Dynamic Failure Mechanisms in Gallstones Using Lithotripsy

Abstract

Gallstones most frequently are twinned, cracked, very large grained crystals of cholesterol that have been grown in a saturated bile solution. In cross section, they are similar to spherulitic polymers although the composition of most gallstones is nearly pure cholesterol. The dynamic failure mechanism of “pulverization” during lithotripsy is at present only partially known. The literature suggest that cavitation next to the stone is the mechanism for stone destruction. Yet in vivo and in vitro observations show that many stones initially break approximately in half suggesting that internal flaws and dynamic loading, not cavitation with its surface removal processes, is the dominant mechanism. It is proposed that the stones break by crack propagation in a dominantly transient, reflected, stress field. Crack propagation and failure in gallstones is complicated by the large number of internal flaws, the low yield strength of the stones and the complex microstructure as it effects stress waves and dynamic fracture mechanisms. The physical properties of gallstones are presented from measurements of the elastic constants using literature wave speeds, hardness strengths and static fracture toughness. The static fracture properties have been measured on stones extracted from patients. These natural gallstones have very irregular shapes and a wide range of mechanical properties. While the surfaces of broken stones are very rough there is partial evidence of fatigue crack propagation. The cyclic crack propagation is probably due to stress intensity differences between compressive and tensile dynamic stresses on pre-existing cracks.