RESUMO
Objectives: To assess the association of skin-to-stone distance (SSD) and stone-free rates following extracorporeal shockwave lithotripsy (SWL) using two statistical methods: logistic regression and a matched-pair analysis approach. Patients and Methods: Patients with a solitary radio-opaque upper ureteral calculus diagnosed on noncontrast computed tomography were included. Patients were treated with a Sonolith I-Sys Lithotripter (focal depth 17 cm). Stone treatment success was defined as stone free (fragments ≤3 mm) at 3 months. Failure was defined as persistent fragments beyond 3 months or requirement for intervention with ureteroscopy. The outcome was assessed by a plain kidney, ureter, and bladder radiograph (KUB) at 2 weeks. Logistic regression was used to determine association of patient and stone factors with treatment failure. The patient cohort was divided into tertiles by SSD, and matched-pair analysis was undertaken between individuals from the top and bottom tertiles (SSD ≤12 cm and SSD ≥14 cm). Matching criteria consisted of age, sex, maximum stone diameter (±2 mm), and stone density (±250 HU). Results: From a database of 2849 patients who underwent SWL, 397 patients were identified who had treatment of a single upper ureteral stone. Age (odds ratio [OR]: 1.03, 95% confidence interval [CI]: 1.01-1.04, p = 0.007), SSD (OR: 1.16, 95% CI: 1.03-1.32, p = 0.02), stone side (OR: 1.65, 95% CI 1.01-2.73, p = 0.05), stone diameter (OR: 1.09, 95% CI: 1.00-1.19, p = 0.05), and multiple sessions (OR: 4.65, 95% CI: 2.61-8.29, p < 0.001) were significantly associated with treatment failure by logistic regression univariable analysis. Multiple sessions was the only factor significantly associated with treatment failure on multivariable analysis (OR: 4.03, 95% CI: 2.18-7.42, p < 0.001). From a cohort of 141 patients with SSD ≥14 cm and 174 patients with a SSD ≤12 cm, 66 matches were identified (132 patients). Forty-nine patients (74.2%) with SSD ≥14 cm were deemed stone free at follow-up vs 51 patients (77.3%) with SSD ≤12 cm (p = 0.85). Conclusion: This study demonstrates by two statistical methods that SWL can provide efficacious treatment of upper ureteral stones in obese patients and that the upper threshold of SSD for SWL with Sonolith I-SYS could be revised to allow these patients the benefits of SWL.
