Fibrous Dysplasia.

Note the bilateral lesions.

Fibrous dysplasia is a developmental dysplastic disorder of bone in which immature woven bone is formed directly from abnormal fibrous connective tissue. It is characterized by expanding fibroosseous tissue within affected bones and predominantly is a lesion of the growing skeleton. It is termed a dysplasia because of the inability of involved tissue to form mature lamellar bone from the immature, woven precursor.

Monostotic fibrous dysplasia affects only one bone, most commonly the ribs, proximal femur, and craniofacial bones. Polyostotic affects many bones, up to 75% of the skeleton. Monostotic fibrous dysplasia is 7-10 times more common than polyostotic fibrous dysplasia. It also can be associated with systemic conditions, including precocious puberty and skin pigmentation (as in McCune-Albright syndrome).

Common locations for lesions are the ribs, craniofacial bones, femoral neck, tibia, and pelvis. Radiographic findings in these and other structures are discussed below.

The usual appearance of fibrous dysplasia includes a lucent lesion in the diaphysis or metaphysis, with endosteal scalloping and with or without bone expansion and the absence of periosteal reaction. Usually, the matrix of the lucency is smooth and relatively homogeneous; classically, this finding is described as a ground-glass appearance. Irregular areas of sclerosis may be present with or without calcification. The lucent lesion has a thick sclerotic border and is called the rind sign.

The lesion may extend into the epiphysis only after fusion. Premature fusion of the ossification centers may occur, resulting in adult dwarfism. The dysplastic bone may undergo calcification and enchondral bone formation.

The frontal bone is involved more frequently than the sphenoid, with obliteration of the sphenoid and frontal sinuses. The skull base may be sclerotic. Single or multiple, symmetric or asymmetric, radiolucent or sclerotic lesions in the skull or facial bones may be present. The external occipital protuberance may be prominent; however, these features are less common in Paget disease, neurofibromatosis, and meningioma.

Most commonly, maxillary and mandibular involvement has a mixed radiolucent and radiopaque pattern, with displacement of the teeth and distortion of the nasal cavities. The diploic space is widened, with displacement of the outer table. The inner table of the skull is spared in fibrous dysplasia, unlike in Paget disease. Cystic calvarial lucencies, which commonly cross the sutures with sclerotic margins, may have a doughnut configuration.

These bones have lucencies, with a diffuse ground-glass appearance and rind lesions. Cystic lesions are common. Protrusio acetabuli is a feature on the pelvic radiograph.

Spinal involvement is common in polyostotic disease and rare in monostotic disease. Well-defined, expansile, radiolucent lesions with multiple internal septa or striations involve the vertebral body and, occasionally, the pedicles and arches. Paraspinal soft-tissue extension and vertebral collapse are rare. Kyphotic deformity and spinal cord compression may occur.

CT is not often required for diagnosis. CT demonstrates the nature of the lesion better by characterizing the matrix of the lesion. It also depicts expansion of the affected bone and its subtle mineral contents. It can demonstrate subtle nondisplaced pathologic fractures. CT is extremely useful in evaluating the extent of disease in complex locations such as the facial bones, pelvis, chest wall, and spine. Usually, attenuation is in the range of 70-130 HU (Hounsfield unit).

In the skull, the outer table always expands outward. Therefore, the lesion is invariably convex; both tables are intact, although they are thinner. In the spine, CT can demonstrate the extent of bony disease and compromise of the spinal canal space. Paraspinal soft-tissue extension can be demonstrated at CT. CT scans may suggest malignant transformation, with the definition of an extraosseous soft-tissue mass and bone destruction

MRI
On T1-weighted MRIs, the lesion has low-to-intermediate signal intensity equal to that of muscle. T2-weighted images also show low signal intensity owing to the high content of collagen and bone. Cartilaginous islands may be present in some lesions, and they appear as areas of high signal intensity on T2-weighted images. In children, T2-weighted images show hyperintense signal greater than that of subcutaneous fat; this finding is characteristic of fibrous dysplasia.

Also, fluid-fluid levels are reported in fibrous dysplasia. On short–inversion time inversion-recovery (STIR) images, the signal intensity of the lesion may be very high. MRI may be useful in assessing malignant change and demonstrating extension of the tumor into the surrounding soft tissues.

For postoperative follow-up, gadolinium-enhanced MRI is useful in demonstrating the proliferation of fibrocellular tissue.