When is an MRI indicated for an acute right knee injury?

MRI Indications for Acute Right Knee Injury

For acute right knee injuries, MRI is indicated after initial radiographs when there is suspicion of internal derangement or occult fractures not visible on X-rays. 1

Initial Imaging Approach

1. Plain Radiographs First

   • Radiographs should be the initial imaging modality for all acute knee injuries with:
     • Focal tenderness
     • Joint effusion
     • Inability to bear weight 1
   • At minimum, two views (anteroposterior and lateral) should be obtained
   • Additional views may include patellofemoral, internal oblique, and external oblique projections

2. Ottawa Knee Rules

   • Radiographs are indicated when any of these criteria are met:
     • Age ≥55 years
     • Isolated tenderness of the patella
     • Tenderness at the head of the fibula
     • Inability to flex the knee to 90 degrees
     • Inability to bear weight immediately and in the emergency department 1

MRI Indications After Negative Radiographs

MRI is indicated in the following scenarios:

1. Suspected Internal Derangement

   • Suspected meniscal tears
   • Suspected ligamentous injuries (ACL, PCL, MCL, LCL)
   • Clinical instability despite negative X-rays 1

2. Suspected Occult Fractures

   • Persistent pain despite negative radiographs
   • High clinical suspicion for fracture not visualized on X-rays 1

3. Significant Mechanism of Injury

   • High-energy trauma
   • Twisting injuries with immediate swelling
   • Giving way or locking sensations 1

Benefits of MRI in Knee Trauma

• Superior detection of bone marrow contusions and occult fractures (sensitivity 79-100%) 1
• Highly accurate for meniscal and ligamentous injuries 1
• Can alter treatment decisions in approximately 21% of cases 2
• Allows for earlier surgical intervention when appropriate 1
• Reduces unnecessary physiotherapy sessions (2.5 vs 5.1 sessions) 3

Special Considerations

• Knee Dislocation: MRI or MRA should be performed to evaluate for multi-ligament injury and can be combined with CTA to assess for vascular injuries 1
• Patellar Injuries: MRI aids in diagnosis of bone and soft-tissue injuries associated with patellar dislocation 1
• ACL Injuries: MRI is particularly valuable when the diagnosis of ACL insufficiency is in question, but less beneficial when the clinical diagnosis is clear 2

Pitfalls to Avoid

1. Premature MRI

   • Ordering MRI before radiographs can lead to unnecessary testing and delay in appropriate care
   • CT may be more appropriate for suspected bony injuries 1

2. Delayed MRI in Significant Trauma

   • Delaying MRI in cases of significant trauma or suspected multi-ligament injury can lead to missed diagnoses and poorer outcomes 1

3. Overreliance on MRI for ACL Tears

   • When clinical examination clearly indicates ACL insufficiency, MRI provides limited additional benefit (only 5% of cases) 2
   • MRI is more valuable for evaluating meniscal and chondral injuries 2

4. Missing Associated Injuries

   • In acute ACL tears, associated meniscal and chondral injuries may be missed without MRI 2

By following these guidelines, clinicians can optimize the use of MRI in acute knee injuries, improving diagnostic accuracy while ensuring appropriate resource utilization.