Introduction: To compare the effectiveness and safety of small- and standard-caliber semirigid ureteroscopes for ureteral stones and evaluate whether caliber correlates with intraoperative surgeon stress.

Material and Methods: In this prospective randomized study, 130 patients underwent ureterorenoscopy using either a 4.5–6.5 Fr (thin) or 8–9.8 Fr (standard) semirigid ureteroscope. The primary endpoint was the stone-free rate (SFR); secondary endpoints included perioperative outcomes and surgeon stress. Stress was assessed using the State–Trait Anxiety Inventory Form TX-1 (STAI-TX1) before and after each procedure, with increased stress defined as ΔSTAI-TX1 > 0. Multivariable logistic regression identified independent predictors of increased stress.

Results: Baseline characteristics were comparable. SFRs did not differ significantly between the thin- and standard-caliber groups (92.3% vs. 86.2%, p = 0.25). Balloon dilation (32.3% vs. 4.6%, p < 0.01), ureteral injury (21.5% vs. 4.6%, p = 0.004), and JJ stent placement (46.2% vs. 26.2%, p = 0.018) were more frequent in the standard-caliber group. Increased surgeon stress occurred more often with standard-caliber ureteroscopes (27.7% vs. 9.2%, p = 0.007). Multivariable analysis confirmed that standard-caliber use was independently associated with increased stress (adjusted OR 3.11, 95% CI 1.09–8.84, p = 0.033), which remained significant in a Firth penalized sensitivity analysis (adjusted OR 2.94, 95% CI 1.06–8.17, p = 0.037).

Conclusion: Small- and standard-caliber ureteroscopes showed no significant difference in SFRs, but standard-caliber instruments were associated with higher rates of balloon dilation, ureteral injury, and JJ stent placement, as well as increased surgeon-reported stress. Ureteroscope caliber may influence both technical outcomes and the surgeon's intraoperative experience, although stress findings remain exploratory.