N.Z. Increased osteoplastic bone resorption with irregular bone formation leads to focally deficient bone. Paget's disease typically involves the sacrum, spine, femur and cranium of older patients. Some have hypothesised a viral etiology. Radiographic features vary with the stage of the disease but include cortical thickening, coarse trabeculae, sclerotic ivory or enlarged picture frame vertebrae, cotton wool appearance of the femur and pelvis, and osteoporosis circumscripta of the calvarium. Late radiographic hallmarks include enlarged bone, thickened cortices, and prominent trabeculae.
Bone scans are also helpful for screening.
Histologically there is mosaic. Malignant degeneration of Paget's disease usually carries a poor prognosis. It is characterised by the production of osteoid directly from malignant spindle cell stroma. The knee and proximal humerus are common sites, and early diagnosis is difficult. A relatively painless mass is a frequent presentation.