Paget’s disease, or osteitis deformans, is a common condition in the middle-aged and elder persons. The disease is present in approximately 10% of patients over the age of 80 years. It is more common in men than in women. Seventy-percent of Paget’s disease is discovered as an incidental finding, patients for the most part asymptomatic. The rest of the patients can present with bone pain (due to microfractures or osteoarthritis), headache or hearing/visual impairment due to impingement on nerves, effects of bony expansion (increase in head size, limb bowing, abnormal curvatures of the spine). Less often, the patients have symptoms of heart failure, although these patients usually have an underlying cardiac disease such as atherosclerotic heart disease that is aggravated by Paget’s disease, which cause hyperemia around the lesion and consequent increase in cardiac demand. Increase in bone resorption characteristic of disease cause increase in calcium that can precipitate in urine to form renal stones. Approximately 1% of Paget’s disease lesions degenerate to form osteosarcoma. Laboratory abnormalities related to Paget’s are increased serum alkaline phosphatase and hydroxyroline and urinary levels of hydroxyproline, which are reflections of increased bone resorption.

The basic underlying pathologic process of Paget’s disease is the excessive and abnormal bone remodeling. The active phase of the disease consists of an intense osteoclastic activity with resorption of normal bone by giant multinucleated cells. Next, excessive and disorganized new bone formation by the osteoblasts occurs. This new bone formation is mostly made up of primitive or woven bone with increased vascularity and pronounced connective tissue reaction. Eventually, the osteoblastic activity decreases, and the disease process enters an inactive, sclerotic phase.

With the basic pathologic processes in mind, one can imagine what these lesions would look like on plain radiographs. The classic osteolytic lesions of the Paget’s disease of the skull is called osteoporosis circumscripta, with sharply delineated advancing lucent lesions of the skull characteristic. The osteolytic lesions of the long bones have been described as having a “blade of grass” or a “flame” shape. After the blastic and sclerotic phases of the disease have taken over, the bones affected by Paget’s have the classic triad of 1) bone expansion, 2) cortical bone thickening, and 3) trabecular bone thickening.

Etiology of Paget’s disease still remains elusive. Early HLA association studies have shown some association of HLA-DQW1 and HLA-DR2 alleles to Paget’s, but later genetic linkage studies have not fully elucidated those earlier associations. Linkage to chromosome 18, especially at 18q, have been heavily investigated, since the disease familial expansile osteolysis localized to 18q have a very similar appearance to Paget’s. Later finding of osteosarcoma tumor suppressor gene in the 18q makes the case stronger for the “Paget’s disease of bone gene” to be somehow located in 18q. Viral etiology has been suggested (more specifically, with paramyxovirus such as measles and canine distemper virus) due to the paramyxovirus-like inclusion bodies within the abnormal osteoclasts, but no adequate animal model exists to confirm that hypothesis.