Intraocular lens implant power calculations: investigations controlling for lens type.

Many second generation intraocular lens power calculation formulas have recently been introduced. This study explores the performance of these formulas while controlling for a potential source of variation--the lens type. For this study, all 1,157 cases studied used the Cilco CPLU posterior chamber lens. All surgeries were performed using similar phacoemulsification techniques by only two physicians (R.M.C. and S.C.G.). The SRK, SRK II, Holladay, and Binkhorst formulas were compared among themselves and also with a piece-wise nonlinear regression formula ("best fit") developed specifically from these data by the authors. Performance of the SRK II, Holladay, and best fit were better than the older SRK and Binkhorst for most axial length ranges. For these data, the Holladay and best fit formulas performed marginally better overall than the other formulas. It was also found that manipulation of specific surgeon constants significantly affected the performance of the Binkhorst formula, but had little effect on the other formulas.