Bones in your body continuously break down and form again in a natural and tightly balanced process called bone remodeling. This bone remodeling also occurs in response to stress or injury placed on the bone. For example, weight-bearing exercise leads to increased bone formation.
In Paget's disease, more bone breaks down than usual and more new bone forms than usual. These changes in the bone can lead to bone enlargement and deformity. The new bone growth tends to be softer and more fragile than normal bone, and can develop in a haphazard pattern. Because of this, the bone can fracture. The long bones, especially the legs, tend to bow, and the skull may enlarge, particularly over the forehead.
Paget's disease is the second most common bone disorder in people over 50, after osteoporosis. It is rarely diagnosed in young adults.
Although the cause is unknown, genetics may play a role because the disease sometimes runs in families. Research suggests a slow-growing virus may trigger the disease; some cells in the bone of people with Paget's disease look like they are infected with a virus. However, no virus has been identified.
The condition is rare in certain parts of the world, such as Japan, but more common in the United States and Australia. In the United States, an estimated 10% of people older than age 80 have Paget's disease.
In about 20% of people with Paget's disease, only one area of the body – the spine, pelvis, thighs, lower legs or skull – is affected. The rest have many areas involved. In serious cases, complications can include deafness, congestive heart failure (caused by the extra blood flow required by the diseased bone), an elevated calcium level and cancer of the bone.