A 40-year-old man has hip pain. MRI shows subchondral T1 hypointense, T2 hyperintense crescent-shaped area in the femoral head without femoral head collapse. The 'double line sign' is seen on T2-weighted MRI. What does the double line sign represent and what is the diagnosis?
- A Avascular necrosis (AVN) of the femoral head — inner T2 bright line (granulation tissue/hyperemia) and outer T1/T2 dark line (sclerotic margin) ✓
- B Transient osteoporosis — reactive edema; double line is the physeal scar
- C Pigmented villonodular synovitis — hemosiderin deposition
- D Subchondral stress fracture — fracture lines
Explanation
The double line sign on T2-weighted MRI is pathognomonic of avascular necrosis (AVN/osteonecrosis) of the femoral head: an inner bright line (T2 high) representing reactive hyperemia/granulation tissue at the interface between viable and necrotic bone, and an outer dark line (T1/T2 low) representing the sclerotic margin or chemical shift artifact. This curvilinear band follows the contour of the femoral head subchondral zone. Before collapse occurs (pre-Ficat stage II–III), MRI can detect AVN and guide management (core decompression). Transient osteoporosis shows diffuse edema without a distinct double line. PVNS shows hemosiderin blooming on GRE/SWI.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.