Best Diagnostic Test for Avascular Necrosis
MRI without contrast is the gold standard diagnostic test for avascular necrosis (AVN) due to its superior sensitivity and specificity compared to other imaging modalities. 1, 2
Diagnostic Approach to Avascular Necrosis
Initial Imaging
- Radiography should be performed as the initial imaging study for suspected AVN to exclude other causes of pain such as fracture, primary arthritis, or tumor 1
- Radiographs are less sensitive for detection of early AVN but can show findings of secondary osteoarthritis in late-stage disease 1
- Anteroposterior, lateral, and oblique views are recommended to exclude subchondral collapse in cases of epiphyseal osteonecrosis 1
Advanced Imaging
- MRI without contrast is the preferred diagnostic method for AVN, especially in patients with normal or equivocal radiographs 2, 3
- MRI has demonstrated superior sensitivity (100%) compared to bone scintigraphy (81%) for detecting early AVN 4
- MRI allows for better assessment of lesion size and location, which has been shown to relate to prognosis and need for treatment 3
- MRI can detect characteristic segmental patterns of decreased signal within the femoral head on T1-weighted images, even when scintigraphy and radiography are negative 4
Other Imaging Modalities
- CT without contrast can be valuable for determining the extent of necrosis, which is the main determining factor in outcome, particularly in Freiberg's infraction 1
- CT is well-suited to assess trabecular bridging at fracture sites, nonunion, malunion, and the sclerosis and fragmentation of avascular necrosis 1
- Bone scintigraphy has been largely replaced by MRI due to poor spatial resolution, low specificity, and inability to quantify the size of necrotic lesions 1, 5
- Single-photon emission CT (SPECT) may improve the accuracy of bone scintigraphy but is not commonly performed for detection of AVN 1
Comparative Effectiveness of Imaging Modalities
- MRI has been shown to be better than both CT and radionuclide scanning in controlled statistical studies, with the difference between MRI and radionuclide scanning being statistically significant (p < 0.01) 6
- In cases with early AVN, MRI demonstrated even greater superiority over CT and radionuclide scanning 6
- MRI offers a more specific diagnosis in patients presenting with hip pain of uncertain etiology compared to bone scintigraphy 3
- CT imaging is particularly valuable for preoperative planning, showing the location and extent of articular collapse that may not be visible on radiographs 7
Clinical Considerations
- Bilateral imaging is recommended as AVN can be bilateral even if only one joint is symptomatic 2, 5
- Early diagnosis and treatment of AVN improve prognosis, making the high sensitivity of MRI particularly valuable 3, 5
- Late presentation is a major negative prognostic factor, as untreated AVN inevitably leads to early degenerative joint disease 2, 8
- Lesions involving <30% of the femoral head have <5% progression to collapse, highlighting the importance of accurate assessment of lesion size 7
In conclusion, while radiography should be the initial imaging study for suspected AVN, MRI without contrast is the definitive diagnostic test of choice due to its superior sensitivity, specificity, and ability to detect early disease before irreversible changes occur.