Predictors of lower pole renal stone clearance after extracorporeal shock wave lithotripsy

Lower pole renal stones are well known to show a poor stone clearance rate after extracorporeal shock wave lithotripsy (ESWL) (Dornier Medical Systems, Marietta, Georgia). In the current study we analyzed several anatomical factors as predictors of lower pole stone clearance that may be used to indicate the usefulness of ESWL in such patients.

Between January 1993 and October 2000, 63 patients with a unilateral single lower pole stone of 2 cm. or less were included in the study. Excretory urography was done to determine the lower infundibulopelvic angle, caliceal pelvic height, lower infundibular length and diameter, lower infundibular length-to-diameter ratio and number of lower pole minor calices. Stone-free status was assessed by x-ray.

The overall stone clearance rate was 54% (34 of 63 patients). While caliceal pelvic height, lower infundibular diameter, length, length-to-diameter ratio and number of minor calices were different in the stone-free and residual stone groups on univariate analysis, multivariate logistic analysis revealed that the lower infundibular length-to-diameter ratio, diameter and number of minor calices were independent predictors of successful stone clearance. The 13 patients (20.6%) who showed all 3 favorable anatomical factors (lower infundibular length-to-diameter ratio less than 7, diameter greater than 4 mm. and a single minor calix) achieved an impressive 84.6% stone clearance rate. In patients with only 1 or 2 favorable factors the stone clearance rate was still greater than 60%. In contrast, the stone clearance rate was only 6.7% in the 15 patients who showed none of these factors.

From this study it is apparent that successful ESWL is highly sensitive to the anatomy of the lower pole of the kidney. If a patient with a lower pole stone has at least 1 of the favorable factors identified in this study, ESWL may be advocated as first line therapy with a greater than 60% prospect of a successful outcome. Other treatment options should be considered in those without any of these predictive factors.