The mechanisms of stone fragmentation in ESWL

Currently, several mechanisms of kidney stone fragmentation in extracorporal shockwave lithotripsy (ESWL) are under discussion. As a new mechanism, the circumferential quasistatic compression or "squeezing" by evanescent waves in the stone has been introduced. In fragmentation experiments with self-focussing electromagnetic shock-wave generators with focal diameters comparable to or larger than the stone diameter, we observed first cleavage surfaces either parallel or perpendicular to the wave propagation direction. This is in agreement with the expectation of the "squeezing" mechanism. Because, for positive pulse pressures below 35 MPa and stones with radii of 15 mm or smaller, cleavage into only two fragments was observed, we developed a quantitative model of binary fragmentation by "quasistatic squeezing." This model predicts the ratio of the number of pulses for the fragmentation to 2-mm size and of the number of pulses required for the first cleavage into two parts. This "fragmentation-ratio" depends linearly alone on the stone radius and on the final size of the fragments. The experimental results for spherical artificial stones of 5 mm, 12 mm and 15 mm diameter at a pulse pressure of 11 MPa are in good agreement with the theoretical prediction. Thus, binary fragmentation by quasistatic squeezing in ESWL as a new efficient fragmentation mechanism is also quantitatively verified.