Abstract
Purpose: :
To determine if, and by how much, a plasma treatment alters the oxygen transmissibility (Dk/t) of gas-permeable contact lenses, and to quantify the Dk/t of plasma treatment.
Methods: :
Twenty four (24) Boston XO2 contact lenses (Hexafocon B) were obtained from the Bausch & Lomb Boston Group (Wilmington MA) and 24 Menicon Z contact lenses (Tisilfocon A) from Abba Optical, Inc. (Stone Mountain GA). Lenses of each material were produced from a single lot of buttons with a nominal thickness uniform across the central areas of the lenses. Half of the lenses from each manufacturer were untreated while the other half were plasma treated on both lens surfaces. Their mean thicknesses were determined from 4 caliper measurements. Ten (10) pairs of each lens type were assembled such that the mean thicknesses of the two lenses in a pair were as close as possible given that one lens of each pair was untreated and the other treated. Mean oxygen transmissibility (Dk/t) was derived for each lens from 4 polarographic evaluations per lens (160 in all). Each lens of a pair was measured immediately before or after its mate in a schedule designed to spread the effects of machine drift and other factors.
Results: :
The overall mean thicknesses of the untreated and treated Boston XO2 lenses were 0.1306 and 0.1307 mm, respectively, and the overall mean Dk/t values were 108.0 and 106.4 Fatt Dk/t units. The overall mean difference, 1.63 Dk/t units, was statistically significant (Paired T = 3.093, n = 10 pairs, p = 0.013). When the Dk/t values were adjusted for the effects of small thickness differences between lenses of the same pair, the overall mean difference was similarly significant. The overall mean thicknesses of the untreated and treated Menicon Z lenses were 0.1073 and 0.1082 mm, respectively, and the overall mean Dk/t values were 143.8 and 143.6 Fatt Dk/t units. The overall mean difference, 0.20 Dk/t units, was not statistically significant (Paired T = 0.134, n = 10 pairs, p = 0.896). When the Dk/t values were adjusted for the effect of small thickness differences between lenses, the Dk/t difference remained insignificant.
Conclusions: :
Plasma treatment of Hexafocon B slightly lowered oxygen transmissibility by a statistically significant, yet clinically insignificant, small amount (1.5%). The Dk/t of the dual surface plasma treatment was over 7000 Dk/t units when calculated by the method of additive resistances. Treatment of Tisilfocon A did not have a statistically significant effect. The oxygen discrepancy between treatments shown here could be the result of the recommended plasma concentration, current, and duration of current application that vary between manufacturers and lens materials.
Keywords: contact lens • cornea: clinical science • oxygen