Factors controlling oxygen tension under a hydrogel contact lens.

Corneal edema, folds in Descemet's membrane, and epithelial erosion may accompany hydrogel lens wear. These changes are often caused by corneal hypoxia and can therefore be prevented if the oxygen tension under the lens can be maintained above some critical level. THe two principal factors that control the oxygen tension under a contact lens are the diffusion of oxygen through the lens material (oxygen transmissibility) and the fractional replacement of freshly oxygenated tears under the lens occurring with each blink (tear pumping). Recent studies show that during hydrogel lens wear most oxygen reaches the cornea by diffusion and not by tear pumping. This suggests that if the oxygen level under the lens needs to be increased, the clinician must make changes directed at increasing oxygen transmissibility rather than attempting to increase the tear pump.