Are you at risk for osteoporosis? Although many of us think of our bones as stable, solid structures that do not change with time, the truth is that bone is living tissue that undergoes a constant process of rebuilding.

The word osteoporosis means porous bone. A person with osteoporosis typically has low bone mass, poor bone quality, and fragile bones. This combination, together with the increased risk of falling among older people, leads to painful fractures and other health problems. 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, osteoporosis is expected to become an even greater health problem.

The hallmark of osteoporosis is weakened bones that fracture easily. Osteoporosis results from an abnormality in a natural process known as bone remodeling. As part of its normal cycle, old bone is broken down (resorbed) by cells called osteoclasts, and new bone is formed by cells called osteoblasts to replace the bone removed by resorption.

The body maintains bone mass as long as bone replacement keeps pace with bone resorption. Osteoporosis develops when resorption occurs faster than replacement. The spine, in particular, is susceptible to this imbalance.

Osteoporosis often leads to thinning of the bone and disruption of the struts called trabeculae that make up the bone architecture. In the vertebrae, this can cause vertebral compression fractures. Part of the hip joint, the femoral neck, also is highly vulnerable to fractures from osteoporosis.

Osteoporosis arises largely as a result of increasing age and a decline in the level of sex hormones. Normally, estrogen in women and men, as well as testosterone in men, maintain bone mass. Peak bone mass is reached around age 30. Estrogen production rapidly declines after menopause, placing women at particularly high risk for osteoporosis. Women may lose 20–30% of their bone mass in the 10 years following their menopause.