Paget's disease 

Paget's disease is the second most common bone disease in the United States. Osteoporosis is No. 1. Paget's disease can cause pain, deformities, hearing loss, and limits on activity. The disease, which affects people in different ways, also can cause arthritis and other serious consequences.

In Paget's disease, the process goes awry. In discrete portions of bone, overly large osteoclasts dissolve bone too quickly--as much as 50 times faster than normal. Osteoblasts try to compensate for the increased pace by rapidly depositing new bone. But, in the hurried process, the newly deposited bone is loose and bulky in structure, rather than strong, compact, and neatly arranged.

Over time, pagetic bone becomes weak and soft and can easily bend, actually shortening the part of the body affected: for example, a leg or the spine. The bone may enlarge in diameter, though, and it can become painful and break easily.

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