Skin to Stone Distance Is an Independent Predictor of Stone-Free Status Following Shockwave Lithotripsy

Abstract

In recent years several new parameters such as Hounsefield unit density and skin to stone distance (SSD) have been determined to correlate with successful extracorporeal shockwave lithotripsy (SWL). SSD has been shown to independently predict successful SWL for lower pole calculi. The objective of this study was to determine which stone characteristics on noncontrast computed tomography (NCCT), including SSD, independently predict for SWL success regardless of stone location within the kidney. A retrospective review of all 1282 SWL procedures between 2004 and 2007 at a single institution revealed 83 patients meeting inclusion criteria for current study. All patients were treated using a single electromagnetic lithotriptor (DoLi 50; Dornier Medical Systems, Marrieta, GA) with a single technician. Stone-free (SF) status was determined by NCCT or abdominal plain film radiographs at 6 weeks post-SWL. Logistic regression analysis was computed to assess SF rates using SSD, stone size, location, and Hounsefield unit density as predictors. Of the 83 patients, 51 were SF and 32 had residual stones at a mean follow-up of 2.3 months. The mean SSD in the SF group was 83.3 +/- 21.9 mm in comparison to those in the residual stone group who had a mean SSD of 107.7 +/- 28.9 mm (p < or = 0.05). Multivariate logistic regression analysis revealed that SSD was the only significant independent predictor of treatment outcome. SSD is an easily measured parameter on NCCT that independently correlates with SWL efficacy in treating stones in all locations.