A 38-year-old man with a 15-year history of systemic lupus erythematosus on long-term high-dose prednisolone (20 mg daily for 3 years) and heavy weekend alcohol use presents with progressive right groin pain exacerbated by weight bearing and internal hip rotation. Plain pelvic radiographs are unremarkable with preserved joint space. MRI T2-weighted coronal images show the pathognomonic double line sign of avascular necrosis of the right femoral head. The structure marked **B** in the diagram represents the inner hyperintense line. What tissue does this inner hyperintense line represent, and what is its pathophysiological significance in early-stage avascular necrosis?

Question

Accepted Answer

B. Hypervascular granulation tissue at the necrotic-viable bone interface, representing the host's reparative response and the most sensitive MRI finding for early AVN. ## Why option 1 is correct

The inner hyperintense line (marked **B**) on T2-weighted MRI represents hypervascular granulation tissue at the necrotic-viable bone interface. This is the pathognomonic inner component of the double line sign, which is the single MRI feature most specific for avascular necrosis. The hyperintensity on T2 reflects the increased vascularity and edema within the granulation tissue, which represents the host's reparative response attempting to demarcate and eventually resorb the necrotic bone. This finding is present in early-stage (FICAT Stage II) AVN, before subchondral collapse occurs, and is far more sensitive than plain radiographs for detecting early disease. According to Campbell's Operative Orthopaedics, the double line sign—comprising the outer hypointense sclerotic rim and the inner hyperintense granulation line—is pathognomonic for AVN on T2 imaging.

## Why each distractor is wrong

- **Option 2**: Fibrocartilage scar tissue is a late finding associated with chronic AVN and articular degeneration, not the early reparative granulation tissue. This option misrepresents the temporal stage and tissue type.
- **Option 3**: Calcified necrotic bone marrow would appear hypointense on T2 and is not the inner hyperintense line. This describes the necrotic core itself, not the interface tissue, and does not explain the T2 hyperintensity.
- **Option 4**: Reactive sclerotic bone is hypointense on both T1 and T2 and corresponds to the outer line (**A**), not the inner hyperintense line (**B**). Sclerosis is best seen on T1-weighted imaging as a low-signal rim, not as a T2 hyperintensity.

**High-Yield:** The double line sign on T2-weighted MRI—outer hypointense sclerosis + inner hyperintense granulation—is pathognomonic for AVN and detects disease at FICAT Stage II, before radiographic collapse, allowing intervention to preserve the femoral head.

[cite: Campbell's Operative Orthopaedics, 14th Edition, Chapter 6: Osteonecrosis]

Answer

A. Fibrocartilage scar tissue indicating irreversible necrosis and the need for immediate hip arthroplasty

Answer

C. Calcified necrotic bone marrow with dystrophic mineralization, indicating advanced collapse

Answer

D. Reactive sclerotic bone representing the body's attempt to wall off the necrotic segment, best seen on T1-weighted imaging

Explanation

Why option 1 is correct

Why each distractor is wrong

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