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 × 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 ≥10 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.