Fibrous Cortical Defect

Fibrous cortical defect is a very common, benign, self-limiting developmental lesion of the metaphyseal cortex in children, made up of fibrous tissue with histiocytes and giant cells. It is a small cortical lesion (under 3 cm long, under 50% of the cortical width); larger lesions that extend into the medulla are non-ossifying fibromas.

It is found in 30 to 40% of children aged 4 to 8 years on radiographs, and in a higher share of boys. It occurs in the distal femur (posteromedial metaphysis), the proximal and distal tibia, and the fibula, and is often bilateral or multiple. It is nearly always an incidental finding.

Complications are very rare: a pathological fracture through a large lesion. The lesion fills in with bone and disappears by skeletal maturity, usually by 15 to 20 years, and recurrence or malignant change is not recognised.

Pathophysiology.

The lesion represents a focus of fibrous tissue in the cortex near the physis, perhaps a failure of normal remodelling at the metaphysis. It has the same MAPK signalling changes (KRAS, FGFR1) as non-ossifying fibroma. The growth plate moves away from it with growth, and the lesion ossifies from the periphery inwards, seen as sclerosis.

None in nearly all cases. The lesion is found on radiographs taken for an injury or another problem. Mild pain suggests a different diagnosis or a stress reaction. There is no swelling, deformity or limp.

Examine the limb to exclude another cause of pain, and check for signs of neurofibromatosis (café-au-lait spots) when lesions are multiple or large.

• Radiographs: an eccentric, oval, cortically based lucent lesion with a scalloped, sclerotic margin, its long axis parallel to the bone, in the metaphysis, with no periosteal reaction and no soft tissue mass. Features are diagnostic, and no biopsy is needed.
• MRI or CT is rarely needed; use when the lesion is atypical (periosteal reaction, pain, extension into the medulla, over 4 cm).
• Do not biopsy a typical lesion. Distal femoral cortical irregularity (avulsive cortical irregularity at the medial supracondylar ridge, at the insertion of the adductor magnus) is a normal variant that mimics a tumour.
• Check phosphate and alkaline phosphatase when lesions are multiple and large (a rare link with phosphaturic tumour and hypophosphataemic rickets).

Differential diagnosis.

• Non-ossifying fibroma and avulsive cortical irregularity.
• Osteoid osteoma and Brodie abscess.
• Periosteal desmoid and stress fracture.
• Langerhans cell histiocytosis and osteofibrous dysplasia.

Reassurance and observation. No treatment is needed. Explain that the lesion is benign and will heal by itself. Normal activity and sport are allowed, as the risk of fracture is very low for lesions under 3 cm.

Follow up. Repeat radiographs are not needed for a typical lesion in an asymptomatic child. When the lesion is large, symptomatic or atypical, review with radiographs at 3 to 6 months, and again at 12 months.

Surgery. Not needed. Curettage with bone grafting is considered only for a large lesion at risk of fracture (over 50% of the cortex and over 3 cm) with symptoms; otherwise treat only a fracture or a symptomatic lesion.

Pathological fracture. Cast treatment, with healing of the fracture in the usual time; the lesion often fills in after fracture.

Analgesia (if a fracture or minor pain).

• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily) and ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily), for up to 5 days; avoid ibuprofen with kidney impairment, dehydration or active bleeding.
• Adolescents and adults: paracetamol 1 g orally every 6 hours as needed (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment) and ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily).

Monitoring. None for typical lesions. For atypical lesions, clinical review and radiographs at 3 to 6 months and 12 months, with onward referral for any growth, new pain or periosteal reaction.

Referral and escalation. Referral is not needed for typical lesions. Refer to orthopaedics when the lesion is atypical, large, painful, or complicated by a fracture. Urgent referral for rapid growth, periosteal reaction or a soft tissue mass.

Special groups.

• Children: nearly all lesions resolve; sport is allowed.
• Adults with a persistent lesion: consider another diagnosis (non-ossifying fibroma, bone infarct, enchondroma) and MRI.
• Multiple or large lesions with café-au-lait spots: assess for neurofibromatosis type 1 and Jaffe-Campanacci syndrome.
• Hypophosphataemia: measure phosphate and investigate for a phosphaturic tumour.