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Osteoporosis

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of osteoporosis.

Introduction

Osteoporosis is a disease of the skeleton in which bones become brittle and prone to fracture. In other words, the bone loses density. Osteoporosis is diagnosed when bone density has decreased to the point where fractures occur with mild stress.

Until a healthy person is around age 40, the process of breaking down and building up bone by cells called osteoclasts and osteoblasts is a nearly perfectly coupled system, with one phase stimulating the other. As a person ages, or in the presence of certain conditions, this system breaks down and the two processes become out of sync. The reasons why this occurs during aging are not clear. Some individuals have a very high turnover rate of bone, some have a very gradual turnover, but the breakdown of bone eventually overtakes the build-up.

The Bones The Function of Bones The skeleton has a dual function: It provides structural support for muscles and organs. It also serves as a depot for the body’s calcium and other essential minerals, such as phosphorus and magnesium. The skeleton holds 99% of the body’s calcium. The remaining 1% circulates in the blood and is essential for crucial bodily functions, ranging from muscle contraction to nerve function to blood clotting. Bone Turnover: the Breakdown and Growth of Bones Like other organs in the body, bone tissue is constantly being broken down and reformed again. This turnover is necessary for growth, for repair of minor damage that occurs from everyday stress, and for the maintenance of a properly functioning body. Two essential cells are involved in this process: Osteoblast cells are produced by bone cells and are the bone builders. They rebuild the skeleton, first by filling in the holes with collagen, and then by laying down crystals of calcium and phosphorus. Osteoclast cells are formed from certain blood cells and are responsible for the breakdown, or resorption , of the skeleton. These cells dig holes into the bone and release the small amounts of calcium into the bloodstream that are necessary for other vital functions. Each year, about 10 - 30% of the adult skeleton is remodeled in this way. The bone build up (formation)-break down (resorption) balance is controlled by a complex mix of hormones and chemical factors. If bone resorption occurs at a greater rate than bone build up, your bone loses density and puts you at risk for osteoporosis. In women, estrogen loss after menopause is associated with rapid resorption and loss of bone density. This group, then, is at highest risk for osteoporosis and therefore for fracture.

Primary Osteoporosis

There are two primary kinds of osteoporosis: type I and type II:

• Type I. Type I, or high turnover, osteoporosis occurs in 5 - 20% of women, most often between the ages of 50 and 75. This is because of the sudden postmenopausal decrease in estrogen levels, which results in a rapid depletion of calcium from the skeleton. This is associated with fractures that occur when the vertebrae compress together, causing a collapse of the spine. It is also associated with fractures of the hip, wrist, or forearm caused by falls or minor accidents. Type 1 accounts for the significantly greater risk for osteoporosis in women than in men.
• Type II. Type II, or low turnover, osteoporosis (also known as age-related or senile osteoporosis) results when the process of resorption and formation of bone are no longer coordinated, and bone breakdown overcomes bone building. (This occurs with age in everyone to some degree.) Type II osteoporosis affects both men and women and is primarily associated with leg and spinal fractures. Older women can have both type I and type II osteoporosis.

The determining factor for the actual existence of osteoporosis, whether type I or type II, is the amount of calcium left in the skeleton and whether it places a person at risk for fracture. Someone who has exceptionally dense bones to begin with will probably never lose enough calcium to reach the point where osteoporosis occurs, whereas a person who has low bone density could easily develop osteoporosis despite losing only a relatively small amount of calcium.

Secondary Osteoporosis

Secondary osteoporosis is caused by other conditions, such as hormonal imbalances, diseases, or medications (such as corticosteroids or anti-seizure drugs). Details on the many other causes of secondary disease are included throughout this report.

• Review Date: 10/18/2006
• Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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