Paget disease evolves through 3 stages as follows:
• An early lytic or hot phase
• An intermediate or mixed phase
• A final cold phase, marked by dense bone formation
Paget disease rarely is diagnosed in the initial lytic phase. At this early point of the disease, osteoclastic activity is predominant. Paget disease usually begins at the end of a bone, except when it occurs in the tibia. A characteristic sharply demarcated zone of osteolysis may begin in the subcortical bone and advance along the diaphysis. Osteoblastic activity lags behind, thus radiolucent fibrous tissue replaces normal bone.
The intermediate or mixed phase reveals evidence of osteolytic and disorganized osteoblastic activity. New bone forms abnormally and demonstrates characteristically coarsened trabecula and cortical thickening in the cancellous and compact bone, respectively. Characteristic intracytoplasmic inclusions may be observed microscopically, supporting evidence for the viral etiology theory.
The final or cold stage demonstrates less evidence of continual osseous remodeling. Previously laid down woven bone is converted to dense lamellar bone. Histologic features of disorganized bone are prominent. The intersecting lines of remodeled bone have a characteristic mosaic pattern histologically
Other Problems to be Considered:
Bony sclerosis without osseous enlargement, including blastic metastases, myelofibrosis, renal osteodystrophy, fibrous dysplasia, fluorosis, mastocytosis, and tuberous sclerosis
Coarsened trabecula in the axial skeleton with osteomalacia
Calvarial hyperostosis with hyperostosis frontalis interna, fibrous dysplasia, anemia, and metastatic disease
Hyperphosphatasia (juvenile Paget disease) in young patients