Osteoporosis – The Role of Bone Mineral Density Testing in the Detection of the Disease
There are many known risk factors for the development of osteoporosis. Among these are increasing age, Caucasian or Asian race, small body frame size, family history of osteoporosis, smoking, rheumatoid arthritis, poor calcium intake, lack of estrogen replacement in the post menopausal state, and the use of steroids for a prolong period of time. None of these clinical risk factors can predict who will fracture as accurately as bone mineral density testing.
Bone mineral density testing is so important to the diagnosis of osteoporosis that the World Health Organization has defined osteoporosis in terms of bone mineral density testing. The World Health Organization defines osteoporosis as a bone mineral density that is greater than 2.5 standard deviations below the normal for the age and sex matched young adults. Osteopenia is defined by the World Health Organization as bone mineral density that is 1.0 to 2.4 standard deviations below the average young mean. The relative risk for fracture doubles for each 1 standard deviation below the normal bone mineral density. Thus a bone mineral density of 4 standard deviations below the normal has a relative risk of fracture 16 times the normal.
A high proportion of menopausal females and a significant percentage of patients on corticosteroids tend to rapidly lose trabecular bone, so measurement of bone mineral density of the spine, which is largely trabecular, is the best site to detect early osteoporosis in these individuals. With further aging, there is loss of both trabecular and cortical bone. In the elderly, the hip is the preferred site for one mineral density measurement, as degenerative spurring, aortic calcifications and vertebral compression fractures can falsely elevate spine bone mineral density readings. Additionally, the chance of hip fracture, the most dreaded complication of osteoporosis, correlates better with hip bone mineral testing than testing at other sites. Most osteoporosis centers now measure both hip and spine bone mineral density.
There are multiple methods of measuring bone mineral density, but the most widely accepted method is D.E.X.A. (dual energy x-ray absorptiometry). This method is noninvasive, has minimal radiation exposure, and is highly reproducible The DEXA study can measure multiple sites and this be an effective screening tool for groups of high risk individuals. In addition to establishing the diagnosis of osteopenia or osteoporosis in an individual, the DEXA study is most important in the follow-up of individuals who are being treated to determine whether or not the treatment in the particular individual is effective.