Nephrolithiasis remains a prevalent condition with increasing global incidence. Retrograde intrarenal surgery (RIRS) is commonly employed for lower pole stones, although success rates vary based on factors such as stone burden, density, and lower pole anatomy. Traditionally, the infundibulopelvic angle (IPA) has been used as a predictor of stone-free rates post-RIRS. However, the pelvic stone angle (psa) has recently emerged as a potential alternative, offering enhanced predictive relevance. This study seeks to validate psa as an external predictor for stone-free outcomes in RIRS and to compare it with IPA, RIRS, and the Resorlu-Unsal Score (RUSS) in a clinical setting.

We conducted a retrospective analysis of 111 patients with lower pole stones treated with RIRS at Seoul National University Hospital between January 2022 and January 2024. All procedures were performed by a single experienced urologist (>1000 RIRS cases). Each patient was reviewed three times by five urologists, with averaged PSA, IPA, RIRS, and RUSS scores utilized for analysis. Stone-free status was confirmed using postoperative non-contrast CT imaging within three months. Statistical analyses included univariate logistic regression, with model fitting evaluated through Area Under the Curve (AUC).

Out of 111 patients, 93.6% were stone-free at 1 month post-op. Stone-free patients showed a higher mean PSA (37.03° ± 11.30) compared to non-stone-free patients (31.14° ± 9.54). However, the difference was not statistically significant (p=0.182). IPA was also higher in stone-free patients (62.14° ± 16.66) versus non-stone-free patients (48.98° ± 9.80) which was statistically significant (p=0.042). No significant difference was also observed between stone-free and non-stone free patients in terms of RIRS (p=0.409). On the other hand, RUSS was significantly lower in stone-free patients (1.7 ± 0.66) versus non-stone-free patients (2.4 ± 0.49) (p=0.015). The predictive performance of PSA was: sensitivity= 0.81, specificity= 0.57, AUC=0.69, Odds Ratio (OR)=1.1 95%CI 0.97-1.16, p=0.185. The predictive performance of IPA was: sensitivity= 0.62, specificity= 0.86, AUC=0.74, Odds Ratio (OR)=1.1 95%CI 1.00-1.13, p=0.047. The predictive performance of RIRS was: sensitivity= 0.30, specificity= 0.71, AUC=0.51, Odds Ratio (OR)=0.6 95%CI 0.20-1.92, p=0.409. The predictive performance of RUSS was: sensitivity= 0.11, specificity= 0.71, AUC=0.41, Odds Ratio (OR)=0.2 95%CI 0.06-0.79, p=0.021. No significant difference in predictive performance was observed between PSA and IPA (p=1.000).

In this external validation study, PSA and IPA demonstrated comparable predictive abilities, with RUSS showing greater potential utility for predicting stone-free outcomes in RIRS. These results support the continued use of RUSS and suggest psa may serve as an alternative to IPA in specific clinical cases.