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

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.