A Direct Method for Determining Toricity Ratios of Toric Intraocular Lens Calculators.

This study develops a method to determine toricity ratios used by arbitrary toric intraocular lens calculators. Access to this information allows for the improvement of refractive results. We derive the Sayegh-Gabra formula, which uses input and output parameters in a toric calculator to extract toricity ratios that are typically not disclosed. We illustrate the method on a number of commercial calculators. For each calculator, high, average, and low axial length values are crossed with high, average, and low mean corneal power values to generate a 3 × 3 matrix. A toricity ratio is generated for each axial length and mean corneal power pair. We thus identify several toric lens manufacturers' calculators that use a constant toricity ratio, often 1.46. Some others use a variable ratio centered at 1.46, but varying as axial length and mean K increases over a range of values corresponding to physiological myopia and hyperopia. There is an emerging trend away from constant toricity ratios. Using our methodology, it is possible to extract the toricity ratio used by specific calculators/manufacturers, distinguish those using constant versus variable toricity ratios, and use this information to improve surgical outcomes by refining current and future toric intraocular lens calculators.