Detection of Osteoporosis

Routine X-rays can detect osteoporotic bones only when at least 30% of their bone mass has been lost. At this stage of the disease, the affected bones have a much lighter and thinner appearance than normal bones. An earlier and more accurate assessment of bone loss is accomplished through the use of bone densitometry. Bone densitometers measure the absorption of radiation by the skeleton (skeletal calcium) in order to determine bone mass. Measurements of bone mass are generally considered the most valid estimator of an individual's fracture risk.

Bone density can be measured in several different locations, with different techniques used for different sites. The two most common techniques used in the 1980s were single-photon absorptiometry (SPA), used to measure appendicular sites such as the wrist, and dual-photon absorptiometry (DPA), capable of two-dimensional scanning and used to measure the spine or femur. Both SPA and DPA use radioisotopes for bone imaging. While still available and widely used, SPA and DPA are being replaced by dual energy x-ray absorptiometry (DXA) and quantitated computer tomography (QCT), both of which have more precision and can assess bone mass at any site. These techniques use x-ray sources to image bones, as does radiographic absorptiometry (RA), primarily used to measure the small bones of the hand. Of all the techniques, only QCT has the ability to separate cortical from trabecular bone by providing a three-dimensional image; however, it is more difficult to perform and delivers much more radiation (100 times that of DXA) than the other methods of measurement.

No agreement yet exists on which method is the most effective for diagnosing individual patients and monitoring the progress of their disease. In fact, even the value of densitometry itself in assessing an individual's risk of osteoporosis has been questioned. Many physicians support the use of densitometry in screening high-risk patients to identify those with osteopenia or osteoporosis. Others, however, question the cost effectiveness of large-scale screening, arguing that no studies exist to date showing that the use of bone densitometry in high-risk groups actually results in a reduced incidence of fractures. The usefulness of bone densitometry in research on osteopenia and osteoporosis and for individual clinical decision making, i.e., measuring therapeutic response through interval monitoring, is generally more accepted.

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