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Tiffany Yuen, Andrew D Graham, Annie Lee, Meng C Lin; Evaluation of two methods for measuring post-lens tear exchange: central and peripheral. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4667a.
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To understand the differences in post-lens tear exchange with soft contact lenses (CL) occurring centrally, as measured with fluorophotometry, and peripherally, as measured with a novel slit-lamp technique referred to as the out-in method.
Eleven experienced soft CL wearers were recruited. Subjects wore 3 different CLs in randomized order: Air Optix Night and Day (N&D), Acuvue Oasys (AO) and Biofinity (BIO). Tear exchange was evaluated centrally in one eye with fluorophotometry. 2µL of 4% FITC-dextran was placed onto the posterior lens surface prior to lens insertion. Changes in fluorescence intensity in the post-lens tear film (PoLTF) were monitored to determine the tear turnover rate (TTR, %/min). Seven subjects returned for assessment of peripheral tear exchange in the same eye with the out-in method. After placing 5µL of Fluoresoft 0.35% onto the superior bulbar conjunctiva, the PoLTF at the lens periphery was monitored at the 6 o’clock position through a slit lamp biomicroscope at 20X magnification. Out-in time (OIT, sec) was recorded at the first sign of fluorescein in the PoLTF. All measurements were performed after 20-30 minutes of lens wear.
The mean TTRs determined by fluorophotometry were 12.01 %/min for N&D, 6.29 %/min for AO, and 5.21 %/min for BIO. The variance was similar between AO (6.29) and BIO (5.21), but greater for N&D (12.01). The average OITs were 167.86 sec for N&D, 75.86 sec for AO, and 135.26 sec for BIO. There were differences in variance between the CLs (N&D: 16750; AO: 1511; BIO: 4703). Based on these variance estimates, for a crossover study design the sample sizes required to detect a mean difference between lenses of 25%, with 95% confidence and 80% power, are approximately 45 subjects for TTR, and 71 subjects for OIT.
The dynamics of the PoLTF is not well understood. By assessing both central and peripheral tear exchange, there is potential to gain a better understanding of tear mixing under soft CLs. Variance estimates from this study will help to define appropriate sample sizes for future crossover trials.
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