Severe deformity of the leg caused by Paget's disease (osteitis deformans). Paget's disease is a chronic bone disease usually occuring in the elderly. Affected bones become thickened and deformed by excessive and disorganised resorption and formation of bone. X-rays reveal patchy sclerosis (tissue hardening). The bones most commonly affected are the skull, spine, and pelvis. Pagetic bone may be hard or soft but either type fractures easily. Severe pain may be caused by the bone itself, by an associated fracture or by arthritis in nearby joints. Nerve compression by enlarging Pagetic bone is also common. The cause of Paget's disease is unknown.