Skin to stone distance (SSD) predicts stone-free status in patients after shockwave lithotripsy (SWL), independent of stone size, location, and density, according to researchers.

A team at the Columbia University College of Physicians and Surgeons in New York studied 82 patients who underwent SWL. They used preoperative noncontrast CT (NCCT) to determine stone size, density, and location. They determined stone-free status using NCCT or abdominal plain radiographs at six weeks post-SWL.

Of the 83 patients 51 (61.4%) were stone free and 32 (39.6%) had residual stones at a mean follow-up of 2.3 months. The mean SSD was 83.3 mm in the stone-free group compared with 107.7 mm in the residual stone group, the investigators reported in the Journal of Endourology (2009; published online ahead of print).

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“SSD is an easily measured parameter on NCCT which correlates with SWL efficacy in the treatment of stones in all locations independent of other factors,” the authors concluded. “SSD is a valuable tool, easily accessible to clinicians, which can be used to predict the success of SWL.”

Previous studies have shown that SSD predicts SWL success for lower pole stones, the researchers noted.