How to differentiate avascular necrosis (AVN) and osteoarthritis (OA) of the hip radiologically?

Radiological Differentiation of Avascular Necrosis vs Osteoarthritis of the Hip

Start with plain radiographs in two planes, but proceed immediately to MRI without IV contrast when radiographs are normal or equivocal, as MRI has 93% sensitivity and 91% specificity for detecting AVN and is the definitive modality to distinguish these conditions. 1

Initial Imaging Approach

• Plain radiography (AP and lateral views) is the appropriate first-line imaging study for both suspected AVN and osteoarthritis of the hip 1
• However, radiographs have limited sensitivity for early AVN and may appear normal in early stages, while OA typically shows characteristic changes when symptomatic 1, 2
• If radiographs are normal or suspicious but clinical suspicion for AVN remains, MRI without IV contrast is the next appropriate study with sensitivity and specificity approaching 100% 1

Key Radiographic Distinguishing Features

Avascular Necrosis Radiographic Findings:

• Subchondral crescent sign (pathognomonic for AVN when present) - represents subchondral fracture 1, 3
• Patchy sclerosis and lucency in the femoral head (early finding) 2
• Flattening or collapse of the femoral head in later stages 1
• Sudden onset of symptoms with relatively preserved joint space until late stages 4
• Younger patient age (typically before fifth decade) 4, 5

Osteoarthritis Radiographic Findings:

• Joint space narrowing (cardinal feature) 1
• Osteophyte formation at joint margins 1
• Subchondral bone sclerosis (diffuse, not patchy) 1
• Subchondral cysts 1
• Gradual onset in older patients (typically >40 years) with usage-related pain 1

MRI Distinguishing Features (When Needed)

AVN on MRI:

• "Double-line sign" on T2-weighted images (inner hyperintense line, outer hypointense line) - highly specific for AVN 1, 3
• Geographic, well-demarcated lesion in the subchondral region of femoral head 1, 6
• Bone marrow edema surrounding the necrotic area (when present with double-line sign, indicates worse prognosis) 3
• Preserved joint space until advanced stages 6
• Lesion location and volume can be precisely characterized 1

Secondary OA on MRI:

• Diffuse cartilage loss and joint space narrowing 1
• Bone marrow edema lesions (less well-demarcated than AVN) 1
• Synovitis and joint effusion 1
• No double-line sign or geographic necrotic lesion 1

Critical Clinical Context Clues

• History of sudden onset pain strongly suggests AVN over OA (present in >50% of AVN cases) 4
• Risk factors for AVN: corticosteroid use, alcohol abuse, trauma, sickle cell disease, radiation, Caisson disease 6, 4, 2
• Multiple joint involvement is more common with AVN than primary OA 4
• Pain out of proportion to radiographic findings suggests early AVN 4
• AVN patients have shorter symptom duration at presentation compared to OA 4

Advanced Imaging Considerations

• CT without IV contrast is superior to MRI for showing the exact location and extent of articular collapse once it has occurred, and demonstrates osseous details of secondary OA well 1
• CT is useful for preoperative planning in known AVN with collapse 1
• MRI with dynamic contrast enhancement can differentiate AVN from transient bone marrow edema syndrome: AVN shows a rim of high plasma flow surrounding a subchondral area without flow, while transient bone marrow edema shows subchondral hyperperfusion 1

Common Pitfalls to Avoid

• Do not rely on radiographs alone to exclude AVN - they are insensitive in early stages when treatment is most effective 1, 2
• Do not mistake the bone marrow edema pattern of transient osteoporosis of the hip (diffuse edema throughout femoral head and neck) for AVN (geographic lesion with double-line sign) 1
• Secondary OA can develop from advanced AVN, making differentiation difficult in late stages - look for the underlying geographic necrotic lesion on MRI 1, 6
• Subchondral insufficiency fracture can mimic AVN but shows different enhancement patterns on dynamic contrast MRI 1