Outcomes of zero power intraocular lenses: insights into the keratometric index and axial length.

Abstract

Purpose: To analyze intraocular lens (IOL) power calculation in a series of patients with a zero diopter power implant to bypass the estimated lens plane, an important variable in normal IOL calculation.

Setting: Large multicenter group practice.

Design: Retrospective, observational study.

Methods: A consecutive series of patients undergoing cataract surgery after optical biometry and implantation of a zero (0.0 diopter [D]) power IOL were studied. Complete biometry was performed with an optical low coherence reflectometry biometer. The predicted refraction was calculated with and without recalibrating the axial length (AL) using a sum-of-segments (SOS) approach with the Hoffer Q, Holladay 1, SRK/T, Haigis, Barrett Universal II, and Olsen formulas. A ray-tracing model to back-calculate the corneal power from the observed postoperative refraction and AL was also included.

Results: In 52 eyes of 52 patients, mean prediction errors for the Hoffer Q, Holladay 1, SRK/T, Haigis, Barrett Universal II, and the Olsen formulas were +1.48 D, +1.15 D, +0.91 D, +0.65 D, +0.15 D, and +0.11 D, respectively using standard AL, and after SOS correction of the AL, the mean errors changed to +1.08 D, +0.75 D, +0.52 D, +0.26 D, -0.25 D, and -0.27 D, respectively. The mean back-calculated corneal power was 1.25 D lower than the mean keratometer reading using the standard keratometric index of 1.3375 and corresponded to an effective index of 1.328.

Conclusions: The hyperopic error commonly seen in highly myopic eyes with classic thin-lens formulas is only partly corrected by SOS recalibration of the AL. We suggest the source of the residual error is an incorrect keratometric index.