What happens inside bone in osteoporosis?

In osteoporosis, bone loses density and becomes porous. The name comes from the Greek for "porous bones", and the disorder is defined by low bone mass and a deterioration of the bone's fine inner structure, which raises the risk of fracture.

The cause lies in remodelling, the constant tearing down and rebuilding of bone. Bones undergo continuous remodelling: osteoclasts resorb old bone, and osteoblasts synthesize new bone. With advancing age the rate of resorption exceeds the rate of bone formation, so bones lose density and become more susceptible to fractures. Researchers describe three main routes to osteoporosis: an inadequate peak bone mass during growth, excessive bone resorption, and inadequate formation of new bone.

The cruel part is that there is typically no warning. There are usually no symptoms until a bone breaks, and bones may weaken to such a degree that a break occurs with minor stress or even spontaneously. These breaks are called fragility fractures, because they occur in situations where healthy people would not normally break a bone.

It becomes more common with age. One source puts the prevalence at about 2 percent at 50 and almost 50 percent by 80. It affects women more than men, since the sharp fall in estrogen after menopause accelerates bone loss, though older men lose bone too, with lower testosterone.

The numbers are large. Using the World Health Organization's definition, the International Osteoporosis Foundation estimates that 21.2 percent of women and 6.3 percent of men over 50 have osteoporosis worldwide, around 500 million people. Doctors define it by a DXA scan result 2.5 standard deviations below that of a young adult.