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

Abstract

Purpose: 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.

Methods: 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.

Results: 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 μL/cm²/hr for the open eye and 6.0 and 6.1 μL/cm²/hr for closed eye for the diffusion and EOP‐based models, respectively.

Conclusions: 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 times 10⁻⁹ (cm/sec) (ml0₂/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.

Key words:

• contact lenses
• models
• oxygenation
• oxygen flux