Fibrous cortical defect (FCD) is a previously used term to describe non-ossifying fibromas smaller than 2-3 cm, displaying the same histology and features as the latter. Non-ossifying fibroma is now the recommended term per the WHO classification of soft tissue and bone tumors (5th edition) 6.
In the popular mnemonic for lucent bone lesions FEGNOMASHIC, non-ossifying fibroma accounts for the "N" and FCD can be considered to account for the "F".
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Terminology
The term fibrous cortical defect has been used for bone lesions representing non-ossifying fibromas smaller than 2-3 cm. According to the WHO classification of soft tissue and bone tumors (5th edition), the term is no longer recommended and instead, non-ossifying fibroma (NOF) is preferred 6.
Epidemiology
Fibrous cortical defects typically occur in children (usually 2-15 years) and are one of the most common benign bone lesions, which combined with non-ossifying fibromas are seen in up to 40% of skeletally immature children/adolescents 3. There is a male predilection by a ratio of 2:1 3.
Clinical presentation
They are asymptomatic and self-limiting, completely healing by adulthood.
Pathology
Fibrous cortical defects macroscopically appear as fleshy, fibrous, yellow or tan-brown lesions with variable areas of hemorrhage 3.
Microscopic examination reveals the lesions to be highly cellular containing spindle-shaped cells on a background of stromal tissue in a prominent storiform pattern. Foamy histiocytes and multinucleated giant cells are also seen. Mitotic figures and/or cellular dysplasia should not be seen 3.
During the healing phase, there is an increase in osteoblastic activity as new bone replaces the defect, gradually being remodeled and completely disappearing 4.
Radiographic features
Fibrous cortical defects are benign lytic bone lesions, and, along with fibrous dysplasia share the F in the popular mnemonic FEGNOMASHIC.
They typically occur in the metaphysis or diametaphyseal junction and appear as small (<2-3 cm) lucent defects within the cortex that over time become sclerotic as they heal. They are typically located in the distal femur or proximal or distal tibia. They are much less frequently seen in the upper limb 3,4.
Importantly, there is no associated soft tissue mass 3.
Plain radiograph and CT
lucent intracortical defects
outlined by a thin rim of sclerosis
no involvement of the underlying medullary cavity
MRI
Signal characteristics include:
T1: hypointense
T2: variable, depending on the phase of healing 3
Nuclear medicine (bone scan)
The appearance depends on the phase of the lesion. In general, they are negative; however, mild hyperemia and moderate bone uptake are present during healing. If extensive uptake or hyperemia is present, then an alternative diagnosis or superimposed fracture should be considered 3.
Treatment and prognosis
As these lesions are benign, characteristic in appearance and self-limiting, no treatment, biopsy or follow-up is required in typical cases. If associated with pathological fracture (more common in non-ossifying fibromas) then cast immobilization until the fracture has healed, followed by biopsy with or without curettage, and bone grafting may be necessary 3.
History and etymology
The fibrous cortical defect was first described by Dallas Burton Phemister (1882-1951) 5, an American orthopedic surgeon in 1929 6.
Differential diagnosis
Possible differential considerations include
cortical desmoid: posteromedial aspect of the distal femur
osteoid osteoma: night pains
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