for an extra 290,000 hospital days. In women of this age, there were 2.3 million physician visits for osteoporosis in 1986. At each visit, one-third of the women needed an x-ray, one-fourth required physiotherapy, and most received a prescribed medication for the condition. There were also an estimated 83,000 nursing home stays for osteoporosis-related causes in 1986, with an average duration of stay of one year. Altogether, these direct costs of osteoporosis totaled $5.2 billion for women alone, of which $2.8 billion were for inpatient services and $2.1 billion were for nursing home care.36

Such costs can only rise in the future because the elderly population is growing rapidly. Between 1988 and 2050, the actual number of individuals aged 65 and older will increase from 30 to 67 million in the United States.5 Because the incidence of osteoporosis-related fractures rises with age, this growth in the elderly population will eventually result in a doubling or tripling of the number of hip fractures seen each year, with increases for the other fracture sites as well. It is projected that in only 30 years' time, there could be almost 350,000 hip fractures each year in the United States at an annual cost estimated to be between $31 and $62 billion.7 This alarming situation can be at least partially avoided if methods to preserve bone mass can be refined and exploited for the entire population, young and old.

DETERMINANTS OF FRACTURES

The risk of fracture in any given situation depends on the degree of trauma experienced and the ability of the skeleton to resist such forces. Bone density in the skeleton can decline to such a point that fractures occur spontaneously, and violent accidents can impose loads capable of breaking any bone. Usually, however, both skeletal integrity and trauma are important. For example, the risk of falling (and particularly of falling heavily) generally increases with age, whereas bone strength diminishes. As a consequence, fracture incidence rises dramatically (especially for hip fractures for which the age-related increase is exponential), and the amount of trauma required to produce a fracture declines.27 Although the occurrence of a fracture is a complex event (Figure 6-2), interventions are usually directed either at bone mass or trauma, and risk factors should be considered in that light.

Bone Mass

The skeleton is constantly being renewed through a linked process of bone resorption and bone formation.34 In young adults,



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