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INTRODUCTION

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 almost any skeletal site. Osteoporosis affects almost 10 million individuals in the United States, but only a small proportion are diagnosed and treated.

DEFINITION

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. Although risk is lower in this group, more than 50% of fractures among postmenopausal women, including hip fractures, occur in this group with low bone density, because the number of individuals in this category is so much larger than that in the osteoporosis range. As a result, there are ongoing attempts to identify individuals within the low bone density range who are at high risk of fracture and might benefit from pharmacologic intervention. Furthermore, some have advocated using fracture risk as the “diagnostic” criterion for osteoporosis.

EPIDEMIOLOGY

In the United States, as many as 9 million adults have osteoporosis (T-score <–2.5 in either spine or hip), and an additional 48 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. As the population continues to age, the number of individuals with osteoporosis and fractures will also continue to increase, despite a recognized reduction in age-specific risk. It is estimated that about 2 million fractures occur each year in the United States as a consequence of osteoporosis, and that number is expected to increase as the population continues to age.

The epidemiology of fractures follows the trend for loss of bone density, with exponential increases in both hip and vertebral fractures with age. Fractures of the distal radius have a somewhat different epidemiology, increasing in frequency before age 50 and plateauing 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. 425-1). This distinct epidemiology may be related to ...

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