What Causes Osteoporosis: Osteoporosis

Content provided by the Faculty of the Harvard Medical School

What causes osteoporosis?

Bone loss occurs when the cells that form bone (osteoblasts) cannot keep pace with the cells that are eating away at bone (osteoclasts). If you were to view a microscopic movie taken over time, you would see the osteoclasts going about business as usual, while the osteoblasts' efforts fall short. Although the tunnels dug by the osteoclasts aren't getting any deeper, neither are they being refilled completely (see Figure 3). As tunnels accumulate, the bone becomes thinner, more porous, and weaker than it once was.

Figure 3: When bone is lost Bone is constantly being demolished and rebuilt. If reconstruction lags behind demolition, then bone is lost. Osteoclasts gnaw at bone, releasing calcium and other minerals into the bloodstream and leaving troughs behind (A). If the osteoblasts that build bone cannot keep pace, these tunnels will not be completely refilled (B). The result is bone that is weaker and more susceptible to breaks.

There are no symptoms associated with such bone loss. But if it continues indefinitely, bones will eventually become too weak to bear the load they were designed to carry. The result is usually a fracture of the wrist, hip, or spine.

There are a variety of underlying reasons for osteoporosis. Experts use the following classifications to distinguish among the different causes.

Last updated:	January 23, 2007