A comparison of stone damage caused by different modes of shock wave generation

A standard stone phantom was used to compare stone damage after extracorporeal shock wave administration from electrohydraulic, electromagnetic and piezoelectric lithotripters. For each machine, a low and high shock wave intensity setting was chosen: 18 & 24 kV for electrohydraulic; 16 & 19 kV for electromagnetic; power levels 1 and 4 for piezoelectric. The shock wave was focused either at the front (surface facing the wave source) or back surface of the stone and 50, 100, 200 or 400 shocks were delivered to different stone groups. Effects of varying physical properties in the stone phantom were also investigated. Stone damage was described in terms of volume loss and both depth and width of the resulting damage crater. At the lower intensity settings, all three machines produced stone volume loss which was linearly related to the number of shock delivered. At higher intensity settings, volume loss increased rapidly as the number of shocks increased. With the same number of shocks, stone volume loss was greatest with the electrohydraulic machine, followed by electromagnetic and piezoelectric lithotripters for both low and high intensity settings. Damage craters from the piezoelectric device were narrow and deep; those from the electromagnetic machine were of the shape of a right angle circular cone; whereas those from the electrohydraulic lithotripter were shallow and wide. At the high intensity settings, damage from the piezoelectric and electrohydraulic lithotripters appeared to depend upon the position of the focal point with a higher volume loss when the shock waves were targeted at the front surface of the stone. For the electromagnetic device, a higher volume loss was found when we positioned the focal point at the back surface of the stone phantom. Stone phantoms with lower mechanical strength and acoustic impedance were more easily damaged than those with higher values. Finally, a computer regression model was developed to express volume loss in terms of the intensity setting, focal position and number of shocks for each lithotripter.