Oxygen permeability of the hypertransmissible contact lenses

Oxygen permeability (Dk) was assayed in a masked fashion using the polarographic method for three materials found capable of hypertransmissibility as contact lenses: tisilfocon A, balafilcon A, and lotrafilcon A.

Two sets of tisilfocon rigid lenses were tested in six thicknesses and two sets in nine thicknesses for the hydrogels. The two sets of tisilfocon were from two different lots and of lotrafilcon were from different base curve radii (8.6 and 8.4 mm). The six test sets were measured with the inclusion of rigid reference materials for the purpose of simultaneous calibration.

Oxygen permeability (in barrers) of the two lots of tisilfocon A was 159.6 (95% confidence interval [CI], 146.3-175.5) and 175.1 (95% CI, 154.9-201.6). The Dk values for balafilcon A were 111.3 (95% CI, 103.6-120.3) and 102.0 (95% CI, 90.5-116.7), and the Dk values for lotrafilcon A were 176.1 (95% CI, 165.1-188.7) and 190.2 (95% CI, 177.5-205.0) The coefficients of determination (R2) of the corrected resistance versus thickness plots were 0.98 or more. Paired-comparison t tests were performed between the sets of each test material paired by thickness. These revealed the oxygen transmissibilities (Dk/L) of the lenses made from the two lots of tisilfocon A to be significantly different (t = 2.62; P < 0.047; df = 5) and lenses of the two base curve radii made of lotrafilcon A to be significantly different (t = 4.42; P < 0.002; df = 8). These differences were attributed to a material difference in Dk between lots of tisilfocon A and a thickness difference between lens designs of lotrafilcon A.

Oxygen permeability of tisilfocon was found consistent with that claimed by the manufacturer (163 barrers). Dk of balafilcon was found marginally greater than claimed (99 barrers) and of lotrafilcon, greater than claimed (140 barrers). Preliminary Dk/L values of two of the three test materials were statistically different between Sets 1 and 2, yet they were inside the American National Standards Institute Z80.20-1998 tolerance (+/-20%). Resistance-versus-thickness plots were linear up to a Dk of at least 190 barrers. The Dk/L values over the central zone of minus and low-plus contact lenses made of these materials are predicted to decrease on the upper asymptotic portion of the Dk/L versus equivalent oxygen percentage curve. Hence, the physiologic effects of oxygen variability from lot-to-lot, lens-to-lens, and site-to-site on the same lens will be minimized.