Avascular necrosis (AVN) is diagnosed primarily through imaging, with MRI being the most sensitive and specific tool available. No blood test can confirm AVN on its own, so the diagnostic process typically starts with X-rays and moves to MRI when early-stage disease is suspected or X-rays look normal despite concerning symptoms.
What Happens During the Physical Exam
In early AVN, a physical exam can appear completely normal, which is part of what makes this condition tricky to catch. As the disease progresses, your doctor will typically find tenderness around the affected bone, restricted range of motion, and pain when moving the joint both actively and passively. In the hip, which is the most common site, you may walk with a noticeable limp. Advanced cases can cause visible joint deformity and muscle wasting around the affected area.
Because these findings overlap with many other joint conditions, the physical exam alone can’t confirm AVN. It does, however, help your doctor decide which imaging to order and how urgently.
Why X-Rays Come First
The American College of Radiology recommends a standard X-ray of the pelvis as the first imaging study for anyone at risk for AVN of the hip who presents with hip pain. Two specific views are needed: an anteroposterior (front-to-back) view of the pelvis and a frog-leg lateral view of the hip. Articular collapse or subtle depression of the bone surface sometimes shows up on only one of those angles, so both are necessary.
The limitation is significant. X-rays look completely normal in the early stages of AVN. They only pick up changes once the bone has already started to visibly deteriorate, such as increased density, cysts, or collapse of the joint surface. This means a normal X-ray does not rule out AVN. If your symptoms and risk factors still point toward the condition, the next step is an MRI.
MRI: The Key Diagnostic Tool
MRI without contrast is the gold standard imaging test for AVN. It outperforms CT scans by more than two standard errors and bone scans by more than three standard errors when it comes to detecting early disease. MRI can detect AVN at a stage when X-rays still look perfectly normal, making it indispensable for catching the condition before the joint surface collapses.
On MRI, AVN typically appears as a segmental area of low signal intensity in the bone just beneath the joint surface, surrounded by a distinct low-signal border. On certain MRI sequences, this border can show what radiologists call the “double line sign,” a dark line running alongside a bright line. This pattern is considered a classic indicator of the condition.
Beyond simply confirming that AVN is present, MRI also reveals two critical pieces of information that guide treatment decisions: the size of the necrotic lesion and the likelihood that the joint surface will collapse. Larger lesions carry a worse prognosis, and MRI is the only imaging tool that can reliably quantify lesion size. It also allows doctors to monitor asymptomatic joints over time, which matters because AVN can develop on both sides simultaneously.
Other Imaging Options
Bone scintigraphy (bone scans) was once used to detect AVN but has largely been replaced by MRI. Bone scans suffer from poor spatial resolution, low specificity, and an inability to measure lesion size. A more advanced version called SPECT can improve accuracy somewhat, but its use hasn’t been widely adopted. One remaining role for bone scans is screening for AVN in multiple joints at once, since the test images the entire skeleton in a single session.
CT scans occasionally play a role, particularly in detecting small fractures just below the joint surface (subchondral fractures) that signal a more advanced stage of the disease. But for initial detection and overall evaluation, MRI remains clearly superior.
The Role of Blood Tests
No laboratory test can confirm or rule out AVN. Blood work serves a different purpose in the diagnostic process: identifying underlying conditions that may have caused the bone to lose its blood supply in the first place. Your doctor may check for clotting disorders, lipid abnormalities, or markers of autoimmune diseases like lupus. These tests don’t diagnose AVN directly, but they help explain why it developed and can influence how aggressively it’s treated.
Bone Biopsy: Rarely Needed
Examining bone tissue under a microscope is technically the definitive way to confirm AVN. In practice, bone biopsy is almost never performed purely for diagnosis because MRI provides enough information in the vast majority of cases. When a biopsy does happen, it’s usually obtained during a surgical procedure like core decompression that’s already being done for treatment, not as a standalone diagnostic step.
How AVN Is Staged
Once AVN is confirmed, doctors classify it using the ARCO staging system (revised in 2019) to determine severity and guide treatment. The stages are based on what X-ray, MRI, and CT imaging reveal:
- Stage I: X-rays look normal, but MRI shows a characteristic low-signal band. This is the earliest detectable stage and the window where joint-preserving treatments have the best chance of working.
- Stage II: Both X-rays and MRI show abnormalities, but the bone surface hasn’t fractured or collapsed yet.
- Stage III: A fracture has developed just beneath the joint surface. This stage is subdivided based on how much the femoral head has flattened: 2 mm or less of depression is classified as early (IIIA), while more than 2 mm is late (IIIB).
- Stage IV: Osteoarthritis has developed in the joint, meaning damage has extended beyond the bone into the cartilage and joint space.
Staging matters because treatment options narrow as the disease advances. At Stage I or II, procedures aimed at preserving the natural joint are still on the table. By Stage IV, joint replacement is often the most practical option.
Conditions That Mimic AVN on Imaging
Several conditions can look similar to AVN on X-rays or MRI, and distinguishing between them is important because the treatments and outcomes differ significantly. Transient osteoporosis of the hip, for example, causes bone marrow swelling that can resemble early AVN but resolves on its own over several months. Normal variations in bone marrow, particularly in younger patients, can also create MRI patterns that raise false alarms. Small pits in the femoral neck, called synovial herniation pits, are another benign finding that can be confused with early necrosis. An experienced radiologist can usually tell these apart, but in uncertain cases, a follow-up MRI after a few weeks or months can clarify the picture.