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A. D. Graham, T. N. Truong, M. C. Lin; Survival Analysis of Conjunctival Epithelial Flap With Silicone-Hydrogel and Gas-Permeable Contact Lenses. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4873.
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Conjunctival Epithelial Flap (CEF) is a condition in which sheathing of epithelial cell layers occurs on the conjunctiva. Although CEF appears to be benign from a patient’s perspective, it does involve a disruption of the ocular surface whose long-term effects we do not fully understand. Even basic information, such as the probability of developing CEF, the wearing time involved, and the differences between lens types, has not been reported. In this report, we employ statistical techniques from survival analysis to address these basic questions.
In this study, 42 gas permeable (GP) and 107 silicone-hydrogel (SiH) lens wearers were followed for up to 30 days of overnight wear. The Kaplan-Meier non-parametric estimator was used to construct survival curves and estimate the mean survival time free of CEF. Survival curves were compared using the Mantel-Haenszel chi-squared statistic.
Among the 149 subjects there were 65 occurences of CEF (43.6%). Of these, the GP group accounted for 18 events (42.9%), and the SiH lens group accounted for 47 events (43.9%). Although the point estimates suggest that the probability of CEF was similar in GP and SiH wearers, when censoring was taken into account the mean (SE) survival time free of CEF was 91.3 (10.4) days for the GP group, compared to 70.0 (6.1) days for the SiH lens group. There was some suggestion of a difference in overall survival curves between lens types, although it was not statistically significant (p=0.11).
The incidence density over 30 days of overnight night wear was more than 40% in our study population, slightly lower than the cumulative incidence of more than 50% reported by Lofstrom and Kruse. Interestingly, the time course to development of CEF differs between rigid and soft lenses. Further research is warranted to determine whether this common disruption of the ocular surface could have longer term adverse effects.
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