Bone Density Scan

Bone Density Scan

Medical Author: Jason C. Eck, DO, MS
Medical Editor: William C. Shiel Jr., MD, FACP, FACR

• What is osteoporosis?
• How does osteoporosis occur?
• What is "bone mineral density" (BMD)?
• Why is BMD measurement important?
• What is the relationship between BMD and fracture risk?
• Who should have BMD testing?
• How is BMD measured?
• What are other methods of measuring BMD?
• How often should DEXA scans be repeated to monitor treatment?
• What is the cost of DEXA?
• What about the accuracy of BMD testing in the doctor's office using smaller equipment?
• Summary
• Bone Density Scan At A Glance

What is osteoporosis?

Osteoporosis is a condition that is characterized by bones that are less dense than, and thus not as strong as, normal bone. Osteoporosis increases the risk of breaking bone (fracture) with even minor trauma, such as a fall from standing height, or even from a cough or sneeze. Unfortunately, people often do not realize they have osteoporosis until either they have a fracture or have a screening test ordered by their doctor to check for osteoporosis. Osteoporosis and low bone mass affect an estimated 44 million Americans (National Osteoporosis Foundation 2008). Of those, 10 million have osteoporosis, and the remaining 34 million have a lower than normal bone mass and are at higher risk of developing osteoporosis. Women are four times more likely to develop osteoporosis than men. Other risk factors include older age, family history of osteoporosis, small and thin stature, inactive lifestyle, smoking, alcohol, and use of certain medications including steroids. The treatment and management of osteoporosis is discussed in detail elsewhere on this site. This article will focus on the role of bone mineral density (BMD) evaluation.

How does osteoporosis occur?

In order to understand the role of bone mineral density scanning, it is important to know a little about how osteoporosis occurs. Bone is constantly being remodeled. This is the natural, healthy state of continuous uptake of old bone (resorption) followed by the deposit of new bone. This turnover is important in keeping bones healthy and in repairing any minor damage that may occur with wear and tear. The cells that lay new bone down are called osteoblasts, and the cells responsible for resorption of old bone are called osteoclasts. Osteoporosis occurs as a result of a mismatch between osteoclast and osteoblast activity. This mismatch can be caused by many different disease states or hormonal changes. It is also commonly a result of aging, change in normal hormones as occurs after menopause, and with diets low in calcium and vitamin D. In osteoporosis, osteoclasts outperform osteoblasts so that more bone is taken up than is laid down. The result is a thinning of the bone with an accompanying loss in bone strength and a greater risk of fracture. A thinning bone results in a lower bone density or bone mass.

There are two major types of bone. Cancellous bone (also known as trabecular bone) is the inner, softer portion of the bone, and cortical bone is the outer, harder layer of bone. Cancellous bone undergoes turnover at a faster rate than cortical bone. As a result, if osteoclast and osteoblast activity become mismatched, cancellous bone is affected more rapidly than cortical bone. Certain areas in the body have a higher ratio of cancellous bone to cortical bone such as the spine (vertebrae), the wrist (distal radius) and the hips (femoral neck).

Most of a person's bone mass is achieved by early adulthood. After that time, the bone mass gradually decline throughout the rest of a person's life. There is a normal rate of decline in bone mass with age in both men and women. For women, in addition to age, the menopause transition itself causes an extra degree of bone loss. This bone loss is greatest in the first three to six years after menopause. Women can lose up to 20% of the total bone mass during this time. Since women generally have a lower bone mass to begin with in comparison with men, the ultimate result is a higher risk of fracture in postmenopausal women as compared to men of the same age. Nevertheless, it is important to remember that men may also be at risk for osteoporosis, especially if they have certain illnesses, a low testosterone level, are smokers, take certain medications, or are sedentary. The best method to prevent osteoporosis is to achieve as high a bone mass by early adulthood with a proper diet and regular exercise. Unfortunately, osteoporosis is not often considered during this time in a person's life.

Source: MedicineNet.com
http://www.medicinenet.com/bone_density_scan/article.htm