Teaching Case

A 6-year-old girl presented with pain in her left leg. The patient had a history of trauma to the left lower extremity a month before.

Anteroposterior radiography of the leg showed a solid periosteal reaction on the proximal femoral diaphysis (Fig. 1). Radiography had not involved the knee.
Computed tomography (CT) showed a solid periosteal reaction and fracture on the medial aspect of the proximal femoral diaphysis (Fig. 2).
On CT, an irregular cortical defect was seen on the distal medial femoral metaphysis at the attachment of the medial head of the gastrocnemius muscle (Fig. 3).
On MR, the proximal diaphysis of the femur was hyperintense on T2W images, hypointense on T1W images due to oedema (Fig. 4).
MR imaging showed an oval-shaped, 11x9x6 mm sized lesion on the posteromedial distal femoral metaphysis. The lesion was iso-hypointense to muscle on T1W images and hyperintense on T2W image. On postcontrast T1W series the lesion enhanced (Fig. 5).

Distal femoral cortical irregularity (DFCI) is a benign entity presenting as an irregular, destructive cortical lesion on the posteromedial aspect of the distal femoral metaphysis. DFCI on radiographs of distal femur has been reported in 11.5% of male and 3.6% of female children [1]. DFCI shows a definite predilection for children and adolescents, and certainly a relation to stress [2]. The lesions are bilateral in approximately 35% of cases [3].
Distal femoral cortical irregularity has also been referred to as “cortical desmoid”, “cortical irregularity syndrome” and “distal femoral defect” [4].
The typical radiological appearance is a radiolucent cortical defect with adjacent sclerosis. It is often asymptomatic and is almost an incidental finding [3].
CT imaging shows cortical erosion at the origin of the medial head of the gastrocnemius or fibres of adductor magnus muscle. Small fragments of resorbing bone may be seen in the lesion [4].
On MRI, DFCI is hypointense on T1-weighted images and hyperintense on T2-weighted images. A dark rim may be seen on both sequences. The lesion is typically at or near the sites of the bony attachment of the medial head of the gastrocnemius muscle [5].
Bone scintigraphy with technetium typically shows no increased uptake in the area of DFCI [6].
Because of the immature reactive bone and fibrous tissue involvement, DFCI has been mistaken for osteosarcoma histologically. It is usually possible to identify these lesions radiologically and avoid biopsy [3]. DFCI is a 'do not touch' lesion which is not known by all radiologists and which can lead to unnecessary follow-up imaging.