Osteoporosis is a disorder characterized by low bone mass leading to bone fragility and increased risk of fracture. It is a preventable disease that affects up to 25 million people. Although bone density diminishes with age at a rate of 3% per year after menopause, current treatment regimens retard and can possibly reverse this process. Thus, osteoporosis should be diagnosed as early as possible. Because it supplies a mass-per-area measurement along with age-matched, sex-matched normative data, dual-energy x-ray absorptiometry (DEXA) has emerged as the primary technology for clinical use.

In clinical practice, DEXA is used in three ways. First it is used to diagnose osteoporosis. The World Health Organization (WHO) now accepts a bone mineral density (BMD) measurement that is 2.5 standard deviations below the mean for young normals as indicative of osteoporosis. A measurement of 1 to 2.5 standard deviations below the mean is indicative of osteopenia. A BMD that is more than 2.5 standard deviations below the mean in the presence of a fragility fracture is indicative of severe osteoporosis.

The second way that DEXA is used in clinical practice is to predict fracture risk. Clinical DEXA not only gives an absolute value of bone mass measured in gm/cm2 (with the volume of bone being inferred), but also gives comparison values that help predict fracture risk. It is generally accepted that for every standard deviation below the mean of young normal bone mass, vertebral compression fractures increase 1.9 times and hip fractures increase 2.4 times. For a 65-year-old woman whose BMD is 0 standard deviations below the mean for young normals, the risk for fracture per annum is 2.5%. For the same woman with a BMD of 4 standard deviations below the mean, the risk of fracture changes from 5% to 10% over five years to 25% to 50%.

The third way that DEXA is used in the clinical setting is to monitor therapy. Obviously, a therapy that does not achieve the desired response within a year needs to be changed. Calcitonin and estrogens have been reported to increase bone mass by 1% to 3% per year. Alendronate has been reported to increase bone mass by 6% to 8% per year. Patients maintained off therapy can be monitored, and those for whom estrogens are contraindicated can also be followed up fairly closely.

Other technologies are available for the measurement and diagnosis of osteoporosis, such as radioabsorptiometry and quantitative CT; however, neither of these technologies has emerged as having the ability to reliably monitor patients in therapy with minimal precision error. Quantitative CT cannot provide adequate measurements of hip and spine radioabsorptiometry, which relies on the comparison of BMD to a template on conventional x-ray. Quantitative CT is excellent for screening and diagnosing osteoporosis, but it has not been shown to be an effective technology for the serial determination of BMD for follow-up of patient currently involved in therapy.