Physiology of Bone
Bone Mechanics and Remodeling
Elasticity is the characteristic of a substance that enables it to change shape when subjected to stress but return to its original shape when the stress is removed.
Mature bone is relatively inelastic. A rod of bone can be elongated only about 0.5% of its length before breaking. However, even this much deformity is not perfectly elastic; the deformity is permanent, and the bone will not return completely to its original length if stretched near its breaking point. This characteristic of deforming under stress without returning to the original shape is exaggerated in bone diseases such as rickets.in addition to tension (stretching), bone may be subjected to stresses of compression, shearing, bending, and torsion (twisting). A bone will support considerably more weight in a static situation (supporting weight without moving) than under a dynamic load. A dynamic load results from impact between the bone and another object. For example, the leg bones of a horse bear a static load when the horse is standing quietly, but bear a dynamic load when the horse is running, jumping, or kicking. Compression, bending, and shearing of the leg bones are all stresses produced by this type of activity. When a horse or other animal pivots with one or more feet bearing weight, torsion or twisting is added to the other stresses. This is seen particularly well in the action of cutting horses. Muscles and tendons that run parallel to a bone tend to act like guy wires and reduce stresses, particularly bending and shearing stresses.
Bone, even in a fresh carcass, appears hard, dense, inelastic, and almost lifeless. Actually, bone is quite a dynamic tissue, and all bone is constantly being formed and resorbed. The continuous turnover of bone in mature animals is termed remodeling. Through remodeling, bone can shrink (atrophy), increase in size (hypertrophy), repair breaks, and rearrange its internal structure to best resist stresses and strains.
in both normal and pathologic conditions, bone can reshape itself according to good engineering principles to sustain a maximum of stress with a minimum of bone tissue. Atrophy of bone occurs when pressure is constant and excessive or when there is little or no stress, as in weightlessness in space or when a limb is immobilized and not bearing weight. Proliferation of bone may occur in response to concussion or intermittent pressure. Thus, pressure can cause either atrophy or proliferation, depending on the degree and duration of stress and the maturity of the bone. Excessive pressure on growing bone slows or stops growth, while in mature bone it may stimulate either excessive growth or rearrangement of structure.Calcium of Bone
In 100 cc of bone there is 10 g of calcium, as compared with 6 mg per 100 cc for most tissues and about 10 mg per 100 mL for blood. Thus, bone serves as a reservoir of minerals (especially calcium), which are constantly being either replenished or depleted. Through the action of osteoclasts and osteocytes, calcium can be taken from this reservoir when serum calcium levels are low. The reservoir can be replenished by the action of osteoblasts and osteocytes.
The activity of the cells within bone is subject to regulation by the hormones parathyroid hormone (PTH) and calcitonin. The sources of these hormones and the regulation of their secretion are discussed in Chapter 12. The overall effect of PTH is to increase serum calcium by increasing the net release of calcium salts from bone. This effect is due in part to increased osteoclast activity and inhibition of osteoblast activity. PTH also affects osteocytes in mature bone, and these cells also play a role in the rapid release of calcium salts in response to PTH. However, the mechanism by which osteocytes promote a rapid release of calcium is poorly understood. The primary effect of calcitonin is to reduce osteoclast activity, which tends to lower serum calcium. Thus, the normal regulation of serum calcium concentrations involves a balancing of the effects of pTH and calcitonin on the cells of bone.