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Bone density (or bone mineral density) is a medical term normally referring to the amount of mineral matter per square centimeter of bones. Bone density (or BMD) is used in clinical medicine as an indirect indicator of osteoporosis and fracture risk.
This medical bone density is not the true physical "density" of the bone, which would be computed as mass per volume. It is measured by a procedure called densitometry, often performed in the radiology or nuclear medicine departments of hospitals or clinics. The measurement is painless and non-invasive and involves low radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip. The forearm may be scanned if the hip and lumbar spine are not accessible.
There is a statistical association between poor bone density and higher probability of fracture. Fractures of the legs and pelvis due to falls are a significant public health problem, especially in elderly women, leading to much medical cost, inability to live independently, and even risk of death. Bone density measurements are used to screen women for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.
Bone density tests are not necessary for people without risk factors for weak bones. Unnecessary testing is more likely to result in superfluous treatment rather than discovery of a true problem.
The following are risk factors for low bone density and primary considerations for the need for a bone density test.
Other considerations which related to risk of low bone density and the need for a test include smoking habits, drinking habits, the long-term use of corticosteroid drugs, and a vitamin D deficiency.
For those people who do have bone density tests, two conditions which may be detected are osteoporosis and osteopenia. The usual response to either of these indications is consultation with a physician.
Results are often reported in 3 terms:
While there are many different types of BMD tests, all are non-invasive. Most tests differ in which bones are measured to determine the BMD result.
These tests include:
The test works by measuring a specific bone or bones, usually the spine, hip, and wrist. The density of these bones is then compared with an average index based on age, sex, and size. The resulting comparison is used to determine risk for fractures and the stage of osteoporosis in an individual.
Average bone mineral density = BMC / W [g/cm2]
Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.
The T-score is the relevant measure when screening for osteoporosis. It is the bone mineral density (BMD) at the site when compared to the young normal reference mean. It is a comparison of a patient's BMD to that of a healthy thirty-year-old. The US standard is to use data for a thirty-year-old of the same sex and ethnicity, but the WHO recommends using data for a thirty-year-old white female for everyone. Values for thirty-year-olds are used in post-menopausal women and men over age 50 because they better predict risk of future fracture. The criteria of the World Health Organization are:
|WHO category||Age 50–64||Age > 64||Overall|
The Z-score is the comparison to the age-matched normal and is usually used in cases of severe osteoporosis. This is the number of standard deviations a patient's BMD differs from the average BMD of their age, sex, and ethnicity. This value is used in premenopausal women, men under the age of 50, and in children. It is most useful when the score is less than 2 standard deviations below this normal. In this setting, it is helpful to scrutinize for coexisting illnesses that may contribute to osteoporosis such as glucocorticoid therapy, hyperparathyroidism, or alcoholism.
Use of BMD has several limitations.
There may be associations between some forms of BMD and diet.