MRI of a knee shows a T2 hyperintense, T1 hypointense band parallel and adjacent to the articular surface of the medial femoral condyle with overlying cartilage thinning. This subchondral low-signal band on T1 is called the 'double-line sign' on T2 (inner bright/outer dark). What is the diagnosis?

• A Stress fracture of the condyle
• B Subchondral insufficiency fracture
• C Avascular necrosis (AVN) of the femoral condyle ✓
• D Osteochondritis dissecans

Explanation

The double-line sign on T2 — an outer low-signal rim and an inner high-signal line — is pathognomonic of avascular necrosis on MRI. The inner bright line represents granulation tissue at the interface between viable and necrotic bone, while the outer dark line represents sclerotic reactive bone. This sign differentiates AVN from stress fracture and subchondral insufficiency fracture. Osteochondritis dissecans shows a loose body or fragment at a different condylar location.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.