Abstract
Purpose: :
To calculate corneal ATP generation from oxygen diffusion equations with and without contact lens wear.
Methods: :
A three dimensional corneal model of 11mm diameter was constructed from standard curvature and thickness values. Corneal epithelium was assumed to be a constant thickness of 0.05 mm and all change in thickness toward the periphery was assumed to occur in the stroma. One dimensional oxygen diffusion equations were used to determine partial pressure and thus consumption at individual positions across the cornea. Total oxygen usage was calculated by numerical integration across the surface. Anaerobic metabolism was assumed to be consistent, so all variation in ATP generation was calculated from variation in partial pressure of oxygen through the cornea. Total ATP generation was calculated for open and closed eye conditions with and without contact lens wear to yield daily estimates.
Results: :
The cornea was calculated to generate 6.8, 6.6 and 5.5 x 1020 molecules of ATP per day without contact lenses, with a –3.00D balafilcon A lens in continuous wear and with a –3.00D etafilcon A lens in daily wear, respectively.
Conclusions: :
This methodology provides a format for examining the degree of hypoxia during various contact lens wearing conditions, using the ultimate index, generation of high–energy phosphorylated nucleotides. Numerous assumptions are required but, as additional empirical data become available, further refinement of the model is possible. Critical ATP levels for normal corneal function are unknown; however, the estimates tend to support the notion that there is minimal hypoxia with silicone–hydrogels under continuous wear conditions, but that traditional hydrogel lenses do not provide adequate corneal oxygenation under daily wear conditions.
Keywords: cornea: basic science • contact lens • cornea: clinical science