Flexure and residual astigmatism with Polycon and polymethyl methacrylate lenses on toric corneas

Previous investigators have shown that thin polymethyl methacrylate (PMMA) contact lenses (less than or equal to 0.12 mm) flex on toric corneas, thereby altering the residual astigmatism. The present study was designed to evaluate the flexure and residual astigmatism of gas permeable Polycon contact lenses on toric corneas and compare the results with those obtained with PMMA lenses on the same corneas. Eight subjects (16 eyes) with corneal toricities ranging from 1.25 to 4.25 D wore four pairs of identical Polycon and PMMA lenses. The only lens parameter that varied was center thickness, which ranged from 0.07 to 0.16 mm. Flexure and residual astigmatism were measured on all corneas with all lenses in a double-masked fashion. Our results indicate that Polycon lenses undergo significantly more flexure and alter residual astigmatism more on toric corneas than PMMA lenses at all center thicknesses evaluated. For both materials, flexure and residual astigmatism increased as center thickness decreased. The clinical implications of these findings are discussed.