Physiology, Bone Remodeling

Bones are not inert structures within the human body; they continue to change throughout the lifespan. This process of skeletal change is known as bone remodeling, which both protects the structural integrity of the skeletal system and metabolically contributes to the body's balance of calcium and phosphorus. Remodeling entails the resorption of old or damaged bone, followed by the deposition of new bone material.  The German anatomist and surgeon Julius Wolff developed a law that describes the nature of bone remodeling regarding stresses. Wolff's Law states that bones adapt to the degree of mechanical loading, such that an increase in loading causes the architecture of the internal, spongy bone to strengthen, followed by the strengthening of the cortical layer. Furthermore, a decrease in stress on the bone causes these bone layers to weaken. The duration, magnitude, and rate of forces applied to the bone (in other words, tendons pulling at their attachments) dictate how the integrity of the bone is altered.  There are 2 primary cells responsible for both the resorption and deposition phases of bone remodeling: osteoclasts and osteoblasts; however, osteocytes also have a role in this process. The activity of these cells, particularly the osteoclasts, is influenced directly or indirectly by hormonal signals. This interaction between bone remodeling cells and hormones creates the opportunity for a multitude of pathophysiological consequences.