Maximum Stone-Edge Angle (MSEA): A Novel Approach for Predicting Retrograde Intrarenal Surgery Success in Patients with a Single Lower Pole Stone

Abstract Background Lower pole kidney stones remain challenging to treat with retrograde intrarenal surgery because of unfavorable pelvicalyceal anatomy. Several angle measurements were proposed to quantify this complexity, but they are not fully standardized and show inconsistent predictive value. Additionally, these measurements do not reflect the maximum deflection of the flexible ureteroscope required to reach the entire stone, an important technical challenge during surgery. To address this limitation, we introduced the maximum stone-edge angle (MSEA), assessed its predictive value, and compared it with existing angle measurements. Methods We retrospectively reviewed 116 patients who underwent retrograde intrarenal surgery for a single lower pole stone ≤ 2 cm between January 2023 and March 2026. Computed tomography was used to measure the infundibulopelvic angle, pelvic stone angle, MSEA, infundibular length and width, stone length, volume and density. Stone-free status was defined as no or ≤ 2 mm residual stone. Continuous variables were compared using the independent-samples t test or Mann–Whitney U test, and categorical variables using Pearson’s chi-square test. Discriminative ability was assessed using ROC analysis, and an optimal cut-off value was determined when significant. Univariable and multivariable binary logistic regression analyses were performed to identify predictors of stone-free status. Results Stone-free status was achieved in 76 of 116 patients (65.5%). MSEA was significantly higher in stone-free than non-stone-free patients (29.25° vs 24.50°, p < 0.001), while infundibulopelvic and pelvic stone angles did not differ significantly. MSEA showed significant discrimination for stone-free status (AUC 0.725, 95% CI 0.614–0.836; p < 0.001). The optimal cut-off was 25.75°, with 81.6% sensitivity and 65.0% specificity. In multivariable models, MSEA ≥ 25.75° independently increased the likelihood of stone-free status when stone burden was represented by maximum stone length (OR 9.641, 95% CI 3.483–26.688; p < 0.001) or stone volume (OR 9.107, 95% CI 3.486–23.790; p < 0.001). Greater maximum stone length and stone volume were each independently associated with a lower likelihood of stone-free status in separate models. Conclusions MSEA may provide a simple, stone-oriented anatomical measure that better reflects the deflection required to access the entire lower pole stone than conventional angle measurements. Further validation and reproducibility assessment may support its future clinical use.