Twenty years of shock wave research at the Institute for Surgical Research.

Shock waves are acoustic pressure wave pulses which last only a microsecond and reach a pressure of several hundred up to over 1,000 atmospheres (fig. 1). They are produced outside the body and are coupled on a relatively large area through the skin into the body, propagated in tissue and focused on a stone by means of ultrasound. The history of the Institute for Surgical Research of the University of Munich is tightly linked with the development of extracorporeal shock wave lithotripsy, the fragmentation of stones in the ureteric tract and other locations. Its former director Walter Brendel set up the Institute in the sixties and directed it till his death in 1989. He started research on the clinical application of shock waves in the mid seventies. The following brief review starts with a remark on the historical roots of stone removal and describes the evolution of extracorporeal shock wave lithotripsy. Like any other treatment, shock waves generated side effects which stimulated researchers to take a closer look at. The idea of a causative role of cavitation emerged, a process involving the generation and movement of bubbles in a fluid. This mechanism was supported by a number of experiments. It Fig. 1. Pressure registration of a shock wave of a lithotripter. The peak pressure is shown as positive upstroke on the y-axis and exceeds 90 MPa. The positive pressure wave is followed by a tensile wave which is below the baseline and is in the range of 10 MPa. The time scale on the x-axis is in Ìs. The reading was made with a fibre-optic probe hydrophone which is today the only way to provide an undistorted detection of strong tensile pressures. The principle was invented by Eisenmenger and Staudenraus [32].