The primary abnormality is an increase in osteoclastic bone resorption. The osteoclasts are of enormous size and contain up to one hundred nuclei. The osteoclast nuclei contain inclusion bodies that resemble the those seen in viral diseases. The main reason for linking Paget's disease to a viral aetiology is the morphological observation of these inclusion bodies. There is also an increase in new bone formation which is probably secondary to the osteoclastic bone resorption. The new bone that is formed is immature woven bone. It is disorganised in structure and of irregular pattern. There is also an increase in vascularity of the bone. Lesions occur most frequently in the vertebrae, skull, pelvis and axial skeleton, and in the proximal ends of the long bones and the tibia. They usually occur as discrete but multiple lesions.
There is no abnormality in calcium or phosphate homeostasis.
