Skeletal Properties

Bone: We have seen before that bone is uniquely structured to be lightweight and strong at the same time. This purposeful construction to efficiency is evident the more we look into it. Biomechanically, bone is akin to a biphasic composite structure, of organic and inorganic phases. Organic component gives it flexibility and the inroganic component provides rigidity. The composite structure is far stronger than the original constituents alone. Microscopically bone is composed of cortical and cancellous bone. Cortical bone is stiffer than cancellous bone. 

Bone being viscoelastic shows loading rate sensitivity and becomes stiffer, stronger and more brittle at higher loading rates. Bone responds to load by deformation. This ability depends on its ability to store energy. Older bone has low ductility, is less able to store energy than a young one and fails at a lower level of load. 

Low energy loading results in clean cut fracture. High energy loading results in comminuted fracture.

Muscle contraction is important to regulate bone loading. They can neutralise tensile load and allow bone to carry increased load. Fatigue failure can result from muscle weakness. 

Bone behaviour is also influenced by its geometry. Bone response to bending and torsion follows the same principle, so distribution of bone mass away from the neutral axis is helpful. This is why long bones are tubular in shape. Bending moment is also influenced by its length. The longer a bone, the more its bending moment and resultant stress. 

A long bone can act as a column, supporting compressive load along its long axis. or as a shaft, to resist torsion or as a beam , resisting bending moments. Tibia acts as a column to support body weight , neck of femur resists bending moments. When the foot is twisted, tibia resists torsion. 

Bone structure is dissimilar in longitudinal and transverse directions, this results in anisotropic behaviour. 

Bone behaves differently under different loading conditions. It is stronger in compression than tension than shear. Different types of loading also produces different types of fracture.