Bone Remodelling

“…bone is continually undergoing synthesis and resorption, even after linear growth has ceased. This is reflected in the high flow of both calcium and phosphate into and out of the bone each day… This process of continued bone turnover is termed remodeling. Synthesis and resorption of bone can occur along most of the outer surface of the bone, making the bone thicker or thinner, respectively. In the long bones [such as the shin], synthesis and resorption also occur along the inner surface of the bone shaft, adjacent to the bone marrow cavity. Remodeling reshapes and reforms bone to meet the changing mechanical needs that occur throughout life. A good illustration of such a case would be a cowboy’s bowed legs, in which the curvature of the legs is in part the result of the continued pressure on the legs as he [or she] rides his [or hers] horse.”

Human Physiology, Rhodes & Pflanzer, Saunders College Publishing, 1989, p.812

In 1892, Julius Wolff published his work on bone transformation which is known today as bone remodeling. In his discourse, Wolff explained that bones respond to the presence and absence of forces with changes of size, shape, and density. According to Wolff’s law, the size, shape, and density of a bone change depending on the magnitude and direction of mechanical stress on the bone.

Remodeling includes the sensing of environmental changes, the formation of new bone (synthesis), and the removal of existing bone (resorption). Illnesses such as osteoporosis result from disturbances in the control of remodeling.

The core elements of bone are calcium carbonate, calcium phosphate, collagen, and water. Calcium carbonate and phosphate generally constitute approximately 60% to 70% of bone weight. These minerals give bone its stiffness and strength. In some respects, bone tissue closely resembles reinforced concrete: its hard, inflexible calcium phosphate matrix is reinforced by flexible fibers of collagen, a protein that plays a role similar to that of steel rebar in reinforced concrete. Collagen is a structural protein. Other minerals such as magnesium, sodium, and fluoride are also in charge of vital structural and metabolic roles in bone growth and development.