What is the next step in the workup for a patient with Diabetes Mellitus (DM) and Rheumatoid Arthritis (RA) suspected of having avascular necrosis of the femoral head with uncontrolled hip pain, inability to bear weight, and X-ray findings of sclerosis and flattening of the femoral head?

MRI Without IV Contrast is the Next Step in Workup

For this patient with suspected avascular necrosis of the femoral head based on X-ray findings of sclerosis and flattening, MRI without IV contrast should be performed immediately to confirm the diagnosis, determine the extent of necrosis, and guide treatment decisions. 1, 2

Rationale for MRI as Next Step

Diagnostic Superiority

• MRI is the gold standard for diagnosing avascular necrosis with sensitivity and specificity approaching 100%, far exceeding other imaging modalities 1, 2, 3
• While X-rays have already shown suggestive findings (sclerosis and flattening), MRI is essential to definitively confirm AVN, quantify the necrotic volume, and exclude other causes of hip pain 1
• The American College of Radiology specifically recommends MRI without IV contrast (not with contrast) as the appropriate next imaging study when radiographs show findings suspicious for osteonecrosis 1

Critical Prognostic Information

• MRI quantifies necrotic volume, which is the most important prognostic factor: lesions involving ≥30% of the femoral head progress to collapse in 46-83% of cases, while those <30% progress in <5% of cases 1, 2
• MRI identifies additional poor prognostic features including joint effusion and bone marrow edema extent, both of which predict increased risk of femoral head collapse 1
• This information directly determines whether joint-preserving surgery (core decompression) versus joint replacement is appropriate 2

Bilateral Disease Screening

• Nontraumatic AVN is bilateral in 70-80% of cases, and MRI should image both hips even though symptoms may be unilateral 1, 2, 4
• This patient has multiple risk factors (diabetes, rheumatoid arthritis likely requiring corticosteroids) making bilateral screening essential 1, 4

Why Not Other Imaging Modalities

CT Scan Limitations

• CT is useful for surgical planning and detecting subchondral fractures but is less sensitive than MRI for early AVN detection 3, 5
• CT does not provide the prognostic information about necrotic volume and bone marrow edema that MRI offers 1

Bone Scintigraphy Obsolete

• Bone scintigraphy has been replaced by MRI due to poor spatial resolution, low specificity, and inability to quantify necrotic lesion size 1
• Scintigraphy had only 81% sensitivity compared to 100% for MRI in detecting AVN 6
• The American College of Radiology states that bone scintigraphy is not commonly performed for osteonecrosis detection 1

Treatment Implications Based on MRI Findings

Early-Stage Disease (Pre-Collapse)

• If MRI shows <30% necrotic volume without subchondral collapse, protected weight-bearing and possible core decompression may preserve the joint 2
• Bisphosphonates may prevent bone collapse in early stages 2

Late-Stage Disease (Post-Collapse)

• If MRI confirms articular collapse with the sclerosis and flattening seen on X-ray, total hip arthroplasty becomes necessary 1, 2
• AVN accounts for 10% of total hip replacement indications in the United States 1

Common Pitfalls to Avoid

• Do not delay MRI: The X-ray findings already suggest advanced disease, and immediate MRI is needed to determine if any joint-preserving options remain 1, 2
• Do not order MRI with IV contrast: Contrast adds no diagnostic value for AVN and only increases cost and time 1
• Do not image only the symptomatic hip: Always image both hips given the 70-80% bilateral involvement rate 1, 2, 4
• Do not assume X-ray staging is sufficient: MRI patterns correlate poorly with radiographic stages and provide essential additional prognostic information 7