What is “bone mineral density” (BMD)?
The absolute amount of bone as measured by bone mineral density (BMD) testing generally correlates with bone strength and its ability to bear weight. The BMD is measured with a dual energy x-ray absorptiometry test (referred to as a DEXA scan). By measuring BMD, it is possible to predict fracture risk in the same manner that measuring blood pressure can help predict the risk of stroke.
Figure 1 shows a bone mineral density (BMD) test of a healthy older woman. The X-ray shows normal hipbones. The graph shows the bone mineral density number in the green zone (normal). Figure 2 shows a BMD test of an older woman with osteoporosis. The X-ray shows weaker hipbones. The graph shows the bone mineral density number in the red zone (osteoporosis), placing her at much greater risk for broken bones (fractures).
It is important to remember that BMD cannot predict the certainty of developing a fracture. It can only predict risk. It is also important to note that a bone density scan, or test, should not be confused with a bone scan, which is a nuclear medicine test in which a radioactive tracer is injected that is used to detect tumors, cancer, fractures, and infections in the bone.
The World Health Organization has developed definitions for low bone mass (osteopenia) and osteoporosis. These definitions are based on a T-score. The T-score is a measure of how dense a patient’s bone is compared to a normal, healthy 30-year-old adult.
Normal: A bone BMD is considered normal if the T-score is within 1 standard deviation of the normal young adult value. Thus a T-score between 0 and -1 is considered a normal result.
Low bone mass (medically termed osteopenia): A BMD defines osteopenia as a T-score between -1 and -2.5. This signifies an increased fracture risk but does not meet the criteria for osteoporosis.
Osteoporosis: A BMD greater than 2.5 standard deviations from the normal (T score less than or equal to -2.5) defines osteoporosis.
Based on the above criteria, it is estimated that 40% of all postmenopausal Caucasian women have osteopenia and that an additional 7% have osteoporosis.
Why is BMD measurement important?
Determining a person’s BMD helps a doctor decide if a person is at increased risk for osteoporosis-related fracture. The purpose of BMD testing is to help predict the risk of future fracture. The information from a BMD is used to aid a decision as to whether prescription medicine therapy is needed to help reduce the risk of fracture. Additionally, if a patient has a fracture or is planning orthopaedic surgery, a diagnosis of osteoporosis might affect the surgical plan. A fracture that could potentially heal in a cast with normal bone mass might require either a longer period of casting or even surgery if the patient has osteoporosis.
What is the relationship between BMD and fracture risk?
In patients with low bone mass at the hip or the spine (the two areas traditionally measured with DEXA scanning), there is a two- to threefold increase in the incidence of any osteoporotic fracture. In other words, low bone density at the measured areas of the spine and hip can even predict future osteoporotic fractures at other parts of the body besides the spine and hip. In subjects with a BMD in the osteoporosis range, there is approximately a five times increase in the occurrence of osteoporotic fractures.
Who should have BMD testing?
BMD testing is recommended for all women over the age of 65. Additionally, postmenopausal women under 65 years who have risk factors for osteoporosis other than menopause (these include a previous history of fractures, low body weight, cigarette smoking, and a family history of fractures) should be tested. Finally, men or women with strong risk factors as listed below should discuss the benefit of DEXA scanning with their doctor to see if testing is indicated.
The following are potential risk factors for osteoporosis that might suggest the need for DEXA scanning:
- personal history of fracture as an adult
- history of fracture in first-degree relative
- low body weight or thin body stature
- advanced age
- current cigarette smoking
- use of corticosteroid therapy for more than three months
- impaired vision
- estrogen deficiency at early age
- poor health/frailty
- recent falls
- lifelong low calcium intake
- low physical activity
- alcohol intake of more than two drinks/day
- thyroid disease
- rheumatoid arthritis
- excessive caffeine consumption
- use of oral contraceptive (birth control pills)