The resorption and removal of bone by osteoclasts shapes bones during development and continues throughout life. In the healthy state, bone resorption is coupled with new bone deposition, and the paired process is called bone remodeling. Bone remodeling in the adult frees mineral from the skeleton and removes microscopic areas of damaged bone.
Thus, bone remodeling contributes to both mineral homeostasis and skeletal homeostasis, although mineral homeostasis takes precedence: insufficient amounts of calcium can cause cardiac arrhythmias and sudden death, whereas the transient loss of bone structure, however important, does not typically cause rapid and devastating consequences. Bone remodeling is initiated by both chemical and mechanical signals. The primary chemical signal is PTH, which is mobilized in response to low serum calcium. Estrogen, too, is an important chemical signal. It helps ensure that the actions of osteoclasts are coupled with those of osteoblasts. When an estrogen shortage occurs, the activities of osteoclasts may exceed those of osteoblasts, resulting in decreased skeletal mass over time. This is the phenomenon that underlies perimenopausal osteoporosis. Testosterone also affects bone metabolism via osteoblast function, but its role is less clear than the role of estrogen. Skeletal signals in the form of physical stresses can also stimulate bone remodeling. Bone is deposited in areas where load bearing is high, a process known as Wolff’s law. Bone deposition may be mediated by direct stress receptors on the bone cells or by the electrical potentials created by deforming (ie, bending) the bone under load bearing. The converse of Wolff’s law also applies to bone remodeling: bone will be resorbed in areas not subject to stress. Accordingly, disuse will lead to significant bone loss. This has practical relevance. For example, a bone that is casted for treatment of a fracture will be weaker than normal, even in the areas that were not fractured. Bone remodeling requires biologic substrate (material) and energy (power). Thus, bone remodeling takes place only in areas with an adequate blood supply. If an area of bone infarcts (ie, is cut off from circulation and dies), then that area will not remodel. This occurs in osteonecrosis, a process in which the inability to repair subchondral bone eventually leads to fracture and collapse within the joint.