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 in anteroposterior and frog-leg lateral views, 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 may appear normal in early disease, but can help identify other potential causes of hip pain. However, MRI without IV contrast is the gold standard diagnostic tool with nearly 100% sensitivity and specificity, capable of detecting early changes before radiographic abnormalities appear 1. MRI typically shows a serpiginous line separating normal from ischemic bone with associated bone marrow edema. If MRI is contraindicated, computed tomography (CT) can help assess bone architecture and collapse, while bone scintigraphy may show increased uptake in affected areas. Laboratory tests should include complete blood count, comprehensive metabolic panel, coagulation studies, and specific tests for underlying conditions like sickle cell disease or thrombophilia. In unclear cases, bone biopsy may be considered, though rarely necessary with modern imaging. Early diagnosis is crucial as treatment outcomes are significantly better before femoral head collapse occurs, with options ranging from conservative management to surgical interventions depending on disease stage, as outlined in the American College of Radiology guidelines 2, 1. Some key points to consider in the diagnostic workup include:

• The use of CT without IV contrast may be appropriate in certain cases, particularly for preoperative planning, as it can provide valuable information on the location and extent of articular collapse 1.
• The role of MRI in predicting necrotic volume and guiding treatment decisions, as well as its ability to detect early changes in the disease process 2.
• The importance of considering underlying risk factors and comorbid conditions in the diagnostic workup and treatment planning for AVN of the hip 2.

From the Research

Diagnostic Approach

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

• Imaging studies are crucial in diagnosing avascular necrosis (AVN) of the hip, with MRI and bone scintigraphy being the best methods in the early stages 3
• In later stages, plain X-rays and CT scans can help visualize calcification, new bone formation, and microfractures 3
• The ARCO-classification of the Association for the Research of Osseous Circulation is essential in diagnosis, with different stages showing distinct characteristics 4
• Stage 1 shows MR signal changes, while stage 2 shows native x-ray changes with lower radiolucency reflecting new bone apposition on dead trabeculae 4

Staging and Treatment

The staging of avascular necrosis is important in determining the treatment approach:

• Stage 0 can only be found histologically, while stage 1 is reversible and shows MR signal changes 4
• Established therapy in stage 1 includes core decompression, physiotherapy, and bisphosphonates 4
• In stage 2 and 3, proximal femoral osteotomies and (non)vascularised bone transplantation are performed, while in stage 4, resurfacing or short stem hip arthroplasty can be performed depending on the size and location of the necrotic zone 4
• Conventional total hip arthroplasty (THA) is still the golden standard, but the goal is to preserve the hip as long as possible, especially in young patients 4

Risk Factors and Prognosis

Avascular necrosis of the hip can be caused by various factors:

• Known aetiologies of nontraumatic femoral head necrosis include alcoholism, steroids, sickle cell anaemia, and Gaucher's disease 4
• Other risk factors include chemotherapy, chronic inflammatory bowel disease, systemic lupus erythematosus, and multiple sclerosis 4
• Prognosis depends on the localization and size of the AVN, as well as the number of repair mechanisms, which can be outlined with contrast-enhanced MRI and return of fatty marrow 3