New insights into the mechanisms and process of stone fragmentation in shock wave lithotripsy

Stone fragmentation in shock wave lithotripsy (SWL) is the consequence of dynamic fatigue produced by stress waves and cavitation. Stress waves [longitudinal (or P), transverse (or S), and surface acoustic waves (SAW)] and associated tensile and shear stresses are the primary driving forces to create fracture, initially from pre-existing (or intrinsic) flaws inside the stone. In contrast, cavitation produces pitting on the stone surface, and consequently, introducing new (or extrinsic) flaws to weaken the stone structure during SWL. Stress waves and cavitation act synergistically to produce effective and successful stone comminution in SWL, with cavitation serving as catalysts to enhance the efficiency of stress waves-driven stone fracture. In this talk, contemporary understanding about the mechanisms and process of stone fragmentation in SWL will be summarized, using a heuristic model which incorporates two important lithotripter field parameters (i.e., pressure and dose) that can critically influence the treatment outcome. The effects of stone size, geometry, composition on the transient stress field produced inside the stone, and the potential role of SAW in crack initiation and propagation will be discussed to provide physical insights into improvements in lithotripsy device design and treatment strategy.