The living tissue that makes up the human skeleton. Each of the skeleton’s more than 200 bones is composed of two types of tissue: cortical bone, the dense and hard outer shell that gives the bone most of its strength, and trabecular bone, a fine, lattice-like network found in the inside of the bone. Bone marrow, a soft substance, fills the spaces within the bone. Different bones have different proportions of each type of tissue. For example, bones in the spine (the vertebrae) are mostly trabecular bone, with a thin outer layer of cortical bone. The long bones of the legs and arms are mostly cortical bone, with trabecular bone concentrated at the ends.

Bone tissue is made up of a framework or matrix of protein, most of it a type of protein called collagen. Calcium and phosphorus crystals are embedded in the matrix, held in place by a “glue” composed of other minerals, including potassium, sodium, and fluoride. The calcium and other minerals give bone strength and hardness; the collagen gives it flexibility.

Bone is constantly in a state of flux known as remodeling, a process in which old bone is broken down and new bone is formed. When calcium levels in the blood drop too low, cells called osteoclasts break down bone in order to release calcium into the bloodstream, where it can be used by the body for other purposes. This process is called bone resorption. In bone formation, calcium from the bloodstream and cells called osteoblasts are constantly being incorporated into the bone matrix, forming new bone.

In children, teens, and young adults, bone formation outpaces bone resorption, and the bones grow bigger and stronger. Around age 30, the balance tips the other way, and people begin to gradually lose more bone than they form. In women, the loss of bone mass tends to accelerate after menopause. If the density of bone drops below a certain level — as determined by a test known as a bone mineral density scan — a person is diagnosed with osteoporosis (which literally means “porous bones”). People with osteoporosis are at increased risk of bone fractures, especially in the spine, hips, and wrists. In addition, inflammatory types of arthritis, such as rheumatoid arthritis, can degrade bone and contribute to osteoporosis. Corticosteroid drugs, such as prednisone, can cause bone loss as well.

There are many drugs available that aim to treat or prevent osteoporosis by altering the processes of bone formation and resorption. The most common of these are bisphosphonates, which include alendronate (Fosamax), risedronate (Actonel), ibandronate (Boniva), and zoledronic acid (Reclast). Bisphosphonates work by blocking bone resorption, allowing the bone to remain stronger. Teriparatide (Forteo), a synthetic form of parathyroid hormone, can spur the body to build new bone. Denosumab (Prolia), another osteoporosis drug, blocks a chemical that is involved in bone resorption.

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

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