Flexible ureteroscopy across calyceal locations: impact of ureteroscope caliber and lower-pole anatomy on stone-free outcomes

Abstract Background This study compared the clinical performance of 7.5 F and 9 F digital flexible ureteroscopes in sheathless retrograde intrarenal surgery (RIRS), with a particular focus on how ureteroscope caliber interacts with lower-pole anatomy to influence stone-free outcomes. Methods This retrospective study included patients who underwent sheathless RIRS for single renal stones measuring 10–20 mm between October 2023 and August 2025 using either a 7.5–9 F digital flexible ureteroscope. All procedures were performed by three experienced surgeons under a standardized protocol. Stone-free status (SFR) was defined as residual fragments ≤ 4 mm on 1-month non-contrast CT. Perioperative parameters and complications were compared between groups. A prespecified lower-pole subgroup analysis examined the relationship between IPA and clearance using logistic regression and ROC analysis. Results Seventy patients were analyzed (7.5 F = 37; 9 F = 33). Overall SFRs were 78.4% and 69.7% for the 7.5 F and 9 F groups, respectively (p = 0.42). Complication rates were low and comparable. In the lower-pole subgroup, the 7.5 F scope achieved a significantly higher SFR (64.7% vs. 23.1%, p = 0.03). Multivariable analysis identified ureteroscope caliber (adjusted OR = 5.76, p = 0.036) and IPA (adjusted OR = 0.92 per degree, p = 0.017) as independent predictors of lower-pole clearance, with moderate discriminative ability for IPA (AUC = 0.682). Conclusion In sheathless RIRS, smaller-caliber ureteroscopes were associated with higher stone-free rates for lower-pole stones, while overall outcomes were comparable between groups. Lower-pole anatomy was significantly associated with stone-free outcomes.