From Jonathan Cluett, M.D.,
Your Guide to Orthopedics.
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

What is osteoporosis?
Osteoporosis is a condition that causes thinning and weakening of normal bone. The definition of osteoporosis is a decrease of the density of bone mass. When this occurs, a patient with osteoporosis will have weaker bones and have a higher risk of bone fracture. Osteoporosis is not arthritis, which leads to problems within joints due to cartilage wear. Instead, osteoporosis is a problem of the bone, and its ability to adequately support the weight of your body.

There are two main categories of osteoporosis, Type I and Type II. Type I osteoporosis occurs only in post-menopausal women, and is due to estrogen deficiency. Type II osteoporosis occurs in both men and women (about two times more frequently in women), and is due to aging, and calcium deficiency over many years.

What causes osteoporosis?
As people age, the amount of bone in their body steadily decreases. Women are especially prone to developing thin bone because they don't develop as much bone while younger, and the rate of bone loss in women is greater than men. Because of this, age and gender are the most important risk factors for developing osteoporosis.

Both men and women achieve their "peak bone mass" in the third decade of life. After that time, bone mass gradually, but steadily decreases. In pregnant and lactating women, the rate of bone loss will temporarily increase if the increased calcium demands of pregnancy or breastfeeding are not met by increased dietary intake of calcium. In women, there is also a significant decrease of bone mass in the immediate postmenopausal period.

Other important risk factors that can contribute to developing osteoporosis include Northern European ancestry, hypothyroidism, anticonvulsant medications, and a sedentary lifestyle. Americans are especially prone to developing osteoporosis, the exact cause of this is not known. We do know that this is not entirely related to ancestry, as studies have shown that individuals who immigrate into the United States from other countries develop an American's higher risk of osteoporosis.

How is osteoporosis diagnosed?
Osteoporosis most commonly is found either on routine examination, or following a pathologic fracture (a fracture due to weakened bone). X-rays usually show a generalized loss of bone density. Laboratory studies are not too helpful in showing evidence of osteoporosis, but they are helpful in showing evidence of conditions that can have symptoms similar to osteoporosis. Laboratory studies can show evidence of osteomalacia, kidney failure, parathyroid gland insufficiency, or other problems that can mimic osteoporosis and cause bone weakness.

When screening for osteoporosis, or trying to detect the early stages of the condition, the most useful test is called bone densiometry, or DEXA scan. While this test does require special equipment, it is safe, exposes the patient to small amounts of radiation, and is useful in detecting the early stages of osteoporosis.

Fractures are the most common problem associated with osteoporosis. A pathologic fracture is a broken bone that occurs because the bone is abnormally thin. When a fracture occurs in an area of osteoporotic bone, it is called a pathologic fracture. The most commonly fractured bones due to osteoporosis are the vertebral bones of the spine, the wrist, and the hip.