Influence of the Corneal Posterior-to-Anterior Radius Ratio on Intraocular Lens Power Prediction Using Standard and Total Keratometry.

PURPOSE: To compare the predictive accuracy of standard keratometry (K) and total keratometry (TK) for intraocular lens (IOL) power calculation across eyes stratified by the posterior-to-anterior corneal radius ratio (PAR).

DESIGN

Retrospective evaluation of IOL power calculation accuracy.

METHODS

A total of 1314 eyes of 1314 patients who underwent uneventful phacoemulsification with monofocal IOL implantation were enrolled. Preoperative biometry was performed with the IOLMaster 700 (Carl Zeiss Meditec). Eyes were divided into three subgroups according to the PAR: low (<25th percentile), medium (25th-75th percentiles), and high (>75th percentile). Refractive prediction errors (PEs) were calculated with the Barrett Universal II (BUII) and Kane formulas, using both K and TK values. Main outcome measures were PE, median absolute error, and the proportions of eyes within predefined absolute PE thresholds.

RESULTS

PAR was independently associated with the discrepancy between K- and TK-based PEs in both the BUII and Kane formulas (both P < .001). Compared with standard K, TK produced a consistent myopic shift in eyes with low PAR (-0.10 D for both formulas; both P < .001) and a hyperopic shift in eyes with high PAR (+0.14 D for both formulas; both P < .001), whereas only minimal changes were observed in eyes with medium PAR. Despite these systematic directional shifts, median absolute error remained comparable between K- and TK-based calculations across all PAR subgroups. In the high PAR subgroup, incorporation of TK significantly reduced the proportion of eyes within ±0.50 D of target refraction for the BUII formula (79.9% vs 75.4%, P = .041), whereas no significant difference was observed for the Kane formula.

CONCLUSIONS

PAR is an independent factor associated with the discrepancy between K- and TK-based refractive predictions. Rather than uniformly improving refractive accuracy, TK primarily induces systematic, PAR-dependent directional refractive shifts. These findings suggest that PAR may serve as a clinically useful biomarker for selecting between K- and TK-based calculations instead of supporting the routine use of TK in all eyes.