A model of oxygen flux through contact lenses

Although oxygen transmissibility has been a favored index to describe the physiologic performance of contact lenses, it has been maintained by some that the flux through a contact lens would be a more useful guide. Here, a model is described that allows contact lens oxygen flux to be estimated under open and closed eye wearing conditions.

The equivalent oxygen potential (EOP) was used to approximate the oxygen concentration beneath a contact lens. A logarithmic relation between corneal oxygen consumption and this oxygen level was substituted into Fick's Law to provide a mathematical model. Paired data of EOP and oxygen transmissibility (Dk/t), from a previous empiric derivation, were entered into a nonlinear regression analysis of this model.

The modelling procedure produces a good fit to the selected data. The estimated maximum flux during open eye wear is 7.5 microL/cm2 x h, consistent with previous determinations. Error estimates increased from 0 to 0.55 microL/cm2/h at Dk/t values of 0 and 200 x 10(-9) Barrer/cm, respectively, for the open eye.

This study provides a workable model for estimating the oxygen flux through contact lenses. Varying the underlying relation between the oxygen tension beneath a lens and the oxygen flux produces minimal variation to the result. The model has a number of clinical applications, such as demonstrating the advantages of highly transmissible contact lenses and the limits to which increasing oxygen transmissibility can alter the corneal physiologic environment.