Soft lens movement: effect of blink rate on lens settling

Little is understood about the mechanism by which soft lenses settle on the eye, although it has been suggested that lens base curve, steepening with in-eye lens dehydration or tear film changes may influence lens movement in the initial period of wear. In this study, we investigated the role of postlens tear fluid expulsion by assessing lens movement in 20 subjects wearing 38% water content hydrogel lenses for 10 min under three different conditions: 10 blinks/min, 30 blinks/min and eye closure. Over the 10 min wearing period, the total decrease in median lens movement for the 10 blinks/min condition was 0.07 mm, which was not a significant change (Friedman ANOVA, p = 0.13), while significant decreases occurred with conditions of 30 blinks/min (0.19 mm, p = 0.004) and eye closure (0.43 mm, p = 0.0001). As expected, lens movement under the three conditions was the same at insertion, but was significantly higher thereafter for the slower blink rate condition compared to the faster blink rate or eye closure conditions (Wilcoxon test, Z = 2.8 and -3.0, p = 0.006 and 0.003, respectively). Based on these findings, we postulate the model that the extent of lens settling and the degree of postinsertion lens movement are determined by the timeaverage pressure for postlens tear film expulsion exerted on the lens by the eyelids.