For evaluating knee pathology, should a CT scan or an MRI be ordered?

MRI is Superior to CT for Evaluating Knee Pathology

For evaluating knee pathology, MRI should be ordered over CT in the vast majority of clinical scenarios, as MRI demonstrates superior accuracy for detecting soft-tissue injuries (meniscal tears, ligament injuries, cartilage lesions) and bone marrow abnormalities that are the primary pathologies in knee evaluation. 1

Clinical Decision Algorithm

Initial Imaging: Start with Radiographs

• Always obtain plain radiographs first (anteroposterior, lateral, and patellofemoral views) 1
• This is the standard initial imaging for acute knee trauma or chronic knee pain

After Negative or Equivocal Radiographs: Choose MRI

MRI is the next-line imaging modality because:

• 93.5% of acute knee injuries involve soft-tissue rather than bone 1
• MRI has proven superior accuracy for:
  • Meniscal tears (sensitivity/specificity superior to CT) 1
  • Ligament injuries (ACL, PCL, collateral ligaments) 1
  • Bone marrow contusions and occult fractures 1
  • Articular cartilage lesions 2

Limited Role for CT

CT should only be ordered in specific circumstances:

1. Fracture characterization when fracture is already visible on radiographs

   • CT shows 100% sensitivity for tibial plateau fractures vs 83% for radiographs 1
   • Useful for surgical planning of complex fractures (classification, articular depression measurement) 1

2. When MRI is contraindicated or unavailable

   • Pacemakers, metallic implants, severe claustrophobia
   • CT can detect some ligament injuries (80% sensitivity, 98% specificity for tears) but misses many soft-tissue pathologies 1

3. Patellofemoral maltracking evaluation

   • CT can assess trochlear morphology and tibial tubercle-trochlear groove distance 2

Key Evidence Supporting MRI Superiority

MRI outperforms CT across all soft-tissue structures:

• While CT can detect bony avulsions (80% sensitivity, 98% specificity), it has low sensitivity for meniscal and other soft-tissue injuries 1
• MRI provides multiplanar imaging with superior contrast resolution for bone marrow edema, which CT cannot adequately visualize even with dual-energy techniques 1
• MRI shortens diagnostic workup, reduces additional procedures, and improves quality of life in the first 6 weeks post-injury 1

Common Pitfalls to Avoid

Don't order CT as first-line imaging after negative radiographs - This will miss the majority of clinically significant pathology since most knee injuries are soft-tissue related 1

Don't assume CT "rules out" internal derangement - Even when CT shows no bony avulsion, MRI remains necessary for detecting meniscal injuries preoperatively 1, 3

Be aware of age-related incidental findings on MRI - Meniscal tears are often asymptomatic in patients over 70 years, and prevalence is similar in painful vs asymptomatic knees in patients 45-55 years 2

Special Consideration: Tibial Plateau Fractures

When radiographs show a tibial plateau fracture:

• Order CT first for fracture classification and surgical planning 1
• Then order MRI if soft-tissue injury assessment is needed, as CT can predict but not definitively diagnose meniscal/ligament injuries 1
• CT articular depression >11mm predicts higher risk of lateral meniscus tear and ACL avulsion, indicating need for MRI 1

The bottom line: MRI is the definitive imaging modality for knee pathology evaluation, with CT reserved for fracture characterization and situations where MRI cannot be performed.