The role of shear and longitudinal waves in the kidney stone comminution by a lithotripter shock pulse

Abstract

Shock wave lithotripsy has been in clinical use for 20 years but there is no consensus as to the main mechanism of kidney stone comminution. Experiments show that several mechanisms might be involved, including cavitation, spallation, and dynamic fatigue. Until recently, little attention was paid to shear elasticity of the stone material, i.e., mechanical load was mainly attributed to the longitudinal waves. In a previous numerical study, we found that shear elasticity resulted in tremendous change in the stress pattern inside cylindrical stones. The numerical model has been extended to study elastic waves in asymmetric inhomogeneous stones. Strains and stresses in the stone are calculated based on the Lamé equation for an isotropic elastic medium. Lithotripter shock waves of various temporal and spatial profiles were considered according to several clinical models of lithotripters. Maximum compression, tensile and shear stresses are predicted as a function of stone dimension and shape. The model predicts that both shear and longitudinal waves play an important role in creating the regions of excess stresses where cracks can be formed. The results of modeling are compared with the experimental observations. [Work supported by ONRIFO, CRDF, NIH-Fogarty, RFBR, NIH, and Whitaker Foundation.]