Abstract
Contrary to earlier concepts, it is now recognized that bone takes an active and important part in the general homeostasis of the body. Bone in turn is profoundly affected by changes in the internal and external environment, leading to manifold pathologic conditions, the most common among them being osteoporosis. Regardless of its etiology (disuse, post-menopausal, Cushing's syndrome, old age and others), osteoporosis is characterized by decreased bone substance per unit volume of bone. Microscopically, the bone that remains is essentially normal but new bone formation is usually not evident (42). Of the various types of osteoporoses mentioned above, that of disuse, i.e., osteoporosis following fracture or paralysis, by its very nature lends itself most easily to experimental study. | The mechanism leading to osteoporosis of disuse is not yet fully understood. It is known, however, that any impairment of one of the two functions of the skeleton — i.e., (1) to give structural support to the body and (2) to serve as the body's calcium reservoir -- is reflected in the skeleton as well as the entire body. To attain harmony between these two opposing functions, bone formation and bone resorption rates must balance each other. Thus, under normal conditions in the adult individual, formation and resorption take place at rates which provide for the body's calcium requirements without weakening the skeletal system. With disuse, this balance is upset and osteoporosis sets in. | In the mechanism of disuse osteoporosis several local factors may be operative, such as: mechanical forces (muscle pull and pressure) and nervous and circulatory changes. | Studies dealing with these factors as well as those concerned with bone formation and resorption rates, will be discussed in the following sections.