An overlap in the bone density values in patients with or without hip fractures suggests that factors other than bone mass including bone quality may also contribute to fracture risk. Traditionally, bone quality has been measured through mechanical parameters determined from uniaxial loading. However, evidence is gradually accumulating that in vivo loading of bone is multiaxial and that the fracture of materials subjected to multiaxial loading cannot be predicted from uniaxial tests.

In this investigation, in vivo strain gauge data was analyzed and used for conducting a variety of in vitro fatigue tests to determine the influence of multiaxial loading parameters and physiologically relevant axial-torsional loading on aging human bone. It was found that the damage mechanisms and fatigue behavior of cortical bone varies with the components of physiological loading (tension, compression, and torsion) and the interrelationships (phase angle) between axial and torsional loading. Torsional loading was identified as the most aggressive damage-inflicting component and it’s effects were modeled by a cumulative damage model based on time (creep) and cycle (fatigue) damage mechanisms.

Physiologically relevant multiaxial loading was found to reduce the fatigue life of human cortical bone by 2 to 20 folds. More significantly, bone fragility increased with age under physiologically relevant loading due to a decrease in osteon size and an increase in interstitial bone area that influenced microcracking and increased the susceptibility of older bone to mixed mode fracture. Furthermore, age related differences in the multiaxial fatigue behavior of bone at a variety of load levels including physiological levels were successfully predicted (R²=0.83) based on damage models incorporating shear strain alone and the interaction of normal and shear loading