Shock wave effects in biomechanics

Abstract

Focused shock waves convey energy from a source and deposit it in a localized region remote from the source. This mechanism is used to deliver crushing impulses to kidney stones in patients, but it may also be an undesirable source of tissue injury. Tissue injury occurs in extracorporeal shock wave lithotripsy (ESWL), a popular procedure used around the world to treat kidney stone disease. Shock waves can also be responsible for unexpected symptoms appearing remote from the site of blunt impact trauma. This paper describes two examples of the effects of focused shock waves on tissue. A victim of gunshot wound suffered specific neurological symptoms indicating highly localized injury to the spinal cord remote from the impact point and penetration of the missile. We suggest that wave focusing by thoracic vertebrae can concentrate energy into the spinal canal, with consequent injury to the spinal cord. Impulsive stress in repeated shock waves of order 20 MPa strength administered in ESWL is the mechanical stimulus of injury to the kidney. Research to increase the understanding of the causal relationship between the shock-induced mechanical stimulus and the consequent biomedical symptoms in the shock-wave treatment of kidney stone disease is described. The interaction of focusing shock waves with simple, planar polymeric membranes immersed in tissue-mimicking fluids has been studied. We have explored the nature of membrane failure in both cavitating and non-cavitating fluids. In water, thin nitrocellulose membranes are easily damaged during the passage of a lithotriptor shock wave by the collapse of proximal bubbles generated by cavitation. In uniform non-cavitating liquids, focusing shock waves do not by themselves cause damage, but after passing through tissue or simulated tissue, they do. Shocks with large amplitude and short-rise time (for example, in uniform media) cause no damage in non-cavitating fluids, while long-rise time, dispersed shock waves, though only moderately attenuated, do. A simple model of shearing at small scale provides a framework for accounting for the properties of the scattering medium and the membrane material in dynamic fatigue. The fact that the membranes exhibit fatigue-like behaviour suggests that a definition of dose can be formulated based on Miner's Law. This definition gives the dose at failure in terms of the stress applied to the membrane by each shock wave and the failure stress. This analysis suggests that ESWL shock waves induce strain rates in tissue of order 10000 s⁻¹ and that behind a tissue, phantom membranes fail after a dose of about 1400 effective shock waves, of the same order as the number of shock waves typically used in lithotripsy therapy and research. Above strain rates of about 40000 s⁻¹, ESWL shocks cause virtually no damage.

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