Would an MRI be useful in evaluating new onset pain in a patient with a history of femur shortening 20 years later?

MRI is Useful for Evaluating New Onset Pain in a Patient with History of Femur Shortening

MRI without IV contrast is the most appropriate imaging modality for evaluating new onset pain in a patient with a history of femur shortening from 20 years ago. 1

Rationale for MRI

When evaluating a patient with new onset pain in the context of previous femur shortening, MRI offers several advantages:

1. Superior soft tissue evaluation: MRI is highly sensitive and specific for detecting many abnormalities involving both osseous structures and surrounding soft tissues 1

2. Detection of potential complications: MRI can identify:

   • Bone marrow lesions (BMLs), which are associated with increased pain 1
   • Stress fractures or insufficiency fractures that may be radiographically occult 1
   • Soft tissue abnormalities such as tendinopathy, bursitis, or muscle injuries 1
   • Early signs of osteoarthritis that may develop following femoral shortening 2

3. Comprehensive assessment: MRI can evaluate both intra-articular and extra-articular causes of pain, which is crucial in post-surgical patients 1

Diagnostic Algorithm

1. Initial radiographs: Start with AP pelvis and lateral femoral head-neck radiographs to assess for obvious structural abnormalities or hardware complications 1, 3

2. If radiographs are negative or equivocal: Proceed to MRI without IV contrast of the affected hip/femur 1

   • This follows the ACR Appropriateness Criteria recommendation that "MRI hip without IV contrast is appropriate in patients with chronic hip pain when radiographs are negative, equivocal, or nondiagnostic" 1

3. If MRI is contraindicated: Consider CT without contrast as an alternative, though it has lower sensitivity for soft tissue abnormalities 1

Potential Pathologies to Consider

The new onset pain 20 years after femur shortening could be related to several conditions that MRI is well-suited to detect:

Mechanical/Structural Issues

• Altered biomechanics: Femoral shortening can lead to changes in the mechanical axis of the limb, potentially causing lateral deviation and altered knee alignment 2
• Femoroacetabular impingement (FAI): Changes in femoral geometry may lead to impingement, which MRI can effectively evaluate 3, 4
• Stress fractures: Altered biomechanics may lead to stress injuries that are often radiographically occult initially 1

Soft Tissue Pathology

• Tendinopathy or bursitis: Common in patients with altered biomechanics 1
• Muscle imbalance or compensation injuries: MRI can detect muscle atrophy or hypertrophy related to compensatory mechanisms 1

Degenerative Changes

• Early osteoarthritis: Femoral shortening may accelerate degenerative changes due to altered joint mechanics 2
• Bone marrow lesions: Associated with pain and early degenerative changes 1

Clinical Implications

The findings on MRI will guide treatment decisions:

• Conservative management: For soft tissue injuries, early osteoarthritis, or mild stress reactions
• Surgical intervention: For significant structural abnormalities, advanced osteoarthritis, or unstable stress fractures
• Pain management strategies: Targeted based on specific pathology identified

Pitfalls and Caveats

1. Incidental findings: MRI may reveal asymptomatic abnormalities that are not the source of pain. Correlation with clinical examination is essential 3

2. Hardware artifacts: If the patient has retained hardware from the shortening procedure, metal artifact reduction sequences may be necessary 1

3. Referred pain: Consider that hip pain may be referred from the lumbar spine or knee. If hip MRI is negative, imaging of these regions may be warranted 1

4. Delayed complications: Femoral shortening can lead to long-term biomechanical changes affecting the entire limb. A comprehensive evaluation should consider the hip, knee, and even ankle joints if clinically indicated 2

By following this approach, clinicians can effectively evaluate new onset pain in patients with a history of femur shortening, potentially identifying treatable causes and preventing further complications.