To maintain strength over time and to respond adaptively to physical stress, in adults bone is continually resorbed and reformed. This process is called remodeling. About 25% of trabecular bone and 3% of cortical bone is remodeled each year. Osteocytes act as mechanoreceptors that identify sites for remodeling. Remodeling is carried out by basic multicellular units (BMUs), i.e., the coordinated activities of osteoclasts and osteoblasts.
Three main hormones (parathyroid hormone, vitamin D, and calcitonin) and three lesser hormones (glucocorticoids, thyroid and reproductive hormones) mediate bone homeostasis. Signals mediated by these hormones determine differentiation, activity, and death of osteoclasts, osteoblasts, and osteocytes and how many BMUs are activated, how active and well-balanced the BMUs are, and whether bone mass (density) will be gained, lost, or maintained.
When bone resorption exceeds bone formation, the patient develops osteopenia and, potentially, osteoporosis. Medical management includes weight-bearing exercises, smoking cessation, avoiding excessive use of alcohol, and calcium and vitamin D supplementation. Pharmacological strategies center primarily on the administration of antiresorptive agents (Table 18).17 The risk of a MME is predicated on adverse drug effects and/or other, concurrent variables.39-41
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