Enhancing RGP contact lens performance: comparing back surface options.

Background: In some cases, rigid gas-permeable (RGP) contact lenses may be the best--or only--means of refractive correction. High Dk RGP materials have markedly reduced hypoxia under those lenses. With aspheric lens back surface designs, post-lens circulation may be enhanced as well by maximizing the provision of nutrients and the clearance of metabolic by-products, toxins, and debris, while minimizing the risk of lens binding.
Methods: Performances of four aspheric back surface RGP designs were compared with a spherical optic zone and peripheral curve back surface reference design. Oxygen uptake rates were measured for each of 40 wearing combinations (five lens designs fitted to each of eight eyes) under non-blink and blink conditions for their effectiveness in reduction of post-lens hypoxia. Hypoxic reductions with blinking were scaled in exchange efficiency (EE) units, and used as relative indicators of post-lens tear exchange.
Results: Combined exchange efficiency index (EE) scores (averaged responses across all eight eyes) for each of the five lens back surface designs ranged from a best overall performance of +13.9 EE units down to only +2.5 EE units, with the spherical reference design averaging +8.9 EE units. Tear pump efficiency of each of the eight eyes (averaged responses across all five lens designs) ranged from a high of +12.3 EE units down to -9.8 EE units (i.e., worse than the non-blink condition of 0 EE units). Among the 40 eye-lens back surface design combinations studied here, the highest exchange efficiency score registered was +28.4 EE units, the lowest being -13.8 EE units.
Conclusions: Aspheric lens back surface and/or peripheral curve designs were found to vary significantly in their post-lens exchange efficiency performances, but no "universal problem-solver" design was found among the five we investigated.