Fibrous Dysplasia

Background: Fibrous dysplasia is a benign skeletal developmental anomaly of the bone-forming mesenchyme that manifests as a defect in osteoblastic differentiation and maturation. Virtually any bone in the body can be affected. Unknown cause- probable gene mutation.
Age: 1st-2nd decade (peak 3-15yrs), 75% before 30, M:F=1:1
Pathophysiology: The medullary bone is replaced by fibrous tissue, which appears radiolucent on radiographs, with the classically described ground-glass appearance. Trabeculae of woven bone contain fluid-filled cysts that are embedded largely in collagenous fibrous matrix, which contributes to the generalized hazy appearance of the bone.
Types: Monostotic (70-80%); Polyostotic (20-30%); Craniofacial (10-25% of monostotic, 50% polyostotic)
Variants: Cherubism; McCune-Albright syndrome
Skull: Deformity with cranial nerve compromise. Proptosis. Sclerotic skull base. Widened diploic space with displacement of outer table. Obliteration of frontal and sphenoid sinuses. Inferolateral displacement of orbits. Occipital thickening. Cystic calvarial lesions, often crossing sutures.
MRI: T1WI- marrow lesions hypointense to muscle. T2WI- homogeneous/ mildly heterogeneous marrow lesions hyperintense to fat (60%) or intermediate/ low signal intensity. Fluid-fluid levels.
Complications: 1. Transformation into osteo-/ chondro-/ fibrosarcoma or malignant fibrous histiocytoma (0.5-1%, more often in polyostotic form). 2. Pathologic fractures.
Reference: Dähnert W. Radiology Review Manual, 5th edition, Lippincott, Williams & Wilkins, 2003.
Credit: Dr Marina-Portia Anthony
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