Fig. 2.

Different theories for initial stone fragmentation. (A) Tear and shear forces: Shock waves are transmitted and reflected at the low impedance stone-water interfaces, with pressure inversion splitting off stone material by tensile stress. (B) Spalling: The distal stone surface as an acoustically soft interface generates a reflected tensile wave of the initially compressive longitudinal shock wave pulse propagating through the calculus, with maximum tension within the distal third of the stone (high-speed shadowgraphy by Zhong). (C) Quasi-static squeezing: Stone breakage by tensile stress of the circumferential shock wave resulting from a lower shock wave velocity in the surrounding fluid than within the stone (modified from Eisenmenger). (D) Cavitation: Negative pressure waves of high-speed shocks cause cavitation in liquids surrounding stones and within microcracks or cleavage interfaces by inducing microjets. (E) Dynamic squeezing: Shear waves initiated at the corners of the stone and driven by squeezing waves along the calculus lead to the greatest stress and tension (three-dimensional computer simulation according to a numerical model by Cleveland). Note the different pressure distributions and travelling time of waves inside and along the stone surface. *Blue = compressive phase; green = maximum shear stress (55 MPa); red = maximum tensile stress (80 MPa).