Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses

Abstract

The relationship between corneal edema and hydrogel lens oxygen transmissibility was examined for both daily and extended contact lens wear by measuring the corneal swelling response induced by a variety of contact lenses over a 36-hr wearing period. The relationships derived enable average edema levels that occur with daily and extended wear in a population of normal young adults to be predicted to within +/- 1.0%. The critical lens oxygen transmissibilities required to avoid edema, for the group as a whole, for daily and extended contact lens wear were obtained from the derived curves. It was found under daily wear conditions that lenses having an oxygen transmissibility of at least 24.1 +/- 2.7 X 10(-9) (cm X ml O2)/(sec X ml X mmHg), an Equivalent Oxygen Percentage (EOP) of 9.9%, did not induce corneal edema. This level of oxygen transmissibility can be achieved (1) in standard, low water content, poly-HEMA lenses by using an average lens thickness of 33 microns or less, or (2) in a higher water content material, such as Duragel 75, by using an average thickness of 166 microns or less. The critical hydrogel lens oxygen transmissibility needed to limit overnight corneal edema to 4% (the level experienced without a contact lens in place) was found to be 87.0 +/- 3.3 X 10(-9) (cm X ml O2)/(sec X ml X mmHg)--an EOP of 17.9%. This ideal level of oxygen transmissibility cannot, at present, be provided with hydrogel materials.(ABSTRACT TRUNCATED AT 250 WORDS)