Assessing the effect of lithotripter focal width on the fracture potential of stones in shockwave lithotripsy

This talk presents a combination of computational and experimental study on the effect of lithotripter focal width on the fracture potential of stones treated at various distances from the lithotripter focus. Two representative lithotripter fields are considered: (1) the original Siemens Modularis with a focal width of 7.4 mm and (2) a modified version with a larger focal width of 11.0 mm with comparable acoustic pulse energy of 40 mJ. The interaction of these two lithotripter fields with spherical and cylindrical model stones located at 0 to 12 mm from the shock wave axis is investigated. Specifically, a three-dimensional CFD (Computational Fluid Dynamics)—CSD (Computational Solid Dynamics) coupled computational framework is used to simulate the propagation of stress waves, as well as the initiation and propagation of fractures. The two-scale Tuler-Butcher fracture criterion will be employed: it will be calibrated experimentally, then applied to assessing stress-induced stone damage. An element deletion method will be applied to simulate fracture. Characteristic changes in wave focusing and interference in relation to the buildup of the maximum tensile stress inside the stone will be presented. The physical mechanism(s) responsible for the different fracture patterns observed at different off-axis distances will be discussed.