Osteoporosis:
A generalized, progressive diminution of bone density (bone mass per unit
volume), causing skeletal weakness, although the ratio of mineral to
organic elements is unchanged.
In normal
bone, bone formation and bone resorption are closely coupled. In
osteoporosis, the net rate of bone resorption exceeds the rate of bone
formation, resulting in a decrease in bone mass without a defect in bone
mineralization. In women, osteoclast activity is increased because of
decreased estrogen; as men and women age > 60 yr, osteoblast activity
drops off. Men with prematurely decreased testosterone may have increased
osteoclast activity. These changes result in further net loss of bone. The
amount of bone available for mechanical support of the skeleton eventually
falls below the fracture threshold, and the patient may sustain a fracture
with little or no trauma. Bone loss affects both cortical and trabecular
bone. Histologically, there is a reduction in cortical thickness and in
the number and size of trabeculae of cancellous bone, with normal width of
the osteoid seams. Trabecular bone loss predominates in typical
postmenopausal osteoporosis. A defect in mineralization (osteomalacia) and
osteoporosis can coexist.
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