Bone Remodeling

The process of bone formation and breakdown that occurs continuously in the body. Bone remodeling involves two simultaneous processes. In the first, known as bone resorption, cells called osteoclasts break down bone, releasing calcium and other minerals into the bloodstream for use in other parts of the body. In the second, known as bone formation, other cells called osteoblasts lay down a matrix of a fibrous substance called collagen and become embedded in this matrix to become new bone cells (called osteocytes). As calcium and other minerals from the bloodstream are deposited into the bone matrix, new bone tissue forms.

During childhood and adolescence, bone formation occurs faster than bone resorption, allowing bones to become larger and denser. By the time we’re in our thirties, bone formation has slowed down and the breakdown of bone has accelerated, leading to bone loss. If resorption outpaces formation by too much, osteopenia (diminished bone mass) may result. In time, osteopenia can lead to full-blown osteoporosis, which is marked by bones that are porous, fragile, and more likely to fracture.

Changes to the bone remodeling process can increase or decrease a person’s risk for osteoporosis. For example, rheumatoid arthritis, systemic lupus erythematosus, and other types of inflammatory arthritis can all affect the bone remodeling process, decreasing bone density and increasing osteoporosis risk. So can the use of corticosteroids, smoking, excessive alcohol consumption, inadequate exercise, and inadequate intake of calcium or vitamin D.

Drugs used to treat or prevent osteoporosis also have an effect on the bone remodeling process, but in a positive way. They slow down bone loss and increase bone mass. They include drugs called bisphosphonates, such as alendronate (Fosamax), risedronate (Actonel), and ibandronate (Boniva); raloxifene (Evista); the synthetic hormone teriparatide (Forteo); and the biologic drug denosumab (Prolia).

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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