Introduction and synopsis

Superficially, bones look fairly solid. But looks are deceptive. Most bones are an elaborate construction, made up of an outer shell of dense compact bone, enclosing a core of porous cellular, cancellous, or trabecular bone (trabecula means ‘little beam’ in Latin). Examples are shown in Fig. II.I: cross-sections of a femur, a tibia, and a vertebra. In some instances (as at joints between vertebrae or at the ends of the long bones) this configuration minimizes the weight of bone while still providing a large bearing area, a design which reduces the bearing stresses at the joint. In others (as in the vault of the skull or the iliac crest) it forms a low weight sandwich shell like those analysed in Chapter 9. In either case the presence of the cancellous bone reduces the weight while still meeting its primary mechanical function.

An understanding of the mechanical behaviour of cancellous bone has relevance for several biomedical applications. In elderly patients with osteoporosis the mass of bone in the body decreases over time to such an extent that fractures can occur under loads that, in healthy people, would be considered normal. Such fractures are common in the vertebrae, hip and wrist, and are due in part to a reduction in the amount of cancellous bone in these areas. The degree of bone loss in a patient can be measured using non-invasive techniques, so an understanding of the relationship between bone density and strength helps in predicting when the risk of a fracture has become high.