Bone tissue undergoes a continuous process of remodelling through resorption of damaged tissue by osteoclasts and apposition of new bone by osteoblasts, at the level of basic multicellular units (BMUs). The number of BMUs ultimately defines bone turnover and it is by itself a source of scarcely mineralized bone. In young healthy individuals bone resorption and bone formation are strictly coupled at the level of the individual BMU and thus of the entire skeleton. Bone loss in the elderly is due to both excess resorption over formation and increased turnover. The inhibitors of bone resorption diminishes the number of BMUs and this invariably decreases the rate of bone loss. However their effect at individual BMU is uncertain. Estrogen replacement therapy lessens the rate of bone loss in postmenopausal women but it does not seem to correct the imbalance between resorption and formation. In several studies bisphosphonates have been shown to induce a continuous positive balance, and this might indicate that these compounds are able to correct the basic alteration of bone metabolism leading to age-related bone loss.
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