International Ophthalmology 15: 313-319, 1991. �9 1991 Kluwer Academic Publishers. Printed in the Netherlands.

Corneal endothelial cell morphology under permanent wear of rigid contact lenses

Carla P. Nieuwendaal, 1 Jan H.C. Kok, 1 Erwin A.M. de Moor, 2 Johannes Oosting 2 & Henk W. Venema 2 1 Contact Lens Unit of the Department of Ophthalmology and 2 Laboratory of Medical Physics and Informatics of the Academic Medical Centre, University of Amsterdam, Meibergdreef 9, 1105 A Z Amsterdam, The Netherlands

Key words: corneal endothelium, elliptical geometry, high gas-permeability, pleomorphism, polymegathism, rigid contact lens

Abstract

The influence of the permanent wear of rigid high Dk-lenses (nominal Dk 71 or 92) with elliptical geometry on the corneal endothelium was assessed. For this purpose the endothelium of 16 patients was compared by specular microscopy before and after the permanent wear. The wearing time of the lenses varied between 7 and 24 months. Computerized morphometric analysis of the photographs revealed that the variability in cell size (polymegathism) and in cell shape (pleomorphism) did not change during the wearing period. It is suggested that this is due to the high gas-permeability and the elliptical geometry of the contact lenses.

Introduction

A number of studies have demonstrated morph- ological changes of the corneal endothelium, asso- ciated with the daily wear of polymethyl methacry- late (PMMA) [1, 3], soft [2, 3] and rigid gas-perme- able [2] contact lenses and with the extended wear of soft [4, 5] and rigid gas-permeable [6, 7] contact lenses. These changes include an increased varia- bility in cell size (polymegathism), expressed by an increased coefficient of variation of the cell area, and an increased variability in cell shape (pleo- morphism), expressed by an increased coefficient of variation of the cell angularity (number of sides per cell) and a decreased percentage of hexagons.

The clinical importance of these endothelial changes was demonstrated in intraocular lens im- plantation by Rao et al. [8]. It appeared that cor- neas with increased polymegathism were more likely to develop corneal edema postoperatively, suggesting that corneas [2] with increased polyme-

gathism are less stress-resistant. The increased de- grees of polymegathism and pleomorphism are thought to be induced by chronic hypoxia of the cornea under the contact lens [2, 4, 10, 11]. This hypothesis is supported by the finding that silicone elastomer lenses [9], which have an extremely high gas-permeability (nominal Dk 300), did not induce these changes in the corneal endothelial morphol- ogy.

In this study the influence of the permenant wear of contact lenses with high gas-permeability and elliptical geometry on the morphology of the cor- neal endothelium was assessed. The use of an ellip- tical geometry of a contact lens may improve tear exchange under the contact lens and thereby the oxygenation of the cornea. This elliptical geometry combined with a high gas-permeability of the mate- rial itself (nominal Dk 71 or 92) may prevent the induction of endothelial changes. The aim of this study was to verify this expectation in permanent contact lens wear.

314 C.P. Nieuwendaal et al.

Materials and methods

Three different high gas-permeable (nominal Dk 71 or 92) lens materials (Fluorperm, Equa and Quantum) were used. The precise gas-permeabil- ity of each meterial is listed in Table 1. The geom- etry of the lenses was elliptical, such that the back surface of the contact lens closely matches the ellip- tical topography of the cornea.

Sixteen subjects (mean age 28.2, standard devia- tion 5.4 years, range 18 to 42 years) who were free of ocular disease participated in this study. Nine subjects had not worn any contact lens previously and the other seven subjects had worn PMMA lenses, soft lenses or low gas-permeable rigid con- tact lenses before. The contact lenses worn previ- ously by each subject, if any, are listed in Table 2. The wearing time of the high Dk-lenses was 7 months for 4 subjects, 12 months for 7 subjects and 24 months for 5 subjects. In each subject the lenses were worn on a permanent wear basis; this means that the lenses were worn 24 hours a day, without any lens free days.

Quantum was first high gas-permeable lens ma- terial available in Europe. These lenses were worn by the 5 subjects with a wearing time of 24 months, except for subject 8 who had a wearing time of 8 months. One year later, the Fluorperm and Equa material became available at the same time. The other 10 patients each were provided with one lens made of Equa and the other of Fluorperm (except patient no. 10, Table 2). The type being fitted to either the left or the right eye was determined at random. The lens materials (Quantum, Fluorperm or Equa) and the wearing times are listed in Ta- ble 2.

The corneal endothelium of both eyes of the 16 subjects was photographed with a Nikon non-con- tact specular microscope, before and after the wearing period, using a Kodak T,max 400 ASA film. The approximate location of the specular photograph was the centre of the cornea. The area covered by one photograph was circa 0.04mm 2. About 15 photographs were made for each cornea. Based on cell boundary clarity the best three were selected for further evaluation. The total linear magnification of the final enlargements was 400 x. The exact magnification was estasblished from the photograph of a Nikon 2 mm grid.

For the evaluation of the photographs a graphic tablet (Tektronix 4953) was used, interfaced to an I.B.M. compatible personal computer. The con- tours of the endothelial cells on the enlarged pho- tographs were entered into the computer by digitiz- ing the coordinates of the cell apices with aid of the graphic tablet. For each photograph a pattern of 50 contiguous cells was digitized. Thus 150 cells per eye, before and after the permanent wear, were measured.

For each cornea a number of parameters describ- ing cell size and cell shape was determined. With regard to cell size, the mean cell area was deter- mined, the standard deviation and the coefficient of variation. Regarding cell shape, the mean num- ber of sides of a cell (cell angularity) was deter- mined, the standard deviation, the coefficient of variation, and the percentage of hexagonal cells in each cornea. The coefficient of variation of cell area was used as a measure of the variability in cell size (polymegathism), and the coefficient of varia- tion of cell angularity and the percentage of hexa-

Table 1, Lens material characteristics.

material oxygen permeability (nominal Dk) (according to Fatt)

geometry

Quantum Fluorperm Equa

92 (Fluoro silicone copolymer) 92 (Fluoro silicone acrylate copolymer) 71 (Fluoro silicone acrylate copolymer)

sphero-elliptical elliptical elliptical

Dk units: (x 10 n)(cm2/s)(ml O2/ml • mmHg).

gonal cells was used as a measure of the variability in cell shape (pleomorphism).

Reliability and accuracy of the method were as- sessed by analyzing six repetitions of 25 cells. Both endothelium with little variability in cell area and endothelium with much variability in cell area were tested. The error in the individual cell area (the mean value of the coefficient of variation) was 4%. For the whole photograph (here 25 cells) the error in the mean cell area was less than 1% and that in the coefficient of variation of cell area was less than 5%. These tests showed that the method used in this study is reliable and accurate.

This study met legal requirements governing

Table 2. C o n t a c t lens p a r a m e t e r s p e r sub jec t .

Corneal endothelial cell morphology 315

consent. Permission was obtained from the Med- ical Ethical Committee of the Academic Medical Centre of Amsterdam (project 86/255) and in- formed consent was obtained from each project.

Results

To examine the effect of high gas-permeable con- tact lenses on endothelial cell morphology we as- sessed the corneal endothelial mosaic before and after contact lens wear in 16 subjects. The mean values of the parameters describing cell size and cell shape of the 16 subjects are listed in Table 3.

s u b j e c t / e y e lens m a t e r i a l lens p o w e r w e a r i n g t ime p r e v i o u s cl w e a r ( b o t h eyes)

1 R F l u o r p e r m - 3 .00

L E q u a - 3 .00

2 R Q u a n t u m - 3 .50

L Q u a n t u m - 2 .75

3 R F l u o r p e r m - 3 .00

L E q u a - 3 .00

4 R E q u a - 4 .00

L F l u o r p e r m - 3 .75

5 R Q u a n t u m - 4 .00

L Q u a n t u m - 4 .25

6 R Q u a n t u m - 3 .00

L Q u a n t u m - 3 .50

7 R Q u a n t u m - 12 .50

L Q u a n t u m - 11.00

8 R E q u a - 4 .50

L F l u o r p e r m - 4 .25

9 R E q u a - 10.75

L F l u o r p e r m - 11 .00

i 0 R F l u o r p e r m - 0 .50

L F l u o r p e r m - 2 .75

11 R E q u a - 2 .75

L F l u o r p e r m - 2 .75

12 R E q u a - 3 .75

L F l u o r p e r m - 3 .00

13 R E q u a - 3 .50

L F l u o r p e r m - 3 .75

14 R F l u o r p e r m - 2 .25

L E q u a - 1 .00

15 R Q u a n t u m - 2 .75

L Q u a n t u m - 2 .75

16 R Q u a n t u m - 6 .00

L Q u a n t u m - 5 .50

6 m o n t h s 14 y e a r s P M M A

8 m o n t h s 10 y e a r s g a s - p e r m e a b l e

8 m o n t h s 2 y e a r s sof t

12 m o n t h s 3 y e a r s P M M A

2 y e a r s g a s - p e r m e a b l e

22 m o n t h s 7 y e a r s sof t

24 m o n t h s 1 y e a r g a s - p e r m e a b l e

25 m o n t h s 10 y e a r s P M M A

7 m o n t h s n o n e

11 m o n t h s n o n e

11 m o n t h s n o n e

12 m o n t h s n o n e

12 m o n t h s n o n e

12 m o n t h s n o n e

14 m o n t h s n o n e

22 m o n t h s n o n e

24 m o n t h s n o n e

316 C.P. Nieuwendaal et al.

Figures 1, 2 and 3 show a comparison of the coeffi- cient of variation of cell area, the coefficient of variation of cell angularity and the percentage of hexagons before and after the permanent wear.

To quantify the results, a Wilcoxon matched- pairs test was performed for each parameter with the significance level set at a - - 0.05. The param- eters measured before wearing lenses were com- pared with the parameters measured afterwards. There was no significant change in the mean cell area (p = 0.41), nor a change in the coefficient of variation of cell area (p = 0.69), nor a change in the coefficient of variation of cell angularity (p = 0.33), nor a change in the percentage of hexagons (p = 0.35). A representative example of corneal endothelial cell morphology (patient 15, Table 2), before and after high gas-permeable contact lens wear is shown in Fig. 4. The endothelial cell mosaic of this patient did not change during 22 months of permanent wear.

To assess the possible influence of endothelial changes induced by previous contact lens wear on the response of the endothelium to a new stress situation, that is the wear of the high-Dk lenses, the

Table 3. Mean results of the parameters describing endothelial

group with previous wear (7 subjects) and the group without previous wear (9 subjects) were ana- lysed separately. No significant change in the par- ameters was found in either group (p-values in Table 3).

The initial figures for the group having worn lenses previously (Table 3) showed an increased polymegathism (coefficient of variation of the cell area 0.34 vs 0.27) and a increased pleomorphism (percentage of hexagons 54% vs 63%) in compari- son with the group without previous contact lens wear. The data however did not reach statistical significance.

Discussion

This study did not show any significant change in morphological parameters following permanent high gas-permeable contact lens wear.

Furthermore the trend seen in our study was in keeping with previous reports [1-3] showing that the daily-wear of PMMA soft and low gas-perme- able contact lenses caused morphological changes

cell area and cell angularity before and after permanent wear.

before after p-value

all subjects (32 eyes)

Area:

Angularity:

mean (/zm 2) coefficient of variation (%) mean no. of cell apices coefficient of variation (%) percentage of hexagons (%)

group with previous cl wear (14 eyes)

Area: mean 0zm 2) coefficient of variation (%)

Angularity: mean number of cell apices coefficient of variation (%) percentage of hexagons (%)

group without previous cl wear (18 eyes)

Area: mean (/xm 2) coefficient of variation (%)

Angularity: mean number of cell apices coefficient of variation (%) percentage of hexagons (%)

299 300 0.41 30.6 30.3 0.69

6.0 6.0 11.5 11.5 0.33 59.1 58.7 0.35

292 287 34.7 33.8 0.55 6.0 6.0

12.5 12.6 0.36 54.4 53.0 0.36

306 310 27.4 27.6 0.91

6.0 6.0 10.7 10.6 0.57 62.7 63.2 0.74

| d45 6

35

25

15

�9 �9

i i i

15 25 35 45 Before C.L. wear

16 | --I

6

<

12

10

Corneal endothelial cell morphology 317

, / O ( ~ / J I / �9

o

�9 0 0 S

o o

o~ I I I I

10 12 14 16 Before C.L. wear

Fig. 1. Coefficient of variation of endothelial cell area (polyme- gathism) before and after pe rmanen t contact lens wear. �9 =

subjects with previous wear, 0 = subjects without previous

wear.

Fig. 2. Coefficient of variation of endothelial cell angularity

(pleomorphism) before and after permanent contact lens wear.

�9 = subjects with previous wear, 0 = subjects without previ-

ous wear.

of the corneal endothelium, but the changes were not significant (Table 3).

The corneal endothelium of the group of sub- jects with previous contact lens wear did not sustain further damage from the permanent wear of the high Dk-lenses. On the other hand, no improve- ment of the endothelial morphology was seen.

The results of our study are in contradiction with the results of Orsborn et al. [6]. These authors demonstrated an increase in polymegathism and pleomorphism of the endothelium after three months extended wear of low to moderate gas- permeable spherical contact lenses (nominal Dk 16 to 57). These changes were not demonstrable after three months daily wear. Our study did not show any increase in polymegathism and pleomorphism, even after two years of permanent wear. This might be explained by the higher gas-permeability (nomi- nal Dk 71 or 92) and the elliptical geometry of the lenses used.

Liberman et al. [7] showed an increase of poly- megathism under the extended wear of fluoro-sil- icone methacrylate lenses (nominal Dk 96) for one year. In our study elliptical lenses with the same gas-permeability were fitted. This new geometry of the contact lens may be responsible for improved

tear exchange under the contact lens. We believe that the promising results achieved here may be due as much to the elliptical geometry of the lenses as to their high gas-permeability.

75 | .A 6

~6s

55

45

35 35

o

O0 0 0

�9 o .

p I _ _ r

45 55 65 75 Before C.L. wear

Fig. 3. Percentage of hexagonal cells (pleomorphism) before

and after permanent contact lens wear. �9 = subjects with previ- ous wear, 0 = subjects without previous wear.

318 C.P. N ieuwendaal et al.

Fig. 4. Corneal endothelial cell morphology before

A: area:

angularity:

area:

angularity:

(A) and after (B) 22 months of high gas-permeable lens wear (Quantum).

B:

1. mean (txm 2) 287 2. standard deviation (/zm 2) 68 3. coefficient of variation (sd/mean) 23.5

1. mean (number of cell apices) 6.00 2, standard deviation 0.55 3. coefficient of variation (sd/mean) 9.2 4. percentage of hexagonal cells 68.1

1. mean (/zm 2) 297 2. standard deviation (/.~m 2) 68 3. coefficient of variation (sd/mean) 22.7

1. mean (number of cell apices) 6.00 2. standard deviation 0.57 3. coefficient of variation (sd/mean) 9.5 4. percentage of hexagonal cells 67,7

References

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4. Holden BA, Sweeney DF, Vannas A, Nilsson KT, Efron N. Effects of long-term extended contact lens wear on the human cornea. Invest Ophthalmol Vis Sci 1985; 26: 1489- 1501.

5. Schoessler JP. Corneal endothelial polymegathism associ- ated with extended wear. Inter C L Clinic 1983; 10: 148-56.

6. Orsborn GN, Schoessler JP. Corneal endothelial polyme- gathism after the extended wear of rigid gas-permeable contact lenses. Am J Optom Physiol Opt 1988; 65: 84-90.

7. Liberman G, Mandell RB. Corneal endothelial polyme-

Corneal endothelial cell morphology 319

gathism in high-Dk contact lens wearers. Inter C L Clinic 1988; 15: 282-5.

8. Rao GN, Aquavella JV, Goldberg SH, Berk SL. Pseu- dophakic bullous keratopathy. Relationship to preoper- ative corneal endothelial status. Ophthalmology 1984; 91: 1135-40.

9. Schoessler JP, Barr JT, Freson DR. Corneal endothelial observations of silicone elastomer contact lens wearers. Inter C L Clinic 1984; 11: 337-40.

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Address for offprints: J.H.C. Kok, Contact Lens Unit of the department of Ophthalmology, Academic Medical Centre, University of Amsterdam, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands