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Benign Fibrous Bone Lesions

Avulsive Cortical Irregularity

Avulsive cortical irregularity (cortical desmoid) refers to the common radiographic irregularity over a short segment of cortical surface along the posterior aspect of the medial femoral condyle, just superior to the adductor tubercle. The radiographic appearance is similar to that of a fibrous cortical defect. The margins are variably sclerotic (Figure 63-1). Small osseous fragments in the immediately adjacent soft tissues are sometimes visible on lateral radiographs or CT. Avulsive cortical irregularity is often bilateral and is more common in boys. The typical age range is 3 to 17 years. This "lesion" is apparently related to repetitive stress at the site of attachment of the medial head of the gastrocnemius or adductor magnus muscles.1

Figure 63–1

Avulsive cortical irregularity.

A. There is a faint oval lucency (arrow) in the medial aspect of the distal femoral metaphysis of an 8-year-old boy. A peripheral sclerotic rim is present. B, C. The cortical lesion in this 9-year-old girl is smaller and has minimal adjacent sclerosis. The lateral view confirms the typical posterior location and shallow character.

The only clinical importance of avulsive cortical irregularity is when it is mistaken for a neoplasm, infection, fracture, or other pathology. Patients with this finding typically have no related symptoms. When the diagnosis is in doubt, radiographs of the contralateral knee typically show an identical appearance. Bone scintigraphy is normal or shows minimal increased uptake. On T1-weighted MR images, there is a rim of low signal intensity at the origin of the medial head of the gastrocnemius muscle; the area is sometimes hyperintense on T2-weighted images.2

Fibrous Cortical Defect

A fibrous cortical defect is a common, benign fibrous lesion of bone that is histologically identical to a nonossifying fibroma. This lesion is, by definition, confined to the cortex; most are less than 2 to 3 cm in diameter. The nonossifying fibroma is a closely related lesion that is at least 2 cm in diameter and extends into the medullary portion of the bone. Fibrous cortical defect is composed of fibrous tissue; the fibrous tissue induces a mild adjacent osteoblastic response. Developmentally, the ectopic fibrous tissue likely originates from the physis, with subsequent bone growth causing the lesion to "migrate" into the metadiaphyseal region. Most fibrous cortical defects eventually resolve spontaneously, either by reparative ossification or by gradual extrusion from the cortex.3

Most fibrous cortical defects occur in the extremities, especially the femur and tibia. This common, asymptomatic lesion can be considered a developmental variation of normal; approximately 30% ...

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