RESUMO
BACKGROUND AND PURPOSE: There is no uniform consensus regarding the anatomic factors that influence the clearance rate of caliceal stones after SWL, as different authors have studied various independent factors separately. We correlated both favorable and unfavorable factors into a formula to predict the clearance rate. PATIENTS AND METHODS: A series of 56 consecutive patients (37 male, 19 female) with isolated lower-caliceal single stones (right 29, left 27) treated with SWL using the Dornier Compact Delta lithotripter were analyzed retrospectively. Of these, 40 patients had soft stones, and 16 patients had hard stones. The first stone-free rate (SFR) according to plain films and ultrasonography was analyzed by stone size, and a stone clearance index (SCI) formula was applied to see if there was any change in the SFR, especially for stones >2 cm. RESULTS: The formula, which correlated well with the clearance, is SCI = [(IVA x IW x stone type)/IH] - (stone size in mm(2)/10). Nearly all (90%) of the patients with an SCI of >0 cleared their stones within 3 months, and 87% of the patients with an SCI <0 cleared their stones after 3 months. Positive and negative predictive values were 93.33% and 76.9%, respectively. The accuracy of the correlation is 85.71%. CONCLUSIONS: The success of SWL for lower-caliceal stones can be predicted easily using the SCI. Stones of >200 mm(2) (>2 cm) surface area may still be suitable for SWL if the SCI is positive, whereas alternative treatment modalities should be considered if the SCI is a low negative value (<-7).