Simple and practical nomograms for predicting the stone-free rate after shock wave lithotripsy in patients with a solitary upper ureteral stone

Abstract

Purpose: To develop practical nomograms for predicting the stone-free rate after shock wave lithotripsy (SWL) in patients with a solitary stone in the proximal ureter.

Methods: Between July 2006 and June 2015, 319 patients with a proximal ureteral stone who underwent preoperative non-contrast enhanced computed tomography (NCCT) and subsequently received SWL were identified. Patients' age, gender, laterality, stone size, mean and maximum Hounsfield Unit (HU) of the stone, and skin-to-stone distance (SSD) were assessed. The stone-free status was defined as no radiopacity detected on the 3-month follow-up plain radiography or NCCT.

Results: Mean stone size was 10 mm (range 3-20 mm). Mean and maximum HU of the stone ranged from 115 to 1447 (mean 701) and from 265 to 1881 (mean 1062), respectively. The overall stone-free rate was 70%. Multivariate analyses identified stone size (p < 0.001), maximum HU (p < 0.001), and SSD at 90° (p = 0.038) as independent predictive factors for the stone-free status after SWL. Nomograms could be constructed for predicting the probability of stone-free status after SWL corresponding to SSD of 8, 10, and 12 cm using maximum HU and stone size.

Conclusions: This study demonstrated that stone size, maximum HU of the stone, and SSD at 90° are significant predictors of successful SWL outcome in patients with a proximal ureteral stone. We have developed simple and practical nomograms corresponding to three different SSDs for predicting the stone-free rate after SWL.

Keywords: Hounsfield unit; Lithotripsy; Nomograms; Shock wave lithotripsy; Skin-to-stone distance; Urinary calculi.