Understanding of the physiology of skeletal tissue has advanced remarkably during the past few decades. Studies of the various cell types in bone and their interactions have led to the concept of a system involving not only systemic hormones but also local factors that regulate bone turnover. Also, molecular biology techniques are now being applied to bone cells and have provided rapid advances.

Structurally, bone must provide a framework for locomotion, must protect internal organs and marrow, and must be able to adapt to changing physical stress. Metabolically, the skeleton functions as a storehouse and as a homeostatic buffer system. Presumably, bone evolved to fulfill both its structural and metabolic roles as our ancestors moved from the calcium-rich, buoyant ocean to fresh water, and then to dry land. Although the major reservoir function of bone is to supply calcium and phosphorus, the skeleton also serves as a reservoir and source of other ions, such as magnesium and sodium, and as a buffer to deal with hydrogen ion excess. Also, the ability of the skeleton to take up a variety of trace elements may serve as an important safeguard against their toxicity. To achieve its mechanical functions, the skeleton needs to be selectively responsive to different kinds of strain, light, of high tensile strength, and rigid but not brittle.1,2 This is achieved by an orderly, slightly deformable, mineralized collagen structure distributed as a combination of dense cortical bone and spongy trabecular bone.