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.
As more and more treatments emerge, the pressure is on for technology
to improve and to press forward for early diagnosis and follow-up