What is the diagnostic workup for avascular necrosis (osteonecrosis) of the hip?

From the Guidelines

The diagnostic workup for avascular necrosis (AVN) of the hip should begin with plain radiographs of the hip, followed by MRI without IV contrast as the next imaging study, as recommended by the American College of Radiology 1. The initial evaluation should include a thorough clinical history and physical examination, focusing on risk factors such as corticosteroid use, alcohol consumption, trauma, and underlying conditions like sickle cell disease or lupus. Initial imaging with plain radiographs (anteroposterior and frog-leg lateral views) is essential, although these may appear normal in early disease. If radiographs are normal or suspicious, MRI without IV contrast is the preferred next step, given its high sensitivity for early detection of AVN, exceeding 90% 1. MRI can show characteristic patterns of bone marrow edema and the pathognomonic "double-line sign." If MRI is contraindicated, computed tomography (CT) can help assess bone architecture and collapse, while bone scintigraphy may detect early vascular changes with approximately 80% sensitivity. Laboratory tests should include complete blood count, comprehensive metabolic panel, coagulation studies, and specific tests based on suspected etiology (such as hemoglobin electrophoresis for sickle cell disease or autoimmune markers for lupus). In unclear cases where imaging is inconclusive but clinical suspicion remains high, core decompression with bone biopsy may provide definitive diagnosis while potentially offering therapeutic benefit. Early diagnosis is crucial as treatment outcomes are significantly better before femoral head collapse occurs, allowing for joint-preserving interventions rather than total hip arthroplasty, as highlighted in the 2022 update of the ACR Appropriateness Criteria for osteonecrosis 1. Some key points to consider in the diagnostic workup include:

• The importance of early diagnosis to prevent articular collapse and the need for joint replacements 1
• The role of MRI in predicting necrotic volume and CT in showing the location and extent of articular collapse 1
• The potential benefits of core decompression with bone biopsy in unclear cases 1

From the Research

Diagnostic Approach

To diagnose avascular necrosis of the hip, the following steps can be taken:

• Use the ARCO-classification of the Association for the Research of Osseous Circulation to stage the disease 2
• Stage 0 can only be diagnosed histologically, while stage 1 shows MR signal changes and is considered reversible 2
• In stage 2, native x-ray changes can be seen, including lower radiolucency reflecting new bone apposition on dead trabeculae 2
• Stage 3 is characterized by subchondral fracture, and stage 4 by secondary arthritis of the hip 2

Risk Factors

Several risk factors are associated with avascular necrosis of the hip, including:

• Traumatic or nontraumatic pathogenesis 2
• Nontraumatic causes such as alcoholism, steroids, sickle cell anaemia, caisson, and Gaucher's disease 2
• Other risk factors include chemotherapy, chronic inflammatory bowel disease, systemic lupus erythematosus, and multiple sclerosis, often involving steroid use 2
• Gravidity is also a risk factor, although idiopathic pathogenesis is common 2

Imaging and Staging

Imaging plays a crucial role in diagnosing and staging avascular necrosis of the hip:

• MR signal changes can be seen in stage 1 2
• Native x-ray changes can be seen in stage 2, including lower radiolucency reflecting new bone apposition on dead trabeculae 2
• Subchondral fracture can be seen in stage 3, and secondary arthritis of the hip in stage 4 2