In vitro study to elucidate the physical laws concerning the fragmentation of both solitary and multiple artificial stones.

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 gallstones. All experiments were performed using the lithotripter model MPL9000 (Dornier). The acoustic energy passing stones of 10-30 mm ranged between 8 and 90 mJ per pulse depending on the stone size and energy setting. These results represent the basis for the three following investigations. In the first experiment the relationship between fragmentation and shock wave energy was investigated in a basket with 2 mm mesh size. Thus no layer of small fragments could shadow the acoustic energy for further fragmentation of larger fragments. A constant amount of stone material was found to be fragmented per shock-wave pulse irrespective of stone volume. A low energy threshold (2 mJ/cm3) was observed, below which fragmentation did not occur. In the second experiment, the sieve was covered with a membrane, thus simulating 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 attenuation 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 thickness 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.(ABSTRACT TRUNCATED AT 250 WORDS)