Generation of shear waves as an effective mechanism of dynamic load of the lithotripter shock wave on the kidney stone

Abstract

A number of stone comminution mechanisms have been studied in lithotripsy. Except cavitation erosion, these mechanisms (namely, spallation, dynamic fatigue, shear, and circumferential compression) are associated with stresses generated in the stone by the shock wave. The mechanical load on the stone depends on the waveform and stone structure, size, and shape. We modeled the propagation of lithotripter shock waves through a cylindrical stone with a finite differences simulation based on Lame’s equation. The stone parameters were similar to those of natural kidney stones. A new mechanism of tensile stress generation is predicted that may be 5–10 times more efficient than spalling. Shear elasticity of the stone gave rise to the peak tensile strain in the bulk of the stone; this strain occurs near the stone axis due to coherent arrival of shear waves from the front edges of the stone. The position of the region of maximum strain and direction of corresponding tensile forces is similar to those predicted by the spalling mechanism. The modeling also showed that circumferential compression is not activated by the dynamic load produced by a short shock wave typical for lithotripsy. [Work supported by NIH PO1 DK43881, RO1 DK55674 and FOGARTY, CDRF, ONRIFO, and NSBRI.]