Cavitation bubble cluster activity in the breakage of kidney stones by lithotripter shockwaves

Background and Purpose: There is strong evidence that cavitation bubble activity contributes to stone breakage and that shockwave-bubble interactions are involved in the tissue trauma associated with shockwave lithotripsy. Cavitation control may thus be a way to improve lithotripsy. Materials and Methods: High-speed photography was used to analyze cavitation bubble activity at the surface of artificial and natural kidney stones during exposure to lithotripter shockwaves in vitro. Results: Numerous individual bubbles formed on the surfaces of stones, but these bubbles did not remain independent but rather combined to form clusters. Bubble clusters formed at the proximal and distal ends and at the sides of stones. Each cluster collapsed to a narrow point of impact. Collapse of the proximal cluster eroded the leading face of the stone, and the collapse of clusters at the sides of stones appeared to contribute to the growth of cracks. Collapse of the distal cluster caused minimal damage. Conclusion: Cavitation-mediated damage to stones is attributable, not to the action of solitary bubbles, but to the growth and collapse of bubble clusters.