Curvature changes in dehydrating soft lenses (Part I)

Abstract

Introduction Most practitioners have seen soft lenses tightening on an eye from an initially good fitting. Some contact lens materials may be susceptible to variations in pH and tonicity and may produce parameter changes when placed on the eye. If the storage solution pH in particular is markedly different from the eye pH this phenomenon may be encountered. However, the most probable cause of lens tightening is that of water loss from the hydrogel. This may be caused by evaporation from the lens surface and/or a heating up of the lens when placed on the eye. The problems of dehydrating lenses or lens surfaces is obviously greater in higher water content lenses where there is potentially more water to he lost. Many patients' eyes react adversely when wearing soft lenses in dry and warm environments where the lens dehydration is accelerated. The lenses may not feel comfortable, vision may be poorer (Fatt and Chaston, 1981) and less stable; there may also be some physiological embarrassment to the cornea. Coupled with a patient 's blinking pattern, crystallization of tear components on to the lens front surface may be accelerated by increased evaporation from the lens front surface (Hathaway and Lowther, 1978). This is particularly so if the individual's tear film by its nature is going to encourage evaporation. The tear film over the lens front surface may be quite different from the tear film over the wearer's naked cornea. Hamano et al, 1981 found that the rate of evaporation from a low water content gel lens could be up to 2.5X that of the naked eye. The changes in soft lens properties and parameters when the lens starts drying out on the eye have received scant attention by researchers and the literature is bereft of data on the subject. For this reason the author embarked on the present investigation.