What is the best test to diagnose avascular necrosis of the femoral head?

Best Test to Diagnose Avascular Necrosis of the Femoral Head

MRI without IV contrast is the best test to diagnose avascular necrosis of the femoral head, with sensitivity and specificity approaching 100%. 1

Initial Imaging Approach

• Start with plain radiographs of the hip in two planes as the first-line imaging study for clinically suspected osteonecrosis, despite their limited sensitivity in early disease 1, 2
• Radiographs are specific but significantly insensitive, particularly in early stages when bone marrow changes precede visible structural alterations 3, 4
• If radiographs are normal or show only suspicious findings in a patient with clinical suspicion for AVN, proceed immediately to MRI 1

MRI Without IV Contrast: The Gold Standard

MRI without intravenous contrast is the most sensitive and specific imaging modality for diagnosing osteonecrosis, with sensitivity and specificity nearing 100%. 1

Key Performance Characteristics:

• A meta-analysis of 43 studies reported 93% sensitivity and 91% specificity for early detection of femoral head osteonecrosis 1
• MRI demonstrates 100% sensitivity compared to 81% for bone scintigraphy in high-risk patients 5
• MRI detects bone marrow changes (edema and sclerosis) before structural collapse occurs 4

Critical Diagnostic Capabilities:

• Characterizes the osteonecrosis including location, volume (necrotic angle), and presence of associated bone marrow edema or joint effusion 1
• Detects asymptomatic disease in the contralateral hip, which is crucial since nontraumatic osteonecrosis is bilateral in 70-80% of cases 2, 6
• Differentiates AVN from mimics such as transient bone marrow edema syndrome and subchondral insufficiency fracture 1

Optimal MRI Protocol:

• Coronal STIR (short tau inversion-recovery) sequence alone shows 100% sensitivity 1
• Adding coronal T1-weighted sequence increases confidence and specificity 1
• T1-weighted images show moderately decreased bone marrow signal in segmental patterns within the femoral head 5

Alternative Imaging Modalities: When and Why to Avoid

CT Without IV Contrast:

• Less sensitive than MRI and bone scintigraphy for detecting early osteonecrosis 1
• Reserve CT for specific situations: when MRI is contraindicated or to evaluate articular collapse extent once insufficiency fracture has occurred 1, 4
• CT is superior to MRI only for showing location and extent of articular collapse and osseous details of secondary osteoarthritis 1

Bone Scintigraphy:

• Not beneficial for characterization due to poor spatial resolution, low specificity (75%), and inability to quantify necrotic lesion size 1, 7
• Sensitivity only 77.5-81% compared to MRI's 88.8-100% 5, 7
• Up to 18% false-negative studies in biopsy-proven femoral head necrosis 8
• Sensitivity further diminished when bilateral AVN is present 8
• May be used only to screen for multifocal osteonecrosis in exceptional cases 1, 3

MRI With IV Contrast:

• Not necessary for diagnosis of osteonecrosis itself 1
• Dynamic contrast enhancement is useful only when differentiating osteonecrosis from transient bone marrow edema syndrome or subchondral insufficiency fracture 1
• Transient bone marrow edema shows subchondral spot of marked hyperperfusion, whereas osteonecrosis shows a rim of high plasma flow surrounding a subchondral area without flow 1

Critical Clinical Pitfalls to Avoid

• Always image both hips even when symptoms are unilateral, as 70-80% of nontraumatic cases are bilateral 2, 6
• Do not delay MRI waiting for follow-up radiographs in high-risk patients (corticosteroid use, alcohol abuse, chemotherapy, HIV) 2, 3
• Do not rely on bone scintigraphy as it misses up to 19-23% of cases that MRI detects 5, 7
• Document prognostic factors on MRI: necrotic volume >30% has 46-83% progression risk versus <5% for lesions <30%; also note joint effusion, bone marrow edema extent, patient age >40 years, and BMI >24 kg/m² 2, 9

Algorithmic Approach

1. Obtain plain radiographs first (AP and lateral hip views) 1, 2
2. If radiographs are negative or equivocal AND clinical suspicion remains high: proceed directly to MRI without IV contrast 1, 2
3. If MRI is contraindicated: CT without contrast is an acceptable but less sensitive alternative 1
4. Once AVN is confirmed on MRI: measure necrotic angle and document risk factors for prognosis and treatment planning 9
5. If articular collapse is present on radiographs: either MRI or CT is appropriate for preoperative planning, with CT superior for depicting collapse extent 1