Dehydration, lens movement and dryness ratings of hydrogel contact lenses

Previous studies have shown that soft lenses dehydrate during lens wear. The purpose of this study was to determine the dehydration time course of 38% water content non-ionic Medal st™, 58% ionic Acuvue™ and 74% non-ionic Permaflex™ lenses, and the relationship between dehydration and in vivo diameter, movement and symptoms of dryness. Nineteen subjects randomly wore three pairs of lenses, each for 7 h. Lens movement and diameter were measured in vivo and hydration after lens removal at 1, 3 and 7 h. Dryness was rated by the subjects using a visual analogue scale, A separate experiment was conducted to measure hydration changes after 7 continuous hours of lens wear. The water content of all three lens types decreased significantly over 7 h with Acuvue decreasing more than the Permaflex and Medal st lenses in the interrupted and continuous experiments (ANOVA P < 0.05). Dehydration of Acuvue was significantly greater in the 7 h continuous experiment (9.0 ± 2.6% H2O, ANOVA P = 0.0062) compared to the interrupted experiment. Hydration levels measured for Acuvue lenses on subjects for control purposes at 0, 1, 3 and 7 h showed no difference over time ANOVA P = 0.0711). Movement of Permaflex lenses decreased 0.60 ± 0.57 mm (ANOVA P = 0.0005) over 7 h and the in vivo diameter of Acuvue lenses decreased by 0.12 ± 0.16 mm (ANOVA P = 0.0569). Dryness ratings increased significantly and equally for all three lenses over 7 h (ANOVA P = 0.9833). No correlation was found between lens dehydration, movement, diameter and dryness with the exception of a weak significant (r = 0.5, P < 0.05) correlation between dehydration and dryness for Permaflex. The results of the study suggest that dehydration of soft lenses over a 7 h period does not significantly affect the clinical performance (dryness and movement) of the three lens types.