Evaluation of variation in oxygen transmission in rigid contact lens extended wear

Variation in thickness is known to affect the Dk/L of rigid gas permeable contact lenses. Our study was designed to evaluate the variability of central and average thickness of Oxyflow EW lenses. In addition, the relationship between overnight swelling of the central cornea and Dk/L was examined to determine whether Dk/L calculated with central lens thickness correlated better with lens-induced edema than Dk/L calculated with average lens thickness. The results showed marked variation among lens center thicknesses for all powers (-6 D, -2 D, +2 D, +6 D) examined, and, as expected, more variation for centrally calculated Dk/L with high minus lenses. Average Dk/L did not vary significantly with power. Average and central Dk/L values had a near 1:1 relationship for -2 D lenses for the base curve (7.8 mm) ordered; but only when power was kept constant was a good correlation seen between central and average Dk/L. A one-patient study evaluating the overnight central swelling response to three +6 D and three -6 D (high, medium, and low center thickness) lenses showed that central Dk/L correlated better with lens-induced edema than average Dk/L. Thus, specifying central thickness on lens orders appears to be a primary variable influencing extended wear edema at night. Peripheral Dk/L, however, may influence corneal physiological variables other than central edema and warrants further study.