Bone Mineral Density

Bone mineral density or bone density is measure of mineral content in per square centimeter of bones. Bone mineral density is an indirect indicator of osteoporosis.

Densitometery is the test for measurement of bone mineral density.

As we grow after reaching peak bone mass, we lose some bone mass because of loss of bone minerals from bone causing thinning of the bones or osteopenia.

With further bone loss, osteopenia leads to osteoporosis where along with mineral content loss also has microarchitectural changes occur in the bone. Osteoporosis occurs mostly in elderly males and postmenopausal females. It can also occur in patients on drugs or inactive recumbent patient.

Bone mineral density is important there is a statistical association between poor bone density and higher probability of fracture.

Osteoporotic fractures are a significant public health problem, especially in elderly women and bone mineral density measurements can be used to screen people for osteoporosis risk. The role of densitometry in the management of children at risk of bone fragility is less certain though concern for bone fragility in children and adolescents has led to increased interest in bone densitometry.

Bone Mineral Density Test

A bone mineral density test measures the density of minerals in the bones using a special equipments. 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.

These tests are

• Dual-energy X-ray absorptiometry
• Quantitative computed tomography
• Qualitative ultrasound
• Single photon absorptiometry
• Dual photon absorptiometry
• Digital X-ray radiogrammetry
• Single energy X-ray absorptiometry

DEXA is currently the most widely used test.

Measurement can be affected by the size of the patient, the thickness of tissue.

Indications for Bone Mineral Density 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 – previous bone fracture from trauma, rheumatoid arthritis, low body weight, a parent with a hip fracture
• 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
• Smoking, drinking, the long-term use of corticosteroid drugs, and a vitamin D deficiency are also related to osteoporosis and carry risk of low bone mineral density and the need for a test.

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

T score t is the bone mineral density at the site when compared to the young normal reference mean. Reference is taken as  bone mineral density of 30 years old. Thus

A negative (–) value means that you have thinner bones than an average 30-year-old. The more negative the number is, thinner bones you have.

A positive (+) value means that your bones are thicker and stronger than an average 30-year-old.

WHO Criteria of T Score

-1.0 or higher : Normal

T score -1.0 and -2.5: Osteopenia  [A bone density that is between one to twoand a half standard deviations below the mean]

<-2.5 or lower: Osteoporosis [Bone density that is two and a half standard deviations below the mean of a thirty year old]man/woman.

Z-score

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 bone mineral density differs from the average bone mineral density of their age, sex, and ethnicity.

A negative (–) value means that your bones are thinner (lower bone density) and weaker than most people in your age group. The more negative the number is, the less bone density you have compared with others in your age group.

A positive (+) value means that your bones are thicker and stronger than most people in your age group.