About 10 million Americans -- eight million women and two million men -- already have osteoporosis, and 34 million more are at increased risk because of low bone mass (osteopenia). Osteoporosis-related fractures are estimated to account for $13.8 billion in hospital and nursing home costs each year, and these costs are increasing. As our population continues to age (more than 35% of Americans will be age 50 or older by 2011), osteoporosis is expected to become an even greater health problem.

Many people think of their bones as stable structures that do not change with time. But bone is a living tissue that undergoes a constant process of renewal. In this process, called bone remodeling, old bone is resorbed (broken down) and new bone is formed.

The framework of each bone is a matrix, or scaffold, that is mostly made up of collagen. Although collagen is soft, it hardens with the deposition of calcium and phosphorus, which enter the bone from the bloodstream. A regular supply of these minerals is required to keep bones strong.

In the resorption stage of bone remodeling, special cells called osteoclasts invade the surface of the bone and remove both the matrix and minerals, leaving small cavities in the bone surface. Resorption is followed by the bone formation stage, which is carried out by another set of special cells, called osteoblasts, which fill in the cavities with new bone.

When bone loss outpaces bone formation, the result is low bone mass. When there is less bone, bones become more fragile and are more prone to fracture. Osteoporosis is a disease of severely low bone mass.