Table 2.

Summary of existing theories for stone fragmentation

Hypothesis	Mechanism	Prerequisites	Type of action	Comments
Tear and shear forces [1]	Pressure gradients resulting from impedance changes at the stone front and distal surface with pressure inversion	Shock wave smaller in space extension than the stone	Hammer-like action resulting in a crater-like fragmentation at both ends of the stone	Only relevant for small focus zones
Spallation [9]	Reflected tensile wave at distal surface of the stone with maximum tension at the distal part of the stone	Shock wave smaller in space extension than the stone	Breaking the stone from the inside like freezing water in brittle material	Only relevant for small focus zones No explanation for stone breakage at the front side
Quasi-static squeezing [11]	Pressure gradient between circumferential and longitudinal waves results in squeezing of the stone	Shock wave is broader than the stone Shock wave velocity is lower in the water than in the stone	Nutcracker-like action requiring large focal diameters	Only relevant for large focal zones
Cavitation [10]	Negative pressure waves induce a collapsing cavitation bubble at the stone surface	Low viscosity of surrounding medium	Microexplosive erosion at the proximal and distal ends of the stone	More important during stone comminution Useful for improving the efficiency of shock waves (ie, EHL)
Dynamic squeezing [12]	Shear waves initiated at the corner of the stone are reinforced by squeezing waves along the calculus	Parallel travelling of longitudinal waves Shock wave velocity is lower in the water than in the stone	Nutcracker-like action in combination with spalling	Best theory to explain results of the numerical model

EHL = electrohydraulic lithotripter.