Clinical Article

Contact Lenses and Cornea1 Topography

Michael J. Collins, MAppSc, and Adrian S. Bruce, PhD

We investigated the effect of 3 months of daily soft contact lenswear on corneal topography. Thirty young myopic subjects

who had not previously worn contact lenses were fitted with hydroxyethyl methacrylate (HEMA) soft lenses. Cornea1 topog raphy was measured in each eye before and after the 3-month period using a uideokeratoscope, the Topographic Modeling Sys-

tem (7N.Y). For each TMS image, we recorded the local radius

of curvature at points approximately I, 2, 3, and 4 mm along the four principal meridians. Following 3 months of soft lens wear, there was A significant steepening of the cornea in atI meridians. This steepening was most pronounced in the periph-

eral cornea, averaging 0.09 + 0.13 mm (i.e., about 0.50 *

0.75 D) at 4 mm from the center of the cornea.

Keywords: Cornea; comeal topography; soft contact lenses; vid- eokeratoscope; Topographic Modeling System

Introduction

The shape of the human cornea is best represented by an

ellipse, which progressively flattens in curvature from cen- ter to edge. Accurate measurement of this shape is impor-

tant for an understanding of the optical characteristics of an individual eye and also in the fitting and optics of contact

lenses. As the cornea is the major refracting component of the eye, variations in the shape of the cornea have impor- tant implications for the optical quality of the eye.

Videokeratoscopes permit the measurement of the shape of a large proportion of the cornea1 surface. This measure- ment is achieved in most videokeratoscopes by reflecting

Address reprint requests to Michael Collins at the Centre for Eye Research, School of Optometry, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland, Australia 4001.

Accepted for publication July 1993.

light from a series of concentric rings from the tear/cornea surface, similar to the principle of the Placid0 disk. The

reflected image is usually captured by a CCD camera, dig- itized, and then analyzed by a computer to provide a de- tailed reconstruction of the cornea1 shape, both centrally and peripherally.

Masnick’ and Bailey and Camey’ found little change in

the central cornea1 curvature following hydroxyethyl meth- acrylate (HEMA) lenswear. Harris et al. 3 measured the cen-

tral cornea1 curvature in a group of 27 myopic soft lens wearers over a period of 9 months of soft (HEMA) lens wear. They found a mean central cornea1 steepening of

0.23 D (about 0.05 mm) after the first month of lenswear, which persisted for the following 8 months. These subjects also showed a small increase in with-the-rule cornea1 astig-

matism over the same time period. We have investigated the effect of soft HEMA lenswear

on cornea1 topography. Previous studies of this phenome- non used relatively thick HEMA lenses and were limited by the available instrumentation. However, videokerato-

scopes provide a relatively simple and accurate method of

monitoring changes in cornea1 topography.

Methods

Thirty young myopic subjects participated in this study. The subjects ranged in age from 17 to 32 years, with a mean age of 21 years, and none had previously worn contact

lenses. The refractive errors of the subjects ranged from - 0.75 to - 4.75 D and eight required soft toric correction.

The maximum cylinder power was - 2.00 D. All subjects were fitted with HEMA soft lenses and wore the lenses on a daily wear basis. The lenses were all of one design and had a nominal center thickness of 60 microns for a - 3.00 DS

back vertex power. Cornea1 topography was measured in each eye before and

after the 3-month wearing period using a videokeratoscope,

0 1993 Butterworth-Heinemann /C/X, Vol. 20, September/October 1993 187

Clinical Art&s

the Topographic Modeling System (TMS). 4 For each TMS image, we recorded the local radius of curvature at points approximately 1, 2, 3, and 4 mm along the superior, infe- rior, nasal, and temporal meridians. Location of the pe- ripheral points 1, 2, 3, and 4 mm along each meridian was normally accurate to within 0.05 mm (maximum error 0.11 mm), because the position of the reflected mire rings varies

slightly in spacing depending upon the shape of the indi- vidual cornea along that meridian. Similarly, the O”, 90”, 180”, and 270” meridians can be approximated only to within kO.7” because the TMS measurement cursor incre- ments in 1.4” steps circumferentially.

In some of the analyses, we averaged the local radius of

curvature for each meridian at the same peripheral location (distance from the center) and referred to this average as the mean ring radius. For example, the mean of local radii

at 1 mm peripheral meridian locations along superior, in- ferior, nasal, and temporal was called the 1 mm ring.

For 21 of the 30 subjects, we had complete data for both eyes and averaged the right and left eye data (e.g., match- ing nasal-to-nasal meridians). For the remaining nine sub- jects, only the data from one eye was used, because one or more data points was not available (e.g., the 4 mm superior

point was obscured by the upper eyelid). Averaging the right and left eye data of the 21 subjects with full data for

both eyes served to improve the reliability of the data for those subjects.

Measurements of cornea1 topography were made within a

few minutes of the subjects removing their lenses at the 3-month visit. No attempt was made to standardize the time of day at which the topography measurements were

taken. However, the small diurnal variations in topogra- phy5 that can occur in subjects should not have systemat- ically biased the results.

The calibration of the TMS was checked by taking re- peated measures of three calibration steel balls both before

and after the study. This technique does not permit abso- lute calibration of the instrument, but will disclose relative

changes in the TMS accuracy. There was no significant variation in the TMS calibration over the course of the study.

Results

Following 3 months of soft lenswear, there was a signif- icant steepening of the cornea in all meridians. This steep-

ening was relatively minor in the central cornea (0.02 + 0.06 mm for the central 1 mm ring) and more pronounced in the peripheral cornea in all meridians, averaging 0.09 5 0.13 mm (i.e., about 0.50 -+ 0.75 D) for the ring 4 mm from the center of the cornea (Figures 1 and 2).

We conducted a repeated-measures analysis of variance (ANOVA) on the change in mean ring radius (i.e., posi- tions 1, 2, 3, and 4 mm averaged across the four meridians) as a function of visit (i.e., before and after 3 months lens- wear). There were significant changes in mean ring radius

0.10

0.08

(mm)

Figure 1. Change in comeal topography associated with 3 months of daily HEMA soft lens wear in a group of 30 myopic subjects. The zero position on both the horizontal and vertical axes corre- sponds to the center of the TMS videokeratoscope image.

795 -

790- c g 7.85 -

s 780-

F - a 775

6 770- z p 7%- LT

760 -

755 -

7 - 50

one two three Iour

Rng radius (mm from centre 01 TMS wnage)

Figure 2. Mean radius of curvature for points at a distance of 1, 2, 3, and 4 mm from the center of the TMS videokeratoscope image (averaged from the superior, inferior, nasal, and temporal merid- ians). Visit 1 is the base-line radius of curvature and Visit 2 is approximately 3 months later.

as function of visit (F = 10.9, df = 1, p = 0.003) as

illustrated in Figures 1 and 2. There were also significant changes in mean ring radius as a function of visit and sub-

ject (F = 17.9, df = 3, p = O.OOOl), which suggests that the changes in mean ring radius were significantly different between subjects.

There was a significant intercorrelation (r2 > 0.85 for all correlations) between the change in the mean radius of curvature of individual rings (1, 2, 3, and 4 mm from the center of the cornea). An example of this relationship is presented in Figure 3, illustrating that changes in mean radius in the 1 mm ring were highly correlated with changes in the mean radius of the 4 mm ring.

We conducted a regression on the change in mean cor- neal radius for the 4 mm ring as a function of the mean back vertex power (using the best sphere if a toric lens was worn) of the subject’s soft lenses. There was no significant corre-

188 ICLC, Vol. 20, September/October 1993

Soft CL and comeal topography: Collins and Bruce

mance when the pupil size reaches 68 mm diameter. This pupil size is likely to be achieved only in some subjects, in

low luminance conditions. For these subjects, the induced

change in spherical aberration is unlikely to have a signif- icant effect on visual acuity7-9 or to influence the optimal refraction of the eye. ‘O

The peripheral cornea1 steepening noted in this study may, in part, be the result of errors in the focusing of the

TMS by the operator. If the TMS focusing markers are misaligned (either too close or too far from the eye), the TMS readings will be in error. This error manifests as overly steep or flat readings, with the error being of greater magnitude for peripheral readings. However, this is un- likely to have influenced the results in this study, because

these errors should be randomly distributed and would man- ifest themselves in the statistical variance of the data. The peripheral steepening noted in this study was evident in 24

of the 30 subjects and is therefore unlikely to be an artifact of a random focusing error.

It would have been preferable to have monitored a con- trol group of myopic subjects wearing spectacles over a sim- ilar period as that of the soft contact lens wearers in this

study. However, the degree of midperipheral steepening (0.50 D) noted over 3 months in the soft lens wearers in

this study is far greater than would be expected to occur in a myopic spectacle-wearing control group. There appears to be little information regarding long-term stability of cor-

neal topography in persons not wearing contact lenses. Clark’ ’ monitored cornea1 topography in three subjects for

up to 14 months and found no significant changes. The mechanism by which soft HEMA lenswear causes

the cornea to steepen in the midperiphery is not clear from this study. Since all subjects were myopic, the contact

lenses would have maximum thickness at the edge of the front optic zone, which approximately corresponds to the region of the cornea showing the greatest changes in to-

pography. It is therefore possible that the changes in cor- neal topography in this midperipheral region resulted from localized cornea1 swelling or the mechanical pressure of the lid during blinking, acting differentially through the thick-

est zone of the lens. When the soft lenses are removed, the time course of the

resolution of midperipheral steepening is difficult to pre- dict. If the steepening is edema-related, the resolution of the midperipheral steepening should parallel the resolution of cornea1 swelling and therefore occur in a matter of hours. If the steepening results from cornea1 distortion related to pressure effects (similar to the principle of orthokeratol- ogy), the resolution of steepening will presumably occur

over days or weeks and will continue to influence the spher- ical aberration of the eye during this time.

We have investigated the changes in cornea1 topography associated with 3 months of daily soft (HEMA) lenswear in 30 myopic subjects. We found a slight, but statistically significant, steepening of the cornea in the midperiphery (i.e., the cornea became more spherical). For most sub-

.4i

-3 -.2 -1 0 1 2 3 4

Change in radius 01 ring 4 (mm)

Figure 3. Correlation change in of curvature points all 1 mm from of TMS and ring 4 (4 all 4 mm from of TMS following months of daily HEMA lenswear. change in indicates

radius of curvature following lenswear. linear regression 0.85).

between these variables 0.99). This was prob- ably due to the limited range of lens back

used in the For the 2 1 subjects

change in mean ring (from visit 1 to 2) between the right and left eyes. These correlations

lation to the magnitude change being measured

Discussion

The changes that we measured in the cornea1 topography

of these subjects are likely to be of limited clinical signifi- cance. An average steepening of 0.1 mm in the midperiph-

era1 cornea1 curvature (i.e., the cornea becomes more spherical) is unlikely to substantially influence the fitting characteristics of soft lenses. However, in a few subjects,

the changes were relatively large, steepening by about 0.3 mm (1.5 D) in some subjects and flattening by 0.2 mm (1 D) in other subjects.

These changes in cornea1 topography are also unlikely to

have substantial effects on visual performance. Since the cornea1 curvature appears to remain relatively stable in the central cornea and steepen slightly (about 0.50 D) in the midperiphery (along all meridians), there will be a con-

comitant increase in the spherical aberration of the eye. Peripheral steepening of the cornea leads to an increase in positive spherical aberration. This effect is predicated on the soft lens flexing to maintain alignment with the cornea1 contour, and this would seem a reasonable assumption.6

Spherical aberration of the eye is dependent on pupil

size, and changes in cornea1 topography at 3-4 mm from the center of the cornea will only influence visual perfor-

ICLC, Vol. 20, September/October 1993 189

Clinical Articles

jects, this change is likely to be of limited clinical signifi- cance in the fitting or vision with soft lenses. However, if this trend continued with longer periods of lenswear, these changes would be of importance.

Acknowledgments

We thank Vicki Shuley and Wong Tin for their assis-

tance in data collection.

References

1. Masnick KB: Comeal curvature changes using hydrophilic lenses. Aust _I Optom 1971;54:24C-241.

2. Bailey IL, Camey LG: Cornea1 changes from hydrophilic contact lenses. Am J Optom Arch Am Acad Optom 1973;50: 299-304.

3. Harris MC, Sarver MD, Polse KA: Cornea1 curvature and refractive error changes associated with wearing hydrogel contact lenses. Am _I Optom Physiol Opt 1975;52:313-319.

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11.

Gormley DJ, Gersten M, Koplin RS, Lubkin V: Cornea1 modeling. Cornea 1988;7:3&35. Keily PM, Camey LG, Smith G: Diurnal variations in cor- neal topography and thickness. AmJ Optom Physiol Opt 1982; 59~976-982. Bibby MM: A model for lens flexure-validation and predic- tions. ICLC 1978;7:12+138. Bauer CT: Longitudinal spherical aberration of modem oph- thalmic lenses and its effect on visual acuity. Appl Opt 1980; 19~22262234. Cox 1, Holden BA: Soft contact lens-induced longitudinal spherical aberration and its effect on contrast sensitivity. OP_ tom Vis Sci 1990;67:679-683. Collins MJ, Brown B, Newman SJ, Atchison DA: Tolerance to spherical aberration induced by rigid contact lenses. Oph- thd Physiol Opt 1992;12:24-28. Charman WN, Jennings JAM: The refraction of the eye in relation to spherical aberration and pupil size. BrJ Physiol Opt 1978;32:78-93. Clark BAJ: Time variations in observed cornea1 topography. Awt .I Optom 1973;56:443-447.

Clinical Implications

This article is the first report on cornea1 topographical changes with soft contact lenswear using a

videokeratoscope. Previous studies on the effect of contact lenses on cornea1 topography have been limited by available instrumentation to the central cornea. The videokeratoscope allows more accurate monitoring

of both the central and peripheral cornea. In this study, the myopic subjects showed statistically significant

cornea1 steepening in the midperiphery after only 3 months of lenswear. Although these results may have limited clinical significance as a whole, for some patients, these changes may explain variations in visual acuity or lens fit with long-term soft contact lenswear.

Suit May Ho Cornea1 Biophysics Laboratory

Department of Optometry University of Melbourne

Parkville VIC 3052, Australia

Michael Collins, DipAppSc (Optom), MAppSc, FAAO, is a graduate

of the Queensland University of Technology (QUT). He worked in private practice before joining the faculty of the School of Optometry at QUT in 1981, where he is now a senior lecturer. He is currently responsible for contact lens teaching and is involved in various aspects of contact lens and vision research.

Adrian Bruce graduated in optometry in 1984 from The University of Melbourne. He gained a PhD degree in 1990, under the supervision of Noel Brennan, on the topic of “Diagnostic assessment of cornea1 func- tion during extended wear of hydrogel contact lenses.” Since 1991, Dr. Bruce has been a postdoctoral research fellow at the QUT, Centre for Eye Research.

190 ICLC, Vol. 20, September/October 1993