Impact of Residual Stone Fragments on Risk of Unplanned Stone Events Following Percutaneous Nephrolithotomy

<b><i>Purpose:</i></b> Residual stone fragments are common after percutaneous nephrolithotomy (PCNL), however, there is an unclear relationship between the presence of these residual stone fragments and the risk of unplanned stone events (USE). We investigated how the size of the largest residual stone fragment (LRSF) impacts the risk of USE post-PCNL. <b><i>Materials and Methods:</i></b> We conducted a retrospective cohort study of PCNL between 2018 and 2022. Preoperative computed tomography of the abdomen and pelvis (CTAP) imaging and postoperative CTAP imaging were reviewed. The primary outcome was the incidence of postoperative USE, defined as stone-related emergency department visits or unplanned stone procedures. LRSF were stratified by five thresholds (no postoperative stone fragments <i>vs</i> ≥0 mm, <2 <i>vs</i> ≥2 mm, <3 <i>vs</i> ≥3 mm, <4 <i>vs</i> ≥4 mm, and <5 <i>vs</i> ≥5 mm), and Cox regression was used to compare the impact of these thresholds on the risk of USE. <b><i>Results:</i></b> After exclusions, we identified 138 patients who underwent PCNL during the study period; 42 patients had a USE. When applying a 4 mm threshold, 52% of patients with LRSF ≥4 mm experienced USE <i>vs</i> 21% with LRSF <4 mm (log-rank <i>p</i> = 0.0004); similarly, with a 5 mm threshold, 62.5% with LRSF ≥5 mm had such events compared with 21% with LRSF <5 mm (log-rank <i>p</i> < 0.00001). A larger LRSF was associated with a greater risk of having a USE. <b><i>Conclusion:</i></b> After PCNL, larger residual stone fragments, particularly those ≥4 mm, are associated with an increased risk of USE. These results demonstrate the value of identifying residual fragments in predicting USE.