05)

–0.02 ± 0.03

(p=0.34 )

–0.03 ± 0.02

(p=0.03)

Positive change represents flattening and negative change represents steepening. P-value of <0.05 statistically significant.

Figure 4-4: Mean axial radii of curvature (mm) in the vertical meridians for baseline, spherical lens and back surface toric lens. VK: Videokeratoscope.

4.4.2 Posterior corneal axial curvature

The type of lens and corneal region had a significant effect on the changes in

posterior axial corneal curvature (both p<0.05, repeated measures ANOVA).

Figure 4-5 and Table 4-2 show the group mean changes in posterior axial

corneal curvature (relative to the baseline day) for the two lens types. Overall,

both lenses caused steepening in both the central and peripheral corneal

regions, but the changes were significant only in the peripheral cornea with the

toric lens (p<0.05, pairwise comparison).

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Figure 4-5: Group mean changes in posterior axial corneal curvature (mm) relative to baseline day for the spherical and back surface toric RGP lenses. Positive change represents flattening and negative change represents steepening.

4.4.3 Corneal thickness

The group mean changes in corneal thickness relative to the baseline day for

the two lens types are shown in Table 4-3 and Figure 4-6. Both spherical and

toric lenses resulted in corneal swelling which was greater in the periphery

compared to the central corneal region, but these changes were not significant

with either lens. The degree of corneal swelling did not differ between the

spherical and the toric lenses (p>0.05, pairwise comparison).

Figure 4-6: Group mean change in corneal thickness (mm) relative to baseline day for the spherical and back toric RGP lenses. Details of the lenses are shown in Table 4-1. Positive change represents swelling and negative change represents thinning.

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Table 4-3: Mean corneal thickness changes relative to baseline days with the two contact lens types in central and peripheral corneal regions.

Lens Mean change in central corneal thickness (relative to baseline)

Mean change in peripheral corneal thickness (relative to baseline)

(µm) ± SD (%) (µm) ± SD (%)

Sphere 0.32 ± 3.75 (p=1.0 ) 0.1 5.05 ± 5.73 (p=0.25 ) 0.8

Back

toric 2.19 ± 8.27 (p=1.0 ) 0.4 6.95 ± 9.31 (p=0.35 ) 1.1

Positive change represents swelling and negative change represents thinning.

4.4.4 Refractive power

The change in anterior corneal best fit sphere (M) and with/against-the-rule

astigmatism (J0) were significantly affected by lens type (both p<0.001,

repeated measures ANOVA). The group mean changes in M, J0 and J45

compared to the baseline day for 4 and 6 mm corneal diameters with the two

lens types are shown in Table 4-4. The spherical lens caused a significant

decrease in M for both 4 and 6 mm corneal diameters (both p<0.05). The toric

lens caused similar changes in M that bordered on significance for 6 mm

corneal diameter (p=0.06). There was a decrease in WTR astigmatism

(negative change in J0) with both lens types, but the change was only

significant with the spherical lens for 6 mm corneal diameter (p<0.001). Oblique

astigmatism (J45) showed only small changes with both lenses that were not

statistically different for either lens type.

4.4.5 Ocular wavefront error

The group mean changes in HO RMS, 3rd and 4th order RMS relative to the

baseline day for the two lens types, for the 4 and 5.5 mm pupil diameters are

shown in Table 4-5. Both spherical and back surface toric lenses showed small

increases in HO RMS, but the changes were not significant for any of the

lenses. There were no significant differences between the two lens types.

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Table 4-4: Mean changes in best fit sphere (M), with/against the rule astigmatism (J0) and oblique astigmatism (J45) in dioptres, relative to baseline day with the two lens types for the 4 and 6 mm corneal diameters.

Lens Mean change

in M ± SD (Dioptres)

p-value

Mean change in J0 ± SD (Dioptres)

p-value

Mean change in J45 ± SD (Dioptres)

p-value

4 mm corneal diameter

Sphere –0.26 ± 0.12 0.01 –0.13 ± 0.12 0.14 0.02 ± 0.16 1.0

Back toric

–0.15 ± 0.13 0.11 –0.10 ± 0.12 0.30 –0.04 ± 0.13 1.0

6 mm corneal diameter

Sphere –0.23 ± 0.12 0.02 –0.13 ± 0.03 <0.001 0.03 ± 0.12 1.0

Back toric

–0.17 ± 0.13 0.06 –0.09 ± 0.08 0.11 0.01 ± 0.11 1.0

Negative change in M represents decrease in corneal refractive power (hypermetropic shift). Negative change in J0 represents decrease in WTR astigmatism. Positive J45 represents negative cylinder axis closer to 45° and negative J45 represents negative cylinder axis closer to 135°.

Table 4-5: Mean changes in HO RMS, 3rd and 4th order RMS, relative to baseline day with the two lens types for 4 and 5.5 mm pupil diameters.

Lens Mean HO RMS

change ± SD (µm) Mean 3

nd order RMS

change ± SD (µm) Mean 4

nd order RMS

change ± SD (µm)

4 mm

Sphere 0.12 ± 0.07

(p=1.0 ) –0.01± 0.03

(p=1.0)

0.01 ± 0.02 (p=1.0)

Back toric

0.09± 0.06 (p=1.0 )

–0.002 ± 0.03

(p=1.0)

0.01 ± 0.02 (p=1.0 )

5.5 mm

Sphere 0.22 ± 0.09

(p=1.0) 0.01 ± 0.09

(p=1.0 ) 0.02 ± 0.06

(p=1.0 )

Back toric

0.17 ± 0.06 (p=1.0)

0.002 ± 0.06 (p=1.0 )

0.01 ± 0.04 (p=1.0)

Positive change represents increase and negative change represents decrease.

4.4.6 Position of contact lenses

Table 4-6 shows the mean decentration of the spherical and back

surface toric contact lens centre from the limbus centre at approximately one

second after 3 different blinks. The ranges of the horizontal and vertical

coordinates of the lens position are also described in the table. The most

frequent on-eye resting position of the contact lenses between the blinks for

both lenses was found to be superior-temporal. Both lenses show high degree

of variability in lens position as indicated by the range. The toric lens showed

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slightly more decentration in the superior direction but this difference was not

significant (p>0.05).

Table 4-6: Mean distance of contact lens centre to limbus centre (mm) and range in the horizontal and vertical directions for the two lens types.

Lens Mean horizontal distance ± SD

Range Mean vertical distance ± SD

Range

Sphere 0.24 ± 0.38 –0.24 to 0.82 0.38 ± 1.01 –0.99 to 1.78

Back toric

0.27 ± 0.45 –0.81 to 0.78 0.68 ± 0.77 –0.64 to 1.36

Positive sign represents temporal direction (horizontally) and positive sign represents superior direction (vertically).

4.5 Discussion

We have investigated the effect of back surface toric and spherical RGP lenses

on corneal curvature and found slight flattening of the vertical anterior corneal

meridian with both the lenses in subjects with WTR corneal astigmatism, which

was significant in the periphery. The flattening of the vertical meridian resulted

in a significant decrease in WTR astigmatism (decrease in J0), for the spherical

lens for 6 mm diameter. This reduction in J0 was also seen in Chapter 3 in

subjects wearing spherical lenses on spherical and low astigmatism corneas,

but it was not significant. The toric lens caused slightly less change in J0, most

likely because it was fitted in alignment to the cornea and therefore caused less

flattening along the vertical meridian compared to the spherical lens.

Our results are similar to Mountford et al. (1997) and Cheung et al.

(2009) who found decreases in WTR astigmatism in a group of subjects with

central corneal astigmatism after the use of spherical ortho-k lenses. It has

previously been noted that patients with limbus-to-limbus astigmatism exhibit

less reduction in astigmatism with ortho-k lenses (Mountford et al. 2004). One

of our subjects had limbus-to-limbus astigmatism and this may have affected

the results because of differences in the pressure profiles beneath the lens. We

investigated the changes in WTR astigmatism in individual subjects and found

that there was a similar decrease in WTR astigmatism for all subjects for the 4

mm corneal diameter. But in the 6 mm corneal diameter, the subject with

limbus-to-limbus astigmatism showed the least change (decrease in WTR

astigmatism). Further research with a greater number of subjects with limbus-

to-limbus astigmatism is required to confirm this trend. We analysed the contact

lens centration on the cornea with respect to the limbus centre and found that

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both spherical and back surface toric contact lenses were decentred in the

superior temporal direction at the end of the 8 hour wearing time. This

decentration may have occurred due to the reflex tearing and the subject‟s lack

of adaptation to the lenses. None of the subjects included in the study were

given any adaptation time and topical anaesthesia was also not administered.

Topical anaesthesia was not used in the study in order to avoid any

confounding effects of these drops on corneal thickness or curvature (Herse

and Siu 1992). The contact lens decentration of both spherical and toric lenses

(in the superior temporal direction) does appear to correlate with the anterior

curvature changes in the topographic maps (lens being used on the left eye)

which shows corneal flattening (in the inferior nasal direction) (Figure 4-3).

These corneal changes could be a result of bearing in the mid-peripheral zone

of the contact lens, causing pressure and subsequent corneal flattening in the

inferior nasal corneal quadrant.

The change in the spherical component of the refractive error (M) was

significant compared to baseline with the spherical lens. For both a 4 and 6 mm

analysis diameter, the change in M was 0.26 and 0.23 D respectively with the

spherical lens (after 8 hours of lens wear) and this change is clinically

significant. The change in M associated with the back surface toric lenses were

smaller (0.15 and 0.17 D for 4 and 6 mm analysis diameters respectively),

however, it could be argued that this change is also of borderline clinical

significance, since any change in refraction of 0.12 D or greater could lead to a

clinical change of 0.25 D in the spherical component of refractive error (i.e.

optimal lens power). Of course, a patient wearing the lens could accommodate

to compensate for the hyperopic shift that occurs as a result of this shift in M in

the minus direction. It would be interesting to know if a change in refraction

persists after longer term RGP lens wear. The changes in best fit sphere M with

the similar spherical RGP lens in Chapter 3 caused changes in the same

direction of slightly greater magnitude (mean change of 0.31 D for 4 mm

diameter and 0.34 D in 6 mm diameter).

The higher order aberrations of the eye tended to be larger after wear of

both types of RGP lens, but these differences did not reach significance. Both

lens types led to a slight posterior corneal curvature steepening and slight

corneal swelling. The corneal swelling was slightly greater in the periphery

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compared with the central region of the cornea, but the difference did not reach

statistical significance.

The spherical lens caused posterior corneal steepening and corneal

swelling which were similar in magnitude and direction to that seen in Chapter

3 with the spherical RGP lens of the same diameter on more spherical corneas.

The mean peripheral corneal swelling with RGP/9.5 lens reported in Chapter 3

was 6.26 µm (0.97%) in the periphery compared to 5.05 µm (0.8%) in this study

and the mean posterior corneal steepening was –0.01 mm compared to –0.02

mm in this study. However these findings could not reach statistical

significance. The corneal swelling with the toric lens (6.95 µm or 1.1% in

periphery) was of a slightly greater magnitude compared to the spherical lens in

this study but this did not reach statistical significance. The toric lens also

showed a significant posterior peripheral steepening of about 0.03 mm, and a

weak correlation with peripheral corneal swelling, but this was not significant.

Changes in anterior and posterior corneal curvature and thickness due

to a back surface toric RGP lenses were very similar to those produced by the

spherical back surface RGP lens. The differences in forces applied by the two

lens types were obviously relatively small. The differences caused by the lens

types may have been greater if the lenses had centred more accurately, if the

degree of astigmatism was higher, if the axis of astigmatism was different, if

more of the subjects had limbus-to-limbus astigmatism or if the lenses were

worn for longer periods of time.

4.6 Conclusion

To summarize, we found a greater decrease in corneal refractive power M, and

decrease in WTR astigmatism with the spherical lens compared to the back

surface toric lens for 6 mm corneal diameter. The decrease in astigmatism

found in the subject with limbus-to-limbus astigmatism was smaller than the

other subjects with central astigmatism. These findings provide some evidence

that the pressure distribution on a toric cornea by a spherical lens can vary

from a back surface toric lens, but a study with a greater number of subjects

with astigmatic corneas would help to clarify these issues.

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Chapter 5

Eyelid changes following short-term rigid and soft contact lens wear

5.1 Introduction

In Chapter 3, corneal thickness, curvature, refractive power and ocular

wavefront error changes after the short-term use of a range of lenses including

PMMA, RGP (2 different diameters) and SiHy were reported. In this chapter,

changes in the characteristics and biometrics of the eyelid structures that

interact with the contact lenses, such as the tarsal conjunctiva, the eyelid

margin (lid-wiper) and eyelid position are described in the same group of

subjects.

Previous research has noted that a range of changes in the eyelids and

related structures can be associated with contact lens wear. Blepharoptosis

has been reported to occur from 2 weeks (Fonn and Holden 1988) to longer

use (3 months to 10 years) of RGP and PMMA contact lenses (Van den Bosch

and Lemij 1992; Fonn et al. 1995). These changes in eyelid position can be

unilateral (Uchinuma et al. 1983; Fonn and Holden 1986) or bilateral (Fonn et

al. 1995; Fonn et al. 1996). The ptosis has been attributed to trauma during

lens removal (Van den Bosch and Lemij 1992), lid inflammation or edema from

lens edges or deposits (Fonn and Holden 1986; Levy and Stamper 1992), and

to a thinning or disinsertion of the levator aponeurosis (Van den Bosch and

Lemij 1992). Some reports suggest that long term soft contact lens wear also

increases the risk of developing aponeurogenic ptosis (Reddy et al. 2007;

Wubbels and Paridaens 2009), however other investigators suggest that there

is no evidence of ptosis in soft contact lens wearers compared to non-lens

wearers (Fonn et al. 1996). Although the longer term influence of contact lens

wear upon eyelid position has been well studied, no previous study has

investigated whether the short-term use of rigid or soft contact lenses leads to

changes in the palpebral aperture.

One of the most common and severe forms of contact lens induced

tarsal conjunctival changes is contact lens papillary conjunctivitis (CLPC)

(Allansmith et al. 1977; Allansmith et al. 1978; Korb et al. 1980; Efron 1999). It

has been reported in both soft and hard contact lens wearers mostly after long

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term wear (3 weeks to a few years) (Allansmith et al. 1977; Maldonado-Codina

et al. 2004). Although CLPC occurs with all types of contact lens wear, it is

more common with silicone hydrogel lenses (Skotnitsky et al. 2002).The

characteristic signs of CLPC are enlarged papillae with hyperemia causing

itching, discomfort, foreign body sensation, contact lens displacement and

decentration. There can also be mucoidal discharge causing blurring of vision

(Allansmith et al. 1977).

Distribution of papillae on the tarsal conjunctiva depends on the type of

lens. Hard PMMA contact lenses most commonly result in development of

papillae in zones close to the central tarsal conjunctiva and lid margin whereas

soft contact lenses cause papillae close to the tarsal fold (Korb et al. 1980;

Korb et al. 1981; Korb et al. 1983). This suggests that the site of mechanical

trauma to the conjunctival surface plays a role in development of CLPC

(Elgebaly et al. 1991). Other factors associated with CLPC aside from

mechanical trauma include meibomian gland dysfunction (Martin et al. 1992;

Mathers and Billborough 1992), type I and type IV hypersensitivity (Begley et

al. 1990; Metz et al. 1997) and deposits on the lens surface (Fowler et al.

1985).

As the tarsal conjunctiva is in direct contact with lenses during wear, the

redness and roughness of the tarsal conjunctiva has been investigated by

many researchers using various grading scales (Efron et al. 2001; Skotnitsky et

al. 2002; Maldonado-Codina et al. 2004). Greater tarsal conjunctival redness

was noted in soft contact lens wearers compared to PMMA contact lens

wearers (Korb et al. 1981; Korb et al. 1983). Covey et al. (2001) have reported

no significant difference in palpebral roughness or redness between long-term

high-Dk silicone hydrogel contact lens wearers and non-lens wearers. While

palpebral redness and roughness are well documented in contact lens wearers,

tarsal staining with fluorescein secondary to the use of different types of contact

lenses has not been systematically studied and a relevant grading scale has

not been reported.

The „lid-wiper‟, is a term introduced by Korb et al. (2002), to describe

the portion of the marginal conjunctiva of the upper eyelid that is thought to

make contact with the ocular surface (or contact lens surface) during blinking.

Shaw et al. (2010) showed using carbon paper imprinting, that a region of the

eyelid margin (of ~0.6 mm width) was applying pressure to the ocular surface.

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This region is thought to act as a wiper of the ocular/lens surface by spreading

and rejuvenating the tear film. Lid-wiper epitheliopathy (a condition

characterized by increased staining of the epithelial cells in the lid wiper region)

has been reported in the majority contact lens wearers with dry eye symptoms

and a smaller percentage of asymptomatic soft contact lens wearers using

fluorescein and rose bengal staining (Korb et al. 2002) and using fluorescein

and lissamine green (Yeniad et al. 2010). In both of these studies, the subjects

were long term daily-wear soft contact lens users (at least 1 year and 6 months

respectively). The influence of shorter periods of contact lens wear upon the

presence and severity of lid-wiper staining is not known.

Contact lenses are in contact with the eyelids and tarsal conjunctiva

during wear, creating a source of potential friction and micro-trauma. In this

study we aimed to investigate whether the short-term use (about 8 hours) of a

variety of contact lenses (PMMA, RGP and silicone hydrogel lenses), was

associated with changes in the eyelids that have their origin in a mechanical

interaction with the lens. These potential sequelae included blepharoptosis of

the eyelids, the presence and severity of eyelid wiper staining, and the

presence and severity of tarsal conjunctival staining (using a newly developed

grading scale designed as part of this study).

5.2 Methodology

The study was conducted over a period of 5 days. Measurements were taken

at the start and end of an 8 hours period on each day along with the

measurements for Chapter 3 (see Figures 3-2 & 3-3). On day one, baseline

measurements were taken without any contact lens in the eye, in the morning

(between 8 and 11 am and at least 2 hours after waking) and 8 hours later, in

the afternoon (between 4 and 7 pm). Four different types of contact lenses

were worn by the subjects on days 2, 3, 4 and 5 of the study in the left eye

only.

An approval was obtained from QUT university human research ethics

committee (Appendix A) and the tenets of declaration of Helsinki were followed.

All subjects gave a written informed consent before participation in the study.

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5.2.1 Subjects

The subject group was the same as described in Chapter 3 (fourteen young,

adult subjects aged 20 to 33 years, mean age 27.8 ± 4.0 years) with visual

acuity of 6/6 or better and corneal astigmatism of ≤ 1.5 D. The mean spherical

equivalent refractive error was –0.6 ± 1.3 D. Prior to participation in the study,

all subjects were screened for any anterior segment abnormalities using slit-

lamp biomicroscopy. Subjects were also screened for any tear film

abnormalities using a range of tests including McMonnies questionnaire

(McMonnies and Ho 1987), tear film break up test, Phenol red thread test and

fluorescein and lissamine green staining of the cornea and conjunctiva. None of

the subjects had a history of corneal injury, infection or surgery. None of the

subjects were previous rigid contact lens wearers. Two of the 14 subjects were

regular soft contact lens wearers but were asked to discontinue lens wear one

month prior to the start of the study, to allow the effects of soft lens wear to

largely resolve. It was calculated that a sample size of 14 subjects used in this

study would give 80% power to detect a change of 0.16 mm in lid position, 0.37

grade units in tarsal staining, and 0.57 grade units in lid-wiper staining at the

0.05 level of significance.

5.2.2 Contact lenses

Three different types of custom made rigid contact lenses and one soft lens

were worn by each subject. A contact lens trial fitting was performed with the

rigid lenses before ordering the lenses. The details of the lenses have been

described in Chapter 3 (Table 3-1). The rigid lenses had a spherical BOZR and

an aspheric periphery with a total diameter of 9.5 and 10.5 mm and were not

plasma treated. The soft lens used was Bausch and Lomb, PureVision silicone

hydrogel lens which had a diameter of 14.0 mm. The order of lens wear was

randomised and a recovery period of at least 2-3 days was scheduled after the

use of each lens and before the wear of another lens. Recovery of any

substantial eyelid changes (tarsal and lid-wiper staining or blepharoptosis) was

checked for all subjects before the wearing of a subsequent lens.

5.2.3 Measurements and Instruments

A questionnaire was completed by each subject to monitor the visual tasks

performed during the 8 hours of lens wear. Subjects were found to be engaged

in similar tasks (most commonly computer work) during the period of the study

each day. The morning and afternoon measurements on each of the contact

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lens wearing days were conducted at around same time of day as on day 1

(baseline day), to allow comparison without confounding effects due to diurnal

variations.

Digital photos of the external eyes in primary gaze were captured at the

start and end of each measurement day to compare the position of the upper

and lower eyelid (with respect to the centre of the limbus) of the left eye

(contact lens wearing eye) and the right eye (control eye). On contact lens

wearing days the images were captured with lens in situ, in the morning at

about 30 minutes after lens insertion and in the afternoon (8 hours later) just

before lens removal. A Fujifilm FinePix 9 mega-pixel digital camera (S9500),

with macro mode was used with the camera‟s built-in flash and automatic

aperture/shutter speed mode. The camera was positioned directly in front of the

subject, at approximately 50 cm (Figure 5-1 a), with a ruler in place next to the

eye for calibration of image size during the analysis (Figure 5-1 b). The subject

was positioned in the head rest with the eyes in primary gaze, with the

instruction to fixate the centre of the camera lens. The image was captured 1

sec after a gentle, complete blink and a total of 4 images (both eyes together,

in one image) were captured (Figure 5-2). All images were taken in the same

room with approximately the same humidity (56.3 ± 5.5%) and temperature

(24.1 ± 0.3ºC) and similar ambient lighting conditions with standard overhead

fluorescent lights (approximately 250 lux at the plane of the subject‟s eye) at

approximately the same time of day on each study day (morning and

afternoon).

Figure 5-1: (a) Set up of digital camera to take the photo of external eyes (b) Ruler next to the eye to allow calibration.

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Figure 5-2: (a) Position of eyelids on the baseline day afternoon (no contact lens in eyes) (b) Position of eyelids with contact lens in left eye in the afternoon after 8 hours of lens wear. Palpebral aperture (PA) height is shown in mm. Yellow rings indicate the limbus outline, upper eyelid margin is shown in red and lower eyelid margin is shown with blue.

Digital photos of the eyelid margins and upper and lower tarsal

conjunctiva were also taken at each measurement session to record any

corneal or conjunctival staining using a Canon Digital Rebel EOS 300 D 6.3

mega pixels Digital SLR (Canon Inc Tokyo, Japan) camera. On contact lens

wearing days these images were captured in the morning without the contact

lens in eye (after approximately 40 minutes of lens insertion) and in the

afternoon (8 hours later), immediately after the lens removal. The camera was

attached to a slit lamp biomicroscope and was used in automatic

aperture/shutter speed mode. The images were adjusted for colour balance

and the same colour balance setting was used for all the images. The slit lamp

magnification was kept constant at 10X and slit aperture was kept at maximum

length and width. The images were taken in the same room with approximately

the same humidity (58.0 ± 5.2%), temperature (24.9 ± 1.0 ºC), and ambient

lighting conditions (slit lamp illumination: approximately 990 lux at the plane of

the subject‟s eye) at approximately the same time of day every day (morning

and afternoon). The subjects were positioned in the head rest of the slit lamp

biomicroscope with the eyes in primary gaze and fixating the examiner‟s right

ear for left eye photos and vice versa. The upper eyelid was then everted and

photos were taken first under white light, and then with a cobalt blue light

through a Wratten #12 filter after instillation of sodium fluorescein dye (4-5 sec

after instillation) and then under white light after instillation of lissamine green

dye (4-5 sec after dye instillation). Two images were captured in each condition

and the best quality image of the two was used for staining grading.

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Fluorescein (Fluorets, fluorescein sodium sterile ophthalmic strips) and

lissamine green strips were moistened with a single drop of unpreserved sterile

saline and lightly touched on the upper bulbar conjunctiva with the subject

looking down.

5.3 Data Analysis

5.3.1 Eyelid position (Blepharoptosis)

The digital images of the external eyes were analysed to determine the position

and diameter of the limbus and the position of the upper and lower eyelids,

using custom-written software (Imetrics, developed at Contact Lens and Visual

Optics Laboratory). This analysis involves the operator manually locating the

position of the coordinates of the limbus (8 points), the upper lid margin (8

points) and the lower lid margin (8 points). The software then determines the

best fitting ellipse to the limbus co-ordinates and the best fitting quadratic

function to the upper and lower eyelid coordinates. The vertical palpebral

aperture (PA) height (i.e. the distance between the upper and lower lid margins

with respect to the limbus centre) is then calculated (Figure 5-3). Palpebral

aperture height was compared for the right (control eye, no lens) and left

(contact lens wearing eye) eyes and to the baseline day measurements. The

analysis of the digital images was performed by an independent masked

observer on two images and the mean of the two was taken as the PA. All the

images were calibrated for size by using a ruler next to the eye in each image

frame. Repeated measures ANOVA with lens, time of the day (am/pm) and eye

(right/left) as within-subject factors, was performed to calculate the statistical

significance of the changes. Bonferroni correction was applied to avoid any

error due to multiple comparisons.

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Figure 5-3: External photo of the eye showing the palpebral aperture (PA) height with respect to limbus centre. Markings in yellow indicate the limbus margins, Markings in red indicate the position of upper lid and markings in blue indicate the position of the lower lid.

5.3.2 Tarsal staining

The grading of the digital slit lamp images of the upper tarsal conjunctiva was

performed by an independent masked examiner. Since a grading scale for

tarsal staining using fluorescein is not available and has not been described

before, a grading scale for this purpose was developed as part of this study.

The tarsal conjunctiva staining was graded on a five-point scale of increasing

severity from 0 (none) to 4 (severe) with possible grading increments of 0.1

units, as shown in Figure 5-4. This classification approach was adapted from a

grading scale described by Efron (1998). The five-step grading scale is widely

used in the field of contact lenses (Woods 1989) and grading to nearest 0.1

scale unit allows greater sensitivity and accuracy in detecting a change in the

severity of a complication (Bailey et al. 1991). The data was not assumed to be

normally distributed, so the Wilcoxon signed rank sum test was used to test the

statistical significance of the changes. Bonferroni correction was applied to

avoid any error due to multiple comparisons.

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Figure 5-4: Digital images showing upper tarsal conjunctival staining with fluorescein on a grade of 0 (None) to 4 (Severe).

5.3.3 Lid-wiper epitheliopathy

Lid-wiper epitheliopathy was evaluated for the upper eyelid using a modification

of the grading system described by Korb et al. (2005) (Table 5-1). The grading

of the digital slit lamp images of the lid margin was performed by an

independent masked examiner using both fluorescein and lissamine green

stains. The grading of lid-wiper staining was carried out for both the horizontal

length and sagittal width of the fluorescein and lissamine green staining of the

lid-wiper as shown in Table 5-1. These gradings were then averaged to obtain

the final score for each subject. Thus the final score was the mean of the four

values (grade of horizontal length and sagittal width staining with fluorescein

and grade of horizontal length and sagittal width staining with lissamine green).

The final score determined whether the lid-wiper epitheliopathy was graded as

mild, moderate or severe (Table 5-2). Examples of grading from some

representative images are shown in Figure 5-5. The data was not assumed to

be normally distributed so the Wilcoxon signed rank sum test was used to test

the statistical significance of the changes. Bonferroni correction was applied to

avoid any error due to multiple comparisons.

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Table 5-1: Grades of horizontal length and sagittal width staining of lid-wiper. Grading was done using both fluorescein and lissamine green.

Horizontal length of staining

Grade Sagittal width of staining Grade

< 10% 0 < 2% of the width of wiper 0

10 – 20% 1 25% - < 50% of width of wiper 1

20 – 30% 2 50% - <75% of the width of wiper 2

> 40% 3 ≥ 75% of the width of wiper 3

Table 5-2 Lid-wiper epitheliopathy classification system for final score as described by Korb et al. (2005).

Lid-Wiper Epitheliopathy Classification

Grade 1 (Mild) 0.25 – 1.00

Grade 2 (Moderate) 1.25 – 2.00

Grade 3 (Severe) 2.25 – 3.00

Figure 5-5: Examples showing grading of lid-wiper epitheliopathy from three representative subjects.

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5.4 Results

5.4.1 Eyelid position (Blepharoptosis)

The type of lens (p<0.001, repeated measures ANOVA) had a significant effect

on the changes in the height of the PA. The group mean change in height of PA

relative to baseline, in the right (control) and left eye (contact lens-wearing eye)

in the afternoon is shown in Figure 5-6. Two out of three rigid lenses resulted in

a significant decrease in the PA height in the left eye after 8 hours of lens wear,

compared to the baseline day. The PMMA/9.5 (–0.78 ± 0.67 mm, p=0.008) and

RGP/10.5 (–0.73 ± 0.76 mm, p=0.03) lenses showed significant decreases in

the vertical palpebral aperture height, whereas the reduction with the RGP/9.5

(–0.46 ± 0.52 mm, p=0.06) lens approached significance (Figure 5-6). The SiHy

contact lens did not cause any significant changes in PA height. There was no

significant effect of time of the day (morning or afternoon) on the PA height

either on baseline or on contact lens wearing days. The non-lens wearing right

eye on average also showed a small decrease in PA height in the afternoon

compared with the baseline day, but this was not significant with any of the

lenses. The decrease in PA following RGP/9.5 lens wear in the nonlens-

wearing right eye was greater than in the contact lens-wearing left eye, but this

difference was not significant.

Figure 5-6: Changes in height of palpebral aperture (mm) of the right and left eye relative to baseline afternoon. Negative values mean that palpebral aperture height is less compared to baseline afternoon. Each error bar represents one standard error of the mean. # represents statistically significant p-values (<0.03), * represents p-value approaching significance (p=0.06). Right eye is control eye (no lens) and left eye is the contact lens wearing eye.

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5.4.2 Tarsal conjunctival staining

The group mean changes in tarsal staining relative to baseline, in the left eye

(contact lens wearing eye) in the morning and afternoon are shown in Table 5-

3. The PMMA/9.5 (1.31 ± 0.88, p=0.008), RGP/9.5 (1.44 ± 0.89, p<0.008), and

RGP/10.5 (1.65 ± 0.72, p<0.004) lenses showed significant increases in tarsal

staining in the afternoon (after 8 hours of lens wear) compared to the baseline

day in the afternoon (Table 5-3). The soft lens did not cause any significant

changes. There was also a small increase in tarsal staining in the morning after

45 minutes of contact lens wear (compared to baseline morning) which was

significant with the RGP/9.5 (0.85 ± 0.87, p=0.03) and RGP/10.5 lenses (0.71 ±

0.69, p=0.03) (Table 5-3). A significant increase in tarsal staining was observed

in the afternoon after 8 hours compared to the morning with all the three rigid

lenses and the changes approached significance with the soft lens (Table 5-4).

There was also a slight but significant increase in staining on the baseline day

without lens wear (Table 5-4).

Table 5-3: Changes in upper tarsal conjunctival staining (relative to baseline) in morning and afternoon. Positive values mean that tarsal staining has increased compared to baseline. Note: The increase in staining in the mornings following approximately 45 minutes of lens wear.

Lens Tarsal staining change (relative to baseline) ± SD (p-value)

Morning

(after 45 mins of lens wear ) Afternoon

(~8 hrs after lens wear)

Lens PMMA/9.5 0.57 ± 0.75 (p=0.07) 1.31 ± 0.88 (p=0.008)

Lens RGP/9.5 0.85 ± 0.87 (p=0.03) 1.44 ± 0.89 (p=0.008)

Lens RGP/10.5 0.71 ± 0.69 (p=0.03) 1.65 ± 0.72 (p=0.004)

Lens SiHy/14.0 0.41 ± 0.92 (p=0.49) 0.43 ± 0.81 (p=0.56)

Table 5-4: Changes in upper tarsal conjunctival staining in the afternoon (relative to morning). Positive values mean that tarsal staining has increased compared to morning.

Lens Tarsal staining change in afternoon (relative to

morning) ± SD (p-value)

Baseline 0.49 ± 0.30 (p=0.01)

Lens PMMA/9.5 1.22 ± 0.54 (p=0.01)

Lens RGP/9.5 1.07 ± 0.68 (p=0.01)

Lens RGP/10.5 1.42 ± 0.64 (p=0.01)

Lens SiHy/14.0 0.51 ± 0.67 (p=0.07)

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5.4.3 Lid-wiper epitheliopathy

The group mean lid-wiper staining relative to baseline, in the left eye (contact

lens wearing eye) in the morning and afternoon are shown in Figure 5-7.

Amongst the lenses, all the three rigid contact lenses PMMA/9.5 (0.88 ± 0.90,

p=0.01), RGP/9.5 (0.77 ± 0.91, p=0.02) and RGP/10.5 (0.73 ± 0.94, p=0.04)

caused a significant increase in lid-wiper staining after lens wear compared to

baseline afternoon. The changes with SiHy/14.0 lens (0.57 ± 0.90, p=0.07)

approached significance. The change in lid-wiper staining in the afternoon

(relative to morning) is shown in Table 5-5. All the four lenses, PMMA/9.5 (0.75

± 0.37, p<0.001), RGP/9.5 (0.68 ± 0.35, p<0.001), RGP/10.5 (0.59 ± 0.50,

p=0.001) and SiHy/14.0 (0.71 ± 0.60, p=0.001) caused a significant increase in

lid-wiper staining in the afternoon compared to morning (Table 5-5). There was

also an increase in lid-wiper staining on the baseline day (no contact lens)

(0.32 ± 0.50, p=0.18) in the afternoon compared to morning, but this was not

significant.

Figure 5-7: Mean lid-wiper epitheliopathy grades with different types of contact lenses and on the baseline (BL) day (no contact lens), in the morning and afternoon. p-values indicated for change in lid-wiper compared to baseline. # indicates p-value <0.05. Each error bar indicates one standard deviation.

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Table 5-5: Changes in lid-wiper staining grade in the afternoon (relative to morning). Positive values indicate increased staining compared to morning.

Lens Lid-wiper staining change in afternoon (relative

to morning) ± SD (p-value)

Baseline 0.32 ± 0.51 (p=0.18)

PMMA/9.5 0.75 ± 0.37 (p=0.01)

RGP/9.5 0.68 ± 0.35 (p=0.01)

RGP/10.5 0.59 ± 0.50 (p=0.01)

SiHy/14.0 0.71 ± 0.60 (p=0.01)

5.4.4 Association between blepharoptosis and tarsal conjunctival staining

To study the association between the blepharoptosis (assumed to be

secondary to eyelid swelling caused by mechanical trauma and/or

inflammation) and tarsal conjunctival staining (assumed to be secondary to

mechanical trauma), a Spearman‟s correlation was calculated for these

parameters. There was no significant correlation (R2 = 0.086, p=0.77) between

these parameters.

5.5 Discussion

We found small decreases in the vertical PA height in both right (control eye)

and left (contact lens-wearing) eyes after eight hours of contact lens wear when

compared to baseline day measurements, but only the changes with the

PMMA/9.5 and RGP/10.5 lenses were significant. The SiHy lens also caused a

slight decrease in PA height, but these changes were not significant. Our

results are consistent with previous studies which have shown a reduction in

PA height with long-term (2 weeks to 10 years) rigid contact lens wear (Fonn

and Holden 1988; Van den Bosch and Lemij 1992; Fonn et al. 1996). This is

the first study to report the effect of short-term (few hours) contact lens wear on

PA height.

Decrease in the PA of the contact lens-wearing eye has been well

established but the causes are still unclear. The suggested causes of changes

in PA size due to contact lenses include trauma to the levator muscle due to

squeezing of the eyelids (during lens removal), forceful rubbing (during blinking

or otherwise) (Van den Bosch and Lemij 1992), due to the lens edge (Sobara et

al. 1993) and irritation of the lid, leading to oedema or blepharospasm (Van den

Bosch and Lemij 1992). Epstein and Putterman (1981) reported the presence

of a disinsertion of the levator aponeurosis associated with ptosis due to

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prolonged rigid lens wear, which they surgically corrected. As our lid aperture

changes occurred after only 8 hours of lens wear, it is unlikely that substantial

trauma or changes to the levator muscle could have occurred in this time. The

changes in palpebral sizes noticed in our subjects could therefore possibly be

due to mechanical irritation leading to lid oedema (Van den Bosch and Lemij

1992) as evidenced by the significant increase in tarsal staining associated with

lens wear, although the change in eyelid position was not significantly

correlated with the tarsal staining. While, we did not follow our subjects to

monitor the recovery in the size of the PA, based on previous studies (Fonn

and Holden 1986; Fonn and Holden 1988; Fonn et al. 1995) we speculate that

the PA size in our group of subjects will increase and normalize after lens

removal.

The PMMA contact lens caused a slightly greater decrease in PA height

compared to the RGP lenses (but not statistically significant, p>0.05) and the

changes with the SiHy contact lenses were smaller again (and not statistically

significant). This differential effect between lens types could arise from one or

more characteristics of the lens. The modulus of elasticity of the PMMA contact

lens is approximately 2000 MPa compared to RGP (1500 MPa) and SiHy (1.1

MPa) contact lenses. The diameters of PMMA/RGP lenses (9.5 and 10.5) were

smaller than the soft (14.0) lens. The differences in lens edge design of contact

lenses is another important factor which determines the interaction between

contact lens and eyelids and thus affects the comfort (La Hood 1988; Picciano

and Andrasko 1989; Andrasko 1991; Caroline et al. 1991). It is conceivable that

one or more of these factors, including the modulus of elasticity, total diameter,

and edge design of the lenses, may influence the changes in eyelid position

after lens wear, presumably due to differences in the mechanical interactions

between the lids and the contact lens.

Changes in the PA with lens wear could also potentially be related to

irritation of the cornea or lids leading to blepharospasm due to the presence of

the contact lenses (Van den Bosch and Lemij 1992) or swelling due to

mechanical trauma. Our group of subjects had no previous experience of

contact lens wear and had no adaptation period to lens wear before

commencement of the study. Therefore, it is likely that our subjects were

sensitive to the initial discomfort caused by the lenses and may have adopted a

narrower PA in order to reduce the mechanical interaction of the lenses with

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the eyelids. If the changes are a reflex response (i.e. a blepharospasm), we

would expect the lower eyelid to get higher along with the upper eyelid getting

lower (i.e. since the orbicularis oculi muscle is contracting we would expect

changes in both the upper and lower lid). To investigate this hypothesis, the

mean distance from the limbus to the upper lid and the mean distance from the

limbus to the lower lid was calculated for the right and left eye and compared

for the baseline and the three rigid contact lens wearing days. We found the

distance of both the upper and lower lid from the limbus centre reduced

significantly after lens wear compared to baseline (p<0.05). Thus the origin of

the changes may be partly related to „reflex‟ lid movements associated with

contraction of the orbicularis oculi muscle (changes in the both upper and lower

lid), along with possible inflammation due to mechanical trauma (Van den

Bosch and Lemij 1992) (swelling changes greater in the upper lid compared to

lower lid).

We found a decrease in PA in both right (control eye, no contact lens)

and left eyes (contact lens- wearing eye). This could be attributed to innervation

to the orbicularis oculi which is bilateral (Forester et al. 2008), therefore, the

irritation due to the presence a contact lens in one eye causes blepharospasm

in both eyes (contact lens-wearing as well as control eye).

The tarsal conjunctiva follows the lid-wiper during a blink and rubs

against the ocular/contact lens surface which can result in trauma to this

surface over the course of the day. We investigated the changes in tarsal

conjunctival staining after short-term wear of different contact lenses, increases

in which are likely to indicate micro-trauma to the surface. There was a

significant increase in the amount of tarsal staining after 8 hours of wear of all

three rigid/hard contact lenses (PMMA/9.5, RGP/9.5 and RGP/10.5),

suggesting the relatively stiff lens edge (high modulus of elasticity) and/or lens

edge design of these lenses could be responsible for increased surface trauma

of the palpebral conjunctiva. The soft lens with a lower modulus conforms to

the ocular surface (Holden and Zantos 1981) and has an edge profile that is

thinner and more tapered than standard rigid lens designs and this increase in

tarsal staining over the 8 hours of lens wear only approached significance. The

increase in tarsal staining on the baseline day (without any contact lens) could

be a result of conjunctival surface constantly rubbing against the ocular surface

during blinking and this may be evidence of a diurnal increase in tarsal staining.

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Previous research has revealed that the tear film on a contact lens

surface dries more rapidly compared to that on the cornea (Cedarstaff and

Tomlinson 1983; Thai et al. 2004). The increased tarsal staining we found

could also be partly attributed to an increased friction between the lens and

palpebral conjunctiva compared to that between the natural cornea and

palpebral conjunctiva. Studies of longer-term contact lens users have reported

contact lens-related papillary conjunctivitis to be more common and severe in

soft contact lens wearers compared to rigid contact lens wearers (Alemany and

Redal 1991). This difference could be attributed to the use of contact lenses for

a longer duration resulting in increased deposits or protein on lens surface

(Allansmith et al. 1977; Fowler et al. 1985; Dumbleton 2003). However our

short-term data would argue against the likelihood of any increased micro-

trauma of the palpebral conjunctiva as a contributing factor to increased CLPC

in soft lens wearers. Some previous studies using impression cytology have

demonstrated squamous metaplasia and goblet cell loss in the tarsal

conjunctiva which was seen to increase with the duration of rigid contact lens

wear (Saini et al. 1990). In another study by Anshu et al. (2001), it was found

that symptomatic contact lens wearers showed a decrease in goblet cell

density and these changes were more severe in soft contact lens wearers

compared to rigid contact lens wearers. The authors attributed this difference to

larger diameter, larger surface contact area, and increased deposits on soft

lenses compared to rigid lenses.

We found an increase in tarsal conjunctival staining in the morning

measurements with both the RGP contact lenses compared to the morning of

baseline day (no contact lens). This means that the lenses caused some

amount of tarsal staining even in the short period of lens wear of about 40

minutes before the staining measurements were taken. We also found a

significant diurnal increase in tarsal staining even on the baseline day when no

lenses were worn, suggesting that the tarsal conjunctival surface is in constant

friction with the ocular surface (during blinking) resulting in some minor surface

damage over the course of the normal day. A previous cytological study (Hirji et

al. 1984) has found a decrease in the total conjunctival cell count over 9 hours.

This decrease in cell count is possibly related to the increased tarsal staining

we found at the end of the day.

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A grading scale for tarsal conjunctival staining has been described and

used for the first time in this study. In the future it would be prudent for this

grading scale to be validated for inter- and intra-observer variability. In this

study, the grading was performed by the one experienced and masked

clinician, so the results are unbiased and should have the ability to discriminate

across a range of levels of tarsal conjunctival staining.

In a contact lens-wearing eye, the lid-wiper region of the upper lid

moves across the edge of the lens with each blink as it spreads the tears

across the lens and ocular surface. This may result in micro-trauma to the

surface epithelial cells in the lid-wiper region that can be identified using

fluorescein and rose bengal staining of the lid margin (Korb et al. 2002; Korb et

al. 2005) or fluorescein and lissamine green staining (Yeniad et al. 2010). Lid-

wiper epitheliopathy has been reported in 13% (Korb et al. 2002) and 32 %

(Yeniad et al. 2010) of asymptomatic long-term soft contact lens wearers. The

prevalence of lid-wiper epitheliopathy is much higher in symptomatic contact

lens wearers (Korb et al. 2002; Yeniad et al. 2010) and dry eye patients (Korb

et al. 2005). In the current study, we found a significant increase in lid-wiper

staining in the afternoon compared to mornings with all of the contact lens

types. There was also an increase in lid-wiper staining on the baseline (no

contact lens) day but this was not statistically significant. On average, there

was also an increase in the magnitude of lid-wiper staining in the afternoons of

the contact lens-wearing days compared to the baseline day afternoons.

However only the changes associated with the rigid lenses were significantly

different to the baseline day afternoon. This is the first study to report lid-wiper

epitheliopathy in as little as 8 hours wear of rigid and soft contact lenses.

The increase in lid-wiper staining during the course of the day is most

likely due to the constant friction between the lid-wiper and the contact lens

surface, edge and/or the ocular surface during each blink. The increase in

staining observed on the contact lens wearing days (particularly with the rigid

lenses) suggests that the contact lens edge and surface cause a greater

amount of friction with the lid-wiper region as compared to the ocular surface

alone (Korb et al. 2005). This highlights the need for smoother contact lens

edges and more lubricious surfaces, to minimise the interactions between the

lid margin and contact lens surface during lens wear. Plasma treatment has

been used for both rigid (Yin et al. 2008; Yin et al. 2009) and silicone hydrogel

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133

(Valint Jr et al. 2001; Valint Jr et al. 2001) contact lenses to increase their

lubricity, thus reducing friction and improving comfort, while the addition of a

wetting agent to soft lenses (for example polyvinyl pyrrolidone) is another

method employed to increase lens lubricity.

On further examination of lens edges after the completion of data

collection using a high resolution OCT (Copernicus HR SD OCT, Optopol

Technology SA, Zawiercie, Poland), we found that the lens edges of the PMMA

and RGP lenses were not tapered anteriorly. Figure 5-8 a and b shows the

temporal and nasal lens edge profiles of the RGP/9.5 mm lens. Similar edge

profiles were found with the RGP/10.5 and PMMA contact lens used in the

study. Temporal and nasal lens edge profiles of the soft contact lens are shown

in Figure 5-8 c and d. Lens edge profiles were checked after the lenses were

received from the manufacturer and before the start of the study. However the

OCT technique was not available before the start of the experiment, so we

were unable to make these detailed observations. These lens edge OCT

images suggest that some of the lid changes seen in this experiment could be

attributed to the poor manufacture of lens edges.

Figure 5-8: Lens edge profiles of the RGP/9.5 and soft lenses used in the study. PMMA and RGP/10.5 lenses had similar edge profiles to the RGP/9.5 lens.

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The increase in the lid-wiper staining on the baseline day (no contact

lens) suggests that there could be some natural diurnal changes occurring in

the lid-wiper staining due to friction with the ocular surface during each blink

throughout the day. Given the normal blink rate of 12 to 15 blinks per minute

(Carney and Hill 1982; Moses 1987), this will lead to more than 10,000 blinks

per day.

5.6 Conclusion

To conclude, we found small decreases in the vertical PA height in both

the right (control eye) and left (contact lens wearing) eyes after eight hours of

contact lens wear when compared to baseline day measurements, but only the

changes in the hard/rigid contact lens wearing eyes were statistically

significant. There was a significant increase in the amount of tarsal staining

after 8 hours wear of all the three rigid/hard contact lenses (PMMA/9.5,

RGP/9.5 and RGP/10.5), suggesting that the lens edge (due to higher

modulus), poor edge manufacture or surface friction with the rigid lenses

resulted in increased surface trauma of the palpebral conjunctiva. The

comparatively soft lens did not result in a significant increase in tarsal staining.

We also found a significant increase in the magnitude of lid-wiper staining in

the afternoons of the three rigid/hard contact lens-wearing days. These results

highlight the importance of good lens edge manufacture and reducing the

friction between the lens surface/edge and the palpebral conjunctiva/eye-lid

wiper during blinking.

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Chapter 6

Tear film surface quality with rigid and soft contact lenses

6.1 Introduction

Contact lens wearers report more dryness symptoms compared to non-wearers

(Millodot 1978; McMonnies and Ho 1987; Brennan and Efron 1989; Orsborn

and Robboy 1989; Doughty et al. 1997; Vajdic et al. 1999), which suggests that

contact lens wear exacerbates marginal tear dysfunction. Up to 50% of all

contact lens wearers have been reported to have some symptoms of dry eye

(Doughty et al. 1997). Additionally, the frequency and severity of dryness

symptoms is higher with contact lenses than without lenses in the same set of

subjects (Cedarstaff and Tomlinson 1983; Lemp 1995; Begley et al. 2000;

Begley et al. 2001).

Contact lens wear alters the structure of the tear film by breaking it into

a pre-lens and a post-lens tear film, where the pre-lens tear film is thought to be

composed of superficial lipid and aqueous layer and the post-lens tear film is

composed of aqueous and mucin layers (Asbell and Uçakhan 2006). Contact

lenses are reported to lead to a variety of changes in the tears including a

reduction in tear break up time (Young and Efron 1991; Guillon and Guillon

1994), increased evaporation of the tear film (Tomlinson and Cedarstaff 1982;

Lemp 1995), increased tear osmolarity (Gilbard et al. 1986; Iskeleli et al. 2002)

and a thinner pre-lens tear film (Wang et al. 2003; Nichols and King-Smith

2004).

The average pre-lens tear film thickness measured using interferometry

has been reported to be 3 µm (King-Smith et al. 2004) with the lipid layer often

being thin or absent (Young and Efron 1991; Craig and Tomlinson 1997;

Nichols and Sinnott 2006). The pre-lens lipid layer thickness is less and

thinning rate is higher in RGP contact lens users with dry eye symptoms than

those without the symptoms (Nichols and Sinnott 2006).

The tear film quality is typically assessed clinically in terms of

fluorescein TBUT, as abnormalities in any of the tear film layers (lipid layer,

aqueous layer or mucin layer) results in an unstable tear film causing reduced

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tear break-up time. As the presence of a contact lens divides the tear film into

the pre-lens and post-lens tear film, any break in the pre-lens tear film may not

be readily observed due to the presence of the post-lens tear film in the case of

RGP lenses and absorption of fluorescein by the soft contact lens, resulting in

measurement errors. Therefore, TBUT in contact lens wearers is usually

measured with non-invasive techniques such as the Tearscope (Guillon 1998),

lipid layer interferometry (Nichols et al. 2002; Nichols and King-Smith 2003;

Nichols and Sinnott 2006; Szczesna et al. 2006), wavefront sensing (Thibos

and Hong 1999; Mihashi et al. 2006), meniscometry (Yokoi et al. 2003) and

high-speed videokeratoscopy (Goto et al. 2003). Some of these techniques

have their own limitations such as the Tearscope being a subjective technique,

lipid layer interferometry being sensitive to eye movements, wavefront sensing

depending on pupil size, and high-speed videokeratoscopy often being based

on surface topographic analysis.

Many studies in the past have used videokeratoscopy to estimate the

tear film quality using corneal topography analysis procedures (Nemeth et al.

2002; Goto et al. 2003; Montés-Micó et al. 2004; Iskander and Collins 2005;

Montés-Micó et al. 2005) but these measurements may be inaccurate when the

tear film breaks up (Iskander et al. 2005; Alonso-Caneiro et al. 2008). This is

because videokeratoscopy depends on the specular reflection of the Placido

disc pattern from the surface of tear film and when the tears begin to break up

the topography becomes highly irregular. However, the degree of topographic

irregularity is not closely correlated with the tear stability (Iskander et al. 2005;

Alonso-Caneiro et al. 2008). A newer technique of estimating the TFSQ using

image processing techniques based on the properties of the Placido disk

images has been recently developed and is independent of the surface

topography analysis (Alonso-Caneiro et al. 2008; Alonso-Caneiro et al. 2009).

This method has been used to quantify the TFSQ in eyes with and without soft

contact lenses (Alonso-Caneiro et al. 2009) and has been demonstrated to

exhibit good performance in the detection of patients with dry eye (Szczesna et

al. 2011).

Previous studies have reported no differences in the frequency of

dryness symptoms between RGP and soft contact lens wearers compared to

non-contact lens users (McMonnies 1990; Vajdic et al. 1999). Farris (1986)

suggesting that it is tear disruption which is responsible for dryness symptoms

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irrespective of the type of contact lens. However, the thickness of lipid layer is

associated with stability of tear film (Craig et al. 1995), and this layer is very

thin or absent in rigid contact lens wearers (Guillon 1986). Therefore, TFSQ

with rigid lenses is likely be poorer compared to soft contact lenses.

The reports on the effect of rigid contact lenses on TBUT, which is

affected by lipid layer thickness, are inconclusive with one study reporting a

decrease in TBUT with rigid lenses (Hamano 1981) while another found no

difference in the TBUT of rigid lens wearers and non-contact lens wearers

(Bhatia and Singh 1993). However, in these studies TBUT was measured using

fluorescein which can confound the results due to associated reflex tearing

(Mengher et al. 1986). Rigid (RGP and PMMA) lenses have been shown to

cause a significant reduction in non-invasive TBUT at the 3 and 9 o‟ clock

portions of the conjunctiva (Itoh et al. 1999). Therefore, the aim of this study

was to measure the TFSQ with short-term use of PMMA and RGP contact

lenses and to compare with SiHy contact lenses, using the videokeratoscopy

technique described by Alonso-Caneiro et al. (2009).

6.2 Methodology

6.2.1 Subjects

The same group of fourteen subjects as in Chapter 3 (age range: 20 to 33

years, mean 27.8 ± 4.0 years, 5 females, 9 males), who were mainly students

and staff at QUT participated in this study. All subjects had low corneal

astigmatism (≤ 1.50 D corneal cylinder) and no signs of keratoconus or other

ectatic corneal disorders as seen in corneal topography maps acquired using

the Medmont E300 videokeratoscope. A slit-lamp examination was conducted

to ensure that all the subjects had a normal anterior segment and ocular health.

The subjects were screened for any significant dry eye based on the

McMonnies dry eye questionnaire, fluorescein TBUT, Phenol red thread test

and fluorescein and lissamine green staining of the ocular surface with staining

graded using Efron grading scale (Efron 1998). The group mean results from

these screening tests are presented in Table 6-1.

The subjects did not report the use of any ocular or systemic

medications or the presence of any ocular or systemic disease that may affect

the tear film or prevent the wear of contact lenses. Out the fourteen subjects,

two were soft contact lens wearers, but they discontinued the use of their

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lenses at least one month prior to the start of the study. None of the subjects

had any history of rigid contact lens wear. A written informed consent was

obtained from all subjects after explanation of the procedures. The study

followed the tenets of the Declaration of Helsinki and was approved by the

Queensland University of Technology (QUT) Human Research Ethics

Committee (Appendix A). Sample size calculations revealed that a sample size

of 14 subjects used in this study would give 80% power to detect 0.02 change

in TFSQ at a 0.05 level of significance.

Table 6-1: Dry eye screening tests, screening criterion and mean scores of the study subjects. Subjects who failed 2 or more dry eye tests were not included in the study.

Test Screening

criterion

Mean score ± SD

McMonnies questionnaire Score ≥ 14 4.29 ± 2.52

Fluorescein TBUT < 10 sec 7.38 ± 4.44

Phenol red thread test < 15 mm 19.36 ± 5.30

Corneal fluorescein/Lissamine staining

staining

> 2 0.75 ± 1.05

6.2.2 Instrument

Dynamic high-speed videokeratoscopy was performed using the Medmont

E300, to derive measurements of non-invasive TFSQ. This technique is based

on specular reflection of a Placido disk pattern that is reflected from the surface

of tear film on the cornea or anterior contact lens. The quality of the reflected

ring pattern depends on the regularity of the surface and gives a measurement

of the TFSQ (Kopf et al. 2008; Alonso-Caneiro et al. 2009; Alonso-Caneiro et

al. 2009).

6.2.3 Contact lenses

Subjects wore 4 different types of contact lenses on 4 different days for 8 hours

on each day in the left eye only. Details of the contact lenses used have been

described in Chapter 3 and are shown in Table 6-2. The PMMA and RGP

lenses were custom ordered from Gelflex Laboratories (Perth, Australia). No

surface treatment was ordered for the PMMA or RGP lenses and the lenses

were stored in Boston conditioning solution (Bausch & Lomb Incorporated, New

York, U.S.A.). The SiHy lens used was the commercially available Bausch and

Lomb PureVision lens, which is manufactured with a standard “Performa”

surface treatment. In order to convert the silicone components on the lens

surface into hydrophilic silicate compounds this lens is surface treated in a gas

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plasma reactive chamber (Grobe et al. 1999; Tighe 2004). The exact nature of

this surface treatment is proprietary information. The method of fitting the

lenses is described in Chapter 3.

6.2.4 Protocol

Measurements were performed with the contact lens in eye, in the morning

(between 8 and 11 am) and then repeated in the afternoon (between 4 and 7

pm) just before removal of contact lenses along with the measurements for

Chapter 3 (Figures 3-2 & 3-3). Baseline measurements were also taken both in

the morning and in the afternoon, on a day when no contact lens was worn.

The order of wear of the contact lenses was randomized. Ten to 20 minutes of

lens settling time was allowed before taking the measurements in the morning.

Three sets of 30 - second videokeratoscopy recordings were captured both in

normal (natural blinking) and then suppressed blinking conditions (6 x 30 sec in

total). Each 30 seconds recording consisted of 25 image frames per second.

Thus a total of 750 image frames were captured for each 30 second

measurement.

The subject was positioned in the chin rest of the E300 and was asked

to fixate on the centre of the inner-most ring of the Placido disk. During the

normal blinking condition, the subject was asked to blink normally. During the

suppressed blinking condition, the subject was asked to “take a gentle,

complete blink and then stop blinking for as long as it was comfortable”. The

subject was reminded to take a gentle blink and move back from the headrest

when it became uncomfortable. A gap of 3 - 4 minutes was allowed between

the measurements during which time the subjects could relax and blink

naturally. The measurements were taken in the same room, with approximately

the same humidity (58.0 ± 5.2%) and temperature (24.9 ± 1.0 ºC) and at

approximately the same time of day for all measurement days (both morning 8 -

11 am and afternoon 4 - 7 pm). The main room lights were dimmed during the

measurements.

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Table 6-2: Description of the lenses used in the study.

Parameter Lens 1 Lens 2 Lens 3 Lens 4

Lens PMMA/ 9.5 RGP/ 9.5 RGP/10.5 SiHy/ 14.0

Design (centre) Spherical Spherical Spherical Spherical

Design (periphery) Aspheric Aspheric Aspheric B&L PureVision

Material PMMA RGP (Boston XO) RGP (Boston XO) Silicone hydrogel

Power (Dioptre) –0.50 –0.50 –0.50 –0.50

Total diameter (mm) 9.5 9.5 10.5 14.0

BOZD (mm) 8.1 8.1 8.8 8.9

Modulus (MPa) ≈ 2000 1500 1500 1.1

Manufacturing method

Lathe Lathe Lathe Cast moulding

Surface treatment None None None Performa/Plasma

oxidation

PMMA: polymethyl methacrylate, RGP: rigid gas permeable, SiHy: silicone hydrogel, B&L: Bausch and Lomb, BOZR: back optic zone radius, BOZD: back optic zone diameter, Dk: oxygen permeability, MPa: megapascal, unit of modulus of elasticity, mm: millimetres

6.3 Data Analysis

The mean TFSQ was calculated in both the normal and suppressed blinking

conditions. Matlab-based custom written image processing techniques were

used for analysis of the videokeratoscope images from the inter-blink interval.

The details of this method have been described by Alonso-Caneiro et al.

(2009). In order to allow for the tear film to build-up (Nemeth et al. 2002), a

period of one second after the blink was excluded from the analysis. First, the

region of interest (ROI), within the cornea which has the Placido ring pattern, is

estimated. Then the unaltered ring pattern within this area is identified, this

involves differentiating the Placido ring pattern from the interference. The

interference in the ring pattern can be due to shadows from the eyelashes or

poor tear film surface quality. The interference from the eyelashes is removed

to obtain the area of analysis (AOA), which now consists of the area of

unaltered ring pattern and interference due to poor TFSQ. The TFSQ is

estimated in the form of a number (ranging from 0 to 1) in the area of analysis

using image coherence analysis (Alonso-Caneiro et al. 2008; Alonso-Caneiro

et al. 2009). The coherence, which is a measurement of the pattern‟s local

orientation, is estimated as close to 0, when the pattern is poorly oriented (and

the tear film is disrupted and of poor quality) and close to 1, when the pattern is

well oriented (and the tear film is smooth and of high quality). TFSQ for each

image is the average of the coherence measurement in the area of analysis. An

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average TFSQ value for all the Placido disc images is then calculated. The

steps involved in the analysis are shown in Figure 6-1. Reflections of the

Placido disc pattern immediately after and a few seconds after blink showing

tear break up on a soft lens is shown in Figure 6-2.

The area of the cornea/tear film that was used for analysis was centred

on the VK rings and had a radius of 2.75 mm (or 5.5 mm diameter). This area

was selected for analysis so that there was no interference from the lens edges

or the front optic zone diameter boundary. For normal blinking conditions the

analysis was carried out on all the data for a duration of 30 seconds (excluding

frames during blinking and 1 sec following the blink) to derive the mean TFSQ

value. For the suppressed blinking condition, the first 6 seconds after the final

blink before start of the recording (excluding the first 1 sec) was selected for

analysis and averaged (i.e. 5 X 25 frames) to derive the mean TFSQ

throughout the 6 seconds. If the subject blinked within 6 seconds, these data

were not used, however at least 2 out of the 3 suppressed blinking trials were

available for the 11 subjects included in the analysis.

A repeated measures analysis of variance ANOVA was used to

investigate the statistical significance of changes in TFSQ, with the lens type (4

different types) and time of day (morning and afternoon) as within-subject

factors in normal and suppressed blinking conditions. Degrees of freedom were

adjusted using the Greenhouse-Geisser correction to prevent any type 1 errors,

where violation of sphericity assumption occurred. Bonferroni adjusted pair-

wise comparisons were carried out for individual comparisons.

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Figure 6-1: Steps involved in estimation of TFSQ value on the corneal or contact lens surface. ROI: region of interest. AOA: area of analysis

Figure 6-2: Image frames from high speed videokeratoscopy with an RGP lens on the cornea. Reflections of the Placido disc pattern (a) Immediately after blink (TFSQ value = 0.85) and (b) few seconds after blink showing tear break up (TFSQ value = 0.68). Yellow lines enclose the area of analysis.

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The number of blinks per 30 seconds was estimated using the Matlab-

based software to calculate the blink frequency. This was multiplied by 2 to

obtain number of blinks per minute. The subjects are unaware of the blink

frequency being recorded since during the measurements they were informed

that their tear film quality was being measured and were asked to blink

normally.

6.4 Results

6.4.1 TFSQ in natural blinking conditions

The group mean TFSQ value in natural blinking conditions for the baseline day

and with the four different contact lenses are shown in Figure 6-3. The type of

lens (p=0.001, repeated measures ANOVA) had a significant effect on the

mean TFSQ values in the natural blinking conditions. Mean TFSQ values for

the PMMA/9.5, RGP/9.5 and RGP/10.5 lenses were significantly worse than

the baseline day (no lens) in both morning and afternoon (all p<0.05, pairwise

comparison) (Table 6-3). The SiHy lens also showed significant reduction in

TFSQ values in the afternoon after 8 hours of lens wear. Post hoc testing

showed that there was no significant difference between the mean TFSQ

values of the four contact lenses compared to each other (all p>0.05) (Figure 6-

3).

Figure 6-3: Mean TFSQ values in 30 seconds with the four contact lenses and on baseline day (no contact lens), in the morning and afternoon, in natural blinking conditions. The TFSQ is calculated on a scale of 0 to 1 where 0 is very poor and 1 is very good quality. * indicates significant difference compared to baseline. Error bars represent standard error of the mean.

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Table 6-3: Mean change in TFSQ values relative to baseline, with the four contact lenses in the morning and afternoon, in natural blinking conditions over a period of 30 seconds. Negative values of TFSQ indicate that TFSQ is worse with contact lenses.

Mean change in TFSQ relative to baseline ± SD (p=value)

Lens Morning (am) Afternoon (pm)

PMMA/9.5 –0.06 ± 0.04 (p=0.002) –0.07 ± 0.04 (p<0.001)

RGP/9.5 –0.06 ± 0.05 (p=0.01) –0.08 ± 0.06 (p=0.002)

RGP/10.5 –0.06 ± 0.06 (p=0.01) –0.05 ± 0.04 (p=0.01)

SiHy/14.0 –0.07 ± 0.09 (p=0.10) –0.04 ± 0.04 (p=0.03)

There was a significant difference in the mean change in TFSQ values

in the afternoon compared to morning for RGP/9.5 lens which showed a

decrease of –0.02 ± 0.03 (p=0.04, pairwise comparison) and RGP/10.5 lens

which showed an increase of 0.02 ± 0.02 (p=0.02, pairwise comparison) (Table

6-4).

Table 6-4: Mean change in TFSQ values in the afternoon relative to morning, with the four contact lenses in natural blinking conditions. Negative values of TFSQ indicate that TFSQ is worse in the afternoon.

Mean change in TFSQ values (pm - am) ± SD

Baseline –0.0001 ± 0.03 (p=0.99)

PMMA/9.5 –0.01 ± 0.03 (p=0.41)

RGP/9.5 –0.02 ± 0.03 (p=0.04)

RGP/10.5 0.02 ± 0.02 (p=0.02)

SiHy/14.0 0.03 ± 0.07 (p=0.15)

6.4.2 Blink frequency in natural blinking conditions

The time of day had a significant effect (p=0.01) on the number of blinks per

minute in natural blinking conditions. The group mean number of blinks per

minute (or blink frequency) in natural blinking conditions for the baseline day

and with the four different contact lenses is shown in Figure 6-4. On the

baseline day, the blink frequency was significantly faster in the afternoon

compared to morning, with a mean increase of 4.24 ± 5.39 blinks/min (p=0.01,

repeated measures ANOVA). The blink frequency did not change significantly

during the day with any of the contact lenses. There was also no statistically

significant difference (all p>0.05) in the blink frequency with the different types

of contact lenses compared to baseline, although all lenses caused some

increase in mean blink frequency compared to baseline morning

measurements.

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Figure 6-4: Mean blink frequencies (number of blinks per minute) in natural blinking conditions with and without contact lenses. Error bars represent standard error of the mean.

6.4.3 TFSQ in suppressed blinking conditions

The TFSQ in suppressed blinking conditions was studied for 11 subjects, since

data with at least 2 measurements up to 6 seconds was available only for these

subjects. The type of lens (p=0.01) also had a significant effect on the mean

TFSQ values under suppressed blinking conditions. The group mean TFSQ

values in suppressed blinking conditions for the baseline day and with the four

different contact lenses in the morning and afternoon are shown in Figure 6-5.

The figure illustrates the mean TFSQ values for each second immediately after

blinking for up to 6 seconds. The mean TFSQ values show significant

increases (improvement), in the first second after the blink (build-up time) with

all lenses and during baseline measurement, both in the morning (Figure 6-5 a)

as well as the afternoon (Figure 6-5 b). Mean TFSQ values for the three rigid

lenses (PMMA/9.5, RGP/9.5 and RGP/10.5) as well as SiHy/Soft lens was

significantly worse than the baseline day (no lens wear) both in the morning

and afternoon (all p<0.05, pairwise comparison) (Figure 6-5 and Table 6-5).

Post hoc testing showed that there was no significant difference between the

mean TFSQ values of the four contact lenses compared to each other (all

p>0.05). There were no significant differences in mean TFSQ values in the

afternoon compared to morning on the contact lens-wearing or the baseline

days.

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Figure 6-5: The group mean TFSQ values in suppressed blinking conditions with time for the 6 seconds after a blink, for the baseline day and with the four different contact lenses in the morning (a) and afternoon (b).

Table 6-5: Mean changes in TFSQ values in suppressed blinking conditions relative to baseline.

Mean change in TFSQ ± SD in suppressed blinking conditions, relative to baseline

Lens Morning (am) Afternoon (pm)

PMMA/9.5 –0.04 ± 0.03 (p=0.01) –0.05 ± 0.02 (p<0.001)

RGP/9.5 –0.05 ± 0.04 (p=0.02) –0.06 ± 0.02 (p<0.001)

RGP/10.5 –0.04 ± 0.03 (p=0.04) –0.04 ± 0.02 (p=0.003)

SiHy/14.0 –0.05 ± 0.05 (p=0.04) –0.05 ± 0.04 (p=0.01)

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There was a clear trend towards reduction in mean change in TFSQ

with time over the 5 second period of recording (Figure 6-5) both in the morning

and afternoon.

6.4.4 Trend of TFSQ with time in suppressed blinking conditions

TFSQ generally decreased with time. Subjects showed four different types of

trends in the change in TFSQ over a period of 30 seconds. The different

patterns that were most commonly observed are shown in Figure 6-6 for four

representative subjects. Type 1 pattern was a steady decline in TFSQ with time

till the end of recording time [Figure 6-6 (Type 1) & Figure 6-7]. The second

type of trend showed a slower decrease with time with a small improvement

towards the end of recording time (Figure 6-6, Type 2). The third trend showed

a steep decline in TFSQ following a stabilization of tear film (after the blink) in

the first few seconds forcing the subject to blink well before the end of

recording time (time of last frame from Figure is at 9.84 seconds, Figure 6-6,

Type 3). The fourth and the most unexpected trend was a decline in TFSQ with

time for first 7 to 9 seconds after the blink, but then TFSQ begins to improve

and gets even better than the baseline values by 12 to 13 seconds before

stabilising [Figure 6-6 (Type 4) & Figure 6-8].

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Figure 6-6: The four different types of representative patterns of TFSQ with time over 30 seconds.

Figures 6-7 and 6-8 show the first and the fourth types of TFSQ

patterns respectively over time and the corresponding Placido disk images at 3

different times. The trend shown in Figure 6-8 was seen most commonly with

rigid contact lenses compared to the soft lens and never noticed during

baseline measurements (no contact lens) (Table 6-6).

Table 6-6: Analysis of recordings showing an increase in TFSQ value with time.

Lens Total number of

recordings Number of recordings

with an increase in TFSQ Percentage (%)

Baseline 84 0 0

PMMA/9.5 84 57 68

RGP/9.5 84 55 65

RGP/10.5 84 56 67

SiHy/14.0

84 16 19

A total of 14 subjects, with 3 recording each performed in both morning and afternoon. (Total recording is 84). Pattern of increase in TFSQ is shown in Figure 6-8.

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Figure 6-7: TFSQ over time for a representative subject, showing an increase during the first second post-blink (build-up time) and then a constant reduction in TFSQ over time till the end of the measurement. Corresponding Placido disc maps can be seen at the beginning (clear rings), middle (breaks in the ring pattern) and end (severe distortion of the ring pattern) of the measurement. Yellow lines enclose the area of analysis.

Figure 6-8: TFSQ over time for a representative subject, showing an increase during first second post-blink, then a reduction is seen with time till a certain point after which it shows an improvement and reaches a value more than the baseline. Corresponding Placido disc maps can be seen at the beginning (clear rings), middle (few breaks in the ring pattern) and end (very clear and regular ring pattern) of the measurement. This later period seems to correspond to complete drying of the lens surface which now acts like a mirror to produce a high TFSQ value. Yellow lines enclose the area of analysis.

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6.4.5 Association between TFSQ value and blink rate

To study the association between mean TFSQ values and blink frequency

(number of blinks per minute) in normal blinking conditions, a Pearson‟s

correlation was calculated between these variables. A very weak negative

correlation (R2 = 0.07, p=0.33) between these parameters was found (i.e. there

was a weak tendency for an increase in blink frequency with decrease in

TFSQ) but this was not significant (Figure 6-9).

Figure 6-9: Correlation between mean TFSQ values and blink rates (number of blinks per minute) in the morning and afternoon, for all the lenses combined. (Afternoon measurements only)

6.4.6 Association between TFSQ value and tarsal conjunctival and lid-wiper staining

To study the association between the TFSQ value and tarsal conjunctival

staining Spearman‟s correlation was calculated between changes in these

variables compared to baseline. There was no correlation (R2 = 0.01, p=0.43)

between the mean TFSQ values and tarsal staining for all lenses combined

(Figure 6-10 a). Similarly, there was no correlation between mean TFSQ values

and lid-wiper staining all lenses combined (R2 = 0.03, p=0.51) (Figure 6-10 b).

y = -0.0007x + 0.8472 R² = 0.0789

0.78

0.80

0.82

0.84

0.86

0.88

0 20 40 60

Me

an

TF

SQ

Blink Rate

Mean TFSQ vs blink rate (pm)

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Figure 6-10: Correlation between mean TFSQ values and (a) Tarsal staining (b) Lid-wiper staining for all the lenses combined for morning and afternoon.

6.5 Discussion

This study shows that all types of contact lenses adversely affect the TFSQ.

The mean TFSQ value was worse with all the lenses both in morning and

afternoon compared to measurements on the baseline day and the changes

were statistically significant with all the rigid lenses (PMMA/9.5, RGP/9.5 and

RGP/10.5) during normal blinking conditions. The SiHy lens also showed a

significant reduction in TFSQ value in the afternoon after 8 hours of lens wear.

This is in agreement with previous studies (Kopf et al. 2008; Alonso-Caneiro et

al. 2009) that found significant differences in TFSQ values with both hydrogel

and SiHy contact lenses compared to baseline after one day of lens wear. The

mean TFSQ value with the SiHy lens in this study was 0.83 ± 0.04 in

suppressed blinking conditions and 0.85 ± 0.08 in normal blinking conditions

(after 8 hours of lens wear) compared to 0.84 ± 0.02 (after one day of lens

wear) in suppressed blinking, in the study by Alonso-Caneiro et al. (2009) This

is the first study to use dynamic videokeratoscopy to measure TFSQ with rigid

lenses and demonstrates that similar magnitude reductions in TFSQ occur with

rigid lenses of different materials and designs.

We found no significant differences between the mean TFSQ values

with the different lenses (i.e. all the lens materials caused reductions in TFSQ

of similar magnitude compared to the baseline) and this correlates with

previous studies of ocular symptoms that have reported no differences in the

frequency of dryness symptoms between RGP and soft contact lens wearers

(McMonnies 1990; Vajdic et al. 1999). Less dryness is reported by subjects

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using SiHy contact lenses compared to conventional hydrogel lenses (Fonn et

al. 2000) and this is attributed to less lens dehydration seen with the SiHy

lenses than with conventional hydrogel materials. Whereas in another study, no

differences were found between SiHy and hydrogel lenses, in terms of pre-lens

tear film thinning times (Thai et al. 2004). The method of measuring TFSQ

described in this study may not be sensitive enough to distinguish differences

between lenses but has shown significant differences between lens wear and

baseline (bare eye) conditions.

We also found a reduction in mean TFSQ values in the afternoon

compared to the morning on the baseline day, and with PMMA/9.5 and

RGP/9.5 lenses. On the other hand, RGP/10.5 and SiHy/14.0 lenses showed a

slight improvement in mean TFSQ value in the afternoon compared to morning.

This is in contrast to studies that have shown an increase in severity of

symptoms at the end of contact lens wearing time in contact lens wearers

(Begley et al. 2000; Begley et al. 2001).

The mean change in TFSQ value in the afternoon compared to morning

showed an opposite effect with the RGP/9.5 lens (a decrease of –0.02 ± 0.03)

and RGP/10.5 lens (an increase of 0.02 ± 0.02). This small difference in the

TFSQ values with the two lenses, though not statistically significant could be

partly attributed to the total diameter of these lenses, since it has been reported

that larger diameter lenses are more comfortable than smaller diameter lenses

(Williams-Lyn et al. 1993). The larger diameter contact lenses, being slight

more comfortable could lead to less blinking and reflex tearing and thus could

result in a more stable tear film. An increase in TFSQ value was also seen with

the larger diameter SiHy lens (SiHy/14.0) but this increase was not statistically

significant.

We noticed four different patterns of deterioration in TFSQ values over

the period of 30 seconds in suppressed blinking conditions (Figure 6-6). First a

gradual linear decrease in TFSQ, second a slower rate of decrease with a little

improvement in TFSQ towards the end of recording time, third a rapid decrease

in TFSQ to reach minimum in about 11 seconds and the fourth showing a

decrease till 9 seconds followed by an improvement at about 13 seconds to

result in TFSQ even better than the baseline. This improvement is unlikely to

be a real improvement in TFSQ and appears to be due to the lens surface

evenly drying and acting as a mirror. This difference in the pattern of

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deterioration in TFSQ has not been previously reported since the measurement

time used by (Alonso-Caneiro et al. 2009) was not long enough (8 seconds),

compared to the present study (30 seconds) to reveal this pattern. This pattern

was most frequently noticed with rigid/hard lenses and a few silicone hydrogel

lenses and is something that would be expected to occur in a clinical setting in

RGP contact lens wearers showing a full break-up of the tear film. The results

are related to the differences in thickness of the pre-lens tear film (Nichols and

King-Smith 2003; Wang et al. 2003; Nichols et al. 2005) and pre-lens lipid layer

(Guillon 1986) of contact lenses. Based upon previous results of a mean pre-

lens thinning rate of 6.97 µm/min, for a mean PLTF (pre-lens tear film)

thickness of 2.54 µm of a soft contact lens (Nichols et al. 2005), it will take

about 22 seconds to dry up completely. It is possible that a thinner or absent

lipid layer on the rigid lens will result in the rate of evaporation with these

lenses being much greater and causes the lens surface to dry out completely

and act as a mirror. Since 22 seconds is slightly longer than the 13 seconds

after which we observed complete drying, it is likely that some factors other

than evaporation may also be involved in this.

6.6 Conclusion

In summary, a significant decrease in TFSQ values is shown with all

types of contact lenses compared to baseline (bare eye) in normal blinking

conditions, though no differences were noticed between the lens types. This

indicates the need for better contact lens materials and surfaces and highlights

the importance of plasma coating on the lenses to improve their wettability and

hydrophilicity. An interesting pattern of change in TFSQ over time was found in

suppressed blinking conditions, in which TFSQ was reduced until a certain time

after which it improved to become even better than the baseline. This

phenomenon (seen more frequently with rigid lenses) has never been reported

earlier and could be attributed to tear film drying completely over the surface of

contact lenses. Our technique presented in this study does not measure tear

film thickness, so in order to test the hypothesis that the tears are drying

(thinning) completely further studies are needed to measure tear film thickness

using interferometry techniques in suppressed blinking conditions.

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Chapter 7

Conclusions We have studied the effects of the short-term wear of soft and rigid contact

lenses on various structures of the anterior eye in front of and behind the

contact lenses. This included the tear film, anterior corneal topography,

posterior corneal topography, regional corneal thickness, palpebral conjunctiva,

eyelid margin and eyelid position. These studies have shown that as little as 8

hours of contact lens wear can lead to measurable changes in all of these

structures of the anterior eye. The location and magnitude of corneal changes

were largely determined by the regional oxygen transmissibility of the lens for

both soft and rigid lenses, while mechanical changes involving the eyelids were

generally more common with the higher modulus rigid contact lenses. Figure 7-

1 summarises the structures and parameters of the anterior eye that were

investigated in this thesis.

Figure 7-1: Schematic representation of ocular structures and parameters affected by short-term use of contact lenses presented in this thesis.

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7.1 Changes in ocular structures posterior to the contact lens

Important changes were found in ocular structures posterior to the contact lens

including regional corneal thickness, anterior corneal topography, posterior

corneal topography and wavefront error changes. These changes are

described below and summarised in Figure 7.2.

Figure 7-2: Changes in ocular structures and parameters (posterior to contact lenses) affected by short-term use of contact lenses, in comparison to baseline day changes.

7.1.1 Corneal thickness changes and contact lenses

We systematically studied the effect of soft contact lens power, design and

material on regional corneal thickness after short-term lens wear (Chapter 2).

Soft toric lenses have their thickest areas in the periphery and might therefore

be expected to have greatest influence on the underlying regional corneal

thickness. This is the first study to systematically investigate the effect of short-

term use of soft toric contact lenses on corneal thickness. Using a rotating

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Scheimpflug technique (Pentacam), we were able to study regional changes in

corneal thickness with these lenses and to compare the changes with those

found with spherical soft lenses. We observed greater swelling towards the

periphery after wear, for all soft lenses tested. However, the swelling was

greatest in the regions corresponding to the thick stabilization zones of the

hydrogel toric lens (47 µm or 8.2% directly under the stabilization zone

compared to 19 µm or 2.9% in the centre). These findings indicate that these

corneal regions are suffering the greatest hypoxia and in the long term are

likely to be susceptible to various secondary complications of compromised

corneal metabolism (e.g. neovascularization at the limbus).

The SiHy spherical and toric lenses resulted in small overall changes in

corneal thickness and led to slight central corneal thinning. Clearly the

increased oxygen permeability of SiHy lenses is providing sufficient oxygen to

the underlying cornea and this leads to little change in corneal thickness profile.

The corneal swelling seen in this study after 8 hours of lens wear was similar in

magnitude to that seen after eyelid closure associated with overnight sleep,

which is not likely to have an adverse effect on the cornea. However, the

corneal changes associated with longer term hydrogel toric lens wear are

expected to be larger (Hagan et al. 1998; Schornack 2003). Future research

involving controlled clinical studies of longer term soft toric lens wear of

different designs is required to improve our understanding of the nature and

magnitude of the longer term corneal effects.

In this study we also found significant diurnal changes in corneal

thickness from mid-morning to afternoon, which were usually larger than those

caused by the SiHy lenses. These natural diurnal variations should be

considered while investigating contact lens induced corneal changes (for both

daily and extended wear conditions) since these can have potentially

confounding effects on any analysis of the effects of the contact lenses.

We also investigated the effect of the short-term wear of PMMA and

RGP lenses on corneal thickness (Chapter 3). Previous studies have shown

significant corneal swelling with the wear of PMMA contact lenses (Carney

1974; Fonn et al. 1984; Wang et al. 2003; Moezzi et al. 2004), and our results

are consistent with these findings. The mean central corneal swelling with the

PMMA contact lenses was 27.9 ± 15.49 µm (4.77%) and was much greater in

the centre than in the peripheral cornea (17.78 ± 12.11 µm, 2.71% swelling).

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The corneal swelling was much less pronounced following 8 hours of RGP lens

wear and did not reach statistical significance, however on average greater

swelling was observed in peripheral corneal regions compared to central

regions with both of the RGP lenses (varied in diameter but the same Boston

XO material). Similar results were seen with the spherical and back surface

toric RGP lenses (both Boston XO material) that were studied in a group of

subjects with toric corneas after short-term wear (Chapter 4). For rigid contact

lenses as with the soft lenses, the degree of corneal swelling appeared to be

primarily driven by the regional oxygen transmissibility characteristics of the

lenses.

7.1.2 Anterior corneal curvature changes and contact lenses

The changes in anterior corneal curvature that we observed with the various

types of contact lenses seem to have been mostly the result of mechanical

pressure from the lens, with little association with the underlying corneal

swelling (with the possible exception of PMMA lens wear). SiHy lenses

(Chapter 2) resulted in slight but statistically significant flattening of the

peripheral anterior curvature which caused a decrease in anterior corneal

refractive power of about 0.15 D. The hydrogel lens caused little change in

anterior curvature, even though this lens caused greater overall corneal

swelling. These results are in accordance with previous studies suggesting that

there can be substantial changes in corneal thickness without any significant

change in anterior corneal topography with the use of contact lenses (Bailey

and Carney 1972; Carney 1972; Carney 1975).

The relationship between anterior axial curvature change and corneal

swelling is not straight forward. If the cornea swells predominantly in a

backward direction (i.e. the anterior chamber becomes narrower) as suggested

by many authors (Read and Collins 2009), then we would expect no

association between corneal swelling and anterior corneal curvature change.

Even if the corneal swelling was entirely in the anterior direction (i.e. anterior

chamber depth remains unchanged) then a substantial uniform change in

corneal swelling of say 3.5% (20.3 microns) will only lead to a small flattening

of corneal curvature (7.70 mm axial radius becomes 7.72 mm) (see Figure 7-3).

The standard deviation of typical anterior axial curvature measurements is ±

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159

0.02 mm, so small changes in anterior curvature related to corneal swelling

would be barely detectable with modern videokeratoscopes.

On comparing the RGP and PMMA lenses in Chapter 3, we found that

these lenses have opposite effects on anterior corneal curvature. RGP lenses

resulted in anterior corneal flattening, which caused a decrease in corneal

refractive power ranging from 0.998 to 0.01 D. These changes in anterior

curvature appear to be related to mechanical pressure from the lens, since

there was little sign of corneal swelling with the RGP lenses. Further research

examining long-term changes in corneal refractive power using RGP lenses are

essential to understand how these changes progress with time.

PMMA contact lenses, on the other hand, showed corneal steepening

which was greater in the centre than the periphery of the cornea. Since the

mechanical pressure exerted by these PMMA lenses is expected to have been

identical to the RGP lens we used (identical fitting), the most obvious cause of

the steepened central corneal curvature is the associated central corneal

swelling seen with the PMMA lenses. This would suggest that at least in the

case of the PMMA lenses, the corneal swelling was predominantly in the

anterior direction (i.e. anterior chamber depth remained unchanged) and

because it occurred to a greater extent in the central cornea, it created a

steeper central curvature (see Figure 7-3).

While rigid contact lenses altered the shape of the cornea to produce

substantial changes in corneal refractive power, SiHy contact lens (used for

comparison in Chapter 3) resulted in only small changes in corneal refractive

power, which were clinically insignificant. These results support the view that

soft contact lenses predominantly conform to the shape of the cornea and

therefore produce little refractive power change due to lens pressure.

We were able to demonstrate a moderate correlation between the

location of regions of corneal flattening on the anterior corneal surface and the

regions of minimum fluorescein clearance behind the rigid lens. The

mechanical forces due to a contact lens are potentially distributed across the

cornea based on the clearance between the lens and corneal surface. The type

of fluorescein pattern indicated the expected changes in corneal curvature due

to mechanical forces and this principle is routinely used in orthokeratology lens

fitting. No previous report has quantitatively analysed the fluorescein pattern of

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160

standard rigid contact lens fittings and correlated this to the short-term corneal

topographic changes. Information provided by this analysis may be useful in

predicting the changes in corneal curvature that a rigid contact lens is likely to

produce in clinical practice after longer term wear.

Figure 7-3: Schematic diagram showing difference between central and uniform anterior swelling.

We further explored corneal curvature and refractive changes after

short-term use of back surface toric compared to spherical RGP lenses on

subjects with toric corneas for the first time (Chapter 4). We found corneal

flattening was slightly greater with the spherical lens compared to the back toric

lens, which resulted in a decrease in refractive power (about 0.25 D with the

spherical lens and 0.15 D with the back toric lens). The spherical lens also

caused a significant decrease in WTR astigmatism (0.13 D). This confirmed our

hypothesis that the mechanical effects due to a spherical lens would differ from

those of a back toric lens which more closely aligned to the corneal curvature,

however it should be noted that the magnitude of difference in corneal changes

between the two lens designs was small.

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161

7.1.3 Posterior corneal curvature changes and contact lenses

The magnitude and nature of posterior corneal change associated with daily

wear of various contact lenses has not been reported earlier. Previous studies

have reported posterior corneal changes with contact lenses only after lens

wear in closed eye conditions (Moezzi et al. 2004; Martin et al. 2009). In

Chapters 2 and 3 we have attempted to gain a better understanding of the

effects of soft and rigid contact lens wear on posterior corneal shape. All of the

soft lenses used (Chapter 2) resulted in a steepening of posterior curvature

which is likely to be due to greater peripheral swelling seen with these lenses

compared to the centre of the cornea. The largest magnitude of change was

observed with the HEMA/Toric/–3 lens, with a mean steepening of –0.07 mm of

the central posterior cornea. The steepening of the central posterior curvature

in this study correlated with the corneal swelling. This is in agreement with

previous reports that posterior corneal curvature changes are associated with

corneal edema (Kikkawa and Hirayama 1970; Lee and Wilson 1981; Erickson

et al. 1999; Read and Collins 2009).

PMMA and RGP lenses were observed to have opposite effects on

corneal curvature (Chapter 3), with PMMA lenses showing a posterior flattening

and the RGP lenses showing steepening. These changes correlated well with

the changes in central and peripheral corneal thickness (i.e. greater central

corneal swelling with PMMA and greater peripheral swelling seen with RGP

lenses) (Chapter 3, Figure 3-9). The spherical and back surface toric RGP

lenses on toric corneas (Chapter 4) also caused steepening similar to changes

seen with the spherical RGP lenses in Chapter 3.

7.1.4 Wavefront aberrations and rigid contact lenses

In the past, a number of authors have described the effects of contact lenses

(on-eye) on the ocular wavefront aberrations (Hong and Himebaugh 2001;

Dorronsoro et al. 2003; Lu et al. 2003). The effect of short-term contact lens

wear on ocular aberrations measured after removal of the lens has been

reported for the first time in Chapter 3. PMMA lenses produced a significant

increase in HO RMS, 2nd, 3rd and 4th order RMS wavefront error for a 4 mm

pupil diameter and in 3rd, 4th and HO RMS for a 5.5 mm pupil. These changes

may cause a reduction in ocular image quality after lens removal and may be

partly responsible for “spectacle blur” reported with PMMA lenses (Levenson

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Chapter 7: Conclusions

162

1983; Wilson et al. 1990). RGP and SiHy lenses resulted in small and

insignificant changes in higher order aberrations after lens removal.

7.2 Changes in ocular structures anterior to the contact lens

A variety of clinically and statistically significant changes were also found in

ocular structures anterior to the contact lens including changes in the position

of the eyelids, lid-wiper epitheliopathy, tarsal staining and changes in the tear

film surface quality. The changes are described below and summarised in

Figure 7.4.

Figure 7-4: Changes in ocular structures and parameters (anterior to contact lenses) affected by short-term use of contact lenses, in comparison to baseline day changes. PA: Palpebral aperture.

7.2.1 Lid related changes and contact lenses

Previous studies have shown a reduction in PA height with long-term (2 weeks

to 10 years) rigid contact lens wear (Fonn and Holden 1988; Van den Bosch

and Lemij 1992; Fonn et al. 1996). The small but significant decrease in the PA

height with PMMA/9.5 and RGP/10.5 lenses in Chapter 5 shows that a subtle

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163

blepharoptosis can occur even after short periods of lens wear. This is the first

study to report the effect of short-term (few hours) contact lens wear on PA

height. The difference in the blepharoptosis seen with different lenses is most

likely due to one or more of the factors such as the modulus of elasticity, total

diameter and edge manufacture of the lenses.

The changes in lid position could potentially be related to irritation of the

cornea or lids leading to blepharospasm due to the presence of the contact

lenses (Van den Bosch and Lemij 1992) or eyelid swelling due to mechanical

trauma. On further investigation, we found that changes in the position of both

upper and lower lids contributed to the reduction in PA but the upper lid

changes were larger. This implies that the origin of the PA height changes may

be partly related to „reflex‟ lid movements associated with contraction of the

orbicularis oculi muscle (changes in the both upper and lower lids), along with

possible inflammation and swelling of the lids due to mechanical micro-trauma.

While we did not follow our subjects to record a recovery in size of PA , based

on previous studies (Fonn and Holden 1986; Fonn and Holden 1988; Fonn et

al. 1995), we speculate that PA size in our group of subjects will increase and

normalize after lens removal.

The lid-wiper region of the upper lid moves across the edge and surface

of the contact lens during each blink as it spreads the tears across the lens and

ocular surface. This may result in micro-trauma to the surface epithelial cells in

this region of the marginal conjunctiva. Lid-wiper epitheliopathy has been

reported in long-term soft contact lens wearers and dry eye patients (Korb et al.

2002; Korb et al. 2005; Yeniad et al. 2010) but this is the first study to report

increases in lid-wiper staining in as little as 8 hours wear of rigid and soft

contact lenses. A significant increase in the magnitude of lid-wiper staining was

associated with the short-term wear of all the rigid lenses. The magnitude of lid-

wiper staining increased during the course of the day with all types of contact

lenses (soft and rigid).

The increase in lid-wiper and tarsal staining during the course of the day

is most likely due to the constant friction between the lid-wiper/tarsal surfaces

and the contact lens surface, edge and/or the ocular surface during each blink.

The increase in staining observed on the contact lens-wearing days

(particularly with the rigid lenses) suggests that the contact lens edge and

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Chapter 7: Conclusions

164

surface cause a greater amount of friction in the lid-wiper region as compared

to the ocular surface alone (Korb et al. 2005).

We also examined the tarsal conjunctiva for signs of micro-trauma since

it follows the lid-wiper during a blink and rubs against the ocular/contact lens

surface. We developed a grading scale (Figure 5.4) to investigate the changes

in tarsal conjunctival staining after short-term wear of different contact lenses,

increases in which are likely to indicate greater micro-trauma to the surface. An

increase in the amount of tarsal staining (Chapter 5) with all three rigid/hard

contact lenses (PMMA/9.5, RGP/9.5 and RGP/10.5) was found. However the

SiHy lens did not cause any significant increase in tarsal staining over the 8

hours of lens wear. Previous research has revealed that the tear film on a

contact lens surface dries more rapidly compared to that on the cornea

(Cedarstaff and Tomlinson 1983; Thai et al. 2004), thereby increasing the

friction between the surfaces. The increased tarsal staining we found could

therefore be partly attributed to an increased friction between the lens and

tarsal conjunctiva compared to that between the natural cornea and tarsal

conjunctiva. However later OCT imaging of the PMMA and RGP lens edges

suggest that poor lens edge manufacture had contributed to the increased

tarsal staining with these lenses.

We also noted a significant diurnal increase in tarsal staining even on

the baseline day when no lenses were worn. This suggests that the tarsal

conjunctival surface is in constant friction with the ocular surface during each

blink resulting in some minor surface damage over the course of the normal

day. A larger controlled study in a group of dry eye and non-dry eye subjects is

required to investigate diurnal changes in these parameters to confirm these

effects.

This is the first study to report blepharoptosis, tarsal conjunctival

staining and lid-wiper staining in as little as 8 hours after use of a variety of

contact lenses. The overall findings related to eyelid changes associated with a

short period of contact lens wear highlight that there is still a need for better

and smoother contact lens edges to minimize the interactions between the lid

margin and contact lens edge during lens wear. There is a need to measure

and better understand the lubricity of a range of available contact lenses and

the effect of different types of plasma treatment on the lubricity of contact lens

materials. The lens edges should be inspected by the practitioner if any signs

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165

of lid trauma are observed. There is also scope for the development of more

lubricious contact lens surfaces to reduce lid surface trauma due to friction

especially for patients with dry eye problems.

7.2.2 Tear film surface quality (TFSQ) and contact lenses

The tear film, in addition to providing the first optical surface of the eye is also

responsible for lubrication between the lids and the ocular surface. The

increased evaporation of the pre-contact lens tear film during lens wear

(Cedarstaff and Tomlinson 1983; Thai et al. 2004) potentially increases the

amount of friction thereby, causing increased lid-wiper and tarsal conjunctival

staining. Therefore, a poor TFSQ may increase damage to the ocular surface

because of the contact lens and reduce the optical quality of the eye. Non-

invasive assessment of TFSQ with RGP contact lenses using dynamic

videokeratoscopy has not been reported earlier. We studied the effect of short-

term lens wear on TFSQ (Chapter 6) using a high speed videokeratoscopy

technique and demonstrated that all types of contact lenses adversely affect

TFSQ in both natural as well as suppressed blinking conditions. The mean

TFSQ value was worse with all the lenses (soft and RGP) in the afternoon in

both normal and suppressed blinking conditions The SiHy lens also showed a

significant reduction in TFSQ in the afternoon after 8 hours of lens wear which

is in agreement with previous studies (Kopf et al. 2008; Alonso-Caneiro et al.

2009) that showed significant differences in TFSQ values with both hydrogel

and SiHy contact lenses compared to baseline after one day of lens wear. The

mean TFSQ values with the SiHy lens in our study (0.83 ± 0.04 in suppressed

and 0.85 ± 0.08 in normal blinking conditions) are comparable to that reported

by Alonso-Caneiro et al. (2009) (0.84 ± 0.02 in suppressed blinking conditions).

This is the first study to use dynamic videokeratoscopy to measure TFSQ with

rigid lenses and demonstrates that a similar magnitude of reduction in TFSQ

occurs with rigid contact lenses irrespective of their material (PMMA versus

Boston XO). This agrees with previous studies of ocular symptoms that have

reported no differences in the frequency of dryness symptoms between RGP

and soft contact lens wearers (McMonnies 1990; Vajdic et al. 1999). The

method of measuring TFSQ described in this study has shown significant

differences between the bare cornea and during contact lens wear, but was not

sensitive enough to distinguish any potential differences between lens

types/materials, if differences do exist.

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166

We found an interesting and unexpected pattern of change in TFSQ in

suppressed blinking conditions. TFSQ value was found to gradually reduce and

then improve to a value even better than the baseline bare eye measurements.

This is the first study to report this phenomenon which was seen more

frequently with rigid lenses and is likely to be due to the tear film drying

completely over the surface of the contact lenses to create a perfect “mirror-

like” reflection directly from the lens surface. In order to test the hypothesis that

the tears are drying (thinning) completely, further studies measuring tear film

thickness using interferometry techniques in suppressed blinking conditions

using various contact lenses are required. The overall findings of this study of

tear film quality over the surface of contact lenses in the eye show that there is

a need for better contact lens materials and surfaces with improved wettability

and hydrophilicity in order to improve the pre-lens tear film surface quality.

7.3 Conclusion and clinical implications

Contact lenses are in close contact with the ocular surface and lead to complex

mechanical and physiological effects on the tissue of the anterior eye. In this

series of studies we have examined the short-term effects of various types

(materials and designs) of contact lenses on anterior and posterior corneal

curvatures, thickness, lid-wiper, tarsal conjunctiva and TFSQ. The experimental

paradigm adopted in this study aimed to utilize lenses of identical design or

material and then systematically vary one or more parameters to understand

the influence of lens characteristics on the ocular surface. The lenses that we

designed and used in the various studies reported in this thesis showed that

measurable changes of each of the anterior eye parameters in as little as 8

hours of lens wear.

In general, the soft lenses made with modern SiHy material caused

minimal changes in the anterior eye after short-term wear compared with the

older HEMA lens material. This was particularly evident with the significant

corneal swelling seen beneath the thicker stabilizing zones of the HEMA toric

soft lens, whereas the identical lens design in SiHy material caused very little

corneal change. In the case of the rigid contact lenses, the differences between

the effects of modern lens materials (Boston XO) and the older lens material

(PMMA) was most clearly illustrated in the case of corneal swelling after short-

term lens wear. The PMMA lens led to significant central corneal swelling and

anterior corneal steepening, whereas an identical design in Boston XO material

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167

caused minimal corneal swelling or curvature changes. All the lenses caused

signs of micro-trauma to the eyelid wiper and tarsal conjunctiva, although rigid

lenses appeared to cause more significant changes than the SiHy lens. Tear

film surface quality was also significantly reduced with all types of contact

lenses.

These results show that even following the advances that have been

made in the last few decades, contact lenses still behave as a foreign object in

the eye and affect ocular health. These short-term changes in the anterior eye

are potential markers for further long-term changes. While modern contact lens

materials have clearly improved the impact of the lenses on the ocular surface,

aspects of lens wear such as tear film surface quality and micro-trauma to the

eyelids still show an obvious opportunity for improvements in lens designs and

materials in the future. Improvements in lens materials and designs will help to

reduce the physiological impact of contact lenses on the anterior eye.

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Appendices

Appendix A: Ethics and Consent form

Appendix B: Conference abstracts arising from this thesis

Tyagi, G., Collins, M., Read, S., Davis, B. “Soft contact lens wear and regional changes in corneal

thickness and topography”. American Academy of Optometry, San Francisco, 19th November,

2010.

Tyagi, G., Collins, M., Read, S., Davis, B. “Corneal topography after short term use of RGP

contact lenses”, American Academy of Optometry, San Francisco, 19th November, 2010.

Tyagi, G., Collins, M., Read, S., Davis, B. “Corneal optical changes with short term contact lens

wear”, IHBI Inspires student conference, Gold Coast, 25th November, 2010.

Appendix C: Publications arising from this thesis

Tyagi G, Collins M, Read S and Davis B (2010). "Regional Changes in Corneal Thickness and

Shape with Soft Contact Lenses." Optometry & Vision Science 87(8): 567-575.

Tyagi G, Collins M, Read S and Davis B (2011). “Corneal changes following short-term rigid

contact lens wear.” Contact Lens and Anterior Eye. Under revision.

Tyagi G, Alonso-Caneiro D, Collins M and Read S (2011). ―Tear film surface quality with rigid and

soft contact lenses” Eye and Contact Lens. Under revision.

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l0!ti PARTICIPANT INFORMATION for QUT RESEARCH PROJECT ~---------------1

"Corneal Topography and Contact Lenses"

Research Team Contacts

Description

Garima Tyagi (PhD Candidate) Phone: 07 31385716

Email: [email protected]

Or Scott Read (Associate supervisor) Phone: 07 31385714

Email: sa [email protected]

Prof. Michael Collins (Supervisor) Phone: 07 3138 5702

Email: [email protected] u.au

Brett Davis (Associate supervisor) Phone: 07 3138 5721

Email: [email protected]

This project is being undertaken as part of PhD research project by Garima Tyagi.

The purpose of this project is to investigate the changes in corneal topography (shape) and thickness with the use of different types of contact lenses. The relative influence of different contact lens materials and designs on the shape of the front and back surface of the eye (cornea), the thickness of the cornea and the total optics of the eye will be investigated.

Participation Your participation in this project is voluntary. If you do agree to participate, you can withdraw from participation at any time during the project without comment or penalty. Your decision to participate will in no way impact upon your current or future relationship with QUT (for example your grades, employment or ongoing clinical care).

In this study, you will be asked to wear contact lenses of different materials and designs. Your participation will involve a series of measurements to determine the shape and optical characteristics of your eyes before and after the wear of contact lenses for up to 8 hours. A drop of local anaesthetic (0.4% Benoxinate) may be used to decrease reflex tearing. Benoxinate is very safe; however, it is possible to scratch the eye without feeling sore because of the anaesthetic effect. You are advised not to rub your eyes for at least 45 minutes after drug instillation. The shape of the front and back surface of your eye (cornea) will be measured using the Medmont and Pentacam instruments, a wavefront sensor will be used to measure the total optics of your eye, and the IOL master instrument will be used to measure t he length of your eye. You will be asked to look into each of the instruments as they take their measurements. These measurements will be carried out a number of times over the course of the study. The Medmont, Pentacam, wavefront sensor and IOL master are all standard clinical instruments that do not touch your eyes and pose no risk to the health of your eyes. Digital images of your eye (cornea) may be taken for our records.

Prior to the experiments, we will conduct a screening examination to determine your suitability for the study and ensure that your eyes are healthy. This will involve routine clinical t ests such as the measurement of your visual acuity (with letter chart) and the examination of the front of the eye with a biomicroscope (slit lamp).

In this study, measurements will be taken a number of times over the 8 hour period. Contact lenses will be removed for the measurements. Each measurement session will take up to 30 minutes. All measurements will be conducted at the School of Optometry at QUT. You will be reimbursed for out of pocket expenses following the study.

Expected benefits lt is expected that this project will not benefit you directly. However, we are interest ed in the changes which occur in the shape and optics of the human eye after the wear of contact lenses. Data collected from this study are expect ed to improve understanding of this area and aid in the better design of contact lenses.

Risks There are no greater risks in this study than those associated with routine eye examinations or the wear of contact lenses. There are minor risks associated with wearing contact lenses at any time. Contact lenses often cause some mild discomfort when they are first inserted into the eyes. Occasionally the lens can irritate your eyes if it is not inserted correctly or if the solutions used to clean the lenses have not been properly rinsed from the lens surface. If the contact lenses cause you undue discomfort we will immed iately remove them from your eyes. An optometrist will then give you advice about the health of your eyes and offer any ongoing eyecare that you may need (at no cost).

The instruments used to measure the shape and optical characteristics of your eye are standard clinical instruments.

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Confidentiality The research data we gather from the experiments will not personally identify you by name, or in any way that allows you to be identified. We will use a code, known only to the investigators listed above, to identify your data. Any publication of data arising from

this research will use a code system which does not identify you personally. The data will be stored securely in the School of Optometry.

Consent to Participate We would like to ask you to sign a written consent form (enclosed) to confirm your agreement to participate.

Questions I further information about the project Please contact the researcher team members named above to have any questions answered or if you require further information about the project.

Concerns I complaints regarding the conduct of the project QUT is committed to researcher integrity and the ethical conduct of research projects. However, if you do have any concerns or complaints about the ethical conduct of the project you may contact the QUT Research Ethics Officer on 3138 5123 or email [email protected]. The Research Ethics Officer is not connected with the research project and can facilitate a resolution to your concern in an impartial manner.

Thank you for helping with this research project. Please keep this sheet for your information.

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~~--------C_O_N_S_E_N_T_F_O_R_M_f_o_rQ __ UT __ R_ES_E_A_R_C_H_P_R_O_JE_CT ________ ~

"Corneal Topography and Contact Lenses"

Statement of consent

By signing below, you are indicating that you:

• have read and understood the information document regarding this project

• have had any questions answered to your satisfaction

• understand that if you have any additional questions you ea n contact the research team

• understand that you a re free to withdraw at any time, without comment or penalty

• understand that you can contact the Research Ethics Officer on 3138 51230 or email [email protected] u.au if you have concerns about the ethical conduct of the project

• agree to participate in the project

• understand that the project may include digital video photography of the eye (cornea)

Name

Signature

Date I I ...................................................................

Please return this sheet to the investigator.

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Event Code 105762

Title SOFT CONTACT LENS WEAR AND REGIONAL CHANGES IN CORNEAL THICKNESS AND TOFOGRAFHY

Author Tyagi, Garima B.S. (Optometry) (Queensland University of Technology)

Coauthor( s) Michael J. Collins (Queensland University of Technology), Scott Read (Queensland University of Technology), Brett Davis (Queensland University of Technology)

Topic Cornea and Contact Lens

Day Fri, Nov 19, 2010

Time 9:00AM-5:00PM

Room Third Floor Foyer

Details Purpose: To examine the effect of short term soft contact lens wear on regional corneal thickness and shape while taking into consideration natural diurnal variations. Method: Four dif ferent types of soft contact lenses were worn by 12 young unadapted subjects, on 4 different days. The lenses were of two different materials (silicone hydrogel or hydrogel), designs (spherical or toric) and powers (-3.00 or -7.00 D). The Pentacam HR system was used to measure corneal thickness and topography before and after 8 hours of lens wear. Additionally, measurements were also carried out on two days without lens wear. Results: A significant diurnal corneal thinning was observed on days when contact lenses were not worn, and this was accounted for when calculating the contact lens induced corneal changes. Significant changes in corneal thickness and curvature were observed following contact lens wear. The greatest magnitude of corneal swelling was seen with the hydrogel toric contact lens in the central (20.3 ± 10.0 microns) and peripheral cornea (24.1 ± 9.1 microns) (p < 0.001) with an obvious regional swelling of the cornea beneath the stabilizing zones. The anterior corneal surface generally showed a slight flattening with lens wear. All contact lenses resulted in central posterior corneal steepening and this was weakly correlated with central (R2 = 0.17, p = 0.03) and peripheral corneal swelling (R2 = 0.27, p = 0.01). Conclusions: The hydrogel soft toric lenses caused an obvious regional corneal swelling under the location of the stabilization zones, the thickest regions of the lenses . However, the magnitude of corneal swelling induced by the silicone hydrogel contact lenses over the 8 hours of wear was typically less than the natural diurnal thinning of the cornea over this same period. These natural diurnal variations in corneal thickness observed from mid-morning to afternoon should therefore be considered when studying contact lens induced corneal swelling.

Attachments Rlename Size Attach Date

Key Words Corneal topography, Corneal anatomy/physiology

Disclosures No Conflicts Exists

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Event Code 105032

Hie CORNEAL TORJGRAPHY AFTffi SHORTTffiM USE OF RGPCONTACT LENSES

Author Tyagi, Garirrra B.S. (Optorretry) (School of Optorretry, Queens~nd Unr-tersity of Technology)

Michael Collins PhD, FAAO (School of Optometry, Queensland Unwersity of Technology), Scott Read PhD Coauthor(s) (School of Optometry, Queens~nd Unr-t erstty of Technology), Brett Davis BAppSc (School of Optorretry ,

Queensland Universtty of Technology)

Topic Cornea and Contact Lens

Day Fri, Nov 19, 2010

Time 9:00AM-5:00PM

Room Third Floor Foyer

Details Purpose: The purpose of this study was to investigate the changes in anterior corneal topography after short term use of rigid gas perrreable (RGP) contact lenses and to ex a nine the correlation between these changes with the fluorescein pattern under the lens. Method: Anterior corneal topography was measured ( 4 maps) before and after 8 hours of RGP lens wear using the Medmont E300 video keratoscope system on a group of 12 young heatthy unadapted subjects. Custom software was used to calculate the statistical significance of the changes in corneal topography and corneal w avefront aberrations. The fluorescein pattern observed under the lens was captured with digital sltt-~mp photography through a Wratten 128 fitter. The klcation of the contact lens w tth respect to the limbus was derwed from these images, while the location of the corneal topography map w tth respect to the limbus was also calculated. Results: The topography tangential curvature maps following lens wear demonstrated a ring of mid peripheral f~ttening which was statistically significant for all subjects. A highly significant posttive correlation (R2 = 0.8, p = 0.003) was observed between the location of minimal fluorescein pooling beneath the RGP lens ( i.e. midperipheral bearing) and the location of corneal flattening after lens wear. Some higher order corneal aberrations (Zernike terms up to 4th order) showed significant changes (p s 0.05) after RGP contact lens wear. Conclusions: A region of slight but significant corneal f~ttening was noticed in the mid peripheral region of the cornea after 8 hrs use of RGPcontact lenses. These corneal topographical changes correlated w ith the fluorescein pattern seen with the contact lens on eye. Small but signif ic ant changes also occurred in some of the higher order corneal aberrations after the use of RGP lenses.

Attachments Rlename Size Attach Date

Key Words Corneal topography

Disclosures No Conf licts Exists

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Abstract #7

Corneal optical changes with short term contact lens wear

Tyagi G.', Collins M1, Read S.1 , Oavis 8 1

1Vision Domain, Institute of Health and Biomedical Innovation, School of Optometry, Queensland University of Technology, Brisbane, OLD

Introduction: Anecdotal reports suggest that corneal optics can be influenced by contact lens wear, however the relative influence of different contact lens parameters on these corneal changes is unclear. This study aimed to examine the effect of different types of contact lens materials, designs and powers on corneal optics after a short period of wear, using the Medmont videokeratoscope.

Methods: Four different types of soft contact lenses and one spherical RGP lens were worn by 12 young unadapted subjects, on 5 different days. The soft lenses were of two different materials (silicone-hydrogel or hydrogel), designs (spherical or toric) and powers (-3.00 or - 7.00 D). The Medmont E300 videokeratoscope was used to measure corneal optics before and after 8 hours of lens wear. The change in best fit sphere-cylinder from the corneal refractive power and higher-order corneal wavefront aberrations were calculated for 4 mm (photopic) and 6 mm (mesopic) corneal diameters.

Results: Significant changes in corneal optics were observed following eight hours of spherical silicone-hydrogel (-7) lens wear. A significant hyperopic shift in anterior corneal best fit sphere (M) forthe4 mm (-0.20 ±0.14 D) and 6 mm (-0.16 ± 0.12 D) analyses were found

(both, p < 0.008). Small but significant changes in corneal vertical coma ( c;') were also

found with this lens (p < 0.05). The changes in corneal best fit sphere with the toric silicone­hydrogel lens also approached significance for the 6 mm corneal diameter. The other lenses caused no significant changes in corneal sphere-cylinder or higher-<>rder aberrations.

Conclusions: This study compared the changes in corneal optics caused by a variety of contact lenses and found that the high power silicone-hydrogel lens caused the largest changes.

'Real World Implications': When contact lenses are worn they can cause subtle, temporary changes in the shape and thickness of the underlying cornea that result in changes in corneal optical power. For one of the lens types we examined in this study, these changes were statistically and clinically significant after only 8 hours wear. An optometrist can use this information to modify the lens power to provide optimal vision.

~ ~n~u~fi Healtl1 and Biomedical Innovation