A 45-year-old man on chronic corticosteroid therapy for systemic lupus erythematosus presents with progressive left hip pain and limited internal rotation. MRI of the hip is performed. The structure marked **A** in the diagram shows a characteristic double-line sign on T2-weighted imaging — an inner high-signal reactive band surrounded by an outer low-signal sclerotic band. X-ray radiographs of the same hip appear normal. What is the most likely diagnosis?

Explanation

## Why Avascular necrosis (osteonecrosis) of the femoral head — FICAT stage I is right The double-line sign on MRI is pathognomonic for avascular necrosis (AVN). This sign consists of a high T2 inner reactive band (granulation tissue and hyperemia) and a low T2 outer sclerotic band (fibrosis and necrotic bone interface) — a finding unique to AVN. The patient's chronic corticosteroid use for SLE is the most common non-traumatic cause of AVN. Critically, X-rays are normal in early AVN (FICAT stage I), making MRI the gold standard for early diagnosis. The absence of femoral head collapse on imaging confirms stage I disease. [Apley 10e] ## Why each distractor is wrong - **Osteoarthritis of the hip with early cartilage loss**: Osteoarthritis does not produce the pathognomonic double-line sign on MRI. Early OA shows cartilage thinning and osteophytes, not the reactive/sclerotic band pattern characteristic of AVN. Additionally, X-rays in early OA typically show joint space narrowing or osteophytes, whereas this patient's X-rays are normal. - **Acetabular labral tear with synovitis**: Labral pathology is marked by signal abnormality within the labrum itself (structure **D**), not the femoral head. The double-line sign is specific to the femoral head and reflects bone necrosis and reactive changes, not labral injury. - **Transient osteoporosis of the femoral head**: Transient osteoporosis presents with diffuse marrow edema on MRI but lacks the characteristic double-line sign. It is self-limited and typically resolves within months; it does not show the sclerotic band demarcation seen in AVN. **High-Yield:** The double-line sign (high T2 inner band + low T2 outer band) on MRI of the femoral head is pathognomonic for AVN and appears when X-rays are still normal — this is why MRI is the diagnostic gold standard for early AVN detection. [cite:Apley 10e]