Properties of gallstones and their relationship to destruction mechanisms in acoustic lithotripsy

Abstract

As part of an investigation of the mechanisms of gallstone destruction in extracorporeal shockwave lithotripsy, the effectiveness of lithotripsy has been examined as well as the form of the resulting damage to simulated and biological concretions as a function of several physical properties of the stones: air content, hardness, fracture toughness, flaw size distribution, chemical composition, and crystal structure. Measurements on plaster suggest that increasing air content tends to promote damage suggestive of a cavitation mechanism of destruction, while degassing promotes rearsurface damage (spalling) suggestive of a stress-fracture mechanism. It has been found that some human gallstones contain air in vivo, that all gallstones that have been allowed to dry contain significant quantities of air, and that during lithotripsy stones containing air break up more easily and into finer pieces than fully hydrated stones. The measured hardness of gallstones is roughly ten times smaller than that of kidney stones; this finding suggests that a ductile, rather than brittle, mechanism of fracture or crack growth may be responsible in gallstone lithotripsy. Most gallstones exhibit a preferred radial pattern of crystal orientation, which may have implications for strain-wave propagation within the stone material. [Work supported by the F. V. Hunt Postdoctoral Fellowship of the Acoustical Society of America and the National Institute of Health through U.S.P.H.S. Grant No. DK 39796.]