To determine whether stone attenuation and the skin-to-stone distance (SSD) can predict
for stone fragmentation by SWL independently. Identifying the factors predictive of
shock wave lithotripsy (SWL) outcome would help streamline the care of patients with
A retrospective review was performed of 111 patients undergoing initial SWL for a
solitary, 5-20 mm, renal calculus. Stone size, location, attenuation value, and SSD
were determined on pretreatment noncontrast computed tomography. The outcome was categorized
as stone free, complete fragmentation <5 mm, and incomplete fragmentation ≥5 mm or
unchanged at 2 weeks on kidney/ureter/bladder radiography.
After SWL, 44 (40%) were stone free, 27 (24%) had complete fragmentation, and 40 (36%)
of 111 patients had incomplete fragmentation. The stone attenuation of the successfully
treated patients (stone free and complete fragmentation groups) was 837 ± 277 Hounsfield
units (HU) vs 1092 ± 254 HU for those with treatment failure (incomplete fragmentation;
P < .01). The mean SSD also differed: 9.6 cm ± 2.0 vs 11.1 cm ± 2.5 for the successful
treatment group vs the treatment failure group, respectively (P = .01). On multivariate analysis, the factors that independently predicted the outcome
were stone attenuation, SSD, and stone composition. When patients were stratified
into 4 risk groups (stone <900 HU and SSD <9.0 cm, stone <900 HU and SSD ≥9.0 cm,
stone ≥900 HU and SSD <9.0 cm, and stone ≥900 HU and SSD ≥9.0 cm), the SWL success
rate was 91%, 79%, 58%, and 41%, respectively (odds ratio 7.1, 95% confidence interval
1.6-32 for <900 HU and SSD <9.0 cm group vs other 3 risk groups; P = .01).
The results of our study have shown that a stone attenuation of <900 HU, SSD of <9
cm, and stone composition predict for SWL success, independent of stone size, location,
and body mass index. These factors will be considered important in the prospective
design of a SWL treatment nomogram at our center.