What imaging modalities are recommended for diagnosing knee instability?

Imaging for Knee Instability

MRI is the recommended imaging modality for diagnosing knee instability as it provides superior assessment of ligamentous injuries, meniscal tears, and bone marrow abnormalities that contribute to knee instability. 1, 2

Initial Imaging Approach

1. Plain Radiographs (X-rays)

   • Should be the initial imaging modality to rule out fractures
   • Minimum of two views required: anteroposterior (AP) and lateral views
   • Additional patellofemoral view for suspected patellar fractures or subluxation 1, 2
   • Indications based on Ottawa rules include:
     • Focal tenderness
     • Joint effusion
     • Inability to bear weight 2

2. Advanced Imaging

   • MRI without contrast is the second-line imaging of choice when:
     • Radiographs are negative
     • Clinical instability is present
     • Internal derangement is suspected 1, 2

MRI for Knee Instability

MRI offers several advantages for evaluating knee instability:

• Superior soft tissue contrast for visualization of:

  • Anterior cruciate ligament (ACL) tears (96% sensitivity, 97% specificity at 3T) 1
  • Posterior cruciate ligament (PCL) tears
  • Medial collateral ligament injuries
  • Lateral collateral ligament injuries
  • Posterolateral corner injuries 3

• Detection of associated injuries that contribute to instability:

  • Meniscal tears
  • Bone marrow contusions
  • Chondral injuries 1, 4

• Clinical impact: MRI findings can alter treatment decisions in approximately 21% of knee injuries, particularly for meniscal and chondral pathology 5

CT Imaging Role

• CT is indicated when:

  • Tibial plateau fractures are present (100% sensitivity vs. 83% for radiographs)
  • Bony avulsion fractures are suspected (80% sensitivity, 98% specificity)
  • Detailed fracture characterization is needed 1, 2

• Limitations of CT:

  • Lower sensitivity for soft tissue injuries compared to MRI
  • However, high specificity means positive findings of ligament tears can be considered reliable 1

Special Considerations

1. Multiple Ligament Injuries/Knee Dislocation:

   • MRI shows excellent correlation with surgical findings for location and extent of tears
   • Consider MR angiography (MRA) to evaluate for popliteal artery injury, which occurs in approximately 30% of posterior knee dislocations 1, 6

2. Acute vs. Chronic Instability:

   • MRI can differentiate between acute and chronic ACL tears, which affects treatment planning 4
   • Secondary signs on MRI (bone bruising patterns) help identify mechanism of injury 4

Pitfalls to Avoid

1. Relying solely on MRI for ACL tears: In cases where the clinician is confident of ACL insufficiency, MRI provides useful additional information in only about 5% of cases 5

2. Missing associated injuries: Patellofemoral chondral pathology is frequently missed on MRI, especially in the presence of acute ACL pathology 5

3. Unnecessary imaging: For obvious clinical instability with positive examination findings, proceeding directly to treatment without MRI may be appropriate in select cases 5

MRI remains the gold standard for comprehensive evaluation of knee instability, providing crucial information about ligamentous, meniscal, and chondral structures that guides treatment decisions and surgical planning.