Validation of the SToP formula for calculating intraocular lens power in eyes with previous myopic excimer laser surgery

Purpose: To validate the accuracy of the SToP formula for calculating the intraocular lens (IOL) power from measurements obtained with a rotating Scheimpflug camera (Pentacam) in eyes with previous myopic excimer laser surgery. Setting: IRCCS - Fondazione Bietti, Rome, Italy. Design: Retrospective case series. Methods: Corneal curvature measurements were taken in eyes that had cataract surgery after myopic excimer laser surgery. Values from both versions of the SToP calculator (based, respectively, on the Holladay 1 and the SRK/T formulas) were compared with historical (Masket and Savini) and no-history methods (Barrett True-K, BESSt formula, equivalent K-readings, Triple-S, and Shammas). Results: Fifty eyes of 50 patients with historical data were analyzed. The SToP formula based on the SRK/T achieved the lowest median absolute error (MedAE) (0.31 diopters [D]) and a higher percentage of eyes (62%) with a prediction error in refraction within ±0.5 D. Excellent outcomes were also achieved with the Masket method, the BESSt formula, the Savini method, and the Triple-S method, which led to similar MedAEs (from 0.31 to 0.37 D) and an even higher percentage of eyes with a prediction error within ±0.5 D (from 64% to 76%). The SToP formula based on the Holladay 1 was slightly less accurate. Conclusions: Several methods based on Scheimpflug camera measurements were used to successfully calculate the IOL power in eyes with previous myopic excimer laser surgery. The SToP formula based on the SRK/T was one of the best choices.