How do we choose the optimal length of flexible and navigable suction ureteral access sheaths (FANS): an EAU endourology and AUSET prospective multicenter analysis.

PURPOSE: To evaluate how to assess the optimal length of flexible and navigable suction ureteral access sheaths (FANS) to be used during flexible ureteroscopy (FURS) for kidney stones. METHODS: A prospective multicenter study (16 centers, July 2024–January 2025) enrolled 226 adults with normal renal anatomy undergoing FURS with FANS for renal stones. Three preoperative measurements were analyzed: (1) Subjective on X-ray: T12–pubic symphysis, (2) Objective on CT: upper pole–pubic symphysis, and (3) Dynamic ureteral catheter length with retrograde pyelogram (RPG): upper pole–urethral meatus. Sheath length appropriateness (too short/correct/too long) was assessed intraoperatively using predefined criteria (complete calyceal access, ergonomics, need for ancillary techniques). Secondary outcomes included 30-day stone-free rates (SFR) and complications. RESULTS: Sheath length was deemed correct in 63.7%, too short in 9.7%, and too long in 26.5% of cases. For both genders, dynamic measurement of ureteric catheter length from upper pole calyx to 5 cm beyond the urethral meatus demonstrated the strongest correlation with optimal length (R = 0.7). Gender-specific formulas for optimal FANS length were derived: Male: 0.52 × ureteral catheter length + 26 cm. Female: 1.2 × ureteral catheter length − 2.1 cm. Compensatory techniques for length discrepancies included assistant-held stabilization of FANS (28.3%) and telescoping of penis (3.5%). 96% of sheaths accessed all calyces. The 30-day SFR was 92.5% (Grade A: 79.2% zero fragments; Grade B: 13.3% fragments ≤ 2 mm). Complications were low: sepsis not requiring ICU admission (0.4%) and ureteric stricture (0.6%). Only 2.7% patients were planned for reintervention. CONCLUSION: The selection of an optimal FANS length is critical for optimizing outcomes in FURS. Dynamic RPG-ureteral catheter length measurement best predicts this, enabling gender-specific formulas for easy estimation. Our results provide valuable insights into the clinical relevance of preoperative measurements, intraoperative adjustments, and the impact of sheath length on procedural success and complications.