- Email: [email protected]
Regular replacement of daily wear rigidgas-permeable contact lenses
Journal of the British ContactLensAssociation,Vol. 19, No. 3, pp 83-89,
1996
© 1996 British Contact Lens Association
Printed in Great Britain
REGULAR REPLACEMENT OF DAILY-WEAR RIGID GAS-PERMEABLE CONTACT LENSES Craig A. Woods* and Nathan Efron* (Received 27July 1995; in revisedform 26th September 1995) A b s t r a c t - - A prospective, single-centre, controlled, double-masked randomised study was conducted to determine the benefits, if any, of regular replacement of daily-wear rigid gas-permeable (RGP) contact lenses. Forty-one subjects, divided into two groups, wore Quantum 2 lenses (Bausch & Lomb, Rochester, New York, USA; Dk=120) for 12 months on a daily-wear basis; subjects in group I replaced lenses every 3 months, whereas those in group II were not scheduled to replace lenses. All subjects were existing RGP lens wearers without any contact lens-induced pathology (>grade 1). The integrity of the lenses and the ocular responses to lens wear were monitored in both groups every 3 months. Compared with lenses worn by subjects in group II, lenses worn by subjects in group I displayed significantly less surface drying (at 9 months), less mucus coating (at 12 months), less surface deposition (at 12 months), and less surface scratching (at 9 and 12 months) (p
contact lens, Quantum 2, planned replacement
Introduction dverse problems associated with soft lens wear have been shown to be significantly reduced by the introduction of planned replacement schemes for the lenses. 1The frequency of replacement of soft lenses varies depending on the lens type and the marketing strategy employed by the manufacturer, and typically varies from daily to 6-monthly replacement. Benefits for both the daily and extended wear modalities have been demonstrated1,2; these include less giant papillary conjunctivitis, reduced risk of infective keratitis, improved visual performance and increased comfortY Planned replacement of rigid gas-permeable (RGP) contact lenses has not yet been introduced on clinical grounds, possibly because the perceived benefits are not thought to outweigh the increased costs. 4 The introduction of materials that are highly permeable to oxygen has facilitated a reduction in corneal hypoxia compared with that noted with the lower permeability materials~-7; however, highly permeable materials have a higher reported incidence of deposition and scratching, and a reduced life expectancy. 8,9 One approach to minimise lens surface degradation is to polish the lenses on a regular basis but Walker 1° has strongly warned against the dangers of over polishing fluorosilicone acrylate lenses because this can drastically affect their surface wettability. The introduction of planned replacement for these types of lenses may overcome such disadvantages. The question as to whether there are any clinical benefits of planned replacement of RGP lenses does not appear to have been addressed experimentally. Theoretically, planned replacement may reduce
A
* BSc, MCOptom, DCLP. *BSc, PhD, DSc, MCOptom, FVCO, FAAO.
adverse reactions to RGP lenses as has been shown with soft lenses. Adverse reactions to RGP lenses are similar to those found during soft contact lens wear but also include idiopathic ocular changes such as 3 and 9 o'clock corneal staining, and deUen. Guillon et al. n recently concluded that a replacement frequency of around 3 to 6 months may be of benefit to a significant number of patients in relation to in vivo wettability of high Dk RGP materials but this conclusion was based on a study over 6 months and did not compare regular replacement with conventional use. The present study sets out to investigate whether the planned replacement for RGP lens wear has any clinical benefits. Methods The study was prospective, single-centre, controlled, double-masked and randomised. The duraction of the study was 12 months. Forty-one subjects (10 male, 31 female) were enrolled and randomly assigned to one of two groups: group 1 - - 22 subjects (7 male, 15 female) who replaced lenses every 3 months, and group II - - 19 subjects (3 male, 16 female) who used lenses on a conventional basis (i.e., lenses were replaced only if clinically necessary or los0. All subjects were informed that they were testing a new product for its performance. Enrolment
Subjects were only enrolled in the study if they conformed to all of the following criteria: • existing RGP lens wearer • no previous significant adverse responses to contact lens wear • no present ocular signs of greater than grade 112 83
CRAIG A. WOODS AND NATHAN EFRON
• no active ocular pathology • no experience of extended contact lens wear Details of subjects enrolled in the study are given in Table 1. Table 1. Details ofgroup I and group Il patients
Replacement frequency n
Males/females Age (years) Refraction Sphere Cylinder Length of RGP wear Current pair
Group I
Group II
3-monthly 22 7/15 28_+9.8
As required 19 3/16 27.7+7.6
- 1.94D+4.21 -0.74D_+0.84 8 years_+6.24 1.78 years+l.26
-4.28D+~.08 -0.93D+1.23 8 years+7.5 3.06 years+~2.22
Lenses All subjects were fitted with fluorosflicone acrylate lenses (Quantum 2), designed and manufactured by Bausch & Lomb (Rochester, New York). The material from which these lenses are fabricated has an oxygen permeability of 120 x 10 11 (cm2/sec) (ml O2/ml x mm Hg) at 35°C (adjusted for edge and boundary effects).13 The lenses had a spherical central optic zone and an aspheric periphery. Centre thickness for a -3.00 D lens is 0.17ram. The range of variables of lenses fitted in this study was as follows: Total diameter: 9.00ram to 10.20mm Back optical zone diameter: 7.20mm to 8.40ram Back vertex power: + 8.25D to - 9.50D. The same fitting philosophy of apical clearance was used on all subjects. Lens Care All subjects were instructed in the same protocol regarding lens care and maintenance. Lenses were cleaned daily using a surfactant cleaner (Miraflow, CIBA Vision, Southampton) and were stored in Wetting and Soaking Solution (Bausch & Lomb, Rochester, New York). No enzyme tablets were used by either group. Lens Replacement Setting the frequency of lens replacement for group I subjects at 3 months allowed subjects from both groups to return for routine 3-monthly aftercare visits, thus facilitating masking. Both the investigator and the subjects were masked with respect to the group assignment. At the end of each aftercare appointment the lenses were taken by an unmasked assistant for evaluation. At this point the lenses from the subjects in group I were replaced with new lenses whereas subjects in group II had their original lenses returned. Neither the investigator nor the subject knew if lenses had been replaced.
84
Measurements
At each scheduled 3-monthly visit, data relating to lens and ocular characteristics were collected. Lens Integrity Non-Invasive Dry-Up Time (NIDUT) The Guillon Tearscope was used to view the pre-lens tears film (PLTF) and to assess NIDUT. This instrument utilises a cold diffuse light source that is reflected by the tear film and renders it visible without the instillation of colouring agents. 14The NIDUT is described as the time elapsed between eye opening, following a blink, and the total disappearance of the aqueous phase at the front of the contact lens. 14Assessment of the NIDUT has been shown as an important indicator for lens wettabflity, whereby a longer NIDUT represents better performance. 15 Mucus Coating Once the lens had dried up in the course of a NIDUT measurement, the surface of the lens was exposed and a grading of the presumed mucous coating on the lens surface could be made. 11A grading system from 0-4 in terms of increasing clinical severity as suggested by Woods 12 was used, where grade 0 is normal - - zero signs; grade 1 is minor - clinically insignificant; grade 2 is moderate - - clinical action not necessary; grade 3 is marked - - clinical action usually needed; grade 4 is severe - - clinical action indicated. Surface Deposition A slit-lamp biomicroscope was used to observe the surface of the lenses in situ at x 15 magnification. The overall level of deposits was graded using the clinical severity scale described above, in accordance with the principles outlined by Hurst et al. 16 but modified for in situ assessment. Surface Scratching A slit-lamp biomicroscope was used to observe the surface of the lenses in situ at x 15 magnification. The overall level of scratching was graded using the clinical severity scale described above. Lens Binding This relates to a correctly positioned lens that is immobile. Lens binding was demonstrated to the subjects before the start of the study. They were trained to detect binding and instructed to note in a diary if it occurred during the study period. Swarbrick and Holden 17 have reported this to be a very reliable method for continuously monitoring lens binding; relying on data collected at the aftercare visits would not have provided any guide to the frequency of this phenomenon. Lens Comfort Subjects indicated the level of comfort by placing a mark on a 10cm vertical line. The bottom of the scale (0cm) represented a lens that was unwearable (extremely uncomfortable) and the top of the scale (10cm) represented a lens that elicited virtually no sensation (extremely comfortable).18
REGULARREPLACEMENT OF DAILY-WEARRIGID GAS-PERMEABLECONTACT LENSES
Ocular Response Slit-Lamp Biomicroscope Observations A slit-lamp biomicroscope was used to monitor the following measures of ocular response to lens wear: corneal staining, conjunctival staining, conjunctival hyperaemia, limbal hyperaemia, epithelial microcysts and endothelial polymegethism. The ocular response was graded on a severity scale of 0 to 4 as described previously.
NIDUT
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Refractive Changes A full sphero-cylindrical overrefraction was performed on all subjects at all visits, from which changes in refractive error could be determined.
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Visual Acuity At all visits the visual acuity of all subjects was assessed at 10% and 90% contrast using BaileyLovie acuity charts. 19
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12
(months)
Figure 1. Mean (+SD) change in a non-invasive dry-up time (NIDUT) for the two groups with time. The asterisk denotes a statistically significant difference between the two groups at that point in time.
Statistical Analysis As all subjects enrolled on the study were existing RGP lens wearers, the baseline data were taken to be Mucus Coating Lenses worn by subjects in group II at 2 weeks after dispensing the lenses. This allowed all displayed a significant increase in mucus coating over time (Figure 2). There was a significant difference in the subjects to adjust to the new lens material and design, and for any low-level changes caused by their mucus coating between the two groups over time, and the mucus coating was significantly greater for lenses existing lenses to regress. An analysis of covariance worn by subjects in group II (versus group I) at the 12(ANCOVA) investigated the changes in each group and the differences between the two groups over time, month visit. where time was the covariant. An analysis of variance (ANOVA) was used to assess the significance of any 2.0 Mucus coating differences between the two groups for each visit, with / ....~,,,, Group l J the exception of lens binding, where the X2 test was 1.5 Group II J used to assess differences between the frequency of binding between the two groups. As a general rule, a value of P<0.05 was taken as being statistically ~ 1.0 significant.
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Results Of the 41 subjects enrolled, 38 completed the study. The three discontinuations were due to ill health (one female, group I), recurrent lens binding (one female, group II) and unacceptable visual performance (one male, group I). Data for the remaining 38 subjects were analysed. Over the 12-month period of study, subjects in group I used an average of 8.52_+0.81 lenses and subjects in group II used an average of 2.78_+0.81 lenses. The F statistic and P value relating to all measures of lens integrity and ocular response are given in Table 2, for changes in each group over time, differences between groups over time, and differences between groups at each visit.
Lens Integrity Non-Invasive Dry-Up Time (NIDUT) The change in NIDUT over time for both groups is given in Figure 1. The NIDUT was significantly longer for lenses worn by subjects in group I (versus group II) at the 9-month visit, but curiously there was no difference at the 12month visit.
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Figure 2. Mean (+SD) change in mucus coating for the two groups with time. The asterisk denotes a statistically significant difference between the two groups at that point in time. Surface Deposition Lenses worn by subjects in both groups displayed a significant increase in surface deposition over time (Figure 3). There was a significant difference in surface deposition between the two groups over time, and the surface deposition was significantly greater for lenses worn by subjects in group I! (versus group I) at the 12-month visit. Surface Scratching Lenses worn by subjects in both groups displayed a significant increase in surface scratching over time (Figure 4). There was a significant
85
0.022 0.183 2.609 1.670 0.975 0.193 0.278 0.582 1.078
Ocular response: Corneal staining Conjuncfival staining Conjuncfival hyperaemia Limbal hyperaemia Epithelial microcysts Endothelial polymegathism Refractive change Visual acuity 90% Visual acuity 10% 0.882 0.669 0.109 0.303 0.326 0.661 0.599 0.447 0.301
0.280 0.206 0.000 0.004 0.697
2.281 0.131 1.528 3.865 0.476 0.064 0.437 0.315 0.002
0.127 13.037 14.395 25.721 0.001
0.135 0.718 0.220 0.052 0.492 0.801 0.510 0.576 0.997
0.723 0.000 0.000 0.000 0.979
1.111 0.311 0.065 4.571 0.080 0.009 0.018 0.020 0.541
0.971 4.098 4.388 6.168 0.066
F
0.294 0.578 0.799 0.034 0.775 0.927 0.892 0.888 0.463
0.326 0.044 0.038 0.014 0.798
P
Difference between group I and group II over time
*Dependenthas zerovariance(gradelevel=0for all data), so an F statisticandp valuecouldnotbe computed.
1.179 1.619 12.367 8.480 0.152
P
F
F
P
Change in group II over time
Change in group I over time
Lens integrity: NIDUT Mucus coating Surface deposition Surface scratching Lens comfort
Table 2. Statistical analysis of data
1.072 0.664 0.144 1.572 * 0.005 0.617 0.003 0.004
1.061 0.020 0.319 * 0.031
F
0.307 0.420 0.706 0.218 * 0.943 0.437 0.986 0.644
0.309 0.888 0.576 * 0.860
P
Difference between group I and group II at t=0.5 months
0.209 0.009 1.139 0.095 * 0.103 0.098 0.363 0.007
3.146 0.108 0.114 0.076 1.116
F
0.651 0.924 0.293 0.360 * 0.750 0.756 0.550 0.980
0.085 0.745 0.738 0.785 0.298
P
Difference between group I and group II at t=3 months
0.630 0.582 0.455 0.726 0.805 1.260 3.493 0.248 0.007
0.250 0.006 0.026 0.434 1.215
F
0.433 0.459 0.504 0.399 0.376 0.269 0.070 0.622 0.979
0.620 0.981 0.873 0.514 0.278
P
Difference between group I and group II at t=6 months
0.016 0.486 0.347 0.501 1.229 3.132 0.530 0.007 0.092
5.672 1.240 0.170 4.758 0.709
F
0.901 0.490 0.560 0.484 0.275 0.085 0.471 0.936 0.763
0.022 0.273 0.682 0.035 0.405
P
Difference between group I and group H at t=9 months
8.389 0.043 0.725 2.807 0.814 0.197 0.254 0.006 0.707
0.377 6.001 5.560 4.237 0.096
F
0.006 0.837 0.400 0.102 0.373 0.660 0.617 0.941 0.406
0.543 0.019 0.024 0.047 0.758
P
Difference between group I and group II at t=12 months
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Figure 3. Mean (+-SD) change in surface deposition for the Figure 5. Mean (+SD) change in corneal staining for the two two groups with time. The asterisk denotes a statistically signifi- groups with time. The asterisk denotes a statistically significant cant difference between the two groups at that point in time. difference between the two groups at that point in time.
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Figure 4. Mean (+SD) change in surface scratching for the two groups with time. The asterisk denotes a statistically significant difference between the two groups at that point in time.
Figure 6. Mean (+SD) change in limbal hyperaemia for the two groups with time.
difference in the extent of surface scratching between the two groups over time, and the extent of surface scratching was significantly greater for lenses worn by subjects in group II (versus group I) at both the 9-month and 12-month visits.
ence in limbal hyperaemia over time between the two groups, with a trend towards limbal hyperaemia decreasing in group I and increasing in group II (Figure 6). No other significant differences between groups over time or at specific visits were revealed.
Lens Binding Up to 28% of subjects reported at least one incident of lens binding; however, no significant trends over time, or differences between groups, were detected with respect to this phenomenon.
Refractive Changes No significant trends over time, or differences between groups, were detected with respect to refractive changes.
Lens Comfort Again, no significant trends over time, or differences between groups, were detected with respect to lens comfort.
Visual Acuity No significant trends over time, or differences between groups, were detected with respect to visual acuity at 10% and 90% contrast.
Discussion Ocular Response Slit-Lamp Biomicroscope Observations A significantly greater amount of corneal staining was observed in subjects in group II (versus group I) at the 12-month visit (Figure 5). The ANCOVA revealed a significant differ-
Maintenance of Masking At the end of the study the investigator attempted to nominate which subjects were in each group. Of the 38 subjects who completed the experiment, the investigator could only correctly place 60% of subjects. This
87
CRAIG A. WOODS AND NATHAN EFRON
exercise demonstrated that for the study, using the information available, the investigator fared slightly better than chance allocation (50%), therefore confirming the efficacy of the masking procedure.
Lens Integrity It has been clearly shown that for an RGP lens to be successful then it must have good wettability. The wettability of the lenses in this study was monitored by observing the changes in the NIDUT and lens mucus coatings using the Tearscope. A reduction in the NIDUT and an increase in the level of mucus coating for lenses worn by subjects in group II at the 9-month and 12-month visits, respectively, indicate reduced wettability. The reason for the convergence of data between the two groups at the 12-month visit is unclear, but may be related to the significant increase in surface deposition on lenses worn by subjects in group II at the 12-month visit. As expected, the level of deposits on lenses worn by subjects in group II increased over time. A significant but less pronounced increase in lens deposition was observed in group I; presumably the statistical significance of this finding relates primarily to the overall increase in deposition relative to baseline. The difference in deposit gradings between the two groups started to diverge after the 6-month visit, and became statistically significant at the 12-month visit. This finding suggests that lens deposition on RGP lenses is cumulative and can be alleviated by 6-monthly lens replacement. It is self-evident that the extent of surface scratching of RGP lenses which are handled regularly will increase over time. The present data confirm this notion. As is the case with surface deposition, the difference in the extent of surface scratching between the two groups started to diverge after the 6-month visit, and became statistically significant at the 12-month visit. The absolute rate of increase in lens surface scratching could be affected by a number of factors, such as the abrasiveness of the cleaner used, the cleaning technique adopted and the lens material. Certainly this report demonstrates that the extent of surface scratching on high Dk RGP lenses using an alcohol-based cleaner is cumulative and can be alleviated by 6-monthly lens replacement. The occurrence of lens binding has often been reported during studies of extended wear RGP lenses. 2°-z~ Little has been reported for daily wear. z°26 Swarbrick and Holden 17 concluded that lens binding was subject-independent and thus not mode-of-weardependent. This study demonstrates that lens binding occurs during daily wear at an incidence of up to 28%. Furthermore, the incidence of binding was never below 12% for group II or 5% for group I. The incidence of binding for RGP lenses worn on a daily basis is apparently unaffected by the frequency of lens replacement. It is curious that lens comfort was unaffected by the frequency of lens replacement, because intuitively one would expect that a lens that is more heavily deposited
88
and more extensively scratched would be less comfortable. Clearly, such adverse changes in lens integrity are of insufficient magnitude to elicit a detectable decrease in comfort. An important clinical ramification of this finding is that patient symptomatology - - at least in the form of reports of lens comfort - - cannot be taken by clinicians to be a reliable indicator of lens integrity.
Ocular Response The contact lens oxygen transmissibility for the avoidance of corneal oedema was found by Holden and Mertz 27 for daily wear to be 24 × 10-11 (cm/sec) (ml O2/ml × mm Hg). The Quantum 2 lens exceeds this criterion for most prescriptions, being 77 × 10 -11 (cm/sec) (ml O2/ml × mm Hg) for a -3.00D lens. 13 The use of a lens with such a high oxygen transmissibilty for daily wear should therefore minimise or eliminate hypoxic signs such as epithelial microcysts, stromal oedema and endothelial changes, as indeed was the case in this study. Of all the ocular responses monitored, only corneal staining was significantly reduced by planned replacement; this became evident at the 12-month visit. The present data also suggest a trend towards regular lens replacement alleviating limbal hyperaemia. Both the increased staining and trend towards less limbal hyperaemia with regular lens replacement may be attributed to the avoidance of a degradation of lens integrity after 12 months of lens wear, although a causal relationship has not been established in this study. Throughout this 12-month study the visual acuity recorded by both groups showed little change. Regular lens replacement is known to preserve vision in patients wearing soft lenses on a daily-wear basis 2s, although it should be noted that data pertaining to lenses of up to 4 years old were used in establishing that finding. A period of 12 months' wear may not have been long enough to allow a degradation in the surface of the nonreplaced RGP lenses of sufficient magnitude to reduce the quality of vision, if such a phenomenon were to occur at all.
RGP Lens Replacement Frequency It is clear from these data that adverse changes in lens and ocular integrity can be detected after 9-12 months of daily lens wear if lenses are not replaced on a scheduled basis. The fact that the differences in gradings and trends reported here are statistically significant, does not necessarily infer clinical significance. The assignment of an event as being clinically significant is a subjective opinion that must be made by clinicians. Although most of the observations reported in this paper rarely exceed the 'clinical action not necessary' level (grade 2), the authors feel that the accumulated evidence of changes in both lens integrity and ocular response represents an overall subtle clinical determination in the lens-eye system that could reasonably be assumed to be a precursor to adverse patient symptomatology. Although the data generated in this report relate to
REGULARREPLACEMENTOF DAILY-WEARRIGID GAS-PERMEABLECONTACTLENSES
one particular lens material, Guillon et al. 1~ reported that degradation of the integrity of the surface of RGP contact lenses during wear is not appreciably affected by the type of material. We advocate, therefore, that RGP lenses worn on a daily-wear basis should be replaced every 6 months to avoid adverse changes in lens and ocular integrity that have been demonstrated in this report to begin to occur by 9 months.
Acknowledgements This study was supported by Polymer Technology Corporation. Assistance was provided by Bausch & Lomb in the co-ordination of lenses and supply of solutions.
Address for Correspondence Craig Woods, Department of Optometry and Vision Sciences, UMIST, PO Box 88, Manchester, M60 1QD. email: [email protected]. REFERENCES 1 Poggio, E,C. and Abelson, M. Complications and symptoms in disposable extended wear lenses compared with conventional soft daily wear and soft extended wear lenses. CLAO J., 19, 31-39 (1993). 2 Poggio, E.C. and Abelson, M. Complications and symptoms in disposable daily wear and conventional soft daily wear. CLAO J., 19, 95-102 (1993). 3 GeUatly, K.W. Disposable contact lenses: a clinical performance review. Can. J. Optom., 55, 166-173 (1993). 4 Harmon, F. Annual replacement of high Dk RGPs? Why not? Eyecare Business, 5, 78-79 (1990). 5 Hill, R.M. EOP: have we hit the limits? CL Forum, 13, 39 (1988). 6 Hamano, H., Hamano, T., Iwasaki, N. and Mitsunaga, S. Clinical evaluation of various contact lenses from the incidence of complications.J. Br. Contact Lens Assoc., 11, 25-30 (1988). 7 Schoessler, J. and Hili, R.M. The B&L high Dk material: Pilot studies. Spectrum, 2, 39-40 (1987). 8 Phillips, A.J. and Cascoigne, K. Surface cracking of RGP materials. Clin. Exp. Optom., 71,198-203 (1988). 9 Yokota, M., Goshima, T. and Itoh, S. The effect of polymer structure on durability of high Dk gas-permeable materials. J. Br. Contact Lens Assoc., 15, 125-129 (1992).
10 Walker, J. Overpolishing fluorosilicone acrylates. J. Br. Contact Lens Assoc., 12, 29-32 (1989). 11 Guillon, M., Guillon, J.P., Shah, D., Bertrand, S. and Grant, T. In vivo wettability of high Dk RGP materials. J. Br. Contact Lens Assoc., 18, 9-15 (1995). 12 Woods, R. Quantitative slit lamp observations in contact lens practice.J. Br. Contact Lens Assoc., 11, 42-45 (1989). 13 Brennan, N ~ , El~on, N., Holden, B.A. and Fatt, I. A review of the theoretical concepts, measurement systems and application of contact lens oxygen permeability. Ophthal. Physiol. Opt., 4, 485-490 (1987). 14 Guillon, J.P. and Guillon, M. Tear film examination of the contact lens patient. Contax, May, 14-18 (1988). 15 Guillon, J. and Guillon, M. Tear film examination of the contact lens patient. Optician, 206, 21-29 (1993). 16 Hurst, M.A., Mitchell, S.E. and Douthwaite, W.A. Contact lens opacity grading system (CLOGS). jr. Br. Contact Lens Assoc., 17, 19-24 (1994). 17 Swarbrick, H.A. and Holden, B.A. Rigid gas-permeable lens adherence: A patient-dependent phenomenon. Optom. Vis. Sci., 66, 269-275 (1989). 18 Efron, N. and Brennan, N.A. In: Efron, N. (ed.) How to Manage
Complaints of Discomfort During Contact Lens Wear. W. B. Sannders Co Ltd, London, 7-20 (1993). 19 Bailey, I.L and Lovie, J.E. New design principles for visual acuity letter charts. Am. J. Optom. Physiol. Opt., 53, 740-745 (1976). 20 Benjamin, W. and Boltz, R. RGP lens adhesion is not a benign phenomenon. Int. Contact Lens Clin., 16, 60 (1989). 21 Kenyon, E., Polse, KA. and Mandell, R.B. Rigid contact lens adherence: incidence, severity and recovery. J. Am. Optom. Ass., 59, 168-174 (1988). 22 Levy, B. Rigid gas permeable lenses for extended wear - - a one year study. Am. J. Optom. Physiol. Opt., 62, 889-894 (1985). 2:~ Polse, K.A., Sarver, M.D., Kenyon, E. and Bonanno, J. Gas permeable hard contact lens extended wear. Ocular and visual responses to a six-month period of wear. CLAO]., 13, 31-38 (1987). 24 Swarbrick, H.A. and Holden, B.A. Rigid gas permeable lens binding: significance and contributing factors. Am. J. Optom. Physiol. Opt., 64, 815-823 (1987). 25 Terry, R., Schnider, C. and Holden, B.A. Complications associated with RGP extended wear: two case reports. Clin. Exp. Optom., 72, 19-21 (1989). 2G Backman, H. The adhesion syndrome. Can. J. Optom., 48, 51 (1986). 27 Holden, B.A. and Mertz, G.W. Critical oxygen levels to avoid edema for daily and extended wear contact lenses. Invest. Ophthalmol. Vis. Sci., 25, 1161-1167 (1984). 28 Gellatly, K.W., Brennan, N.A. and Efron, N. Visual decrement with deposit accumulation on HEMA contact lenses. Am. J. Optom. Phys. Opt., 65, 937-941 (1988).
89
1996
© 1996 British Contact Lens Association
Printed in Great Britain
REGULAR REPLACEMENT OF DAILY-WEAR RIGID GAS-PERMEABLE CONTACT LENSES Craig A. Woods* and Nathan Efron* (Received 27July 1995; in revisedform 26th September 1995) A b s t r a c t - - A prospective, single-centre, controlled, double-masked randomised study was conducted to determine the benefits, if any, of regular replacement of daily-wear rigid gas-permeable (RGP) contact lenses. Forty-one subjects, divided into two groups, wore Quantum 2 lenses (Bausch & Lomb, Rochester, New York, USA; Dk=120) for 12 months on a daily-wear basis; subjects in group I replaced lenses every 3 months, whereas those in group II were not scheduled to replace lenses. All subjects were existing RGP lens wearers without any contact lens-induced pathology (>grade 1). The integrity of the lenses and the ocular responses to lens wear were monitored in both groups every 3 months. Compared with lenses worn by subjects in group II, lenses worn by subjects in group I displayed significantly less surface drying (at 9 months), less mucus coating (at 12 months), less surface deposition (at 12 months), and less surface scratching (at 9 and 12 months) (p
contact lens, Quantum 2, planned replacement
Introduction dverse problems associated with soft lens wear have been shown to be significantly reduced by the introduction of planned replacement schemes for the lenses. 1The frequency of replacement of soft lenses varies depending on the lens type and the marketing strategy employed by the manufacturer, and typically varies from daily to 6-monthly replacement. Benefits for both the daily and extended wear modalities have been demonstrated1,2; these include less giant papillary conjunctivitis, reduced risk of infective keratitis, improved visual performance and increased comfortY Planned replacement of rigid gas-permeable (RGP) contact lenses has not yet been introduced on clinical grounds, possibly because the perceived benefits are not thought to outweigh the increased costs. 4 The introduction of materials that are highly permeable to oxygen has facilitated a reduction in corneal hypoxia compared with that noted with the lower permeability materials~-7; however, highly permeable materials have a higher reported incidence of deposition and scratching, and a reduced life expectancy. 8,9 One approach to minimise lens surface degradation is to polish the lenses on a regular basis but Walker 1° has strongly warned against the dangers of over polishing fluorosilicone acrylate lenses because this can drastically affect their surface wettability. The introduction of planned replacement for these types of lenses may overcome such disadvantages. The question as to whether there are any clinical benefits of planned replacement of RGP lenses does not appear to have been addressed experimentally. Theoretically, planned replacement may reduce
A
* BSc, MCOptom, DCLP. *BSc, PhD, DSc, MCOptom, FVCO, FAAO.
adverse reactions to RGP lenses as has been shown with soft lenses. Adverse reactions to RGP lenses are similar to those found during soft contact lens wear but also include idiopathic ocular changes such as 3 and 9 o'clock corneal staining, and deUen. Guillon et al. n recently concluded that a replacement frequency of around 3 to 6 months may be of benefit to a significant number of patients in relation to in vivo wettability of high Dk RGP materials but this conclusion was based on a study over 6 months and did not compare regular replacement with conventional use. The present study sets out to investigate whether the planned replacement for RGP lens wear has any clinical benefits. Methods The study was prospective, single-centre, controlled, double-masked and randomised. The duraction of the study was 12 months. Forty-one subjects (10 male, 31 female) were enrolled and randomly assigned to one of two groups: group 1 - - 22 subjects (7 male, 15 female) who replaced lenses every 3 months, and group II - - 19 subjects (3 male, 16 female) who used lenses on a conventional basis (i.e., lenses were replaced only if clinically necessary or los0. All subjects were informed that they were testing a new product for its performance. Enrolment
Subjects were only enrolled in the study if they conformed to all of the following criteria: • existing RGP lens wearer • no previous significant adverse responses to contact lens wear • no present ocular signs of greater than grade 112 83
CRAIG A. WOODS AND NATHAN EFRON
• no active ocular pathology • no experience of extended contact lens wear Details of subjects enrolled in the study are given in Table 1. Table 1. Details ofgroup I and group Il patients
Replacement frequency n
Males/females Age (years) Refraction Sphere Cylinder Length of RGP wear Current pair
Group I
Group II
3-monthly 22 7/15 28_+9.8
As required 19 3/16 27.7+7.6
- 1.94D+4.21 -0.74D_+0.84 8 years_+6.24 1.78 years+l.26
-4.28D+~.08 -0.93D+1.23 8 years+7.5 3.06 years+~2.22
Lenses All subjects were fitted with fluorosflicone acrylate lenses (Quantum 2), designed and manufactured by Bausch & Lomb (Rochester, New York). The material from which these lenses are fabricated has an oxygen permeability of 120 x 10 11 (cm2/sec) (ml O2/ml x mm Hg) at 35°C (adjusted for edge and boundary effects).13 The lenses had a spherical central optic zone and an aspheric periphery. Centre thickness for a -3.00 D lens is 0.17ram. The range of variables of lenses fitted in this study was as follows: Total diameter: 9.00ram to 10.20mm Back optical zone diameter: 7.20mm to 8.40ram Back vertex power: + 8.25D to - 9.50D. The same fitting philosophy of apical clearance was used on all subjects. Lens Care All subjects were instructed in the same protocol regarding lens care and maintenance. Lenses were cleaned daily using a surfactant cleaner (Miraflow, CIBA Vision, Southampton) and were stored in Wetting and Soaking Solution (Bausch & Lomb, Rochester, New York). No enzyme tablets were used by either group. Lens Replacement Setting the frequency of lens replacement for group I subjects at 3 months allowed subjects from both groups to return for routine 3-monthly aftercare visits, thus facilitating masking. Both the investigator and the subjects were masked with respect to the group assignment. At the end of each aftercare appointment the lenses were taken by an unmasked assistant for evaluation. At this point the lenses from the subjects in group I were replaced with new lenses whereas subjects in group II had their original lenses returned. Neither the investigator nor the subject knew if lenses had been replaced.
84
Measurements
At each scheduled 3-monthly visit, data relating to lens and ocular characteristics were collected. Lens Integrity Non-Invasive Dry-Up Time (NIDUT) The Guillon Tearscope was used to view the pre-lens tears film (PLTF) and to assess NIDUT. This instrument utilises a cold diffuse light source that is reflected by the tear film and renders it visible without the instillation of colouring agents. 14The NIDUT is described as the time elapsed between eye opening, following a blink, and the total disappearance of the aqueous phase at the front of the contact lens. 14Assessment of the NIDUT has been shown as an important indicator for lens wettabflity, whereby a longer NIDUT represents better performance. 15 Mucus Coating Once the lens had dried up in the course of a NIDUT measurement, the surface of the lens was exposed and a grading of the presumed mucous coating on the lens surface could be made. 11A grading system from 0-4 in terms of increasing clinical severity as suggested by Woods 12 was used, where grade 0 is normal - - zero signs; grade 1 is minor - clinically insignificant; grade 2 is moderate - - clinical action not necessary; grade 3 is marked - - clinical action usually needed; grade 4 is severe - - clinical action indicated. Surface Deposition A slit-lamp biomicroscope was used to observe the surface of the lenses in situ at x 15 magnification. The overall level of deposits was graded using the clinical severity scale described above, in accordance with the principles outlined by Hurst et al. 16 but modified for in situ assessment. Surface Scratching A slit-lamp biomicroscope was used to observe the surface of the lenses in situ at x 15 magnification. The overall level of scratching was graded using the clinical severity scale described above. Lens Binding This relates to a correctly positioned lens that is immobile. Lens binding was demonstrated to the subjects before the start of the study. They were trained to detect binding and instructed to note in a diary if it occurred during the study period. Swarbrick and Holden 17 have reported this to be a very reliable method for continuously monitoring lens binding; relying on data collected at the aftercare visits would not have provided any guide to the frequency of this phenomenon. Lens Comfort Subjects indicated the level of comfort by placing a mark on a 10cm vertical line. The bottom of the scale (0cm) represented a lens that was unwearable (extremely uncomfortable) and the top of the scale (10cm) represented a lens that elicited virtually no sensation (extremely comfortable).18
REGULARREPLACEMENT OF DAILY-WEARRIGID GAS-PERMEABLECONTACT LENSES
Ocular Response Slit-Lamp Biomicroscope Observations A slit-lamp biomicroscope was used to monitor the following measures of ocular response to lens wear: corneal staining, conjunctival staining, conjunctival hyperaemia, limbal hyperaemia, epithelial microcysts and endothelial polymegethism. The ocular response was graded on a severity scale of 0 to 4 as described previously.
NIDUT
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Refractive Changes A full sphero-cylindrical overrefraction was performed on all subjects at all visits, from which changes in refractive error could be determined.
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Visual Acuity At all visits the visual acuity of all subjects was assessed at 10% and 90% contrast using BaileyLovie acuity charts. 19
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Figure 1. Mean (+SD) change in a non-invasive dry-up time (NIDUT) for the two groups with time. The asterisk denotes a statistically significant difference between the two groups at that point in time.
Statistical Analysis As all subjects enrolled on the study were existing RGP lens wearers, the baseline data were taken to be Mucus Coating Lenses worn by subjects in group II at 2 weeks after dispensing the lenses. This allowed all displayed a significant increase in mucus coating over time (Figure 2). There was a significant difference in the subjects to adjust to the new lens material and design, and for any low-level changes caused by their mucus coating between the two groups over time, and the mucus coating was significantly greater for lenses existing lenses to regress. An analysis of covariance worn by subjects in group II (versus group I) at the 12(ANCOVA) investigated the changes in each group and the differences between the two groups over time, month visit. where time was the covariant. An analysis of variance (ANOVA) was used to assess the significance of any 2.0 Mucus coating differences between the two groups for each visit, with / ....~,,,, Group l J the exception of lens binding, where the X2 test was 1.5 Group II J used to assess differences between the frequency of binding between the two groups. As a general rule, a value of P<0.05 was taken as being statistically ~ 1.0 significant.
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Results Of the 41 subjects enrolled, 38 completed the study. The three discontinuations were due to ill health (one female, group I), recurrent lens binding (one female, group II) and unacceptable visual performance (one male, group I). Data for the remaining 38 subjects were analysed. Over the 12-month period of study, subjects in group I used an average of 8.52_+0.81 lenses and subjects in group II used an average of 2.78_+0.81 lenses. The F statistic and P value relating to all measures of lens integrity and ocular response are given in Table 2, for changes in each group over time, differences between groups over time, and differences between groups at each visit.
Lens Integrity Non-Invasive Dry-Up Time (NIDUT) The change in NIDUT over time for both groups is given in Figure 1. The NIDUT was significantly longer for lenses worn by subjects in group I (versus group II) at the 9-month visit, but curiously there was no difference at the 12month visit.
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Figure 2. Mean (+SD) change in mucus coating for the two groups with time. The asterisk denotes a statistically significant difference between the two groups at that point in time. Surface Deposition Lenses worn by subjects in both groups displayed a significant increase in surface deposition over time (Figure 3). There was a significant difference in surface deposition between the two groups over time, and the surface deposition was significantly greater for lenses worn by subjects in group I! (versus group I) at the 12-month visit. Surface Scratching Lenses worn by subjects in both groups displayed a significant increase in surface scratching over time (Figure 4). There was a significant
85
0.022 0.183 2.609 1.670 0.975 0.193 0.278 0.582 1.078
Ocular response: Corneal staining Conjuncfival staining Conjuncfival hyperaemia Limbal hyperaemia Epithelial microcysts Endothelial polymegathism Refractive change Visual acuity 90% Visual acuity 10% 0.882 0.669 0.109 0.303 0.326 0.661 0.599 0.447 0.301
0.280 0.206 0.000 0.004 0.697
2.281 0.131 1.528 3.865 0.476 0.064 0.437 0.315 0.002
0.127 13.037 14.395 25.721 0.001
0.135 0.718 0.220 0.052 0.492 0.801 0.510 0.576 0.997
0.723 0.000 0.000 0.000 0.979
1.111 0.311 0.065 4.571 0.080 0.009 0.018 0.020 0.541
0.971 4.098 4.388 6.168 0.066
F
0.294 0.578 0.799 0.034 0.775 0.927 0.892 0.888 0.463
0.326 0.044 0.038 0.014 0.798
P
Difference between group I and group II over time
*Dependenthas zerovariance(gradelevel=0for all data), so an F statisticandp valuecouldnotbe computed.
1.179 1.619 12.367 8.480 0.152
P
F
F
P
Change in group II over time
Change in group I over time
Lens integrity: NIDUT Mucus coating Surface deposition Surface scratching Lens comfort
Table 2. Statistical analysis of data
1.072 0.664 0.144 1.572 * 0.005 0.617 0.003 0.004
1.061 0.020 0.319 * 0.031
F
0.307 0.420 0.706 0.218 * 0.943 0.437 0.986 0.644
0.309 0.888 0.576 * 0.860
P
Difference between group I and group II at t=0.5 months
0.209 0.009 1.139 0.095 * 0.103 0.098 0.363 0.007
3.146 0.108 0.114 0.076 1.116
F
0.651 0.924 0.293 0.360 * 0.750 0.756 0.550 0.980
0.085 0.745 0.738 0.785 0.298
P
Difference between group I and group II at t=3 months
0.630 0.582 0.455 0.726 0.805 1.260 3.493 0.248 0.007
0.250 0.006 0.026 0.434 1.215
F
0.433 0.459 0.504 0.399 0.376 0.269 0.070 0.622 0.979
0.620 0.981 0.873 0.514 0.278
P
Difference between group I and group II at t=6 months
0.016 0.486 0.347 0.501 1.229 3.132 0.530 0.007 0.092
5.672 1.240 0.170 4.758 0.709
F
0.901 0.490 0.560 0.484 0.275 0.085 0.471 0.936 0.763
0.022 0.273 0.682 0.035 0.405
P
Difference between group I and group H at t=9 months
8.389 0.043 0.725 2.807 0.814 0.197 0.254 0.006 0.707
0.377 6.001 5.560 4.237 0.096
F
0.006 0.837 0.400 0.102 0.373 0.660 0.617 0.941 0.406
0.543 0.019 0.024 0.047 0.758
P
Difference between group I and group II at t=12 months
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Figure 3. Mean (+-SD) change in surface deposition for the Figure 5. Mean (+SD) change in corneal staining for the two two groups with time. The asterisk denotes a statistically signifi- groups with time. The asterisk denotes a statistically significant cant difference between the two groups at that point in time. difference between the two groups at that point in time.
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Figure 6. Mean (+SD) change in limbal hyperaemia for the two groups with time.
difference in the extent of surface scratching between the two groups over time, and the extent of surface scratching was significantly greater for lenses worn by subjects in group II (versus group I) at both the 9-month and 12-month visits.
ence in limbal hyperaemia over time between the two groups, with a trend towards limbal hyperaemia decreasing in group I and increasing in group II (Figure 6). No other significant differences between groups over time or at specific visits were revealed.
Lens Binding Up to 28% of subjects reported at least one incident of lens binding; however, no significant trends over time, or differences between groups, were detected with respect to this phenomenon.
Refractive Changes No significant trends over time, or differences between groups, were detected with respect to refractive changes.
Lens Comfort Again, no significant trends over time, or differences between groups, were detected with respect to lens comfort.
Visual Acuity No significant trends over time, or differences between groups, were detected with respect to visual acuity at 10% and 90% contrast.
Discussion Ocular Response Slit-Lamp Biomicroscope Observations A significantly greater amount of corneal staining was observed in subjects in group II (versus group I) at the 12-month visit (Figure 5). The ANCOVA revealed a significant differ-
Maintenance of Masking At the end of the study the investigator attempted to nominate which subjects were in each group. Of the 38 subjects who completed the experiment, the investigator could only correctly place 60% of subjects. This
87
CRAIG A. WOODS AND NATHAN EFRON
exercise demonstrated that for the study, using the information available, the investigator fared slightly better than chance allocation (50%), therefore confirming the efficacy of the masking procedure.
Lens Integrity It has been clearly shown that for an RGP lens to be successful then it must have good wettability. The wettability of the lenses in this study was monitored by observing the changes in the NIDUT and lens mucus coatings using the Tearscope. A reduction in the NIDUT and an increase in the level of mucus coating for lenses worn by subjects in group II at the 9-month and 12-month visits, respectively, indicate reduced wettability. The reason for the convergence of data between the two groups at the 12-month visit is unclear, but may be related to the significant increase in surface deposition on lenses worn by subjects in group II at the 12-month visit. As expected, the level of deposits on lenses worn by subjects in group II increased over time. A significant but less pronounced increase in lens deposition was observed in group I; presumably the statistical significance of this finding relates primarily to the overall increase in deposition relative to baseline. The difference in deposit gradings between the two groups started to diverge after the 6-month visit, and became statistically significant at the 12-month visit. This finding suggests that lens deposition on RGP lenses is cumulative and can be alleviated by 6-monthly lens replacement. It is self-evident that the extent of surface scratching of RGP lenses which are handled regularly will increase over time. The present data confirm this notion. As is the case with surface deposition, the difference in the extent of surface scratching between the two groups started to diverge after the 6-month visit, and became statistically significant at the 12-month visit. The absolute rate of increase in lens surface scratching could be affected by a number of factors, such as the abrasiveness of the cleaner used, the cleaning technique adopted and the lens material. Certainly this report demonstrates that the extent of surface scratching on high Dk RGP lenses using an alcohol-based cleaner is cumulative and can be alleviated by 6-monthly lens replacement. The occurrence of lens binding has often been reported during studies of extended wear RGP lenses. 2°-z~ Little has been reported for daily wear. z°26 Swarbrick and Holden 17 concluded that lens binding was subject-independent and thus not mode-of-weardependent. This study demonstrates that lens binding occurs during daily wear at an incidence of up to 28%. Furthermore, the incidence of binding was never below 12% for group II or 5% for group I. The incidence of binding for RGP lenses worn on a daily basis is apparently unaffected by the frequency of lens replacement. It is curious that lens comfort was unaffected by the frequency of lens replacement, because intuitively one would expect that a lens that is more heavily deposited
88
and more extensively scratched would be less comfortable. Clearly, such adverse changes in lens integrity are of insufficient magnitude to elicit a detectable decrease in comfort. An important clinical ramification of this finding is that patient symptomatology - - at least in the form of reports of lens comfort - - cannot be taken by clinicians to be a reliable indicator of lens integrity.
Ocular Response The contact lens oxygen transmissibility for the avoidance of corneal oedema was found by Holden and Mertz 27 for daily wear to be 24 × 10-11 (cm/sec) (ml O2/ml × mm Hg). The Quantum 2 lens exceeds this criterion for most prescriptions, being 77 × 10 -11 (cm/sec) (ml O2/ml × mm Hg) for a -3.00D lens. 13 The use of a lens with such a high oxygen transmissibilty for daily wear should therefore minimise or eliminate hypoxic signs such as epithelial microcysts, stromal oedema and endothelial changes, as indeed was the case in this study. Of all the ocular responses monitored, only corneal staining was significantly reduced by planned replacement; this became evident at the 12-month visit. The present data also suggest a trend towards regular lens replacement alleviating limbal hyperaemia. Both the increased staining and trend towards less limbal hyperaemia with regular lens replacement may be attributed to the avoidance of a degradation of lens integrity after 12 months of lens wear, although a causal relationship has not been established in this study. Throughout this 12-month study the visual acuity recorded by both groups showed little change. Regular lens replacement is known to preserve vision in patients wearing soft lenses on a daily-wear basis 2s, although it should be noted that data pertaining to lenses of up to 4 years old were used in establishing that finding. A period of 12 months' wear may not have been long enough to allow a degradation in the surface of the nonreplaced RGP lenses of sufficient magnitude to reduce the quality of vision, if such a phenomenon were to occur at all.
RGP Lens Replacement Frequency It is clear from these data that adverse changes in lens and ocular integrity can be detected after 9-12 months of daily lens wear if lenses are not replaced on a scheduled basis. The fact that the differences in gradings and trends reported here are statistically significant, does not necessarily infer clinical significance. The assignment of an event as being clinically significant is a subjective opinion that must be made by clinicians. Although most of the observations reported in this paper rarely exceed the 'clinical action not necessary' level (grade 2), the authors feel that the accumulated evidence of changes in both lens integrity and ocular response represents an overall subtle clinical determination in the lens-eye system that could reasonably be assumed to be a precursor to adverse patient symptomatology. Although the data generated in this report relate to
REGULARREPLACEMENTOF DAILY-WEARRIGID GAS-PERMEABLECONTACTLENSES
one particular lens material, Guillon et al. 1~ reported that degradation of the integrity of the surface of RGP contact lenses during wear is not appreciably affected by the type of material. We advocate, therefore, that RGP lenses worn on a daily-wear basis should be replaced every 6 months to avoid adverse changes in lens and ocular integrity that have been demonstrated in this report to begin to occur by 9 months.
Acknowledgements This study was supported by Polymer Technology Corporation. Assistance was provided by Bausch & Lomb in the co-ordination of lenses and supply of solutions.
Address for Correspondence Craig Woods, Department of Optometry and Vision Sciences, UMIST, PO Box 88, Manchester, M60 1QD. email: [email protected]. REFERENCES 1 Poggio, E,C. and Abelson, M. Complications and symptoms in disposable extended wear lenses compared with conventional soft daily wear and soft extended wear lenses. CLAO J., 19, 31-39 (1993). 2 Poggio, E.C. and Abelson, M. Complications and symptoms in disposable daily wear and conventional soft daily wear. CLAO J., 19, 95-102 (1993). 3 GeUatly, K.W. Disposable contact lenses: a clinical performance review. Can. J. Optom., 55, 166-173 (1993). 4 Harmon, F. Annual replacement of high Dk RGPs? Why not? Eyecare Business, 5, 78-79 (1990). 5 Hill, R.M. EOP: have we hit the limits? CL Forum, 13, 39 (1988). 6 Hamano, H., Hamano, T., Iwasaki, N. and Mitsunaga, S. Clinical evaluation of various contact lenses from the incidence of complications.J. Br. Contact Lens Assoc., 11, 25-30 (1988). 7 Schoessler, J. and Hili, R.M. The B&L high Dk material: Pilot studies. Spectrum, 2, 39-40 (1987). 8 Phillips, A.J. and Cascoigne, K. Surface cracking of RGP materials. Clin. Exp. Optom., 71,198-203 (1988). 9 Yokota, M., Goshima, T. and Itoh, S. The effect of polymer structure on durability of high Dk gas-permeable materials. J. Br. Contact Lens Assoc., 15, 125-129 (1992).
10 Walker, J. Overpolishing fluorosilicone acrylates. J. Br. Contact Lens Assoc., 12, 29-32 (1989). 11 Guillon, M., Guillon, J.P., Shah, D., Bertrand, S. and Grant, T. In vivo wettability of high Dk RGP materials. J. Br. Contact Lens Assoc., 18, 9-15 (1995). 12 Woods, R. Quantitative slit lamp observations in contact lens practice.J. Br. Contact Lens Assoc., 11, 42-45 (1989). 13 Brennan, N ~ , El~on, N., Holden, B.A. and Fatt, I. A review of the theoretical concepts, measurement systems and application of contact lens oxygen permeability. Ophthal. Physiol. Opt., 4, 485-490 (1987). 14 Guillon, J.P. and Guillon, M. Tear film examination of the contact lens patient. Contax, May, 14-18 (1988). 15 Guillon, J. and Guillon, M. Tear film examination of the contact lens patient. Optician, 206, 21-29 (1993). 16 Hurst, M.A., Mitchell, S.E. and Douthwaite, W.A. Contact lens opacity grading system (CLOGS). jr. Br. Contact Lens Assoc., 17, 19-24 (1994). 17 Swarbrick, H.A. and Holden, B.A. Rigid gas-permeable lens adherence: A patient-dependent phenomenon. Optom. Vis. Sci., 66, 269-275 (1989). 18 Efron, N. and Brennan, N.A. In: Efron, N. (ed.) How to Manage
Complaints of Discomfort During Contact Lens Wear. W. B. Sannders Co Ltd, London, 7-20 (1993). 19 Bailey, I.L and Lovie, J.E. New design principles for visual acuity letter charts. Am. J. Optom. Physiol. Opt., 53, 740-745 (1976). 20 Benjamin, W. and Boltz, R. RGP lens adhesion is not a benign phenomenon. Int. Contact Lens Clin., 16, 60 (1989). 21 Kenyon, E., Polse, KA. and Mandell, R.B. Rigid contact lens adherence: incidence, severity and recovery. J. Am. Optom. Ass., 59, 168-174 (1988). 22 Levy, B. Rigid gas permeable lenses for extended wear - - a one year study. Am. J. Optom. Physiol. Opt., 62, 889-894 (1985). 2:~ Polse, K.A., Sarver, M.D., Kenyon, E. and Bonanno, J. Gas permeable hard contact lens extended wear. Ocular and visual responses to a six-month period of wear. CLAO]., 13, 31-38 (1987). 24 Swarbrick, H.A. and Holden, B.A. Rigid gas permeable lens binding: significance and contributing factors. Am. J. Optom. Physiol. Opt., 64, 815-823 (1987). 25 Terry, R., Schnider, C. and Holden, B.A. Complications associated with RGP extended wear: two case reports. Clin. Exp. Optom., 72, 19-21 (1989). 2G Backman, H. The adhesion syndrome. Can. J. Optom., 48, 51 (1986). 27 Holden, B.A. and Mertz, G.W. Critical oxygen levels to avoid edema for daily and extended wear contact lenses. Invest. Ophthalmol. Vis. Sci., 25, 1161-1167 (1984). 28 Gellatly, K.W., Brennan, N.A. and Efron, N. Visual decrement with deposit accumulation on HEMA contact lenses. Am. J. Optom. Phys. Opt., 65, 937-941 (1988).
89