Study of an Extracorporeal Lithotripter Using a Synthetic Stone Model*

Abstract

One hundred synthetic stones were molded from a commercial filler compound. Seven samples were stress tested, revealing a mean maximum compressive stress of 9.8 MPa (range 8.8–11.1 MPa) and a mean elastic modulus of 190 N/mm² (range 167-218 N/mm²). Seventy-four stones of mean mass 1.86 g (range 1.77–1.92 g) were selected for fragmentation studies, exploring treatment options on an extracorporeal piezoelectric lithotripter. Stones were sealed in a 5 x 3-cm polythene bag containing 7.5 ml of 0.9% saline and subjected to 1000 shocks. Fragments were sieved, and the amount fragmented to <2 mm was calculated. Increases in the power settings resulted in disproportionate increases in fragmentation. At the lowest three power settings, fragmentation was similar when shocks were delivered at either 55/min or 110/min, but at the highest power setting, fragmentation was significantly less p < 0.001) with treatment at the faster rate. Lowering the ultrasound imaging probe from its maximum level to half height produced significantly greater fragmentation (p < 0.001). Placing a 6.25-cm layer of tissue equivalent material between the piezoelectric bowl and the focus did not influence fragmentation. The synthetic stone has physical and fragmentation characteristics similar to those of urinary tract calculi, suggesting that it is a useful model for studying extracorporeal shock wave lithotripsy.