Abstract
Abstract: :
Purpose: The relationship between oxygen permeability (Dk) and water content (WC%) of 34 conventional hydrogel materials was analyzed at eye temperature (35°C). The relationship was compared to that established when water content was derived at room temperature (22°C). Methods: Calibrated, corrected polarographic Dk values were previously determined at 35°C for 34 conventional hydrogel materials available as contact lenses. Refractive index was measured with an Abbe'–type refractometer at room temperature and converted to water content by gravimetric adjustment of a volumetric equation derived from the Principle of Gladstone & Dale in the manner of Young MD, Benjamin WJ: Eye & Contact Lens 29(2):126–133, 2003. Similarly, water content was determined with the arbor of the refractometer at eye temperature. The contact lenses were equilibrated at the respective temperatures in Unisol 4 (Alcon Laboratories, Inc., Ft. Worth, TX) having pH 7.37 and tonicity 301.7 mOsm/Kg preceding index measurement. Results: Water contents at the two temperatures were significantly different and strongly correlated. A paired–comparison t–test between water contents gave t = 8.683, p < 0.0000 (n = 34 pairs) and the linear regression was (WC%35) = 0.9493(WC%22) + 0.0536 with coefficient of determination (R2) = 0.979. Dk values were strongly correlated in a semilogarithmic manner with water content at eye temperature as they had been at room temperature. The linear regressions were Log(Dk) = 0.01754(WC%22) + 0.3897 with R2 = 0.945, and Log (Dk) = 0.01806(WC%35) + 0.4101 with R2 = 0.916. Conclusions: Water content determined at eye temperature was less than at room temperature. The difference was 1.873% water for a material composed of 38% water, 2.735% water for a material of 55%, and 3.495% water for a material of 70%. Hence, water content dropped by approximately 5.0% of its value across the range of water contents available in conventional contact lenses, when equilibration was raised from room to eye temperature. The semilogarithmic relationship of Dk and water content was again evident when water content was determined at the higher temperature.
Keywords: contact lens • metabolism • cornea: tears/tear film/dry eye