Biomechanics of Bone

This chapter focuses on the biomechanical properties of bones. Bone is a physiologically dynamic tissue whose primary functions are to provide a mechanical support system for muscular activity, provide for the physical protection of organs and soft tissues, and act as a storage facility for systemic mineral homeostasis. The resulting structure of the skeleton is influenced heavily by mechanical principles, acting both as constraints and as driving forces in its architecture. The mechanical behavior of bone may be studied at two levels, material and structural. The material, or tissue, level properties of bone are evaluated by performing standardized mechanical tests on uniform bone tissue samples. They may also be estimated in vivo by using densitometric projections, but these are less accurate than actual mechanical testing. The assessment of bone mechanical properties can be made using techniques ranging from noninvasive imaging to in vitro mechanical tests of excised specimens or whole bones. Microcomputed tomography (CT), magnetic resonance imaging (MRI), and peripheral qualitative CT (pQCT) methods of imaging, especially in use with finite element models, continue to improve in their accuracy of mechanical property assessment. The strength and rigidity of a bone are typically greater in the direction of customary loading, particularly in cortical bone, where osteons are oriented in a longitudinal direction as indicated by the bone's loading history. Cortical bone is stronger in compression than in tension, and after maturation the tensile strength and the modulus of elasticity of femoral cortical bone decline by approximately 2% per decade. Another important mechanical property exhibited by bone is known as viscoelasticity, which is largely due to its water content, whereas material properties, such as strength and toughness, come from its solid-phase components.