Osteoporosis is a systemic skeletal disorder characterized by low bone mass, disruption of the microarchitecture of bone tissue, and compromised bone strength that leads to an increased risk for fracture.
Osteoporosis is common among menopausal women but is usually clinically silent until a fragility fracture occurs. Hip fractures are the most devastating of these in terms of medical, psychosocial, and financial consequences. The lifetime probability of sustaining a hip fracture for a 50-year-old white woman is 14%. The overall percentage of white women at age 50 or older who will sustain an osteoporotic fracture in their remaining lifetime is ∼40%. Osteoporosis is being recognized with increasing frequency in older men, who account for about one third of all hip fractures in the United States. The 1-year mortality of men after a hip fracture approaches 30%. Finally, patients receiving long-term glucocorticoid therapy are at increased risk for osteoporosis and should have prevention and treatment approaches implemented.
- Reduced bone mineral density.
- Decreased bone strength.
- Fragility fractures.
Bone loss in women begins before the onset of menopause, typically in the late third and early fourth decades, and then accelerates for the 5–10 years after the menopause. Postmenopausal osteoporosis is thought to result from an estrogen-deficiency–induced imbalance between bone formation and resorption such that resorption predominates over formation. Following the increased rate of bone loss immediately surrounding the menopause, a less aggressive phase of bone loss ensues that continues into the eighth and ninth decades. Estrogen deficiency and factors related to aging (reduced osteoprogenitor population, nutritional deficiencies, and malabsorption) play a role in this later phase of bone loss.
Clinically, osteoporosis is diagnosed when bone mineral density (BMD) is reduced or when fragility fractures occur. Such fractures are operationally defined as occurring after little or no trauma such as falling from a standing height. The most common osteoporosis-related fractures involve the thoracic and lumbar spine, hip, and distal radius.
Bone densitometry by dual energy x-ray absorptiometry (DXA) is the best standardized technique for diagnosing osteoporosis and monitoring responses to therapy. DXA assessments have been used by the World Health Organization (WHO) to define osteopenia and osteoporosis. Their criteria are based on a large body of data on postmenopausal white women (Table 58–1).
Table 58–1. WHO Definition of Osteoporosis for Postmenopausal Women Based on DXA Measurements. |Favorite Table|Download (.pdf)
Table 58–1. WHO Definition of Osteoporosis for Postmenopausal Women Based on DXA Measurements.
|T-score||Number of SD above or below peak bone mass (“young normal”) according to race or ethnicity|
|Z-score||Number of SD above or below age-matched bone mass according to gender and race or ethnicity|
|Normal||BMD T-score ≥ –1|
|Low bone mass (osteopenia)||BMD T-score < –1 and > –2.5|
|Osteoporosis||BMD T-score ≤ –2.5|