1. What is osteoporosis?

* The terms "type I" and "type II" sometimes used to describe osteoporosis are not used in this document as they relate more to descriptive patterns than to differences in clinical management. The term "established osteoporosis" was used formerly to distinguish those with markedly low bone density who had suffered a fracture from those without a fracture. This is no longer a useful distinction.

The structure of the skeleton is a balance between efficient weight distribution, strength and calcium homeostasis. The skeleton is designed to provide the strength needed for normal daily stresses at the minimum bone mass. The outcome is a light and strong structure of cortical and/or trabecular bone. Throughout life, bone remodelling occurs at discrete sites (basic multicellular units, or BMUs), and bone mass or density is the net result of the balance between the amount of bone resorbed by osteoclasts and that laid down during formation by osteoblasts.

With advancing age there is progressive loss of both trabecular and cortical bone, due to an imbalance between bone resorption and formation. At menopause, bone turnover increases; there are more BMUs activated and there is a net imbalance in each. In women, trabecular bone loss occurs by thinning, perforation and loss of connectivity. In men, there is no midlife acceleration of bone turnover; trabecular bone loss proceeds by thinning due to reduced bone formation. Cortical thinning is the result of endocortical bone resorption. A variety of other medical conditions, either by themselves or in relation to therapy, can lead to further and accelerated bone loss (secondary osteoporosis). A particular example of this is corticosteroid osteoporosis.

Although osteoporotic fractures are often thought of more specifically in terms of wrist, hip or spinal fractures, it is important to remember that virtually any bone can fracture in an individual with osteoporosis. Fractures of bones in the upper and lower limbs, ribs, spine and pelvis are more numerous and generally occur in younger individuals than do hip fractures. Although osteoporotic fractures heal normally, from middle age onwards each is associated with significant morbidity and varying degrees of long term disability, since the skeletal deformity may be permanent. Moreover, the occurrence of one fracture is associated with an increased risk of further osteoporotic fractures.

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