Bone density (or bone mineral density) is a medical term referring to the amount of matter per square centimeter of bones. 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. The forearm is scanned if either the hip or the lumbar spine can't be. Average density is around 1500 kg m

Terms

Results are often reported in 3 terms:

1. Measured 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
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.