What is the most accurate test to diagnose avascular necrosis in a patient with risk factors such as corticosteroid use, alcohol abuse, or trauma?

Best Test to Diagnose Avascular Necrosis

MRI without intravenous contrast is the gold standard diagnostic test for avascular necrosis, with sensitivity and specificity approaching 100%. 1

Diagnostic Algorithm

Step 1: Initial Plain Radiographs

• Obtain anteroposterior and lateral hip radiographs as the first-line imaging study, despite limited sensitivity in early disease 1
• Radiographs exclude other causes of pain including fracture, primary arthritis, or tumor 2
• Plain radiography will miss AVN in approximately 38% of cases, particularly in early stages 3

Step 2: Proceed Immediately to MRI When Radiographs Are Normal or Equivocal

• If clinical suspicion remains high and radiographs are negative or show only suspicious findings, proceed directly to MRI without IV contrast 1
• MRI demonstrates 93% sensitivity and 91% specificity for early detection of femoral head osteonecrosis based on meta-analysis of 43 studies 1
• A coronal STIR sequence alone shows 100% sensitivity for detecting AVN 1
• Adding a coronal T1-weighted sequence increases confidence and specificity 1

Step 3: Image Both Hips

• Always image bilateral hips even when symptoms are unilateral, as nontraumatic osteonecrosis is bilateral in 70-80% of cases 1
• MRI detects asymptomatic disease in the contralateral hip, with approximately 5% of at-risk patients having positive MRI findings without symptoms 4, 2

Why MRI Is Superior to Other Modalities

MRI Advantages

• MRI is the most sensitive imaging technique for early diagnosis of avascular necrosis, significantly outperforming both CT and bone scintigraphy 5
• MRI characterizes the osteonecrosis including location, volume (necrotic angle), and presence of associated bone marrow edema or joint effusion 1
• MRI differentiates AVN from mimics such as transient bone marrow edema syndrome and subchondral insufficiency fracture 1
• MRI can detect AVN prior to the appearance of radiological or scintigraphic abnormalities 6

CT Limitations

• CT without IV contrast is less sensitive than MRI and bone scintigraphy for detecting early osteonecrosis 1
• CT is superior to MRI only for showing location and extent of articular collapse and osseous details of secondary osteoarthritis 1
• Use CT only when MRI is contraindicated, recognizing it is an acceptable but less sensitive alternative 1

Bone Scintigraphy Limitations

• Bone scintigraphy has been largely replaced by MRI due to poor spatial resolution, low specificity, and inability to quantify the size of necrotic lesions 2
• Bone scintigraphy fails to detect the early ischemic stage of AVN 6
• Scintigraphy showed only 77.5% sensitivity and 75% specificity compared to MRI's 88.8% sensitivity and 100% specificity in comparative studies 7

Critical Clinical Considerations

When to Suspect AVN

• Persistent hip pain worsened by weight-bearing in patients with risk factors including corticosteroid use (especially ≥30 mg prednisolone daily), alcohol abuse, trauma, hemoglobinopathies, HIV infection, or hypercoagulability states 4, 1
• Early diagnosis is critical because symptoms indicate the disease process is already underway 1

Prognostic Information from MRI

• Lesions involving <30% of the femoral head have <5% progression to collapse, while lesions >30% have 46-83% risk of progression 4, 1
• Joint-preserving interventions like core decompression are only effective before subchondral collapse occurs 4

Common Pitfalls to Avoid

• Do not rely solely on plain radiographs in high-risk patients with persistent hip pain, as this will miss early-stage disease in over one-third of cases 3
• Do not order bone scintigraphy as it has been superseded by MRI for AVN diagnosis 2
• Do not image only the symptomatic hip; bilateral imaging is essential given the high rate of bilateral disease 1
• Do not add IV contrast to MRI routinely, as it is unnecessary for diagnosis of osteonecrosis itself 1