PROPAGATION OF SHOCK-WAVES IN ELASTIC SOLIDS CAUSED BY CAVITATION MICROJET IMPACT .2. APPLICATION IN EXTRACORPOREAL SHOCK-WAVE LITHOTRIPSY

To better understand the mechanism of stone fragmentation during extra corporeal shock wave lithotripsy (ESWL), the model developed in Part I [P. Zhong and C. J. Chuong, J. Acoust. Soc. Am. 94, 19-28 (1993)] is applied to study cavitation microjet impingement and its resultant sho ck wave propagation in renal calculi. Impact pressure at the stone bou ndary and stress, strain at the propagating shock fronts in the stone were calculated for typical ESWL loading conditions. At the anterior s urface of the stone, the jet induced compressive stress can vary from 0.82 approximately 4 times that of the water hammer pressure depending on the contact angles; whereas the jet-induced shear stress can achie ve its maximum, with a magnitude of 30% approximately 54% of the water hammer pressure, near the detachment of the longitudinal (or P) wave in the solid. Comparison of model predictions with material failure st rengths of renal calculi suggests that jet impact can lead to stone su rface erosion by combined compressive and shear loadings at the jet im pacting surface, and spalling failure by tensile forces at the distal surface of the stone. Comparing responses from four different stone ty pes suggests that cystine is the most difficult stone to fragment in E SWL, as observed from clinical experience.