Which feature influences on-eye power change of soft toric contact lenses: Design or corneal shape?

Purpose This study investigates how both the peripheral zone design and corneal shape affect the behaviour of soft contact lenses on-eye. Methods In this study, soft contact lenses of varying nominal cylindrical powers and peripheral zone designs-a single-prism gravity-based stabilised lens (G1P), two-prism blink-based stabilised lens (B2P) and four-prism blink-based stabilised lens (B4P)-were generated as finite element models. The on-eye simulation results were analysed to identify the impact of each peripheral zone design (Each with different volume ratios) on the effective power change (EPC) when worn by a subject. Topographies of three eyes of varying average simulated anterior corneal curvature (flat, average & steep) were used in this study. Results The volume of the lens's peripheral zone as a ratio of the total lens volume (V-p) recorded very weak correlations with the effective power change (EPC) among the three investigated designs when they were fitted to the flat eye (R = -0.19, -0.15 & -0.22 respectively), moderate correlations with the average eye (R = 0.42, 0.43 & 0.43 respectively) and strong correlations with the steep eye (R = 0.91, 0.9 & 0.9 respectively). No significant differences were noticed among the three investigated designs and none of the cylindrical lenses designed with axis 90 degrees recorded EPC values outside the acceptance criteria range (ACR) of +/- 0.25 D. No significant differences in EPC were recorded among the three designs G1P, B2P and B4P (p>0.6) when they were designed with three axes at 90 degrees, 45 degrees and 0 degrees. Moving the toric lens axis away from 90 degrees dragged the EPC to the negative side and most of the investigated lenses with axes at 45 degrees and 0 degrees recorded EPCs outside the +/- 0.25D range. Conclusions In all cases, the shape of the cornea had a more dominant effect on EPC when compared to the peripheral zone design. Corneal shape influences the soft toric contact lens's on-eye power change more than the lens design.