He first described the disease's characteristics in 1876. Many years later, scientists realized Paget's disease is a disruption in the normal activity of bone tissue. Bone is constantly being broken down by cells called osteoclasts and rebuilt by cells called osteoblasts. This is called bone turnover, and throughout the entire skeleton, this process is normally in precise balance. In Paget's disease, the process goes awry.
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. The bone may enlarge in diameter, though, and it can become painful and break easily. Any bone can be affected, but the most common sites are the spine, skull, pelvis, and legs. Some patients may have only one affected bone, while others may have two or more. The disease usually does not spread to unaffected bones.