Surface acoustic waves and Mach stem formation produced by lithotripter shock wave-stone interaction

Lithotripter shock wave (LSW)-stone interaction may vary significantly due to respiratory motion of the patient. In this work, we have investigated the variations of the LSW-stone interaction and associated stone fracture pattern using photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to the T-shaped fracture pattern observed in a disk-shaped stone under symmetric loading, we have observed a gradual transition of the fracture pattern to a tilted L-shape under asymmetric loading. Importantly, our model simulations reveal the generation of surface acoustic waves (SAWs)—a leaky Rayleigh wave on the anterior and a Scholte wave on the posterior surface of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral are produced by SAWs on the stone boundary under asymmetric loading, which drive the initiation and extension of surface cracks into the bulk of the stone that has been confirmed by microCT analysis.