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Findings

Radiograph right femur: minimally expansile, lytic lesion with ground glass density and sclerotic borders extending from femoral neck into diaphysis with cortical thinning especially of the inferior aspect of the femoral neck.

MRI: low T1, high T2 signal mass centered in the medullary canal with cortical thinning but no cortical disruption. Adjacent soft tissues are normal.

Radiograph tib/fib: additional mixed lytic and sclerotic lesion longitudinally oriented in tibia.
 

Diagnosis

Polyostotic fibrous dysplasia.

Discussion

Fibrous dysplasia is a non-hereditary developmental anomaly of unknown origin causing replacement of medullary bone by fibrous tissue. Eighty percent of cases are monostotic, however, 20%, as in this case, are polyostotic. Any bone can be affected, but there is a predilection in the polyostotic form for the femur, tibia, pelvis, ribs and facial bones. Patients usually present in late childhood with pain or pathologic fracture.

On plain radiographs, fibrous dysplasia is characteristically a diaphyseal or metaphyseal lucent lesion with a homogenous, smoky radiodensity termed a "ground glass" appearance. Sclerotic borders, endosteal scalloping, variable degrees of central sclerosis and an absence of periosteal reaction are other common features. On MRI, the findings are relatively nonspecific: the lesion demonstrates low T1 and variable T2 signal within the medullary canal.

The differential diagnosis includes non-ossifying fibroma (eccentric, cortically based lytic lesions with sclerotic rim); solitary bone cyst (lytic, intramedullary lesions usually without a sclerotic rim or central sclerosis, and usually more round or oval shaped); aneurismal bone cyst (usually quite expansile with marked cortical thinning; fluid-fluid levels on MRI); enchondroma (solitary form usually has chondroid matrix; polyostotic form (Ollier¿s disease) has long, lytic lesions with vertical striations); osteomyelitis (generally more destructive with cortical disruption and soft tissue findings, periosteal reaction).

In this case, the disease was polyostotic, which raises the additional possibility of syndromic involvement. McCune-albright syndrome is characterized by fibrous dysplasia, café-au-lait spots and precocious puberty. Mazabraud¿s syndrome consists of polyostotic fibrous dysplasia and intramuscular myxomas. Other endocrinopathies including hyperthyroidism, Cushing's disease, hypophosphatemia, and hyperprolactinemia have been associated with fibrous dysplasia

Complications of fibrous dysplasia include pathologic fractures and malignant degeneration, most commonly into osteosarcoma or fibrosarcoma. In the femur, the ¿shepherd¿s crook¿ deformity can occur, which is varus angulation at the femoral neck. In this case, the patient developed hip pain due to impending pathologic fracture, and a pre-emptive reconstruction nail with interlocking screws was placed through the femoral neck.

References

Resnick D, Niwayama G. Diagnosis of Bone and Joint Disorders. 2nd ed. Philadelphia, Pa: WB Saunders;. 1988: 4057-70.

MJ Kransdorf, RP Moser, Jr, and FW Gilkey. Review: Fibrous dysplasia. RadioGraphics 1990; 10: 519.

WH Jee, KH Choi, BY Choe, JM Park, and KS Shinn. Fibrous dysplasia: MR imaging characteristics with radiopathologic correlation Am J Roentgenol., Dec 1996; 167: 1523 - 1527.
 


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