ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

4aPA3. Stress wave fracturing of biological stones with lithotripter pulses.

Sheryl M. Gracewski

Girish Dahake

Adam Fitzgerald

Stephen J. Burns

Mech. Eng., Univ. of Rochester, Rochester, NY 14627

Stress waves contributions to stone fragmentation during lithotripsy are investigated both theoretically and experimentally. A two-dimensional finite difference scheme is developed to analyze the time evolution of the strain fields inside irregularly shaped solids subjected to ultrasonic pulses that simulate lithotripter shock waves. The reflections and superposition of stress waves inside the stones are analyzed to better understand the effects of stone parameters and geometry on the induced internal strains and fragmentation during lithotripsy. Numerical results show the focusing effect of the concave backsurface of a spherical stone, with the subsequent formation of focal zones (caustics). The focusing is reduced when a section of the back surface of the stone is removed. Principal strain contours depict the time evolution of the stress waves as they refract and reflect at the stone boundaries. Locations of maximum stresses are calculated and compared to locations of crack initiation in experiments with stones of similar geometry. The calculated time evolution of strain at fixed points within a stone is compared to imbedded silicon strain gauge measurements. Fracture characteristics of synthetic stones show internal crack initiation and subsequent propagation to external stone surfaces, indicative of internal stress fragmentation mechanisms. [Work supported by NIH.]