Introduction:
Avascular necrosis (AVN), also called osteonecrosis, is a condition where part of the bone (usually femoral head) dies due to loss of blood supply.
- Common in young adults
- Often bilateral
- If not treated early, it can lead to collapse of the femoral head and hip arthritis
🎯 Common Risk Factors:
- Long-term steroid use
- Alcohol abuse
- Trauma (e.g., hip dislocation)
- Sickle cell disease
- Systemic lupus erythematosus (SLE)
- Radiation, decompression sickness (Caisson disease)
🧠 Why MRI?
MRI is the best imaging test for early detection of AVN.
It can pick up bone changes before they are visible on X-ray or CT.
🩻 MRI Protocol:
- T1-weighted images (coronal, sagittal)
- STIR or T2 fat-sat (to detect marrow edema)
- Proton Density (PD) optional
- Contrast-enhanced MRI in some cases
- DWI for research/early detection (optional)
🔍 Typical MRI Findings in AVN Hip:
1. T1-weighted images:
- Shows low signal area in the femoral head (due to dead bone)
2. T2-weighted or STIR images:
- Double line sign:
- Outer dark line (sclerosis)
- Inner bright line (granulation tissue)
- ⚠️ Very specific for AVN!
3. Bone marrow edema:
- Seen as bright signal on STIR/T2
- Common in painful, early stages
4. Subchondral fracture (crescent sign):
- Seen as a low signal line under the cartilage
- Suggests impending collapse
📊 MRI Staging of AVN (Based on ARCO):
| Stage | MRI Appearance | X-ray Appearance |
|---|---|---|
| I | Bone marrow edema or necrotic area only | Normal |
| II | Clear low-signal area in head, “double line sign” | Sclerosis |
| III | Subchondral fracture (crescent sign), no collapse | Flattening may begin |
| IV | Collapse of femoral head, secondary arthritis | Joint space narrowing, osteophytes |
📐 Extent of Necrosis – Kerboul Angle:
- Combines necrotic arc seen on coronal + sagittal T1 images
- Helps predict risk of collapse:
| Angle | Risk of Collapse |
|---|---|
| < 200° | Low |
| > 250° | High |
📎 Other Modalities (for comparison):
| Modality | Usefulness |
|---|---|
| X-ray | May be normal in early AVN |
| CT | Shows bone contour, late collapse |
| Bone Scan | Shows increased uptake; not specific |
| MRI | Most sensitive, detects early changes |
✅ Conclusion:
MRI is the gold standard for diagnosing early AVN of the hip.
It shows marrow changes, extent of necrosis, and early fracture signs, helping guide treatment and predict collapse.
Look for key signs like:
- Double line sign (T2)
- Low signal necrotic area (T1)
- Subchondral fracture (crescent sign)
