Corneal response to rigid and hydrogel lenses during eye closure

Corneal changes were monitored in 14 subjects following 3 hr of eye closure while wearing selected oxygen permeable rigid and hydrogel lenses. The mean increase in corneal thickness ranged from 82.5 to 29.5 microns for rigid lenses with oxygen transmissibilities (Dk/L) between 0.2 X 10(-9) and 57.0 X 10(-9) (cm/sec) (ml O2/ml X mmHg), respectively, and ranged from 82.5 to 23.5 microns for hydrogel lenses with Dk/L between 2.5 X 10(-9) and 70.0 X 10(-9) (cm/sec) (ml O2/ml X mmHg), respectively. No differences in the amount of swelling between rigid and hydrogel lenses of the same oxygen transmissibility were observed (t-test, P greater than 0.20). Combining the swelling data for both types of lenses shows that a minimum lens oxygen transmissibility of approximately 75 X 10(-9) (cm/sec) (ml O2/ml X mmHg) is necessary during eye closure to prevent contact lens induced edema. The estimated oxygen tension under a lens with this Dk/L value is 40 mmHg. Recovery of the cornea to baseline thickness follows a nonlinear time course, with the rate of dehydration decreasing as the cornea thins. For initial swelling of 40-54 microns, 55-69 microns, and 70 microns and above, the time to reach baseline thickness was 1.5, 2.0, and 2.5 hr, respectively. Effects on vision, corneal curvature, distortion, and epithelial integrity were not clinically significant during this short period of eye closure.