When is an MRI (Magnetic Resonance Imaging) indicated for the knee?

MRI Indications for Knee Evaluation

MRI of the knee without IV contrast is usually appropriate as the next imaging study after radiographs do not show fracture, for evaluation of suspected occult knee fractures or internal derangement following acute trauma to the knee. 1

Initial Assessment and Radiographic Evaluation

Before considering MRI, initial radiographic evaluation is essential:

• Radiographs are the first-line imaging for knee trauma and pain evaluation 1, 2
• Radiographs should include at least one frontal projection, tangential patellar view, and lateral views 2
• Radiographs are particularly indicated when at least one of the following is present: focal tenderness, effusion, or inability to bear weight 1

Specific Indications for Knee MRI

MRI is indicated in the following scenarios:

1. Post-Radiographic Evaluation:

   • When radiographs are negative but clinical suspicion remains high for internal derangement 1
   • For suspected occult fractures not visible on radiographs 1, 2

2. Soft Tissue Injury Assessment:

   • Suspected meniscal tears (medial or lateral) 2, 3
   • Ligamentous injuries (ACL, PCL, MCL, LCL) 2, 3
   • Cartilage damage assessment 4

3. Special Clinical Scenarios:

   • Persistent knee pain despite 6-8 weeks of conservative treatment 2
   • Mechanical symptoms (catching, locking, giving way) 2
   • Clinical instability requiring surgical planning 2
   • Bone marrow contusions or edema assessment 2

4. Post-Trauma Evaluation:

   • After tibial plateau fracture diagnosis to evaluate additional bone or soft tissue injury 1
   • Following knee dislocation to assess soft tissue, osseous, and neural injuries 1

MRI vs. Other Imaging Modalities

MRI offers distinct advantages in specific scenarios:

• MRI vs. CT: MRI is preferred for soft tissue injuries with near 100% sensitivity for fracture detection and excellent detection of bone marrow contusions 2

• MRI vs. Arthroscopy: While arthroscopy remains the gold standard for intra-articular pathology, MRI serves as an excellent non-invasive alternative with high accuracy (87.5-95% for various structures) 3

Clinical Pearls and Pitfalls

• Age Considerations: In patients over 50 years with suspected osteoarthritis, MRI should only be ordered if symptoms are not explained by OA alone or if surgical planning requires it 5

• Avoid Unnecessary MRI: MRI may reveal clinically silent changes in the knee that don't necessarily alter management 6

• Timing Considerations: When MRI is indicated after trauma, it should ideally be performed within a reasonable timeframe to guide appropriate management 3

• Vascular Assessment: In cases of knee dislocation, MRA may be performed simultaneously with MRI to evaluate vascular injuries with less morbidity compared to conventional angiography 1

By following these evidence-based guidelines for MRI utilization, clinicians can optimize diagnostic accuracy while avoiding unnecessary imaging, ultimately improving patient outcomes through appropriate management decisions.