Gallstone movement during lithotripsy: mechanisms and effects on fragmentation.

Abstract

We sought to examine the mechanisms of gallstone movement and its effect on gallstone fragmentation in vitro. Two experiments were performed in four specially constructed phantoms that allowed decreasing degrees of movement during the application of shock waves. Shock waves caused displacement of the stone from the focus, but the stone and its fragments were returned to the focus by streaming movements in the coupling liquid when the volume of surrounding fluid was small. Streaming movements were ineffective in large volumes. Restraining movements of the gallstone did not improve the results of fragmentation. We conclude that radiation force and the streaming motion of the surrounding liquid account for movements of the stone and fragments during lithotripsy. Lithotripsy is more effective when smaller volumes are used because streaming brings fragments back to the focus of the lithotripter. Total immobilization of the stone in the focus of the lithotripter, however, offers no benefit, probably because it inhibits rotational movement of the stone.