Bone mineral density
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. Average density is around 1500 kg/m3 or 1.5 kg/L .
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.
Indications for testing
The following are risk factors for low bone density and primary considerations for the need for a bone density test.
- females age 65
- males age 70
- people over age 50 with any of the following:
- Individuals with vertebral abnormalities.
- Individuals receiving, or planning to receive, long-term glucocorticoid (steroid) therapy.
- Individuals with primary hyperparathyroidism.
- Individuals being monitored to assess the response or efficacy of an approved osteoporosis drug therapy.
- Individuals with a history of eating disorders
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.
Overtesting and treatment
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:
- Measured areal density in g cm−2
- z-score, the number of standard deviations above or below the mean for the patient's age, sex and ethnicity
- t-score, the number of standard deviations above or below the mean for a healthy 30 year old adult of the same sex and ethnicity as the patient
Use of BMD has several limitations.
- Measurement can be affected by the size of the patient, the thickness of tissue overlying the bone, and other factors extraneous to the bones.
- Bone density is a proxy measurement for bone strength, which is the resistance to fracture and the truly significant characteristic. Although the two are usually related, there are some circumstances in which bone density is a poorer indicator of bone strength.
- Reference standards for some populations (e.g., children) are unavailable for many of the methods used.
- Crushed vertebrae can result in falsely high bone density so must be excluded from analysis.
Types of tests
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:
- Dual-energy X-ray absorptiometry (DXA or DEXA)
- Quantitative computed tomography (QCT)
- Qualitative ultrasound (QUS)
- Single photon absorptiometry (SPA)
- Dual photon absorptiometry (DPA)
- Digital X-ray radiogrammetry (DXR)
- Single energy X-ray absorptiometry (SEXA)
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]
- BMC = bone mineral content = g/cm
- W = width at the scanned line
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:
- Normal is a T-score of -1.0 or higher
- Osteopenia is defined as between -1.0 and -2.5
- Osteoporosis is defined as -2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a thirty-year-old man/woman.
|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.
There may be associations between some forms of BMD and diet.