Effects on IOL power calculation and expected clinical outcomes of axial length measurements based on multiple vs single refractive indices

Abstract

Purpose: To compare axial length measurements based on multiple specific refractive indices for each segment of the eye to those obtained using a single refractive index for the entire eye and to evaluate the subsequent effects on IOL power calculation. Setting: One site in Lynwood, CA. Design: Single-arm, non-interventional, non-randomized retrospective chart review. Methods: Eyes undergoing cataract surgery where biometry and IOL power calculations were based on axial length calculated with multiple specific refractive indices (multiple) were evaluated. A simulated axial length based on using a single refractive index was calculated for each case (single). The expected residual refractions based on different IOL formulas were calculated for both single and multiple groups. Formulas were then optimized, and the mean prediction errors (MPE) and mean absolute prediction errors (MAE) were calculated, based on the difference between the (optimized) expected value and the actual refractive outcome. Results: A total of 595 eligible eyes were evaluated. Differences between the axial lengths determined in the single and multiple groups ranged from +0.28 mm to −0.14 mm, with a significant correlation between the difference in AL and average AL (r2 = 0.73, p < 0.001). AL differences between groups were statistically significant in long and short eyes (p < 0.001) but not in average eyes or overall (p > 0.25). In nearly all cases, the average MPE in the multiple group was lower than that for the single group across all axial lengths and formulas. When larger differences in MAE were present, the multiple group results were more often lower (better). Conclusion: Differences were found between axial lengths calculated using a single refractive index and multiple refractive indices, mainly in the short and long eyes. Differences had some effect on IOL power calculation. Such effects may become increasingly important as the precision of formulas increases.