A new fragmentation mechanism in extracorporeal shock wave lithotripsy and a first clinical study in China

Abstract

With ESWL-focus diameters of the order of the stone dimension or larger, fragmentation in planes perpendicular and parallel to the wave plane is observed. This is explained by circumferential pressure or ‘‘squeezing’’ of the stone by the wave propagating at the outside of the stone in the liquid or tissue. Since the pressure zone propagates with the sound velocity in the liquid which is below the propagation velocity in the stone, it causes an evanescent pressure zone in the stone resulting in tensile stress in planes perpendicular and parallel to the wave plane. A quantitative model predicting the ratio of pulses needed to fragment the stone to 2-mm particle size in relation to the number of pressure pulses needed for the first fragmentation is well in accord with experiments, supporting the ‘‘squeezing mechanism with binary fragmentation.’’ On the basis of these results it now appears possible to optimize the pressure pulse parameters measured using the fiber-optic probe hydrophone (FOPH). With correspondingly optimized shock wave generator systems, a clinical study of the concept ‘‘wide focus and low pressure’’ ESWL was performed in scientific cooperation between the Physical Institute of the University of Stuttgart and the Xixin Medical Instruments Co., Ltd. in Suzhou, China. [W. Eisenmenger, ‘‘The mechanism of stone fragmentation in ESWL,’’ Ultrasound Med. Biol. 27, 683–693 (2001); Eisenmenger, Du, Tang et al., ‘‘The first clinical results of wide focus and low pressure ESWL,’’ ibid. 8, 769–774 (2002) (in press).]