Stone Damage Modes During Piezoelectric Shock Wave Delivery

Abstract

Using stone phantom models made of plaster of Paris, different modes of stone damage were observed during piezoelectric lithotripter shock wave delivery. A stone phantom in the configuration of a square slab (80 mm by 80 mm by 8 mm) was positioned in water with its horizontal midsurface placed at the geometric focal plane of a Wolf Piezolith 2300 lithotripter. After shock wave exposure, damage was seen at both the lower surface (the surface facing the wave arrival) and the upper surface (the surface distal from the wave arrival) of the stone. The fracture surfaces of the residual pieces of plaster were examined by scanning electron microscopy at 10X, 100X and 1000X magnification. Two different modes of stone damage were observed: damage due to cavitation microjets and damage secondary to spalling. At the surface of the phantom directly facing the incident wave, damage of a cavitation type was observed. The cavitation damage was a deep crater (approximately 3 mm diameter) surrounded by an annular zone (approximately 6 mm diameter) of flake-off failure. Under magnification, surface erosion with scattered pits (ranging from 10 to 300 microns) was observed. The surface erosion was caused by repeated loading, whereas the pits were caused by the high velocity penetration of microjets formed from freshly collapsed cavitation bubbles. At the distal surface of the stone phantom, damage of a spalling type was observed. The spalling damage was characterized by separation of a spherical cap from the pellet surface. Irregular but fine grain texture was found on the fracture surface, a pattern commonly seen on brittle materials after tensile failure. The incident compression wave was reflected as tensile wave at the distal surface of the phantom due to the lower wave impedance of the neighboring water. The separation of the cap occurred when the reflected tension exceeded the tensile strength of the stone.

Spalling may be an undesirable effect from a clinical view point, since the size of stone fragments caused by this phenomenon may be too large to pass spontaneously. The findings of this study suggest that the geometry of the stone, combined with the focal position of the shock wave will determine whether fragmentation will occur by cavitation effect or by a spalling phenomenon, and the type of fragmentation may predict the size of the stone fragments produced.