Paget disease of bone involves localized, uncontrolled formation of highly active osteoclasts. Initial bone resorption is followed by an intense increase in osteoblastic activity. The bone that is formed is chaotic and excessive. The resulting bone has a mosaic pattern with loss of lamellar structure and therefore is weakened. The marrow is fibrotic with increased vascularity. The increase in osteolytic, osteoblastic, and vascular activity make it intensely "hot" on bone scan.

Paget disease may affect up to 3% of adults older than 50 years of age. It is most prevalent in the white population and has a familial prevalence. It occurs in men and women equally; however, men tend to be more symptomatic. The disease usually presents after age 50 and progresses slowly. The pelvis, femur, spine, skull, and tibia are most commonly affected.

Paget disease is most often asymptomatic, but may cause pain due to microfractures. Patients with symptomatic Paget disease often have bone pain at the site of involvement or at adjacent joints due to secondary arthritis. Bowing of the legs is common and pathologic fractures can occur. Spinal involvement often results in kyphosis or cord compression. Hearing loss is the most common consequence of Paget's disease of the skull. Malignant changes have been reported in around 1% of patients with Paget disease. The incidence of osteosarcoma is greater than in the normal population but remains less than 1%. Other tumors such as fibrosarcomas, chondrosarcomas, and giant cell tumors are seen less frequently.

This condition is often discovered because of an elevated serum alkaline phosphatase on routine screening or because of a radiograph for an unrelated problem. The diagnosis should be considered in elderly patients with bone pain or deformities. A bone scan can be performed to localize the involvement, but radiographs must be obtained to confirm that Paget's disease is the cause of the increased uptake.

The indications for treatment are pain or deformities that are likely to cause fracture or neurologic involvement. Early treatment in young asymptomatic patients is acceptable to prevent progression of disease. Modern therapy includes the use of calcitonin and bisphosphonates to inhibit osteoclastic activity.