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Osteoporosis

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of osteoporosis.

Diagnosis

About 20 - 30% of Caucasian women in the U.S. can expect to be affected by osteoporosis, including having a spinal fracture, after age 60. Hispanic, Asian, and Native American women have an even higher risk. Nearly all of them are unaware of the condition and so fail to seek a diagnosis. Even worse, studies continue to report inadequate evaluation for osteoporosis even after a fracture.

Candidates for Bone Density Screening or Testing

Evidence suggests that screening for osteoporosis can help prevent fractures. Expert groups now recommend bone density screening for the following people:

• All women over age 65.
• Any postmenopausal women under 65 years with risk factors for osteoporosis (being thin, being a smoker, having a family history of osteoporosis, corticosteroids use, or any serious high-risk condition, such as hyperthyroidism or early menopause).
• Any older men or women who suffer a fracture. (Unfortunately, studies suggest that only a minority of these patients are evaluated and treated for osteoporosis. Men are especially less likely to be tested.)

Whether perimenopausal women should be screened is unclear. (Perimenopause is the period that extends a few years before and after menopause, usually ages 50 - 59.) Some experts believe that women as young as 21 who have strong risk factors for osteoporosis (such as anorexia or absence of menstruation due to over-exercising) should consider being tested. It is also important that older women continue to get bone density tests. A 2006 study found that only 10% of women over age 75 receive bone density screenings, even though they are the age group most likely to have hip fractures.

Techniques Used for Measuring Bone Density

Bone Densitometry. The standard technique for determining bone density is a form of bone densitometry called dual-energy x-ray absorptiometry (DEXA). DEXA is simple and painless and takes 2 - 4 minutes. The machine measures bone density by detecting the extent to which bones absorb photons that are generated by very low-level x-rays. (Photons are atomic particles with no charge.) Measurements of bone mineral density are generally given as the average concentrations of calcium in areas that are scanned.

Bone density is usually measured at the hip rather than the spine or wrist, which appears to be the most predictive of hip fracture. (Hip fractures are the most dangerous fractures, particularly in women older than sixty.) The BMD in the spine may also be measured. (Spinal BMD in older people however may be misleading. Bone density in this group may increase because of compression on the spinal bones from arthritic changes in the spine. Therefore, bone density measurements may be normal or even high, but the patient may actually be at risk for fracture.)

Ultrasound. Ultrasound techniques measure bone density in the heels, fingers, and leg bones. In early studies, advanced ultrasound techniques, such as quantitative ultrasound (QUS), are promising for improving accuracy in predicting fractures when used with DEXA. Ultrasound itself is less expensive than DEXA and uses no radiation. Ultrasound bone tests are sometimes given at health fairs or other non-medical settings. It should be noted that these results typically vary widely from measurements of the hipbone and are not reliable when used alone.

Quantitative Computed Tomography. Quantitative computed tomography (QCT) scans, a form of CT scans, can provide highly detailed information about spinal density. Radiation doses from this technique are higher than the others. Whether QCT predicts fracture risk accurately is, however, unknown.

Determining Osteoporosis and the Risk for Fracture

Osteoporosis is diagnosed when bone density has decreased to the point where fractures will happen with mild stress, the so-called fracture threshold. This is determined by measuring bone density and comparing the results with the norm. However, low scores on bone density are not very accurate in determining fracture risk without consideration of other risk factors for fracture.

In general, doctors take the following steps to determine osteoporosis:

• Bone mineral density (BMD) is measured, typically in the hipbone, using bone densitometry.
• Measurements of BMD are given as mg/cm ² . This is the average concentration of bone mineral in the areas that are being scanned. In general, bone is normal if results are greater than 833 mg/cm ² . Low bone density (osteopenia) is between 833 and 648 mg/cm ² . Osteoporosis is diagnosed with results below 648 mg/cm ² .

These measurements still do not always indicate the true risk for fracture. The doctor also assesses risk factors and other considerations. The next step is to compare the patient's BMD to normal bone density, which is defined as the average BMD in the hipbones of premenopausal Caucasian women. (This group is used as the basis for the norm because of their high risk and greater proportion in the American population.)

The health professional then uses this comparison to determine her standard deviation (SD) from this norm. SD results are given as Z and T scores:

• A T score gives the SD of the patient in relationship to the norm in young adults. Doctors often use the T-score and other risk factors to determine the risk for fracture.
• A Z score gives the SD of the patient in relationship to the norm in her own age group. Z scores may be used to monitor the effects of treatments in women who have been diagnosed with osteoporosis.

For example, the lifetime risks for a younger woman with a specific T-score would be higher than the same scores in an older woman because the younger woman would have a longer time to lose bone density. In general, the T scores in a 55-year-old woman suggest the following degrees of risk for hip fracture.

• One standard deviation (SDs) or less below the norm indicates normal BMD. (This carries a lifetime chance for a hip fracture of up to about 20%, depending on age and other risk factors.)
• Between 1 and 2.5 SDs below normal defines osteopenia, which is low bone density. This carries between a 20 - 50% lifetime risk for fracture.
• More than 2.5 SDs predicts osteoporosis and over a 60% chance for hip fracture. Additional risk factors increase the risk. They include low weight, smoking, risks for falling, and especially a history of previous fractures. For example, in women 65 years old with low bone density but no adverse factors, the risk for fracture is 4.3% in 1 year and 28.6% over 5 years. In similar women with a previous fracture, the probability of fracture at 1 year is 11% and at 5 years is 71.8%.

Not all older women with osteoporosis develop fractures. There is some evidence that the body partially compensates after menopause by increasing bone strength, which can help offset the risk for fracture. Techniques to measure bone strength may better identify women at higher or lower risk.

Note: Because the standards are based on Caucasian women, they do not necessarily apply to men, children, or to non-Caucasian women. For example, men have a lower risk for fracture at the same SDs as women. Researchers are attempting to establish risk guidelines for these groups as well.

Laboratory Tests

Laboratory blood or urine tests for identifying certain markers of bone loss may prove to be useful in certain cases:

• High levels of the chemicals deoxypyridinoline and C-telopeptide in the blood may indicate increased risk for hip fracture. These substances are produced when bone is broken down.
• A urine test detecting a substance called N-telopeptide may indicate bone loss (although it is not associated with any risk for fracture).

• Review Date: 10/18/2006
• Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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