What is the preferred initial imaging modality for outpatients presenting with knee locking?

Preferred Imaging for Knee Locking in Outpatients

For outpatients presenting with knee locking, MRI without IV contrast is the preferred initial imaging modality after standard knee radiographs. 1, 2

Diagnostic Algorithm for Knee Locking

Step 1: Initial Radiographic Assessment

• Standard knee radiographs should be the first imaging study for all patients with knee pain or locking 1
• Purpose: To rule out fractures, degenerative changes, osteochondral defects, and effusions 3
• Views typically include: AP, lateral, and sunrise/merchant views

Step 2: Advanced Imaging

• If radiographs are negative for fracture but knee locking persists:
  • MRI without IV contrast is the recommended next imaging study 1, 2
  • MRI is considered the gold standard in diagnostic imaging for locked knee 2
  • Provides excellent evaluation of:
    • Meniscal tears (most common cause of knee locking)
    • Anterior cruciate ligament ruptures
    • Loose bodies
    • Other internal derangements

Evidence Analysis and Clinical Considerations

The American College of Radiology explicitly recommends MRI without IV contrast as the next imaging study after radiographs in patients with suspected internal derangement 1. This is particularly relevant for knee locking, which is most frequently caused by meniscal tears, ACL ruptures, or loose bodies 2.

MRI offers superior soft tissue contrast resolution and multiplanar imaging capability without ionizing radiation, making it ideal for evaluating internal knee structures 4. The British Journal of Hospital Medicine (2024) confirms that MRI is the gold standard in diagnostic imaging for locked knee 2.

Important Clinical Pearls:

• Knee locking is considered an orthopaedic emergency requiring prompt diagnosis 2
• CT may be superior for fracture detection but is not routinely used as initial imaging for knee locking 1
• MRI sensitivity for meniscal tears may be affected by concurrent osteoarthritis 5
• In cases where MRI is contraindicated or unavailable, diagnostic arthroscopy may be necessary as it remains the gold standard for management 2

Common Pitfalls to Avoid:

• Relying solely on physical examination findings for diagnosis of meniscal tears, especially in patients with concurrent knee osteoarthritis 5
• Delaying appropriate imaging for persistent mechanical symptoms like locking, as these may indicate the need for surgical intervention 1
• Overlooking the possibility of false positive MRI results, particularly in patients with normal or minimally abnormal radiographs 5

By following this evidence-based approach of radiographs followed by MRI without contrast, clinicians can efficiently diagnose the cause of knee locking and guide appropriate management decisions.