39 Bone is in a constant state of turnover, which is kept in balance by opposing actions of osteoblasts (bone formation) and osteoclasts (bone resorption). Osteoporosis results when, irrespective of the cause, this balance is disturbed and shifts in favour of resorption. It is defined as a progressive systemic skeletal disorder characterized by low bone mineral density (BMD), deterioration of the microarchitecture of bone tissue, and susceptibility to fracture (Fig 39.1). The World Health Organization (WHO) proposed a clinical definition based on measurements of BMD: a patient is osteoporotic based on a BMD measurement 2.5 standard deviations below typical peak bone mass of young healthy white women. This measurement of standard deviation from peak mass is called the T score. A T score of −2.5 (and below) is osteoporosis, while a score of −1 to −2.4 is osteopenia.
Osteoporosis
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