Piezoelectric extracorporeal shockwave lithotripsy of distal ureteric calculi : assessment of shockwave focusing with unenhanced spiral computed tomography

Objective To assess the effects of the interposition of pelvic bones and abdominal gas in the shockwave pathway during piezoelectric extracorporeal shock wave lithotripsy (ESWL) of distal ureteric stones. Patients and methods The study included 35 patients who were evaluated with unenhanced spiral computed tomography (CT), used according to their positioning during ESWL. The shockwave pathway was simulated on the sagittal and coronal views crossing the ureteric calculi, allowing a theoretical evaluation of the effective shockwave focusing (with no bone or gas interference). Vertical and oblique approaches were statistically compared for bone and gas interposition. Results Overall, the effective shockwave focusing during in situ piezoelectric ESWL of distal ureteric stones was 71% of the theoretical area. The interposition of bone and gas was significantly lower for an oblique access than for a vertical approach in the sagittal plane (P < 0.001 and 0.03 on the sagittal and coronal views, respectively). Using stepwise logistic regression, the difference between vertical and oblique accesses in the sagittal plane was mainly affected by the bladder volume (P < 0.001). On the coronal views, the interposition of bone and gas affected 31 patients (89%). Such interference was eliminated in 73% of the patients with a contralateral inclination of the shockwave axis in the coronal plane. Conclusion The interposition of pelvic bones and abdominal gas in the shockwave pathway can affect the performance of piezoelectric ESWL of distal ureteric stones. While awaiting clinical confirmation of these theoretical data, we recommend that patients are treated with the bladder full and that the shockwave generator is inclined in both the coronal and sagittal planes.