Paget Disease Imaging 

Paget disease of the bone (osteitis deformans) is a metabolic disorder characterized by abnormal osseous remodeling. Sir James Paget first described Paget disease in 1877 as a chronic inflammatory remodeling disease of bones. He termed the condition osteitis deformans.^{[1, 2]} Paget disease is depicted in the images below.

Paget disease evolves through 3 stages: (1) an early lytic or hot phase; (2) an intermediate or mixed phase; and (3) a final or 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 phase 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.

Insufficiency fractures in patients with Paget disease may present with pain that can last up to several weeks. If pain is focal and severe, it may be a sign of an impending, complete fracture, and radiographic evaluation is warranted. Insufficiency fractures most frequently affect the femur and tibia. Involvement in critical weight-bearing locations may lead to fracture or severe secondary arthritis.^{[3, 4, 5]} See the images below.

Preferred examination

The radiographic findings of Paget disease are diagnostic in many patients. The lytic stage most commonly is observed in the skull and long bones. The typical appearance in the long bones is osteolysis, which begins in the epiphysis and advances along the diaphysis. Trabecular coarsening and distortion and cortical thickening are observed in the sclerotic phase, typically involving the axial skeleton.

Limitations of techniques

Radiographic findings in Paget disease often are pathognomonic, particularly in the lytic phase. However, given the variable imaging appearance of Paget disease in different stages, as well as the many different bones involved, the differential diagnosis may vary substantially among patients.

Characteristic radiographic findings include the following:

In the skull, the lytic phase (osteoporosis circumscripta) typically involves the frontal or occipital bones and progresses to a mixed pattern with multifocal sclerotic patches in the intermediate stage of the disease, referred to as a cotton wool appearance. See the images below.

The vertebral bodies typically become enlarged with a prominent cortical margin (picture frame vertebrae) or become densely sclerotic, mimicking lymphoma or metastatic disease (ivory vertebra). See the images below.

In the pelvis, typical findings include thickening of the iliopectineal line in early stages, progressing to patchy sclerosis and lucency in later stages. See the image below.

Weakening of the pagetic acetabular bone may lead to protrusio acetabuli and insufficiency fracture. See the image below.

In the long bones, early involvement consists of lysis of the subarticular bone, which advances along the diaphysis with the characteristic shape of a blade of grass. Long bones are affected first in the epiphyseal region, with the exception of the tibia, where Paget disease frequently begins in the tubercle.

Later stages of disease show development of enlarged, sclerotic, deformed bones with thickened coarse trabeculae. The weakened femur and tibia eventually may become bowed under the stress of weight bearing. Insufficiency fractures may occur, characteristically involving the convex cortical surface. Conversely, Looser zones of osteomalacia typically occur on the concave cortical surface. See the images below.

Development of secondary sarcoma in pagetic bone is the most lethal complication of Paget disease, occurring in 1% or fewer of patients with Paget disease (see the image below). These sarcomas are aggressive and may be multicentric. Short-term interval follow-up and/or cross-sectional imaging may prevent diagnostic errors and initiate prompt attention to a newly developing lesion.^{[6, 7, 8]}

Cross-sectional MRI and CT demonstrate enlarged bones with trabecular coarsening and increased cortical thickness. The anatomy is well demonstrated by cross-sectional imaging in complex structures, such as the spine, where spinal or nerve root compression may be an issue. Cross-sectional imaging also helps delineate the pathology in complicated Paget disease, which includes nerve or spinal cord compression, as well as basilar invagination at the skull base and osseous encroachment involving cranial nerve foramina.

Secondary sarcomatous development also is better evaluated with cross-sectional imaging. Additionally, should biopsy be indicated for the diagnosis of sarcoma, CT typically is the guidance modality of choice. See the images below.

See CT Scan section.

Skeletal scintigraphy is useful. Radionuclide bone scans are more sensitive than radiographs for the diagnosis of Paget disease. Additionally, bone scans help survey the different sites of involvement with polyostotic disease.

Characteristically, a marked uptake of radiopharmaceutical in the involved bones is observed. However, late-stage involvement may not reveal intense radiopharmaceutical uptake, and osteoporosis circumscripta may demonstrate only a peripheral rim of increased uptake. Scintigraphy tends to follow the physiologic activity of disease and may monitor treatment.

Polyostotic Paget disease often can be distinguished from multiple metastatic lesions, although occasional difficulties occur. Perform radiographic correlation when this situation arises. Furthermore, the diagnosis of fracture or sarcoma may be challenging, often requiring multimodality correlation. See the images below.