Bone physiology and the osteoporotic process.

Bone continually undergoes modelling (during growth) or remodelling (during adult life), brought about by the co-ordinated action of osteoblasts and osteoclasts. Osteoblasts form new bone, whereas osteoclasts are responsible for bone resorption. Both types of cell are under hormonal regulation. Osteoporosis, a reduction in bone mass, predisposes to fracture. The most important cause of osteoporosis is oestrogen deficiency which results in increased bone turnover in which resorption exceeds formation. Corticosteroids can also induce osteoporosis in which trabecular bone is particularly affected. This mainly results from suppression of osteoblastic activity.