MRI for Knee Pain: Diagnostic Approach
MRI of the knee without IV contrast is the standard imaging modality when radiographs are normal or show only effusion but pain persists, and this approach is safe in patients with kidney disease since no contrast is required. 1, 2
Initial Imaging Algorithm
Before proceeding to MRI, confirm that adequate plain radiographs have been obtained 3:
- Anteroposterior (or Rosenberg/tunnel view), lateral, and tangential patellar views are mandatory 3
- Approximately 20% of patients inappropriately undergo MRI without recent radiographs (within past year) 3
- Weight-bearing radiographs are particularly important in patients ≥40 years to identify degenerative joint disease 4
When to Order MRI
MRI without IV contrast is indicated when: 1, 2
- Initial radiographs are normal but pain persists
- Radiographs show only joint effusion without clear diagnosis
- Soft tissue pathology is suspected (meniscal tears, ligament injuries, tendinopathy)
- Bone marrow lesions or stress fractures are clinically suspected
- Early cartilage damage needs evaluation despite normal radiographs
Renal Function Considerations
The standard knee MRI protocol does not require IV contrast, making it completely safe for patients with impaired renal function or kidney disease. 2 This is a critical advantage since:
- Meniscal tears, ligament injuries, cartilage evaluation, bone marrow edema, and tendinopathy are all adequately evaluated without contrast 2
- Subchondral insufficiency fractures and stress fractures are well-visualized on non-contrast sequences 2
- Post-surgical assessment can be performed without contrast 2
Rare Situations Requiring Contrast
IV contrast is only needed in specific circumstances: 1, 2
- Quantifying synovitis when precise grading is clinically necessary 2
- Evaluating pigmented villonodular synovitis (PVNS) for extent and vascularity 2
- Characterizing suspected neoplastic masses 2
- Diagnosing Hoffa's disease, deep infrapatellar bursitis, or adhesive capsulitis where contrast may improve accuracy 1
In these contrast-requiring situations, patients with significant renal impairment would need alternative imaging or risk-benefit discussion regarding gadolinium use.
Critical Interpretation Caveats
Age-related findings must be interpreted cautiously: 1, 3
- In patients >70 years, the majority have asymptomatic meniscal tears 1, 3
- In patients 45-55 years, meniscal tears occur with similar frequency in painful and asymptomatic knees 1, 3
- The presence of a meniscal tear on MRI does not automatically establish it as the pain source 3
What MRI Detects in Knee Pain
MRI accurately identifies: 1
- Meniscal tears and articular cartilage damage 1
- Bone marrow lesions (BMLs), which correlate strongly with pain, especially in males or those with family history of osteoarthritis 1
- Subchondral insufficiency fractures (often radiographically occult initially, most common in medial femoral condyle of middle-aged to elderly females) 1
- Ligament injuries (ACL, PCL, collateral ligaments) 5
- Synovitis and joint effusions 1
- Patellar tendinopathy, fat pad impingement syndromes, iliotibial band syndrome 1
- Tumors, ganglion cysts, osteonecrosis 1
Common Pitfall to Avoid
Do not skip evaluation for referred pain sources: 3
- Hip pathology must be excluded when knee radiographs are unremarkable—obtain hip radiographs if clinical suspicion exists (groin pain, limited hip range of motion, positive impingement signs) 3
- Lumbar spine pathology can refer pain to the knee—obtain lumbar radiographs if radiculopathy or neurogenic claudication patterns are present 3
Cost-Effectiveness Consideration
In patients ≥40 years with >50% joint space loss on weight-bearing radiographs, MRI was considered unnecessary in 95% of cases and did not contribute to treatment recommendations in 48% of all prereferral MRIs. 4 This emphasizes the importance of obtaining adequate weight-bearing radiographs first.