Refining the Predictive Accuracy of Membranous Urethral Length for Post-Prostatectomy Incontinence: A Standardized Approach in the Korean Population

Background/Objectives: Preoperative membranous urethral length (MUL) is a predictor of post-prostatectomy urinary incontinence. However, measurement inconsistencies have hindered the establishment of ethnically specific clinical thresholds. We identified precise and internally validated MUL cutoff values for persistent incontinence at 6 and 12 months in a Korean cohort using a standardized measurement protocol. Methods: We retrospectively analyzed 151 patients who underwent robot-assisted radical prostatectomy (RARP) between 2022 and 2024. Preoperative MUL was measured using a 3-axis cross-reference system (CRS) on 3.0T mpMRI. Continence was defined as 0 or 1 safety pad per day. Independent predictors were identified via multivariable logistic regression, and the optimal cutoff values were determined using the Youden index with 1000-iteration bootstrap validation. Results: Preoperative MUL was significantly longer in continent than in incontinent patients at 6 (16.7 vs. 13.7 mm) and 12 months (16.7 vs. 11.3 mm; both p < 0.001). In the multivariable analysis, MUL was the only significant independent predictor for persistent incontinence (6 months: OR 0.798, p < 0.001; 12 months: OR 0.586, p < 0.001). The univariable AUROCs for predicting persistent incontinence were 0.707 (6 months) and 0.875 (12 months), whereas the multivariable AUROCs were 0.756 and 0.883, respectively. Optimal cutoff values from bootstrap were 14.00 mm (95% confidence interval [CI], 11.43–16.68) for 6-month and 12.60 mm (95% CI, 11.43–14.37) for 12-month persistent incontinence. Conclusions: Using a standardized CRS protocol, this study provides validated population-specific MUL thresholds for predicting persistent incontinence after RARP in Korean men, offering a pragmatic framework for preoperative risk stratification and evidence-based patient counseling.