Accuracy of the O Formula Based on OCT and Ray-Tracing for Intraocular Lens Power Prediction.

Purpose: To evaluate the predictive accuracy of the O formula, based on the ray-tracing method, compared with the latest formulas included in the ESCRS website.

Design: Retrospective consecutive case series at a single center.

Methods: Records of consecutive patients who underwent routine cataract surgery implanted with acrylic IOLs between August 2021 and December 2022 were retrospectively reviewed. The prediction accuracy of the O formula was compared with that of the Barrett Universal II (BUII), Emmetropia Verifying Optical 2.0 (EVO), Kane, Pearl-DGS formulas based on the website of ESCRS, and the SRK/T formula. Swept-source optical coherence tomography (SS-OCT)-based biometry and anterior segment SS-OCT were performed preoperatively, and manifest refraction was assessed 1-month postoperatively. Primary outcomes were root-mean-square absolute error (RMSAE), median absolute error (MedAE), and the proportions within ±0.50 D and ±1.00 D.

Results: A total of 325 eyes (258 patients) were included. The RMSAE of the O formula, BUII, EVO, Kane, Pearl-DGS, and SRK/T were 0.396, 0.468, 0.441, 0.449, 0.550, and 0.425, respectively. In GEE models, mean absolute error was significantly lower with the O formula than with BUII, EVO, Kane, and Pearl-DGS; no difference versus SRK/T was detected. The percentage of eyes within ±0.50 D was 78.7% for the O formula, 71.4% for BUII, 73.6% for EVO, 73.9% for Kane, 61.6% for Pearl-DGS, and 76.3% for SRK/T. The O formula achieved significantly higher percentages of eyes within both ±0.50 D and ±1.00 D of the predicted refraction compared to Pearl-DGS (P < 0.001).

Conclusions: The O formula demonstrated high refractive accuracy without A-constant optimization and performs favorably against widely used formulas. It can be used as a fail-safe in IOL power calculation, as it is less susceptible to prediction errors exceeding 1.0 D.