rigid gas permeable contact lens and corneal topography

Rigid gas permeable contact lens and cornealtopography

E. Yebra-Pimentel1, M. J. GiraÂ ldez1, F. L. Arias4, J. GonzaÂ lez1,J. M. GonzaÂ lez1, M. A. Para®ta2 and M. Febrero3

1Department of Applied Physics (Optometry Group), School of Optics and Optometry, University ofSantiago de Compostela, 15706 Galicia, Spain, 2Department of Ophthalmology, School of Opticsand Optometry, University of Santiago de Compostela, Galicia, Spain, 3Department of Statistic andO. R., Faculty of Mathematics, University of Santiago de Compostela, Galicia, Spain and4Department of Optometry, University AutoÂnoma of Aguascalientes, Mexico, Mexico

Summary

We investigated the effect of high Dk daily wear rigid gas permeable contact lenses on cornealtopography. Eight young myopic subjects wore hard contact lenses for 21 days. Cornealtopography was monitored using the EyeSys system. It was measured every day during thenext 21 days after the ®tting. We recorded the corneal radius of curvature at 16 peripheral pointsapproximately 1, 2, 3 and 4 mm along the four principal meridians (nasal, superior, inferior andtemporal). Our study showed that daily wear RGP Forum 210 does not produce signi®cantalterations of the corneal curvature as a function of time. q 2001 The College of Optometrists.Published by Elsevier Science Ltd. All rights reserved.

Introduction

The shape of the human cornea is best represented by an

ellipse, which progressively ¯attens in curvature from

centre to edge. As the cornea is the major refracting compo-

nent of the eye, accurate measurement of the corneal shape

is important to understand the optical characteristics of an

individual eye and in the ®tting and optics of contact lenses.

Variations in the shape of the cornea have important impli-

cations for the optical quality of the eye.

Contact lens wear may alter corneal shape and induce

corneal distortion (Wilson and Klyce, 1994). Rengstorff

showed that rigid contact lenses can induce transient

changes in corneal curvature (Rengstorff, 1969a). Poly-

methylmethacrylate lenses (PMMA) produce initial corneal

steepening in association with central corneal edema, parti-

cularly during the adaptation period, followed by a slowly

progressive ¯attening of the central corneal curvature, as

measured by keratometry (Rengstorff, 1969b). Corneal ¯at-

tening has also been reported with long-term wear of rigid

gas-permeable lenses (RGP) on daily and extended wear

schedules (Briceno-Garbi, 1984; Polse et al., 1987; Iskeleli

et al., 1996). Gradual reductions in corneal toricity

(Briceno-Garbi, 1984; Polse et al., 1987; Iskeleli et al.,

1996) and corneal asphericity (Kame and Kennedy, 1987)

have also been reported.

The effect of the base curve ®tting relationship on corneal

topography has been evaluated by Carney (1974) and

Carney (1975), who found that during ®tting, ¯at, steep

and alignment ®tting lenses all reduced the asphericity of

the cornea. After ®tting, the steeply ®tted group continued to

show a decrease in asphericity; the ¯at-®tting group,

however, actually showed an increase in asphericity, with

the alignment-®tting group between the other groups. In

some cases the inferior quadrant was more affected, espe-

cially in the case of ¯at-®tting lenses. These changes are a

result of the moulding force of the lens against the eye or are

caused by contact lens interference with corneal metabolism

or, perhaps, by a combination of both factors (Calossi et al.,

1996).

Videokeratoscopes permit the measurement of the

shape of a large proportion of the corneal surface (Horner

et al., 1998). This measurement is achieved in most

236

Ophthal. Physiol. Opt. Vol. 21, No. 3, pp. 236±242, 2001q 2001 The College of Optometrists. Published by Elsevier Science Ltd

All rights reserved. Printed in Great Britain0275-5408/01/$20.00

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PII: S0275-5408(00)00018-1

Received: 23 July 1999

Revised form: 22 December 1999

Accepted: 19 March 2000

Correspondence and reprint requests to: E. Yebra-Pimentel. Tel.: 134-

981-56-31-00; fax: 134-81-59-04-85.

E-mail address: [email protected] (E. Yebra-Pimentel).

videokeratoscopes by re¯ecting light from a series of

concentric rings from the cornea/tear surface, similar in

principle to the Placido disk. A CCD is used to capture

the re¯ected image, which is then digitized and analyzed

by computer to provide a detailed reconstruction of the

central and peripheral corneal shape.

The purpose of this study was to investigate the effect of

daily wear high Dk rigid gas permeable contact lenses on

corneal topography for a period of 21 days after ®tting.

Materials and methods

Eight young myopic subjects were enrolled in this study.

Of these patients, 4 were females and 4 were males. Their

ages ranged from 20 to 30 years and their refractive errors

ranged from 22.00 and 28.00 D. The maximum cylinder

power was 22.00 D. Two patients had previously worn

hydrogel contact lenses, but they stopped using them one

month before the study. No patient had any ocular or

systemic disease and none had undergone eye surgery.

The study participants did not use any local eye medications

during contact lens wear.

All subjects were ®tted with rigid gas permeable contact

lenses (RGP) and wore the lenses on a daily wear basis. The

responses of 8 right eyes to RGP lens wear were investi-

gated. The contact lenses used are manufactured by Bausch

& Lomb (Spain). The trade name is Forum 210w and the

generic name is Si¯uorfocon-A. They are made of a copo-

lymer in which the major constituent is a monomer with

silicone and ¯uorine. The Dk value of this material is 210 £10211 cm2

=s (ml O2/ml mm Hg) at 358. We used a trial set of

lenses in which base curves ranged from 7.30 to 8.70 mm in

0.10 mm steps. The power of these lenses was 22.00 D, the

diameter was 9.80 mm and the optic zone diameter was

8.40 mm. The optimal ®t was one that achieved a central

alignment ¯uorescein pattern with moderate-to-minimum

edge lift and adequate movement. Lenses were cleaned

and stored according to the manufacturer's recommended

procedures when not being worn.

Corneal topography was measured every day during a 21-

day period after the ®tting. Sixteen corneal points were

measured on the right eye every day after 10 hours of lens

wear (20:00 pm). Measurements were taken 1 min after the

removal of the lens. During the measurements, the patient

blinked naturally, and readings were taken immediately

after completion of a blink. The time of day at which the

topography measurements were taken was standardized to

avoid the effect of diurnal variations in corneal topography

(Kiele et al., 1982).

The EyeSys Corneal Analysis Systemw videokeratoscope

was used to record the local radius of curvature at 16 periph-

eral corneal points approximately at 1, 2, 3 and 4 mm along

the superior, inferior, nasal and temporal main meridians

(with maximum and minimum power). To decide where

the 1, 2, 3 and 4 mm points were located and the radii values

at these points, the data was presented in tabular form.

Location of the peripheral points 1, 2, 3 and 4 mm along

each meridian was usually accurate within 0.05 mm (maxi-

mum error 0.14 mm), because the position of the re¯ected

mire rings varies slightly in spacing depending upon the

shape of the individual cornea along that meridian.

The calibration of the videokeratoscope was checked by

taking repeated measurements of four calibrated steel balls

both before, during and after the study. There was no signif-

icant variation in the videokeratoscope calibration over the

course of the study.

In this study, we analyzed results using the SPSS Profes-

sional Statistics 6.1 program (Statistical Package for the

Social Sciences). Two statistical analyses were used to

examine changes in corneal curvature over time: analysis

of autocorrelation and analysis of regression. Both analyses

were applied for a total of 21 corneal radii measurements

from 16 corneal points per eye. However, less than 21

measurements were obtained for the most peripheral corneal

points in some cases. In the vertical meridian this was possi-

bly due to interference from eyelids, being most noticeable

in the superior meridian. The 4 mm nasal point was prob-

ably obscured by the nose shadow.

Results

The statistical analysis of the data in this study was

performed using data corresponding with 16 corneal points

of 8 right eyes. We analyzed results from each eye sepa-

rately. We based this decision on the assumption that each

eye ®tted with Forum 210 lens would respond indepen-

dently to the particular pressures generated as a result of

the individual lens-cornea ®tting relationship achieved.

Following 21 days of RGP lens wear, no signi®cant

differences were found in corneal curvature at any of the

temporal, nasal, superior and inferior locations studied in

each of the right eyes of the 8 subjects. Several outliers data

points were observed only in one eye, but they have no

in¯uence in the estimation of the slope. Data from this

eye, the only one that shows outliers, are presented as an

example in this paper.

Before a regression analysis can be performed, we must

ensure that the data are independent and there are no

outliers. If there is any type of dependence it is well

known that the estimation of the regression coef®cients

will not be consistent. In our case the measurements were

taken over 21 consecutive days and the suspicion of depen-

dence is strong. To analyze dependence over time the most

suitable tool is the autocorrelation function, usual tool in the

time series methodology (see, for example, Box and

Jenkins, 1976). Let xi be the measure at the day i. The

autocorrelation coef®cient at lag k (hereafter r k) is the

usual correlation between pairs of values {xi, xi1k}. The

plot of r k vs the lag k, is called the autocorrelation function.

When the data are independent r k is equal to zero for every

Rigid gas permeable contact lens and corneal topography: E. Yebra-Pimentel et al. 237

238 Ophthal. Physiol. Opt. 2001 21: No 3

Figure 1. Autocorrelation function for lags k � 1;¼; 5 on x-axis (bars), the con®dence intervals for everycoef®cient (solid line) and the probability obtained by the Ljung-Box test (at the bottom of each ®gure) on thechange in corneal radius at 1, 2, 3 and 4 mm from the center along (a) temporal, (b) nasal, (c) inferior, and(d) superior main meridians as a function of time. Data obtained from one eye are shown as an example.


Figure 1. (continued)

k $ 1: We have tested the hypothesis that the ®rst ®ve auto-

correlation coef®cients are simultaneously equal to zero

using the Ljung-Box test. Indeed, we have calculated con®-

dence intervals at 95% for every autocorrelation coef®cient.

Figure 1 shows the autocorrelation function for lags k �1¼5 (bars), the con®dence intervals for every coef®cient

(solid line) and the probability obtained by the Ljung-Box

test (at the bottom of each ®gure) for the eye taken as an

example. The autocorrelation function also shows that there

is no trend in the data (when the data are affected by a trend

the autocorrelation function decreases slowly and there are

signi®cant coef®cients). Also the probability obtained by the

Ljung-Box test is always greater than 0.05. Consequently

the results obtained in the regression analysis, later in this

paper, are not surprising.

Another thing we must check before doing a regression

analysis is the absence of outliers. In Figure 2 the box-

plots of the residuals from regression analysis are shown.

In this case there are several outliers in the data corre-

sponding to the measurements taken on Monday of the

third week. These may be caused by interference from

the eyelids or irregularities in the tear ®lm (Hough and

Edwards, 1999) or due to a mechanical effect of the contact

lens. But the central position of this outlier means that this

has no in¯uence in the estimation of the slope. To measure

the in¯uence of a datum point, we could use the Mahala-

nobis Distance �Mi � �xi 2 �x�2=s2x� which takes into

account the distance of a datum from its average. The

Mahalanobis Distance of this datum is 0.00502 and the

scale of this distance goes from 0.002 to 2.709. There

were no signi®cant changes in corneal topography at 1,

2, 3 and 4 mm from the center of the cornea as a function

of time in the eight patients from the study.

Analyses of regression were also carried out at each

corneal point independently to investigate trends in corneal

curvature at 1, 2, 3 and 4 mm from the center along superior,

inferior, nasal and temporal main meridians as a function of

time. The statistics of this analysis are presented in Table 1.

The determination coef®cient indicates that the variation of

the curvature is not dependent on time. The slope in all

regression lines is close to zero, with a p value much greater

than 0.05.


Fig. 2. Box-plots of the residuals from regression analysis corresponding to 1, 2, 3 and 4 mm from the center alongtemporal, nasal, superior and inferior main meridians. Data obtained from one eye are shown as an example. Oneor more peripheral data points were not available possibly due to interference from eyelids and nose shadow.

Discussion

Several reports have indicated that PMMA contact lens

wear often leads to a signi®cant increase in central corneal

curvature (Miller, 1968; Rengstorff, 1971; Montenegro et

al., 1993). After the initial adaptation period, the curvature

returns to the baseline value (Rengstorff, 1969b). After

about one year, the cornea shows ¯attening beyond the

original baseline values (Saks, 1966). The ®rst of these

changes is attributed largely to localized corneal edema

caused by non-permeable PMMA material (Montenegro et

al., 1993; Hovding, 1983). Mechanical molding of cornea

may also be partly responsible for the altered corneal curva-

ture with PMMA lenses (Montenegro et al., 1993; Hovding,

1983; Carney, 1975).

In the present study we examined the corneal curvature

changes that occurred in eight RGP lens wearers and discov-

ered that there are no statistically signi®cant �p . 0:05�corneal curvature changes over a 21-day period after ®tting.

The high oxygen transmissibility of Forum lenses can

prevent corneal hypoxia, oedema, and the corneal curvature

changes seen with PMMA lenses. Moreover, our results

show that the mechanical effects of lid pressure do not

provoke corneal curvature variation.

Sevigny (1986) compared the corneal curvature and

refractive changes produced by Boston II lenses on daily

wear on one eye and Boston IV lenses on extended wear on

the opposite eye. He observed that corneal curvature

showed only a 0.07 D average ¯attening with daily wear.

DeRubeis and Shily (1985) found that patients ®tted with

Boston II lenses did not show any change in the radius of

curvature of the horizontal meridian, and the vertical meri-

dian ¯attened by 0.31 D. Obviously these lenses are low Dk

materials. However recent studies with higher Dk materials

have demonstrated a much more moderate keratometric and

refractive changes, often not exceeding 0.25 D. It appears

that the reduction in oedema may make the cornea less

susceptible to the mechanical force of the lens, and less

corneal change results.

Our study corroborates previous studies concerning the

minimal effect of high Dk RGP lenses on corneal curvature.

Previous studies have shown that during the ®rst three

months of lens wear, various types of silicone±acrylate

lenses, including extended wear lenses, did not produce

signi®cant changes in corneal curvature (Lydon and

Guillon, 1984; Rengstorff and Odby, 1986). Minimal

corneal variations were also found for longer periods of

lens wear (Odby and Rengstorff, 1985). Sanaty and Temel

(1996) found that the effect of ¯uorosilicone acrylate lenses

(Quantum, Bausch & Lomb) on central curvature was not

statistically signi®cant. Similar results were found by Pole

and Lowther (1987) when studying the effect of Paraperm

EW and Equalens lenses on corneal radii.

In summary we have demonstrated that Forum 210 lenses

do not induce signi®cant changes in corneal topography

over a 21-day period of lens wear. The implications of

this ®nding in terms of corneal health and optical quality

of the eye warrant further investigation over longer periods

of Forum 210 lenses wear.

The authors have no ®nancial interest in the manufacture,

supply or distribution of the contact lens used in this study.

Acknowledgements

This work was supported by CICYT research project ref.

PM 98-0225.

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Table 1. Statistics for the analysis of regression for thecorneal radius at 1, 2, 3 and 4 mm from the center alongthe temporal, nasal, superior and inferior main meridians asa function of time. Data obtained from one eye are shown asan example

Location Distance Slope Constant R2 Sig. F

Temporal 1 20.0025 8.2729 0.0612 0.293Temporal 2 20.0019 8.2436 0.0395 0.4008Temporal 3 20.0012 8.2533 0.0205 0.5473Temporal 4 20.0011 8.3327 0.0167 0.5869Nasal 1 20.0025 8.3295 0.0476 0.3847Nasal 2 20.0025 8.3610 0.0498 0.3734Nasal 3 20.0016 8.4950 0.0325 0.4738Nasal 4 0.0019 8.6128 0.0180 0.5952Superior 1 20.0016 8.1443 0.0397 0.4945Superior 2 20.0017 8.1010 0.0375 0.5069Superior 3 20.0019 8.2103 0.0051 0.8088Superior 4 20.0012 8.4043 0.0213 0.6187Inferior 1 20.0005 8.1320 0.0025 0.8379Inferior 2 20.0005 8.1790 0.0031 0.8215Inferior 3 20.0005 8.1976 0.0036 0.8068Inferior 4 20.0033 8.3500 0.0687 0.2783

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rigid gas permeable contact lens and corneal topography

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