This paper analyses the fragmentation of kidney stones by direct impact with Swiss Lithoclast Master(R). Based on a previous work, the impact of the probe of the Lithoclast is modeled with a displacement control of the nodes inside the impacted area. Different computations are carried out focusing the efforts on new tentative designs of the cross section of the probe, as well as the influence of multiple impacts. The results show that a hollowed probe could improve the results obtained with the existing massive probe. Likewise, it is shown that there is an optimal internal radius of the hollowed probe for which the damage caused to the kidney stone is maximized. Finally the results demonstrate the existence of a synergetic effect of the application of multiple impacts at a high frequency. (C) 2009 Elsevier Ltd. All rights reserved.