Corneal oxygenation during contact lens wear: comparison of diffusion and EOP-based flux models

The aim of this study was to compare corneal oxygen flux values derived from an oxygen diffusion model, with estimates from a model in which equivalent oxygen percentage (EOP) values were substituted for the post-lens tear film oxygen tension in Fick's law.

A previously described five-layer corneal oxygen diffusion model was found to artefactually allow theoretical oxygen consumption, when the predicted oxygen tension fell to zero. Consequently, an eight-layer diffusion model was constructed, with consumption set to zero at points within the cornea, where predicted oxygen tension falls to zero. Post-lens tear layer thickness was corrected to more contemporary estimates. The eight-layer and EOP-based anterior corneal oxygen flux estimates were compared across the range of commonly encountered contact lens Dk/t values.

The eight-layer model overcomes deficiencies in the five-layer model and provides predicted values that are remarkably similar to the EOP-based model. Open and closed eye anterior corneal oxygen flux in the absence of contact lens wear was estimated at 7.8 and 7.6 microL/cm2/hr for the open eye and 6.0 and 6.1 microL/cm2/hr for closed eye for the diffusion and EOP-based models, respectively.

The diffusion model supports the EOP model in that there is minimal oxygen benefit to be gained by increasing Dk/t above the Holden-Mertz criteria of 24 and 87 x 10(-9) (cm/sec)(ml0(2)/ml.mmHg) during open and closed eye wear, respectively. The eight-layer model is suitable for further definition of corneal oxygenation during contact lens wear.