Transrectal Biplane Ultrasound Combined with Cotton Swab Marking of the External Urethral Orifice for Evaluating the Postoperative Position of Transobturator Mid-Urethral Sling: A Study on Standardized Ultrasound Measurement and Inter-Observer Reliab

Objectives: To assess the technical feasibility and interobserver reliability of transrectal biplane ultrasound with external cotton-swab marking of the urethral orifice after transobturator mid-urethral sling surgery, and secondarily to describe dynamic measurements and exploratory associations with chart-derived postoperative leakage. Methods: In this single-center retrospective cross-sectional imaging study, 112 consecutive women examined between January 2024 and February 2026 underwent a standardized protocol. The swab tip was placed lightly against, but not inserted into, the external urethral orifice. Sling width (W), distance from the sling upper edge to the internal urethral orifice (D-I), and distance from the sling lower edge to the external urethral orifice at rest (D-O) and during Valsalva (V-D-O) were measured. Forty randomly selected examinations were independently measured by two blinded readers. Agreement was evaluated using ICC(2,1), Bland–Altman bias and 95% limits of agreement (LoA), standard error of measurement (SEM), and minimal detectable change (MDC95). Missingness patterns and age-adjusted exploratory logistic models were examined. Results: W, D-I and D-O were measurable in all 112 women; V-D-O was available in 95 (84.8%). Postoperative leakage status was available in 101 women (62 without and 39 with leakage). V-D-O was shorter than D-O in paired examinations (median difference, −3.00 mm; 95% CI, −4.90 to −1.00; p < 0.001). ICCs were 0.845 for W, 0.942 for D-I, 0.936 for D-O and 0.970 for V-D-O. Bland–Altman biases (reader A minus reader B) were −0.18, −0.93, 0.70 and −0.23 mm, respectively; corresponding MDC95 values were 1.53, 2.96, 3.72 and 2.33 mm. Missing V-D-O was associated with a shorter postoperative interval after Holm correction. None of six age-adjusted ultrasound models was associated with chart-derived leakage after Holm correction. Conclusions: The protocol permitted highly available static measurements and good-to-excellent interobserver reliability. Because the study lacked an unmarked comparator, intraobserver repeats and a standardized clinical endpoint, it supports technical feasibility and measurement reliability, not superiority, diagnostic validity or prognostic utility.