Pathophysiology of age-related bone loss and osteoporosis.

Aging is associated with significant bone loss in women and in men [1]. Fig. 1, which draws on numerous cross-sectional and longitudinal studies using areal bone mineral density (aBMD) by dual energy x-ray absorptiometry (DXA), depicts the overall pattern of bone loss in both sexes. The menopause in women is associated with a rapid loss of trabecular bone, as is present in the vertebrae, pelvis, and ultra-distal forearm. There is a less dramatic loss of cortical bone (present in the long bones of the body and as a thin rim around the vertebrae and other sites of trabecular bone) following the menopause. Approximately 8 to 10 years following menopause, slow, age-related phase of bone loss in trabecular and cortical bone becomes apparent and continues throughout life. Because men lack the equivalent of a menopause, they generally do not exhibit this rapid phase of bone loss. Men, however, have a very similar pattern of slow, age-related bone loss as is present in women. Although DXA has provided important insights into the patterns of agerelated bone loss, its utility is limited by the fact that it cannot clearly separate trabecular from cortical bone or provide information on possible changes in bone size/geometry with age. In recent studies, Riggs and colleagues [2] used central and peripheral quantitative CT (QCT) along with new image analysis software [3] to better define age-associated changes in bone volumetric density, geometry, and structure at different skeletal sites. As shown in Fig. 2, there were large decreases in volumetric BMD (vBMD) at the spine over life (predominantly trabecular bone), which