Considerations in Contact Lens Use Under Adverse Conditions Proceedings of a Symposium (1991) / Chapter Skim
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Tear Evaporation Considerations and Contact Lens Wear
Tear Evaporation Considerations and Contact Lens Wear
Pages 38-43
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From page 38... ...
. Contact lenses can be classified as rigid lenses, elastomeric lenses, and hydrogel lenses.
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From page 39... ...
through high-water thin hydrogel lenses and from silicone elastomeric contact lenses (Refojo and Leong, 1981~. The tear film that spreads over rigid contact lenses by the blink retracts immediately upon opening the eyelids.
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From page 40... ...
However, the thinner lenses become proportionately more dehydrated than the thicker lenses and reach a steady state of dehydration on the eye more quickly than the thicker lenses (Andrasko, 1983~. Effect of Abnormally Fast Lens Drying When the surface of a hydrogel lens dehydrates faster than the bulk moisture of the lens can diffuse to the surface, the polymer at the surface contracts and the lens roll up due to the stresses created in the lens network by the nonuniform swollen state of the lens.
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From page 41... ...
found that hydrogel contact lenses dehydrate in the eye to about 20 percent below normal hydration of the lens `'in the bottle." When a lens dehydrates, the lens develops a water imbibition pressure, which increases exponentially with the degree of dehydration. The water imbibiiion pressure for hydrogels, particularly of low hydration is very high, on the order of 4,000-5,000 mmHg for a hydrogel of equilibrium swelling 40 percent H2O, which has dehydrated only to about 5 percent below its normal hydration (Refojo, 1976~.
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From page 42... ...
It is important to recall here that parameter changes on dehydration of hydrogel contact lenses in the eye might be minimal with low-water lenses, but substantial parameter changes will take place in lenses of high hydration (Gundel and Cohen, 1986~. The critical variable in the dehydration of hydrogel lenses in all conditions of wear is blinking, and this is particularly true for lens wearers flying in an aircraft (Corboy and Tannehill, 19731.
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From page 43... ...
Leong 1981 Water pervaporation through silicone rubber contact lenses: a possible cause of complications. Contact and Intraocular Lens Medical Journal 7:226-233.
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