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Errors in IOL Power Study Methodology

We read with interest the article by Rodriguez-Raton et al (1) regarding the effect of pupil dilation on optical biometry measurements and especially its effect on anterior chamber depth (ACD) and its influence on the accuracy of the Haigis formula, which depends on the preoperative measurement of the ACD (epithelium to lens). In their description of their methodology, they explain that not only do they assess the correct prediction error, which is the difference between the formula’s prediction of the postoperative refractive error and the actual refractive error obtained, but also assess what they call the “lowest myopic sphero-equivalent residual refraction.” The latter is unnecessary and incorrect. They base their results on the change in selected intraocular lens (IOL) power implanted rather than on the median absolute error of each formula after the mean error has been zeroed out by adjusting the lens constant for the individual formula, which is required in any formula accuracy study. The authors state, “Stock IOL are manufactured in 0.5-D intervals for most ranges so ideal IOL power is not generally available and an approximation must be made.” But this is true in every IOL power study ever done and is of no consequence when comparing accuracy of formulas. In their Introduction, they mention the errors of applanation contact ultrasound compared to the immersion method, but do not reference the article published by Shammas (2) that first pointed out this fact. Regarding the testing of the Haigis formula (3), it is wellknown that tropicamide dilates the pupil, which is usually associated with a posterior movement of the pupillary complex, thus producing a deepening of the ACD. The Haigis formula makes direct use of the preoperative ACD as a predictor of the effective lens position (ELP) used in the following formula: