Tear oxygen under hydrogel and silicone hydrogel contact lenses in humans.

PURPOSE: To determine the tear oxygen tension under a variety of conventional and silicone hydrogel contact lenses in human subjects. METHODS: Three hydrogel and five silicone hydrogel lenses (Dk/t = 17 to 329) were coated on the back surface with an oxygen sensitive, bovine serum albumin-Pd meso-tetra (4-carboxyphenyl) porphine complex (BSA-porphine). Each lens type was placed on the right eye of 15 non-contact lens wearers to obtain a steady-state open eye tear oxygen tension using oxygen sensitive phosphorescence decay of BSA-porphine. A closed-eye oxygen tension estimate was obtained by measuring the change in tear oxygen tension after 5 min of eye closure. In separate experiments, a goggle was placed over the lens wearing eye and a gas mixture (PO2 = 51 torr) flowed over the lens to simulate anterior lens oxygen tension during eye closure. RESULTS: Mean open eye oxygen tension ranged from 58 to 133 torr. Closed eye estimates ranged from 11 to 42 torr. Oxygen tension under the goggle ranged from 8 to 48 torr and was higher than the closed eye estimate for six out of the eight lenses, suggesting that the average closed eye anterior lens surface oxygen tension is <51 torr. For Dk/t >30, the measured tear oxygen tension is significantly lower than that predicted from previous studies. CONCLUSIONS: The phosphorescence decay methodology is capable of directly measuring the in vivo post lens PO2 of high Dk/t lenses without disturbing the contact lens or cornea. Our data indicate that increasing Dk/t up to and beyond 140 continues to yield increased flux into the central cornea.

Corneal oxygen distribution with contact lens wear.

PURPOSE: Since 1991, multilayer mathematical in vivo oxygenation models have been created to predict normal corneal oxygenation with contact lens wear. From these models, there have been assertions that most hydrogel contact lenses allow 97%-98% of normal corneal oxygenation compared to no contact lens wear. In light of hydrogel lens-induced neovascularization and limbal hyperemia, to clinicians, this finding seems counterintuitive. This work seeks to validate or refute those preexisting models and estimate the impact of contact lens wear on the oxygen distribution profile across the cornea. To this end, to estimate the impact of contact lens wear on the 3-dimensional (3-D) oxygen distribution profile within the cornea as a function of the oxygen permeability of the contact lens, a two-dimensional axisymetric finite element analysis (FEA) model was constructed for contact lenses, on the cornea, both having varying thickness profiles. METHODS: A two dimensional (2-D) axi-symetric finite element analysis (FEA) model of a -3.00 D contact lens on eye was constructed. The model included the varying thickness profiles of the contact lens and cornea. By symmetry, this 2-D model is equivalent to a full 3-D model. The oxygen permeability, material thickness profile, and oxygen consumption coefficients from Brennan (Optometry and Vision Science, June 2005) were used for this validation. Several different oxygen consumption profiles were also considered. Oxygen partial pressure, flux, and consumption profiles were generated. RESULTS: Profiles of the oxygen partial pressure, flux, and consumption were generated from the central cornea to the limbal junction. CONCLUSION: This FEA model reproduced Brennan 8-layer model (BEL model) results at the central cornea. However, BEL model parameters yielded regions of oxygen deficiency in the corneal periphery, even in the open eye with no contact lens. If the BEL model cannot account for oxygenation across the whole cornea, it may be incorrect or incomplete. This assertion calls into question any conclusions from the BEL model regarding the minimum contact lens transmissibility needed to fully oxygenate the eye.