The mechanical effects of focused shock waves on tissuelike structures

Abstract

Shock waves focused to strengths sufficient to fracture kidney stones in extracorporeal shock wave lithotripsy (ESWL) injure soft tissue. In a complex organ like the kidney, the injury occurs in a sequence of processes beginning with the mechanical stimulus during or shortly after the passage of the shock wave, followed by a complex series of bio-mechano-chemical responses, terminating with altered function and histology of the organ. This paper presents preliminary results of an invitro study of the initial mechanical stimulus. A planar nitrocellulose membrane of order 10 μm thick immersed in liquid is used as a simple model of tissue membrane. Use of thoroughly degassed water, glycerin, and castor oil at the focus of an electrohydraulic lithotripter controls cavitation, and addition of 20-μm-diam hollow glass spheres to the test liquid simulates the acoustic nonuniformity of tissue. In water, small (∼1-mm diameter) circular punctures are generated by bubble collapse after only one or two shock waves. In noncavitating, uniform liquids the membrane only fails after of order 100 shock waves, by cavitation. With the addition of acoustic nonuniformity in order to increase shear induced by the shock, a different mode of failure occurs; the membrane fails in long (∼1 cm) tears after about 20 shocks. [Work supported by NIH Grant P01 DK43881-01A3.]