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

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. 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. 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. 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.