Efficacy and Safety of 6.3 Fr Versus 7.5 Fr Single-Use Flexible Ureteroscopes for Upper Urinary Tract Stones: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

<i>Background and Objectives</i>: Retrograde intrarenal surgery (RIRS) is a cornerstone in managing upper urinary tract stones, with a growing trend towards instrument miniaturization. The introduction of the ultra-slim 6.3 Fr single-use flexible ureteroscope presents a potential advancement over the standard 7.5 Fr device, but clinical evidence remains scarce. This systematic review and meta-analysis aims to synthesize data from randomized controlled trials (RCTs) to compare the efficacy and safety of the 6.3 Fr versus the 7.5 Fr ureteroscope. <i>Materials and Methods</i>: A systematic search of PubMed, Scopus, CENTRAL, and Web of Science was conducted for RCTs published up to September 2025. The primary outcomes were the stone-free rate and procedural success rate. Secondary outcomes included operation duration and postoperative complications. Risk ratios (RRs) for dichotomous data and mean differences (MDs) for continuous data, with 95% confidence intervals (CIs), were pooled for analysis. <i>Results</i>: Three RCTs involving 140 patients were included. There was no significant difference between both groups in stone-free rates (RR: 1.06, 95% CI [0.96, 1.18], <i>p</i> = 0.22) or success rates (RR: 1.06, 95% CI [0.97, 1.16], <i>p</i> = 0.17). However, the 6.3 Fr ureteroscope was associated with a significantly shorter operation duration (MD: -6.66 min, 95% CI [-11.29, -2.03], <i>p</i> < 0.001). No significant differences were found in laser operating time (MD: -1.46 min, 95% CI [-3.93, 1.01], <i>p</i> = 0.25), length of hospital stay (MD: -0.09 days, 95% CI [-0.23, 0.05], <i>p</i> = 0.19), or postoperative complications (Clavien I: RR 0.86, 95% CI [0.30, 2.43], <i>p</i> = 0.77; Clavien II-III: RR 0.67, 95% CI [0.12, 3.84], <i>p</i> = 0.65). <i>Conclusions</i>: Based on low-certainty evidence, the 6.3 Fr ureteroscope does not significantly improve stone-free rates but may reduce overall operation duration compared to the 7.5 Fr scope, with a comparable safety profile. These findings are limited by the small number of available studies, highlighting a clear need for larger, high-quality RCTs to confirm these preliminary results.