Biomechanical effects of ESWL shock waves

Abstract

Impulsive stress in the repeated shock waves administered in ESWL is the mechanical stimulus of injury to the kidney. In order to better understand the mechanical origins of injury, the interaction of focused shock waves with simple, planar polymeric membranes immersed in tissue-mimicking fluids was studied. The ESWL shocks in uniform, noncavitating liquids do not cause damage, but after passing through tissue or simulated tissue they do. This result suggests that the acoustic inhomogeneity of tissue may contribute to injury from ESWL. Shocks with large amplitude and short rise time (e.g., in uniform media) cause no damage in noncavitating fluids, while long-rise-time, dispersed shock waves, though only moderately attenuated, do. A continuum model is described which incorporates the mechanical properties of the tissue and accounts for the effect on its strength of microscopic inhomogeneities. It is shown that when transient tensile stress is applied it takes a finite time for failure to occur, so if the pulse is not long enough the material survives, in qualitative agreement with the behavior described above. A definition of dose at failure is derived in terms of the stress applied to the membrane by each shock wave, the strain rate, and the material failure stress.