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Fibrous dysplasia (FD) is an aberration of normal bone development and results from a defect in osteoblastic differentiation and maturation originating in a mesenchymal precursor. The disease is characterized by foci of abnormal fibro-osseous proliferation that affecting any area of the calvarium and presents in three distinct clinical patterns: (a) monostotic, the most common form with single bone involvement, (b) polyostotic with multiple bone involvement, and (c) as part of McCune-Albright syndrome, a rare variant of the polyostotic form with pigmentation and endocrinologic abnormalities. The disease commonly presents in the first three decades of life, particularly in late childhood and early adolescence.
FD is considered an abnormal overgrowth of bone. Recent evidence has shown that activating mutations of G proteins in osteoblastic cells results in increased activation of adenylate cyclase resulting in overproduction of cyclic adenosine monophosphate (cAMP) culminating in increased cell proliferation and aberrant cell differentiation. Interleukin-6 may also increase intracellular cAMP, which can result in osteoclast proliferation thus contributing to the bone lesions seen in FD. Microscopically, FD characteristically appears as woven bone interspersed between areas of fibrous tissue.
On skull x-ray and computed tomography (CT) scans, there is a characteristic “ground glass” appearance. The lesions may appear sclerotic (35% of cases), cystic (25% of cases), or a mix of the two (40% of cases). In thinner bones of the cranium (e.g. temporal, frontal, and maxillary bones), the bone undergoes rapid expansion resulting in lytic, cavitary lesions. Thicker bones of the skull (e.g. sphenoid) tend to undergo a more diffuse sclerotic reaction. CT is an effective imaging tool for detection of the disease although MRI may be a useful adjunct for identifying affected neurovascular structures.
Clinically, the lesion progresses as a painless, nonmobile mass, and orbitocranial swelling can reach significant proportions resulting in severe cosmetic deformities. Lesions involving the petrous portion of the temporal bone can result in conductive hearing loss by stenosis of the external auditory canal. Nasal obstruction may result from involvement of the frontal, ethmoid, sphenoid, or maxillary bones. Skull base involvement may result in diplopia, facial pain or numbness, and headaches. Visual occurs when the optic canal is involved.
Treatment of FD ranges from observation to aggressive surgical intervention. Medical treatment, primarily with bisphosphonates, has been successfully described 15,31. When feasible, surgical resection with cosmetically acceptable reconstruction is advocated. In cases of optic canal involvement (particularly with visual loss), early and aggressive surgical treatment is warranted and some authors advocate prophylactic enlargement of the optic canals. Radiotherapy for FD has been strongly discouraged as the incidence of developing a secondary malignancy is high. FD has also been reported to spontaneously degenerate to more malignant subtypes such as osteosarcoma, and less commonly, fibrosarcoma, chondrosarcoma, or malignant fibrous histiocytoma.