Effect of stone motion on in vitro comminution efficiency of Storz Modulith SLX

Background and Purpose: During SWL, stone motion secondary to ventilatory motion can be as much as 50 mm. This is much larger than the 4- to 15-mm diameter of the focal regions on most clinical machines. The goal of this study was to determine the effect of stone motion on the fragmentation efficiency of a clinical lithotripter with a small (4-mm) focal spot. Materials and Methods: A model stone (6.5 x 7.5 mm) made of gypsum cement was used as an in vitro target for a Storz Modulith SLX lithotripter with a custom-designed waterbath. A motorized positioner was used to translate the stone in order to simulate ventilatory motion. The excursion was variable up to 48 mm (+/-24 mm about the focus). After treatment by 400 shockwaves, the remnants (>2 mm) were dried and weighed. Results: Fragmentation efficiency was reduced (P < 0.05) for motion of greater than or equal to10 mm. Similar results were found with different energy levels and firing rates. The reduction in fragmentation efficiency was consistent with calculations of the time the stone was outside the focal region. Conclusions: Clinically relevant stone motion has a dramatic effect on in vitro comminution. Motion of 10 mm led to a significant reduction in comminution, and for motion >20 mm, it appeared that three-quarters of the shockwaves missed the stone. These data imply that ventilatory gating or stone tracking may result in fewer shockwaves being required for successful treatment with this lithotripter.