Optimizing the Scope–Sheath Compatibility in RIRS: Matching of Reusable and Single-Use Flexible Ureteroscopes with FANS

<b>Background/Objectives</b>: Adoption of single-use ureteroscopes (SU) and flexible and navigable suction ureteral access sheaths (FANS) have improved flexible ureteroscopy (fURS) efficiency and safety. However, the impact of scope-sheath pairing is less studied. This study aims to compare four scope-sheath configurations using reusable ureteroscopes (RU) and SU with either 11/13Fr or 12/14Fr FANS. <b>Methods</b>: We retrospectively evaluated 184 patients undergoing fURS for kidney solitary stones of 10-25 mm. Patients were manually matched across four groups: RU-11/13FANS, RU-12/14FANS, SU-11/13FANS, and SU-12/14FANS (46 patients in each). The endpoints were 30-day stone-free rate (SFR), operative time, surgeon-reported visibility (image clarity and procedural continuity) and postoperative complications. <b>Results</b>: Operative time was significantly shorter in single-use scope groups (<i>p</i> < 0.001). Visibility scores were highest in SU-12/14FANS and lowest in RU-11/13FANS across all subdomains. SFR was higher in SU groups. SU-11/13FANS had a significantly higher SFR than RU-12/14FANS (<i>p</i> = 0.027). In the reusable group, the use of 12/14Fr FANS was associated with a lower SFR compared to the 11/13Fr configuration. Complication rates were low (8.2% overall) and comparable among groups. <b>Conclusions</b>: Pairing SU with 12/14Fr FANS provided optimal visibility and good stone clearance without increasing complications. In contrast, RU paired with 12/14Fr FANS showed slightly reduced efficacy, possibly due to impaired maneuverability. The scope-sheath interaction influenced outcomes differently across scope types, underlining the importance of their matching in fURS.