Calculated in situ tear oxygen tension under hybrid contact lenses

We calculated corneal surface oxygen tension under hybrid contact lenses (CLs) by extending existing models of CL oxygen diffusion to the circumstances of hybrid CL designs with tear vaults.

Theoretical oxygen tensions at the corneal surface, if tear mixing and exchange are excluded, are calculated for hybrid CL (modern, high oxygen-permeable rigid center) designs with a single chamber corneal model using a computer software spreadsheet. Several specific in vivo instances of hybrid CL tear vaults are measured by both slitlamp biomicroscopy and optical coherence tomography (OCT).

No significant difference was found between central thickness of hybrid CL rigid portions measured by Vigor gauge and OCT. The amount of central tear vault thickness was found to be significantly greater, however, when interpolated from biomicroscope images (500-1,554 μm, simulating a poorly fit lens) than from either OCT images (0-91 μm) or direct measurement through OCT software (0-96 μm). Using all measurements, excluding any potential contributions from tear exchange or mixing, we predict corneal surface tear pO2 values under several hybrid CLs to range from zero under a lens with an excessive vault to 112 mm Hg under a lens with a minimal vault.

Tear vaults beneath the gas permeable portion of the hybrid CLs of 100 μm or less resulted in acceptable corneal surface oxygen values. Where tear vaults are greater than about 100 μm, predicted anterior corneal surface pO2 values are less likely to be adequate for optimal corneal physiology. Any significant tear exchange, however, should increase these values. Although measuring tear thickness, we noted that the photographic biomicroscope and the OCT methods provided inconsistent results, and thus limited the reliability of our primary result.