Experimental and numerical evaluation of the effect of stone size on fracture by burst wave lithotripsy

Burst wave lithotripsy (BWL) is an experimental treatment to noninvasively fragment kidney stones using bursts of focused ultrasound. Preliminary simulations with a linear elastic model showed that resonance creates concentrated stresses, which may help predict locations of fractures in the stones. In this study, we aimed to demonstrate this correlation by comparing simulations to experimental data. Cylindrical stones of variable size (4–14 mm diameter, 10 mm length) made from BegoStone plaster were treated in a water bath for 10 min using a 170 kHz focused transducer at a focal pressure of 6.5 MPa. Locations of first fractures in the stones correlated well with the location of peak stress predicted in the linear elastic model. Simulated peak surface stress in the stones decreased as stone diameter increased, with the exception of a spike near d = 12 mm, which matches the half longitudinal wavelength in the stone. Experimentally, a corresponding overall increase in time to first fracture was observed with diameter, except for a drop at d = 12 mm. The results are encouraging that the model may help direct further optimization of BWL. [Work supported by NIH through DK043881, DK104854, EB007643, and NSBRI through NASA NCC 9-58.]