As important as the structural properties of bone is the role bone plays in the maintenance of the ionic composition of the blood and interstitial fluids of the body. All vertebrates possessing true bone exhibit body-fluid calcium ion concentrations of approximately 50 mg per litre (1.25 millimoles) and phosphorus concentrations in the range of 30–100 mg per litre (1–3 millimoles). These levels, particularly those of calcium, are extremely important for the maintenance of normal neuromuscular function, interneuronal transmission, cell membrane integrity and permeability, and blood coagulation. The rigid constancy with which calcium levels are maintained, both in the individual and throughout all higher vertebrate classes, attests to the biological importance of such regulation. Approximately 99 percent of total body calcium and 85 percent of total body phosphorus reside in the mineral deposits of bone; thus, bone is quantitatively in a position to mediate adjustments in concentration of these two ions in the circulating body fluids. Such adjustments are provided by three hormonal control loops (control systems with feedback) and by at least three locally acting mechanisms. The hormonal loops involve parathyroid hormone (PTH), calcitonin (CT), and vitamin D and are concerned exclusively with regulation of calcium ion and phosphorus ion concentrations.

PTH and vitamin D act to elevate ionized calcium levels in body fluids, and C…

In the language of control mechanics, remodeling depends upon two control loops with negative feedback: a homeostatic loop involving the effects of PTH and CT on resorption and a mechanical loop that brings about changes in skeletal mass and arrangement to meet changing structural needs. The PTH-CT loop is basically a systemic process, and the mechanical loop is local; however, the two loops interact significantly at the level of the cells that act as intermediaries in both processes. A large number of other factors, including minerals in the diet, hormonal balance, disease, and aging, have important effects on the skeleton that interact with the control system.

The controls exerted by mechanical forces, recognized for over a century, have been formulated as Wolff’s law: “Every change in the function of a bone is followed by certain definite changes in its internal architecture and its external conformation.” Of the many theories proposed to explain how mechanical forces communicate with the cells responsible for bone formation and resorption, the most appealing has been postulation of induced local electrical fields that mediate this information exchange. Many crystalline or semicrystalline materials, including both bone collagen and its associated mineral, exhibit piezoelectric properties. Deformation of macroscopic units of bone by mechanical force produces a charge in the millivolt range and current flow on …