Bone Density - definition, testing and disolders

What is Bone Density

The amount of mineral, primarily calcium, the bones contain that gives them their mass. BONE density is important to give the SKELETON enough structure to support the body. Insufficient bone density results in the bone loss conditions OSTEOPENIA and OSTEOPOROSIS. Numerous hormones participate in maintaining bone density. Among them are estrogen, TESTOSTERONE, CALCITONIN, vitamin D (in the form of calciferol), and PARATHYROID HORMONE. Though calcium is the mineral most commonly associated with bone structure and bone density, other minerals that also are important, including magnesium and phosphorus.

Bone density naturally diminishes with increasing age, beginning at about age 35, at a rate of about 2 percent per year. Because estrogen is particularly essential for maintaining bone density in women, bone density drops precipitously at MENOPAUSE when a woman’s estrogen production drops to nearly nothing. Because men’s bodies are larger, they inherently have greater bone mass. Testosterone contributes to this mass, as it does a man’s greater MUSCLE mass. The natural decrease in bone density is usually not a health concern until a man reaches his middle to late 60s.

Disorders of Bone Density

Most health problems related to bone density arise from diminished bone mass, which presents increased risk for bone FRACTURE. Spontaneous fracture (fracture that occurs without trauma or other cause) is possible when bone density is very low. The spine and the hip are at particular risk. The most common of these conditions are osteopenia (bone loss that places the individual at increased risk for fracture) and osteoporosis (bone loss that places the individual at significant risk for fracture). Compression fractures of the spine, in which the vertebrae collapse, can endanger the SPINAL CORD; HIP FRACTURE IN OLDER ADULTS is a key cause of disability and death. Medications are available that stimulate bone growth, helping restore lost bone mass. The doctor may prescribe such medications, along with lifestyle measures such as RESISTANCE EXERCISE, to increase bone density. Excessive bone mass is far less common though may occur in conditions such as OSTEOPETROSIS.

Bone Density Testing

A number of tests can measure bone density. Among them are

• dual energy X-RAY absorptiometry (DEXA or DXA), which uses low-DOSE X-ray to measure the bone mass in the spine and hip
• peripheral dual energy X-ray absorptiometry (pDEXA or pDXA), which uses low-dose X-ray to measure the bone mass in the wrist or heel
• single-energy X-ray absorptiometry (SXA), which uses low-dose X-ray to measure the bone mass in the wrist or heel
• quantitative ULTRASOUND, which uses sound waves to measure the bone mass in the heel, patella (kneecap), and tibia (long bone in the lower leg)
• quantitative computed tomography (QCT), which uses X-ray to measure the bone mass in the spine
• peripheral quantitative computed tomography (pQCT), which uses X-ray to measure the bone mass in the wrist

DEXA provides the most detailed information and is simple to perform. QCT and pQCT are variations of COMPUTED TOMOGRAPHY (CT) SCAN that use somewhat higher doses of X-ray and are more complex to perform but can be more reliable in people who have already had fractures of the spine or hip. Mobile clinics and even some pharmacies use peripheral methods for screening. All testing methods to measure bone density are painless, noninvasive, and take 20 minutes or less to complete.

Bone density tests report two scores:

• The T-score compares an individual’s bone density to a figure that represents the bone density of a healthy adult in his or her mid-20s, the period when bone density is at its highest. The comparison is gender specific.
• The Z-score compares an individual’s bone density to a figure that represents other adults of the same age. The Z-score comparison is also gender specific.

Because older adults have lower bone mass, the Z-score is less significant than the T-score for assessing the presence of osteopenia and osteoporosis. Generally the T-score and the Z-score correlate; a person who has a low T-score also has a low Z-score.

The difference between an individual’s bone density score and the representative standard score is the standard deviation (SD), reported as a positive (+) or negative (–) figure. Each full SD represents about 10 percent of normal bone mass. The lower the T-score, the greater the percentage of bone loss. A T-score that is 2.5 SDs or more below the norm (–2.5) is the diagnostic marker for osteoporosis.

See also ESTROGENS; HORMONE; OSTEOMALACIA.

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