Fibrous Cortical Defect

Fibroxanthoma, nonossifying fibroma (NOF), fibrous cortical defect (FCD), and less commonly, benign fibrous histiocytoma, have all been used interchangeably in the radiology literature.

Currently, fibroxanthoma is the preferred term for the NOF lesion because it more accurately reflects the underlying pathologic findings. In the literature, FCD has a longer list of pseudonyms, including metaphyseal fibrous defect, metaphyseal supracondylar cortical defect, and developmental defect. Both are considered developmental defects and nonaggressive.

The distinction between FCD and fibroxanthoma is based on the size and natural history. FCDs are asymptomatic, small, (<3 cm), eccentrically located, metaphyseal cortical defects; most of these spontaneously disappear. However, some evolve and enlarge into fibroxanthomas. Conversely, fibroxanthomas (>3 cm) are larger, eccentric, intramedullary lesions that abut the cortex; they have a typical, superficial, scalloping pattern in the adjacent cortex.

FCDs occur in younger patients, with presentation in those aged 4-8 years. Usually, FCDs are incidental findings on radiographs that are obtained for indications other than the evaluation of FCD.

The imaging features of fibroxanthoma are pathognomonic. Typically, a metadiaphyseal, eccentric, ovoid or round, intramedullary geographic radiolucency, with a sharply marginated sclerotic rim of reactive bone and a scalloped contour, is present. The medullary rim typically is thicker than the cortical rim. While they initially appear near the physeal plate, they eventually appear further down within the diaphysis, following a normal growth pattern. The epiphysis is never affected. Larger lesions can appear multiloculated, with dimensions as large as 4-7 cm, with erosion of the endosteal surface of the cortex.

FCDs usually are round or oval geographic radiolucencies that measure 1-3 cm and they are located within the posteromedial aspect of the distal femoral cortex along the medial supracondylar ridge just proximal to the adductor tubercle. A sharply marginated rim of reactive bone is identified where the lesion impinges on the medullary cavity. No associated soft tissue mass is present. FCDs are best visualized in the anteroposterior (AP) oblique projection of the femur, with the limb in 20-40° external rotation.

Differential Diagnosis :

Chondromyxoid fibroma

Fibrous dysplasia

Osteoid Osteoma