Fracture of cortical bone under controlled crack propagation and combined axial-torsional fatigue loading

Fatigue fractures of cortical bone, clinically known as stress fractures, result from increased levels of physical activities. Such fractures are known to occur in bones of the lower limb, including the metatarsals, calcaneus and tibia due to prolonged walking, marching or running. The tibia is of particular interest as it displays all four types of commonly found fatigue fractures, that is compression, oblique, transverse and longitudinal. On radiographs, each of these shows a different direction of crack propagation, or callus formation, depending on the activity, the age of the subject and the type of loading involved.

In this investigation in vitro controlled crack propagation and fatigue tests were conducted to determine the response of cortical bone to various types of loading.

A fracture mechanics approach has been applied to the mechanism of crack initiation and propagation in cortical bone and to study the influence of microcracks on the fracture behaviour of bone. Based on the velocity of crack propagation and detailed scanning electron microscopic evidence in human and bovine bone, a model of crack propagation has been proposed and subsequently validated by tests on red deer antler bone. Furthermore, the failure of individual microstructural constituents of bone has been quantified by counting the associated microcracks due to mode I (tension) and II (shear) type of loading. These tests have produced an understanding of the relative roles played in the fracture of bone by the microstructural constituents.

In vitro fatigue tests conducted on cortical bone for the first time under combined axial-torsional loading indicated that torsional loading influences the fatigue life of cortical bone. Depending on the relative level of torsional to axial loads, superposition of torsional loading on axial loading leads to a reduction in the fatigue life of cortical bone. The magnitude of reduction is independent of the level of axial load and depends only on the magnitude and phase of the applied torsional loading.