External Validation of the S.T.O.N.E. Score in Predicting Stone-Free Status After Rigid Ureteroscopic Lithotripsy

BACKGROUND: The Size, Topography, Obstruction, Number, and Evaluation of Hounsfield units (S.T.O.N.E.) scoring system has been proposed as a novel prognostic surgical classification for urolithiasis in predicting success rate and complications. OBJECTIVE: We carried out an externally validated S.T.O.N.E. score on rigid ureteroscopic lithotripsy (rURS). MATERIALS AND METHODS: The data of patients who had undergone rURS between 2012 and 2019 at a tertiary referral center were audited retrospectively. The S.T.O.N.E. score was calculated based on factors determined through preoperative computed tomography images and was analyzed in association with stone-free rate (SFR), operating time, surgical complications, and length of stay (LOS). RESULTS: A total of 155 patients were included in the study with a median stone size of 10 mm (7–12) and a median S.T.O.N.E. score of 9 (8–10). The overall SFR was 89.68%. SFRs were 100.0%, 97.83%, and 77.42% in low (5), moderate (6–9), and high (10–13) score groups, respectively. The S.T.O.N.E. score (p = 0.002) and stone size (p = 0.037) were predictive factors for SFR in multivariate analysis. Moreover, there was a significant correlation between the S.T.O.N.E. score and operative time, LOS, and presence of complications (r = 0.22, p = 0.006; r = 0.30, p < 0.001; and r = 0.27, p < 0.001, respectively). The area under the curve of the receiving operator characteristics’ curve for the S.T.O.N.E. score was 0.815. CONCLUSION: The S.T.O.N.E. scoring system is simple and effective in predicting postoperative outcomes; therefore, this score would be a valuable tool in clinical planning for every patient who undergoes rURS.