The effect of fat and nonfat components of the skin-to-stone distance on shockwave lithotripsy outcome

Background: Few studies have addressed the effect of skin-to-stone distance (SSD) on the success of extracorporeal shockwave lithotripsy (SWL). Nevertheless, the effect of the two components of SSD, that is, the fat SSD (FSSD) and nonfat SSD (NFSSD) components, was not previously investigated. Methods: In this prospective study, all patients (n=113) who had single radio-opaque kidney stones and underwent SWL for the first time between January 2006 and June 2007 were recruited. SSD, FSSD, and NFSSD were measured by noncontrast CT scan at 0°, 45°, and 90° and the average was calculated. The outcome was defined as successful (completely stone free or residual fragments ≤3mm) or unsuccessful (residual fragments >3mm or complete failure of fragmentation). Results: Sixty-nine (61%) patients had successful treatment. On univariate analysis, SSD, FSSD, and NFSSD were significantly lower in the successful group compared with those with unsuccessful outcome (71.9±13.3 vs. 86.2±25.1mm [p=0.001], 27.2±10.3 vs. 36.1±17.3mm [p=0.011], and 44.7±7.2 vs. 50.1±13.9mm [p=0.02], respectively). The muscle component of the NFSSD was also lower in the successful group (21.5±4.1 vs. 25.2±10.0mm [p=0.01]). On multivariate analysis, factors that independently predicted treatment success were SSD, stone attenuation, and stone size but not the FSSD or NFSSD. Conclusions: Although the total SSD appeared to be a significant predictor of SWL success, its fat and nonfat components did not independently predict the final outcome and only appeared to be important through their contribution to the total SSD.