Measurement of bone mineral mass in clinical perspective

It has now became possible to measure the bone mineral content in the axial as well as the peripheral skeleton. Moreover, with the use of computed tomography a selective assessment can be made of cancellous (trabecular) versus cortical bone mineral density. These technical achievements have led to a better understanding of the pathophysiology of osteoporosis and provided information on the effects of therapeutic interventions. Despite these sophisticated methods for bone mineral assessment the diagnosis of osteoporosis remains based on the occurrence of non-traumatic fractures and for that purpose an ordinary X-ray will be sufficient. For investigational use several non-invasive methods for measuring bone mineral mass have been developed, although only photonabsorptiometry (Single and Dual energy: SPA and DPA. respectively) and Quantitative Computed Tomography (QCT) are operational in large scale clinical practice. The advantages of photonabsorptiometry and the more recently developed Dual Energy Xray Absorptiometry (DEXA) over QCT are the lower radiation exposure, lower costs, better accuracy and precision and easier operation. The great advantage of QCT is the unique possibility to measure cancellous and cortical bone separately. With these non-invasive methods of bone mineral assessment is has been shown that women will lose during their lifes about 35 percent of their cortical and about 50 percent of their cancellous bone. We studied this pattern of age-related bone loss cross-sectionally in 171 healthy Dutch women and observed an accelerated bone loss around the menopause at all measurement sites (see Chapter 5). Further analyses showed that the onset of cortical bone loss as measured by SPA occurs on the average at least a decade later than the onset of cancellous bone loss which already manifests itself before the menopause. This pattern of cancellous and cortical bone loss during aging shows a parrallelism with the observed patterns of incidence of age-related fractures. The incidence of Colles fractures in women rises soon after the menopause and a plateau is reached around the age of 65. This type of fracture (distal forearm) occurs at a site containing a relatively high proportion of cancellous bone. On the other hand, the incidence of hip fractures increases slowly with age, which rise accelerates late in life in both men and women. This type of fracture characteristically is one of cortical bone. Somewhere between these two types of fractures the vertebral compression fractures take position. They occur soon after the menopause and the incidence apppears to rise over the two decades after menopause. The vertebral body contains about equal amounts of cancellous and cortical bone.