Case 19

Discussion:

Fibrous dysplasia (FD) is a non-neoplastic process, congenital, skeletal disorder manifested by developmental anomaly of bone formation in which the marrow is replaced by fibrous tissue. It is typically unilateral with monostotic forms more common (70 to 80%) than polyostotic. Its etiology is unknown. The lesions tend to arise during periods of bone growth in older children and adolescents and slowly enlarge. They are found in the proximal femur, tibia, humerus, ribs, and craniofacial bones (in decreasing order of incidence).

Polyostotic disease can be associated with abnormal skin pigmentation (ipsilateral to the osseous lesions) and endocrinopathies. The constellation of polyostotic fibrous dysplasia, skin pigmentation and precocious puberty has the eponym McCune Albright Syndrome. Mazabraud Syndrome is fibrous dysplasia associated with soft tissue myxomas. Malignant degeneration is rare but know complication.

Radiographically, Fibrous dysplasia appears as a well circumscribed intramedullary lesion with a ground glass matrix and a narrow zone of transition with no periosteal reaction, cortical thickening or soft tissue mass. Bony deformity can result in so called “shepherd’s crook” deformity of long bones.  On MRI Fibrous dysplasia shows variable appearance depending on the amount of fibrous and ossified tissue present but typically show areas of low signal on both T1 and T2 WI.