A bone density test, or bone density scan, is designed to check for osteoporosis, a disease that occurs when the bones become thin and weak. Osteoporosis occurs when the bones lose calcium and other minerals that keep them strong.
Patient undergoing a bone density scan. Computer read-out of a bone density scan.
A bone density scan measures the strength of an individual's bones and determines the risk of fracture. An observation of any osteoporosis present can be made.
In 2008, the National Osteoporosis Foundation estimated that 10 million people in the United States over age 50 had osteoporosis, and another 34 million were at risk for developing the disease. Women are four to five times more likely than men to develop osteoporosis between ages 50 and 65. They havesmaller, thinner bones than men to begin with, and they lose bone mass more rapidly after menopause (usually around age 50) when they stop producing the female reproductive hormone estrogen, which has a bone-protecting effect. In the five to seven years followingmenopause, women can lose about 20% of their bone mass. By age 65, however, men and women lose bone mass at about the same rate. About half of all of men and women over the age of 75 have osteoporosis. As an increasing number of men live to an older age, there is more awareness that osteoporosis is an important health issue for them as well as for women. Although people of any ethnic background can develop osteoporosis, it is especially common among white and Asian women over age 50.
Many people are not aware that they have osteoporosis until they fracture (break) a bone. Typically, this happens in a fall that would not have caused a fracture in a young adult. Osteoporosis is estimated to be responsible for 2 million fractures annually. The National Institutes of Health (NIH) estimates that after age 50, half of all women and one out of every eight men will experience an osteoporosis-related fracture. These fractures can occur in any bone, but the most common locations are the hip, spine, and wrist. Breaks in the hip and spine are of special concern because they require hospitalization and often surgery and commonly cause a decrease in mobility or a permanent disability. Hip fractures are a leading cause of nursing home admissions in the elderly. Only about 15% of people who fracture a hip are able to walk across a room unaided six months later, and about one-fourth of people over age 50 who have a hip fracture die within one year.
Most bone density scans are done with a machine that uses a technology called Dual Energy X-ray Absorptiometry (DEXA). This machine takes a picture of the bones in the spine, hip, total body, and wrist and calculates the density of these bones. If a DEXA machine is not available, bone density scans can also be done with dual photon absorptiometry (measuring the spine, hip, and total body) and quantitative computed tomography scans (measuring the spine). Bone density scanners that use DEXA technology to just measure bone density in the wrist (called pDEXA scans) provide scans at some drugstores. These wrist scan tests are not as accurate as those that measure density in the total body, spine or hip.
Not all doctors routinely schedule this test. If the following factors apply to an individual, they may need a bone density scan and can discuss this with their doctor. Factors include if the individual:
To take a DEXA bone density scan, the individual lies on a bed underneath the scanner, which has a curving plastic arm that emits x rays. These low-dose x rays form a fan beam that rotates around the body. During the test, the scanner moves to capture images of the individual's spine, hip, or entire body. A computer then compares the individual's bone strength and risk of fracture to that of other people in the United States of the same age and to young people at peak bone density. Bones reach peak density at about age 30 and then start to lose mass. The test takes about 20 minutes and is painless. Some insurance companies and Medicare cover the cost. pDEXA wrist bone scans in drugstores are available at little cost.
The DEXA bone scan exposes the individual to only a small amount of radiation-about one-fiftieth that of a chest x ray, or about the amount a person is exposed to from taking a cross-country airplane flight.
The individual, when compared with people at "young normal bone density" (called the T-score), has the same (or higher) bone density as a healthy 30-year-old. T scores above 1 mean that an individual has a healthy bone mass. Scores from 0 to -1 mean that the individual has borderline bone mass and should repeat the test in two to five years.
The individual has two to four times the risk of a broken bone as other people in the United States at the same age and those at peak bone density. If an individual's T score ranges from -1 to -2.5 they have low bone mass and are at risk for osteoporosis. A T score below -2.5 means osteoporosis is already evident. These individuals should have a repeat bone density scan every year or two.