Damage Mechanisms in Shock Wave Lithotripsy

Graduate Aeronautical Laboratories
Caltech

Abstract-
Shock wave lithotripsy is a `non-invasive' therapy for treating kidney stones. Focused shock waves fragment stones to a size that can be passed out naturally. There is, however, considerable tissue injury associated with this treatment, and the mechanisms of stone fragmentation and tissue injury are not well understood. This work investigates potential tissue damage mechanisms, with an aim towards understanding these mechanisms, and modifying the wave-field parameters, so as to enhance stone fragmentation and minimize tissue damage.

In-vitro experiments implicate cavitation, due to the negative pressure generated by the lithotripter pulse, as a damage mechanism. However, in-vivo, cavitation is highly constrained due to enclosing tissue and lack of nuclei. Our experiments suggest that shearing due to a non-uniform shock plays an important role. Experiments with tissue models, such as polymer films, aluminum foils and red blood cells (RBC's), have demonstrated that in absence of cavitation, a focused shock wave causes significant cell damage and deformation/damage of the foils. The deformation of a RBC and the stresses in the cell membrane due to a focused shock and an exploding bubble are analytically estimated. These are shown to exceed the critical value needed for cell membrane rupture. Based on these observations and model predictions, a simple modification to the lithotripter wave-field is suggested.

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