SOFT LENS MOVEMENT - EFFECT OF BLINK RATE ON LENS SETTLING

Little is understood about the mechanism by which soft lenses settle o n the eye, although it has been suggested that lens base curve, steepe ning with in-eye lens dehydration or tear film changes may influence l ens movement in the initial period of wear. In this study, we investig ated the role of postlens tear fluid expulsion by assessing lens movem ent in 20 subjects wearing 38% water content hydrogel lenses for 10 mi n under three different conditions: 10 blinks/min, 30 blinks/min and e ye closure. Over the 10 min wearing period, the total decrease in medi an lens movement for the 10 blinks/min condition was 0.07 mm, which wa s not a significant change (Friedman ANOVA, p = 0.13), while significa nt 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 movemen t under the three conditions was the same at insertion, but was signif icantly 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 fi ndings, we postulate the model that the extent of lens settling and th e degree of postinsertion lens movement are determined by the time-ave rage pressure for postlens tear film expulsion exerted on the lens by the eyelids.