Bone remodelling in osteoporosis.

Bone remodelling, a highly regulated succession of events, is the temporal sequence of osteoclastic bone resorption and osteoblastic bone formation. Bone loss with age and in osteoporotic patients is due to a disequilibrium between both processus. Bone histomorphometry was the method used to measure these events. Its shows clearly that, with age, the quantity of bone formed in one remodelling unit (so called mean wall thickness) decreases. In osteoporotic women, compared to control women of the same age the amount of bone formed is also decreased. Concordant data on this point have been obtained in different laboratories. By contrast, the cellular mechanism underlying this decreased amount of bone formed is largely controversial: a decreased osteoblast recruitment or life span or capacity to synthesize collagen have been suggested. Bone loss with age is associated with an increase in the amount of bone resorbed. This observation is the result of an indirect measurement founded on the distance between trabeculae. As the decreased bone formation, this processus is exaggerated in postmenopausal osteoporosis. The respective importance of decreased bone formation and increased bone resorption is however difficult to assess. Some osteoporotic patients have increased bone resorption surfaces compared to control women on their bone biopsy; however, it does not seem that these patients form a definite subgroup of osteoporotic patients as the extent of resorption surfaces changes with time in an untreated osteoporotic. In conclusion, the observed changes in bone remodelling in osteoporotic post-menopausal women are an exaggeration of those observed during ageing. These changes should be a basis for a coherent therapy of bone loss in osteoporotic patients.