Clin Investig (1993) 71:882-887 Original ArticNe 61inical Investigator © Springer-Verlag 1993 In vitro study to elucidate the physical laws concerning the fragmentation of both solitary and multiple artificial stones* H. Lobentanzer 1, M. Neubrand 2, H. Hermeking 1, T. Sauerbruch 2 1 Dornier Medizintechnik GmbH 2 Medizinische Klinik II, Klinikum Groghadern, Ludwig-Maximilians-Universit/it Miinchen Summary. These in vitro studies define the basic physical laws regarding work and energy for the successful fragmentation of human gallstones. For this purpose a standardized stone model was used consisting of plaster and glass microspheres with physical properties similar to those of human gall- stones. All experiments were performed using the lithotripter model MPL9000 (Dornier). The acous- tic energy passing stones of 10-30 mm ranged be- tween 8 and 90 mJ per pulse depending on the stone size and energy setting. These results repre- sent the basis for the three following investigations. In the first experiment the relationship between fragmentation and shock wave energy was investi- gated in a basket with 2 mm mesh size. Thus no layer of small fragments could shadow the acoustic energy for further fragmentation of larger frag- ments. A constant amount of stone material was found to be fragmented per shock-wave pulse irre- spective of stone volume. A low energy threshold (2 mJ/cm 3) was observed, below which fragmenta- tion did not occur.In the second experiment, the sieve was covered with a membrane, thus simulat- ing the in vivo situation. The presence of a layer of small fragments hindered the further disintegration of the larger fragments. The attenuation depended to a large extent on original stone volume and acoustic energy per pulse. The corresponding at- tenuation factor increased with the original stone volume. Thus the fragmentation of a stone with a diameter of 30 mm was attenuated twice as much as a stone of 20 mm size. The critical layer thick- ness at which no further disintegration took place was 2.5 mm at 18 kV, 4.2 mm at 22 kV, and 5.0 mm at 26 kV. In the third series of experiments we com- pared the disintegration of one large stone with that of several smaller stones having the same total stone volume at a constant acoustic energy. In this system single stones underwent more complete fragmentation than multiple stones, probably due to the higher threshold energy for intact stones * Dedicated to Prof. Dr. G. Paumgartner on the occasion of his 60th birthday compared to that of fragments of the same size.The information gained in the model system will be of use in the interpretation of clinical results and will contribute to the long-term goal of improving the efficacy of extracorporeal shock-wave lithotripsy. Key words: Lithotripsy - Gallstones - Fragmenta- tion - Extracorporeal shock-wave lithotripsy - Physics of shock waves Extracorporeal shock-wave lithotripsy is today an integral part of clinical treatment for calculi. Indeed for kidney stones it is the method of choice. Extra- corporeal shock-wave lithotripsy of gallstones is also increasingly being viewed as a routine proce- dure, at least in well-selected patients with radiolu- cent, preferably solitary stones in combination with an oral bile acid therapy [2, 5-8]. Adequate disinte- gration of the stones is crucial for complete clear- ance of the gallbladder. Treatment results become worse with increasing stone volume and stone number, due mainly to insufficient stone disintegra- tion [7]. No physical knowledge exists about the energy necessary to fragment stones into small pieces. To obtain more information about the fun- damental laws governing fragmentation we investi- gated disintegration of artificial stones in model systems in vitro. Material and methods Model stones The experimental stones consisted of a mixture of plaster and glass microspheres. They had a specific weight of 0.93 g/cm 3, and the velocity of sound in this material was 2400 m/s. The resulting acoustic impedance of 2.23 x 106 kg m -2 s-1 was in the range of gallbladder stones (1.9-3.1 x 106 kg m -2 s-l). Spherical stones with a diameter between 12 and 25 mm were used. Each experiment was performed in triplicate. All stones were saturated with water.