More about Osteoporosis
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 $19 billion in hospital and nursing home costs each year, and these costs are increasing. As our population continues to age (more than 35 percent of Americans will be age 50 or older by 2011), osteoporosis is expected to become an even greater health problem.
Bones are not static, solid structures that do not change over time, although that’s what many people think. Bone is a living tissue that undergoes a constant process of renewal and rebuilding. 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 (scaffold) that is primarily composed of collagen. Collagen is soft but hardens as calcium and phosphorus are deposited. A regular supply of these minerals, which enter the bone from the blood, 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. This stage is followed by bone formation, which is carried out by another set of special cells, called osteoblasts, which fill in the cavities with new bone.
The entire process takes three to six months, which is why broken bones take some time to heal—and why it takes time to assess whether an osteoporosis treatment is working. When the rate of bone loss exceeds the rate of bone formation, the result is low bone mass.
Because bone mass is difficult to measure noninvasively, the closely related value of bone mineral density (BMD) is used instead. BMD can be measured with a variety of x-ray techniques.
Bone quality also plays a role in bone strength. Bone quality refers to the overall architecture of bone, which can vary from person to person. Because no noninvasive procedure currently can determine bone quality, measuring BMD is the best way to identify osteoporosis and predict a person’s risk of fractures.
How Osteoporosis Weakens the Bones
The hallmark of osteoporosis is weakened bones that fracture easily. This 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 the age of 30. Production of estrogen rapidly decreases after menopause, placing women at particularly high risk for osteoporosis. Women may lose 20 to 30% of their bone mass in the 10 years following their menopause.