A New Theoretical Formula for the Intraocular Lens Power Calculation by the Ray Tracing Method

Abstract

To study the mathematical properties of the intraocular lens(IOL) power calculation with the Ray Tracing method and to investigate its clinical results. We first obtained a ray focusing equation with the Ray Tracing method, and then derived an emmetropizing equation and an equation for postoperative refraction. The exact calculation of these equations was investigated. A new IOL power calculation formula was consequently developed and named "formula H", combining it with the predictive formula for the postoperative anterior chamber depth of pseudophakic eyes in another report by us. A series of 150 phacoemulsificated eyes with capsulorhexis and in-the-bag fixation of IOL were studied. The cases were divided into 3 groups; 16 short eyes(< 22 mm), 103 normal eyes(22 < or = and < 24.5 mm), and 31 long eyes(> or = 24.5 mm). The predictive error of formula H was compared with that of SRK-T, Holladay, and SRK II in each group. The exact calculation of emmetropizing power in taking account of a IOL design was mathematically impossible, or generally difficult. In the latter case, however, we could calculate it by solving a cubic or quartic equation. Exact and easy predictive calculation of postoperative refraction for any IOL design was possible if we knew such data as anterior and posterior curvature and thickness of each power of IOL. The mean absolute error of formula H was 0.38, 0.34, 0.36, 0.44 diopter, in all eyes, short eyes, normal eyes, and long eyes, respectively. Formula H was significantly more accurate than all other formulas in all eyes, in short eyes and normal eyes. For clinical use, predictive calculation of postoperative refraction should be recommended but calculation of emmetropizing power should be abandoned. But emmetropizing calculation is useful and important for the fundamental study of IOL power calculation. Formula H was more accurate than all other formulas and very useful for clinical use.