Mauro Campos, Ester Sakae and Rossen Hazarbassanov

Federal University of São PauloBrazil

None of the authors have any financial interest in the methods and products included here.

BackgroundBiomechanic properties of the cornea have been

studied with increasing interest in the present time (1)

Corneal hysteresis (CH) is a function of the corneal viscous damping properties. It reflects the capacity of the corneal tissue to absorb and release energy (2)

Corneal pathologies such as Fuchs, keratoconus and Lasik may decrease CH values (3)

Scheimpflug images of the eye allow quantification of structural density (4)

Purpose and Study Design

To correlate biomechanical properties, corneal density and vision in a model of corneal opacity

Prospective consecutive interventional case series

MethodsPatients with recalcitrant corneal opacities after epidemic

keratoconjunctivitis that failed to respond to medical therapy

All eyes underwent phototherapeutic keratectomy with 0.002% mitomycin C to improve corneal transparency and surface irregularities

Pre and postoperative evaluation included visual acuity measurements, slit lamp exam and Goldmann tonometry, corneal biomechanical indexes (CH, corneal resistance factor – CRF, corneal compensated ocular pressure –IOPCC and Goldmann calculated ocular pressure – IOPG as produced by ORA (Reichert Instruments) and Scheimpflug quantification of corneal density and central corneal thickness - CCT (Pentacan, Oculus)

MethodsPreoperative evaluation of a representative patient and details of densitometry

Data from rotational scheimpflug camera was evaluated at 0, 45, 90 and 135 degrees

Linear density was given placing the cursor over a single point

Area density was performed using standardized rectangular area with the size distributed equally on the visual axis

MethodsTrans epithelium PTK set to a mean of 68 microns based on opacities depth followed by 0.002% mitomycin application (details previously published, reference 5)

Preoperative Postoperative

Statistical Methods

Box plot representation of BSCVA, corneal biomechanical indexes (CH, CRF, IOPG and IOPCC), corneal density and corneal thickness (CCT)

Pearson linear correlation between BSCVA and density, CH and density and CH and CCT.

Student t-test for the comparisions of pre and postoperative values (BSCVA, CH, CRF, IOPG and IOPCC and CCT)

SPSS (SPSS Inc, version 15) software for data analysis

LogMAR units were used for vision analysis

Results Thirty-one eyes from 23 patients included Mean follow-up was 10.2±3.2 months100% followed for 3 months87% followed for 6 months74% followed for 12 months

Results, visual acuity

Mesopic Contrast Sensitivity

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

1,5 3 6 12 18

Cycle/degree

Lo

g M

ea

n V

alu

e

Pre-op

Post-op 1M

Post-op 3M

Post-op 6M

For illustration only, six months data, contrast sensitivity included

UCVA

46,7%

81,5%

40,0%

18,5%13,3%

0,0%0%

20%

40%

60%

80%

100%

PREOP PO 6M

Time

Per

cen

tag

e o

f p

atie

nts

y

20/20 - 20/40

20/50 - 20/80

> 20/80

BSCVA

0,0% 0,0%

13,8%20,7%

65,5%

0,0% 0,0%4,2%

16,7%

79,2%

0%

20%

40%

60%

80%

100%

Lost 2 or >lines

Lost 1 line No change Gain 1 line Gain 2 or >lines

Time

Per

cen

tag

e o

f p

atie

nts

PO 3M PO 6M

Preop Last visit Preop Last visit Preop Last visit Preop Last visit

BSCVA logmar 0.35 0.094 0.40 0.100 0 0.0 1 0.4

Densitometry                

AREA 9.20 6.57 8.37 6.40 6.7 5 15.2 9.6

LINEAR 67.30 50.94 68.70 44 37.5 22.1 96 95.5

Histeresis mmHg

IOPG 13.08 10.96 12.65 11.8 7.5 3.2 20.2 15.8

IOPCC 15.17 13.89 15.15 14.8 8.3 6 25.3 15.8

CH 9.22 8.7 9.2 8.5 5.1 6.2 14 12.5

CRF 8.69 7.6 8.85 7.4 5.2 3.8 11.7 11.1

CCT μm 491.2 461.4 483 456 408 376 568 555

Table 1: Summary of all variables before and after PTK. .

Mean value Medium Minimum value Maximum value

Results

Linear and area density decreased significantly after treatment

Better correlation between BSCVA and area density after PTK compared with linear density

Results

Correlations between area and linear density and CH were weak and negative, without significance

Preoperatively, better correlation between CCT and CRF than CCT and CH

Postoperatively, better correlation between CCT and CH compared with CCT and CRF

Conclusions and selected references

According with the data presented here, as corneal density decreases, BSCVA increases

Increased corneal density decreases correlation among biomechanical indexes and other variables such as CCT

1. Kotecha A et al. IOVS 2006;47(12):5337-472. Liu J et al,. JCRS 2005;31(1):146-553. Luce DA. JCRS 2005:31(1):156-624. Dobbs RE et al. Ophthalmology 1987;94(7):881-905. Sakae, E et al. AJO 2011;151(1):35-43