Bones are primarily made up of bone mineral matrix and marrow (where red and white blood cells are produced). Bone Density is a medical term referring to the amount of mineral matter per square centimeter of bones. There is an association between poor bone density and higher probability of fracture. Fractures of the back (vertebrae) 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. The tests (there are several types) to measure bone density are painless and non-invasive. DEXA is currently the most widely used of these tests to measure bone density. 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.
Results are generally scored by two measures, the T-score and the Z-score. Scores indicate the amount 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 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 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. 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.
Hip fractures and vertebral body fractures increase in frequency as the bones get weaker and T-scores increase beyond 1.0. By definition fractures that occur with minimal or no trauma at ground level are pathologic osteoporotic fractures.
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.