Many factors have been demonstrated to influence flexure of rigid contact lenses, but the contributions of surface tension and eyelid forces to flexure are not well understood.
We placed lenses on a model eye consisting of a polymethyl methacrylate (PMMA) base which could be flexed, and measured resultant flexure with a videokeratoscope. We varied the base toricity, sequence of measurement, and lens base curve. The effects of evaporation of the postlens fluid were also observed.
Clinically significant flexure (>0.50 D) occurred when two conditions were met: (1) the volume of the postlens space would increase if the lens unflexed, and (2) there was a paucity of fluid available to fill that space. Flexure was minimal (≥0.50 D) when ample fluid was present.
Surface tension forces serve more to maintain rather than create rigid lens flexure. Our model helps to explain why steep-fitting lenses flex more and leads to several predictions for flexure, which appear generally to be obeyed.