Osteoporosis, a condition characterized by decreased bone strength, is prevalent among postmenopausal women but also occurs in men and women with underlying conditions or major risk factors associated with bone demineralization. Its chief clinical manifestations are vertebral and hip fractures, although fractures can occur at any skeletal site. Osteoporosis affects >10 million individuals in the United States, but only a small proportion are diagnosed and treated.
Osteoporosis is defined as a reduction in the strength of bone that leads to an increased risk of fractures. Loss of bone tissue is associated with deterioration in skeletal microarchitecture. The World Health Organization (WHO) operationally defines osteoporosis as a bone density that falls 2.5 standard deviations (SD) below the mean for young healthy adults of the same sex—also referred to as a T-score of –2.5. Postmenopausal women who fall at the lower end of the young normal range (a T-score <–1.0) are defined as having low bone density and are also at increased risk of osteoporosis. More than 50% of fractures among postmenopausal women, including hip fractures, occur in this group with low bone density.
In the United States, as many as 8 million women and 2 million men have osteoporosis (T-score <–2.5), and an additional 18 million individuals have bone mass levels that put them at increased risk of developing osteoporosis (e.g., bone mass T-score <–1.0). Osteoporosis occurs more frequently with increasing age as bone tissue is lost progressively. In women, the loss of ovarian function at menopause (typically about age 50) precipitates rapid bone loss so that most women meet the diagnostic criterion for osteoporosis by age 70–80.
The epidemiology of fractures follows the trend for loss of bone density. Fractures of the distal radius increase in frequency before age 50 and plateau by age 60, with only a modest age-related increase thereafter. In contrast, incidence rates for hip fractures double every 5 years after age 70 (Fig. 354-1). This distinct epidemiology may be related to the way people fall as they age, with fewer falls on an outstretched hand and more falls directly on the hip. At least 1.5 million fractures occur each year in the United States as a consequence of osteoporosis. As the population continues to age, the total number of fractures will continue to escalate.
Epidemiology of vertebral, hip, and Colles' fractures with age. (Adapted from C Cooper, LJ Melton III: Trends Endocrinol Metab 3:224, 1992; with permission.)
About 300,000 hip fractures occur each year in the United States, most of which require hospital admission and surgical intervention. The probability that a 50-year-old white individual will have a hip fracture during his or her lifetime is 14% for women and 5% for men; the risk for blacks is lower (about one-half those rates). Hip fractures are associated with a high incidence of deep vein thrombosis and pulmonary embolism (20–50%) and a mortality rate between 5 and 20% during the year after surgery.
There are about 700,000 vertebral crush fractures per year in the United States. Only a fraction of them are recognized clinically, since many are relatively asymptomatic and are identified incidentally during radiography for other purposes (Fig. 354-2). Vertebral fractures rarely require hospitalization but are associated with long-term morbidity and a slight increase in ...