mineral density in the proximal femur, rising wrist fracture incidence with decreasing bone mass in the radius, and increasing vertebral fracture incidence and prevalence with falling bone density in the spine.17 Several prospective studies show, in addition, that bone mass measurements predict the risk of fractures generally.15,49 Altogether, roughly 20 to 25 million individuals in the United States have sufficiently low bone density that they are at increased risk of fracture.35

Although bone mineral density accounts for most of the ultimate strength of bone tissue,26 other skeletal factors may be important as well. Biomechanically, the body compensates for age-related bone loss by an increase in the diameter of limb bones, which raises their resistance to bending and twisting. However, this compensatory process is less marked at the ends of limb bones or in the vertebrae, sites at which the fractures associated with osteoporosis typically occur. Strength may also be compromised by age-related increases in the fragility of bone tissue.26 Although bone architecture and bone quality are important determinants of bone strength, neither can be accurately assessed by noninvasive means for use in risk prediction, and neither can be intentionally altered by a specific therapy. Consequently, they are not considered further.

Trauma

Some fractures are spontaneous, whereas others (especially vertebral fractures) result from everyday activities. Most of the fractures related to osteoporosis, however, occur in conjunction with falls. Falling increases the chance of fracture at any given level of bone mass, as does the inability to dissipate the kinetic energy produced by a fall.26 The annual risk of falling rises with age, from about one of five women aged 60 to 64 to one in three aged 80 to 84,6 but only about 5 to 6 percent of falls lead to a fracture of any kind, and only 1 percent lead to a hip fracture.11

The pathophysiology of falling is not well understood, and many of the risk factors that have been recognized, such as gait and balance disorders, diminished reflexes and strength, or reduced vision,48 are difficult to correct. Once bone mass has fallen to such a point that fractures begin to occur, however, there are few other therapeutic options than to try to prevent falls. Among elderly women, for example, who have lost most of the bone they will lose over life and who may have bone density so low that they are at high risk of fracture, the emphasis in prevention has to be on falling. Therefore, programs to reduce the fractures associated with osteoporosis must



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