Ultrasound in Med. & Biok Vol. 17, No. 3, pp. 239-243. 1991 0301-5629/91 $3.00 + .00 Printed in the U.S.A. © 1991 Pergamon Press plc OOriginal Contribution THE MECHANISMS OF STONE DISINTEGRATION BY SHOCK WAVES WOLFGANG SASS, t MARTIN BRAUNLICH,* HANS-PETER DREYER,* EIKE MATURA, § WALTER FOLBERTH, § HANS-GEORG PRIESMEYER II and JURGEN SEIFERT t *Department of Experimental Surgery, University of Kiel, Michaelisstr. 5, D-2300 Kiel 1, Germany; *Division of Scientific Film, D. Weigert Film GmbH, Rottmannstr. 5, D-8000 Munich 2, Germany; *Division of Electron Microscopy, Institute of Zoology, University of Kiel, Olshausenstr. 40-60, D-2300 Kiel-1, Germany; ~Department of Medical Research and Development, Siemens AG Henkestr. 127, D-8520 Erlangen, Germany; Iqnstitute of Applied and Nuclear Physics, University of Kiel, Otto-Hahn-Platz 1, D-2300 Kiel 1, Germany (Received 12 July 1990; in final form 22 October 1990) Abstract--Through interpretation of high-speed films at 10,000 frames per second of shock wave action on kidney stones and gallstones, the mechanism of stone destruction was analyzed in detail. This shows that the interaction of the shock wave with the targets firstly produces fissures in the stone material. Liquid then enters these small cracks. The actual disintegration is caused later by the enormous violence of imploding cavitation bubbles within these small split lines. That cavitation acts inside the stone and causes fragmentation even within the human gallbladder could furthermore be demonstrated by using scanning electron microscopy. These results should lead to a different process in gallstone lithotripsy leaving intervals between the shock wave treatments. This will allow the viscous bile fluids to occupy the fissures of the stones more completely and, therefore, should increase the cavitational activity on the subsequent treatment with shock pulses. Key Words: Shock wave action, Gallstone destruction, High-speed films, Biliary lithotripsy, Biliary mucus. INTRODUCTION The use of extracorporeally induced shock waves has become a leading procedure for the removal of renal calculi worldwide (Chaussy et al. 1980; Assimos et al. 1989). This treatment is also becoming accepted for gallstone disease (Sauerbruch 1989; Brendel and Enders 1983). Medical shock waves are generated by lithotripters, which produce acoustic pulses with pres- sure rise times of several nanoseconds duration (Cole- man and Saunders 1989). As the mechanism of stone destruction does not appear to be completely under- stood, it has recently been supposed that repeated ten- sile wave reflection and spalling (Lubock 1989) or even cavitation (Crum 1988) might play an important role in this process. Since biliary lithotripsy is bur- dened with a high rate of retreatments due to an in- complete fragmentation (Marberger et at. 1988), an increased understanding of the procedure of stone disintegration should improve the success of fragmen- tation results. In order to obtain a more detailed analy- sis of shock wave effects on human stones, high-speed films at 10,000 frames per second of shock wave ac- tion on kidney stones and gallstones were used. These observations were extended by using scanning elec- tron microscopy of human gallstones after shock wave treatment. MATERIALS AND METHODS High-speedfilm procedures All stones were placed into a transparent water- basin with 20 L of degassed water at room tempera- ture. The fixation of targets was achieved by using a thin string which was fastened by candle wax or adhe- sive glue on the stone surface. Drying of the stones was not done. The targets were adjusted to the shock wave focus of a commercial electromagnetic source creating a focal peak pressure of 640 bar. The acousti- cal lens had an aperture of 185 mm and formed a focal shock wave angle of 73 o. The distance of the lens aperture to the focus was 125 mm exactly. The shock wave generator was triggered together with a NAC-E- 10 high-speed prism camera guaranteeing the release of the shock wave at the maximum speed of 10,000 frames/s precisely. This allowed a high-speed time of 0.95 s. Eight spot lights each of 1000 watt illuminated the stones during high-speed sequence. Water was 239