Clinical Article

Ocular Protection by Contact Lenses from Mechanical Trauma

Karen E. Ritzmann, B. Ralph Chou, MSc, OD, and Anthony P. Cullen, OD, PhD

Many lost-time industrial eye injuries are due to small high- speed particles striking the cornea. To investigate whether con- tact knses would complicate such injuries, we exposed porcine eyes fitted in vitro with rigid, hydrogel, or no contuct kns to iron filings suspended in a h’ h Ig -speed air jet. Postexposure ewalua- tions of I1 control eyes and 23 contact kns-wearing eyes re- vealed more extensive and severe corneal damage in the control group. Of the 23 contact lens-wearing eyes, fewer and less severe corned injuries were observed among those wearing rigid lenses. Contact lenses do not increase the risk of corneal injury due to small high-speed particks striking the eye; instead, they provide additional protection from mechanical injury.

Keywords: Hydrogel contact lenses; rigid gas permeable contact lenses; ocular injury; eye protection

Introduction

Contact lenses are frequently banned from industrial set- tings on the assumption that they would complicate injuries resulting from mechanical trauma. 1-3 This allegation per- sists despite the publication of many case reports indicating that contact lenses have, indeed, offered some protection to the cornea from mechanical injury.+13 Rengstorff and Black” surveyed contact lens practitioners to determine whether contact lenses cause a greater risk of ocular damage in the event of an accident involving the eyes. In a report that detailed over 100 different cases of mechanical trauma

Address reprint requests to Dr. B. Ralph Chou at the School of Optometry, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl.

Accepted April 20, 1992.

in contact lens-wearing patients, it was found that the con- tact lens minimized or completely protected the eyes from more serious injuries. In many cases, the lens cracked or even fractured when struck with the foreign body, but the cornea sustained no damage or, at worst, minor abrasions that healed quickly and completely. One incident involved a steel pellet from a BB gun. The lens fractured but there was no cornea1 damage. A common finding in patients involved in auto accidents was extensive periocular lacer- ations and a broken contact lens, but in almost all cases, the cornea was not even abraded.7s’0V”

Apart from these case reports, only one controlled study has demonstrated the protective value of contact lenses against airborne particles. Nilsson et al. 14,15 simulated the hazards associated with the operation of a grinding wheel, exposing anesthetized contact lens-wearing rabbit eyes to showers of burning grit particles. They found that hard lenses effectively protected the covered parts of the cornea since all grit particles were observed to rebound off the surface of the lens. Although most of the grit particles were also observed to rebound off the surface of the soft lenses, they usually left behind areas of physical damage to the lens. Occasionally, a particle perforated the soft lens to damage the epithelium superficially.

It would appear that contact lenses present no further hazard to the eye when worn behind industrial safety spec- tacles and may actually provide some additional protec- tion.‘617 Despite this observation, their use is often for- bidden in the industrial setting as occupational health and safety personnel fail to recognize the level of risk for eye injury or the possible hazards involved when some patients do not wear their contact lenses. The latter applies to pa- tients who, due to cornea1 distortion or some underlying

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Ocular protection from mechanical trauma: Ritmnn et al.

pathological condition (see Table I ), do not acquire ade- quate vision with a spectacle correction. The decision of whether or not to restrict the use of contact lenses in in- dustry should include the consideration of reduced specta- cle acuity as well as a clear understanding of the lack of evidence to support an increased risk to industrial contact lens wearers. I8

The purpose of this study was to evaluate the ocular protection afforded by rigid and gel contact lenses from mechanical damage due to small high-speed particles.

Materials and Methods

I. Determination of Average Pig Eye Dimensions and Contact Lens Sekction

An initial 13 pig eyes were obtained from a local abattoir less than 1 hour post-mortem. Mean cornea1 curvature for each eye was determined from five readings made with a Bausch & Lomb keratometer. Mean cornea1 curvature of the adult pig eye was calculated from the keratometric data following the method of Harris. l9 Maximum and minimum comeal diameters were measured with a ruler. These values were used to select the base curve and diameter parameters of the contact lenses to give an “on K fit.”

The 23 lenses selected for the study included 12 rigid gas permeable (RGP) and 11 hydrogel (gel) lenses. The rigid lenses tested were either silicon acrylate or fluorosilicone acrylate and had center thicknesses between 0.11 and 0.2 1 mm. The hydrogel lenses tested ranged from medium-to- high water content and had center thicknesses between 0.05 and 0.16 mm (Table 2). The lenses were first-quality surplus lenses obtained from the Centre for Contact Lens Research, School of Optometry, University of Waterloo.

II. Procedure

Eyes were prepared as follows: The adnexa were dissected away and the front surface of the eye lubricated with a drop of artificial tear solution (Tears Plus, Allergan). Since the absence of intraocular circulation produces a very flaccid post-mortem eye (i.e., IOP less than 5 mmHg), it was necessary to raise the IOP to a physiological level and maintain it at that level while the particles collided with the eye. This was done by inserting a catheter into the posterior chamber. The catheter was connected to a ma- nometer and water was introduced into the eye until an IOP of 17 mmHg was obtained. The pressures obtained

Table 1. Some Ocular Conditions Requiring Contact Lens Wear

Irregular astigmatism Keratoconus Aphakia Bullous keratopathy Recurrent epithelial erosion Aniseikonia

Table 2. Test Lens Parameters

CL No. Material

Base Curve (mm)

Diameter (mm)

Center Thickness

(mm)

Al A2 A3 A4 A5 A6 A7 Bl B2 B3 B4 B5 B6 B7 B8 Cl c2

:: C5

E’; C8

Gel 8.80 14.50 0.05 Gel 8.90 14.00 0.10 Gel 8.90 14.50 0.16 RGP 8.20 9.60 0.21 RGP 8.61 9.20 0.17 RGP 8.45 9.20 0.20 RGP 8.30 9.40 0.13 Gel 8.70 14.50 0.10 Gel 8.80 14.50 0.05 Gel 8.80 14.50 0.05 Gel 8.90 14.00 0.10 RGP 8.25 9.20 0.14 RGP 8.11 9.40 0.19 RGP 7.82 9.40 0.11 RGP 7.72 9.40 0.12 Gel 8.60 14.00 0.10 Gel 8.80 14.50 0.05 Gel 8.80 14.50 0.05 Gel 8.90 14.00 0.10 RGP 7.72 9.40 0.15 RGP 7.72 9.40 0.15 RGP 7.70 9.60 0.18 RGP 7.50 9.60 0.19

with this manometer system were verified using a Tonomat gauge. The catheter remained inserted while the eye was exposed to the spray of particles.

After the contact lens was placed onto the cornea, the holder was mounted in the exposure system. This consisted of a ventilated box with the eye holder mounted 40 cm from the air gun nozzle aperture. The air gun was connected to a 276 kPa air compressor. When the air gun trigger was depressed, a 0.100 g sample of fine (40 mesh) iron filings (Fisher Scientific, I-57) was drawn up a tube into the air gun and sprayed at high velocity onto the far wall of the box. The distribution of particle impacts on the back wall of the box was recorded using an adhesive covered board. Because of the dispersive characteristics of the air jet, only a small portion of the iron filings struck the eye.

Damage to the contact lens was assessed in situ by biomi- croscopy. The lens was removed from the eye and the cor- nea examined for evidence of injury. A dissecting micro- scope was used to inspect the posterior surface of the con- tact lens for signs of penetration by the iron filings.

Results

Results of this study are summarized in Tubk 3. All eyes without contact lenses sustained severe cornea1 damage (Figure I). The injuries usually involved full thickness ep- ithelial displacement with particle penetration extending to the level of the anterior stroma. In Eye No. 3 1, a particle was observed to penetrate through most of the thickness of the stroma.

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Clinical Articles

Table 3. Results

Eye No. CL No. Lens

Damage Lens

Penetration

Cornea1 Abrasion Observed

1 2

: 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Al (gel) A2 (gel) A3 (gel) A4 (RGP) A5 (RGP) A6 (RGP) A7 (RGP) Control Control Control B1 (gel) B2 (gel) B3 (gel) B4 (gel) B5 (RGP) B6 (RGP) B7 (RGP) B8 (RGP) Control Control Cl (gel) C2 (gel) C3 (gel) C4 (gel) C5 (RGP) C6 (RGP) C7 (RGP) C8 (RGP) Control Control Control Control Control Control

+++ +++ ++ +++ + ++ +

+++ +++ +++ ++ ++ ++ + +

+++ +++ +++ +++ + + + +

- - - - - - -

+++ - - + + - - -

- - +

- - -

- -

- - - - +++ +++ +++ + - - + + - - - +++ +++ - - + + - - - - +++ +++ +++ +++ +++ +++

- none; + mild; + + moderate; + + + severe.

In all cases, the contact lenses, whether RGP or gel, afforded some protection to the underlying cornea from mechanical damage caused by the high-velocity iron fil- ings. Only four of the 23 contact lens-wearing eyes sus- tained mild cornea1 damage.

Many of the soft lenses were severely damaged by the high-speed iron particles (Figure 2). In only three of the 11 gel lenses tested, however, had a particle penetrated the full thickness of the lens. Unlike the gel lenses, the RGP lenses sustained very little damage from the impact of the filings (Figure 3).

For the RGP lens-wearing eyes, the exposed regions of the cornea often were damaged by penetrating particles while the cornea1 region covered by the contact lens was undamaged. This suggests that had the contact lens not been on the eye the central area would have also contained penetrating particles. In Eye No. 17, one particle struck the contact lens at the lens edge. The area of the cornea con- tacted by the particle was deeply penetrated while the area

Figure 1. An iron filing is imbedded in the superficial cornea of a control eye following collision. As shown by specular reflection, the epithelium is displaced to the left side of the particle (Eye No. 10).

Figure 2. Gel lenses struck by iron filing projectiles often sus- tained a high degree of physical damage. The posterior surface of this lens, however, was not penetrated by the particle (Eye No. 22).

under the contact lens was not affected by the impact (Fig ure 4). Thus, the protective function of the rigid lens was demonstrated.

Discussion

Of the 23 contact lens-wearing eyes, five corneas showed evidence of mild epithelial damage. Of these, four were soft lens-wearing eyes and only one case of cornea1 damage was found under a hard lens. Because of the nature of the poly- mer cross-linking, the soft contact lens is much more de- formable than is the rigid lens. Thus, when a high-speed

164 ICLC, Vol. 19, July/August 1992

Figure 3. Unlike the gel lenses, rigid lenses were not severely damaged when struck by the iron filings. In this case, the particle adhered to the front surface of the lens, causing only a small area of superficial damage to the lens (Eye No. 16).

Figure 4. This photograph demonstrates the protective function of a rigid lens when an iron filing struck the contact lens at the lens edge. Where the filing hit the cornea, there is deep penetra- tion of the cornea. Where the filing struck the rigid lens, how- ever, there is only superficial lens damage with no damage to the underlying cornea (Eye No. 17).

particle collides with a soft lens, there is a greater energy transfer to the underlying tissue than when a similar parti- cle hits a rigid lens. A soft lens is therefore relatively less effective in preventing mechanical injury from a high-speed small particle than is a hard lens.

Center thickness also seems to affect the amount of im- pact protection offered by a contact lens. Ail the soft lenses that were penetrated had center thicknesses of either 0.05 or 0.10 mm, whereas the 0.16 mm-thick gel lens showed no evidence of perforation. A high-speed particle colliding with a thick lens has a greater distance available for decel- eration than it would in a thin lens. Fewer cases of perfo- ration would therefore be expected in a thick lens as op- posed to a thin lens.

Ocular protection from mechanical trauma: Ritzmann et al.

These results compare well with those of Nilsson et al. I4 despite differences in methodology. In their study, rabbit eyes were exposed to burning grit particles produced by two grinding discs (2770 rpm) and one cutting disc (8000 rpm). Rigid and soft lenses (low and high water content, respec- tively) were tested. The testing distance was 20 cm and the exposures lasted 2-3 seconds. Because the particles were hot, damage induced to the contact lens and ocular tissues was not solely due to the mechanical trauma, but also due to thermal effects. They also concluded that the presence of the contact lens imposed no additional risk to the eye, but, rather, protected the cornea from damage, with rigid lenses offering more protection than did soft.

The findings of this study also serve to verify clinical findings in cases of physical trauma in contact lens-wearing individuals, such as those described by Rengstorff and Black. lo They concluded from their survey of contact lens practitioners and analysis of published case reports that contact lenses minimized or completely protected the eyes from more serious injuries in individuals who were exposed to mechanical trauma while wearing their contact lenses. In comparison to a survey of spectacle lens injuries, Reng- storff and Black added that the protection offered by con- tact lenses is obvious in cases of physical trauma.

Conclusions

Postexposure examinations of 23 contact lens-wearing eyes and 11 control eyes revealed fewer and less severe cornea1 injuries in the eyes wearing the contact lenses. Inspection of the lenses postexposure showed that soft lenses generally incurred more damage and were more fre- quently penetrated by the iron filing projectiles than the rigid lenses were. We conclude, therefore, that contact lenses can provide additional protection to the eye from high-velocity projectiles, when worn with standard safety eyewear in many industrial settings.

Acknowledgments

This study was supported by grants from the Natural Sciences and Engineering Research Council of Canada, Beta Sigma Kappa International Optometric Honor Frater- nity, and the University of Waterloo Science Incentive Fund.

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Karen E. Ritzmann has recently completed her final year at the School of Optometry, University of Waterloo. She was a member of the Beta Sigma Kappa Student Chapter at the University of Waterloo and she received a BSc in Vision Science and her OD in May 1992. She plans to enter private practice.

Dr. Chou received his BSc in astronomy from the University of To- ronto in 1973. His OD and MSc in physiological optics are from the University of Waterloo. A member of the faculty of the School of Optometry at Waterloo since 1982, he presently holds the rank of associate professor. He serves as chairman of the Registration Commit- tee of the College of Optometrists of Ontario and academic editor of the Canadian Journal of Optometry. He also acts as a consultant to several provincial optometric association industrial vision programs. His research interests include the optical properties of ophthalmic lens materials, optical radiation effects on ocular tissues, impact resistance of spectacle lens materials, and occupational/environmental safety ap- plications of ophthalmic materials, including contact lenses.

Dr. Cullen received his optometric education at The City University, London, graduating with honors in 1960. He completed a master’s degree in the Department of Ophthalmology at the University of Sas- katchewan in 1970. He received an OD from the Pennsylvania College of Optometry in 1978 and his PhD from The City University in 1979. Following appointments with the Royal Air Force, the Hospital Eye Service (UK), and the Universities of Saskatchewan and Houston, he is now a professor and director of the School of Optometry at the University of Waterloo. Dr. Cullen is a fellow and diplomate in Con- tact Lens Practice of the British College of Optometrists and a fellow and secretary-treasurer of the American Academy of Optometry.

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