bone density

A scanner used to measure bone density with dual energy X-ray absorptiometry.

Bone density (or bone mineral density) is a medical term referring to the amount of matter per square centimeter of bones.^[1] Note that this is not a true "density", which would be measured in mass per cubic area. 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 minimal radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip.^[2] The forearm is scanned if either the hip or the lumbar spine can't be. Average density is around 1500 kg m^-3

Terms

Results are often reported in 3 terms:

1. Measured density in g cm^-3
2. z-score, the number of standard deviations above or below the mean for the patient's age, sex and ethnicity
3. 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

Limitations

The technique has several limitations.

1. Measurement can be affected by the size of the patient, the thickness of tissue overlying the bone, and other factors extraneous to the bones.
2. 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.
3. Reference standards for some populations (e.g., children) are unavailable for many of the methods used.
4. Crushed vertebrae can result in falsely high bone density so must be excluded from analysis.

Candidates

The National Osteoporosis Foundation recommends BMD testing for the following individuals:^[3]

• All women aged 65 and older regardless of risk factors
• Younger postmenopausal women with one or more risk factors.
• Postmenopausal women who present with fractures (to confirm the diagnosis and determine disease severity).
• Estrogen deficient women at clinical risk for osteoporosis.
• 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

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)

DEXA is currently the most widely used, but ultrasound has been described as a more cost-effective approach to measure bone density.^[4]

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

Interpretation

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.

T-score

The T-score is a comparison of a patient's BMD to that of a healthy thirty-year-old of the same sex and ethnicity. This value is used in post-menopausal women and men over aged 50 because it better predicts risk of future fracture.^[5] The criteria of the World Health Organization are:^[6]

• Normal is a T-score of -1.0 or higher
• Osteopenia is defined as less than -1.0 and greater than -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 woman.

Z-score

The Z-score 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.^[5]

References