What could be causing right knee pain when the X-ray (X-radiation imaging) shows normal results?

Right Knee Pain with Normal X-ray: Diagnostic Approach

MRI without IV contrast is the next indicated examination when knee radiographs are normal or show only joint effusion, as it can detect soft tissue injuries, bone marrow lesions, early cartilage damage, and stress fractures that are invisible on plain films. 1, 2

Most Common Causes Not Visible on X-ray

Soft Tissue Pathology

• Meniscal tears are a leading cause of knee pain despite normal radiographs, though they occur with similar frequency in both painful and asymptomatic knees in patients aged 45-55 years 1, 2
• Ligamentous injuries including cruciate and collateral ligament tears are only visible on MRI 3
• Patellar tendinopathy and various fat pad impingement syndromes cause anterior knee pain not detected on radiographs 1
• Popliteal (Baker's) cysts can rupture or cause posterior knee pain and are accurately depicted on MRI 1, 4

Bone Marrow and Cartilage Abnormalities

• Bone marrow lesions (BMLs) appear as increased edema-like signal on MRI and are strongly associated with knee pain, especially in males or those with family history of osteoarthritis 1, 2
• Subchondral insufficiency fractures most commonly affect the medial femoral condyle in middle-aged to elderly females and are radiographically occult initially, though radiographs may later show articular surface fragmentation and collapse 1, 2
• Early articular cartilage damage can be present despite normal radiographs, particularly in younger patients with anterior knee pain who show elevated T2 mapping values on 3T MRI 1, 2
• Tibial stress fractures are detected on MRI but not on initial radiographs 1

Inflammatory and Synovial Pathology

• Synovitis and joint effusion may indicate the origin of knee pain in osteoarthritis and require MRI evaluation 1, 2, 5
• Hoffa's disease (inflammation of the infrapatellar fat pad) with enhancing synovitis thicker than 2mm correlates with peripatellar pain 1
• Pigmented villonodular synovitis requires contrast-enhanced MRI for diagnosis 1
• Medial plica syndrome can be evaluated with ultrasound or MRI 1, 2

Critical Diagnostic Algorithm

Step 1: Confirm Adequate Radiographic Views

• Verify that knee radiographs included anteroposterior (or Rosenberg/tunnel view), lateral, and tangential patellar views before proceeding to MRI 2
• Approximately 20% of patients inappropriately undergo MRI without recent radiographs (within past year), which should be avoided 2, 4

Step 2: Exclude Referred Pain Sources

• Hip pathology must be evaluated when knee imaging is normal, as hip disease commonly refers pain to the knee 2, 4
• Examine the hip for range of motion limitations, groin pain, or positive impingement signs 2
• Obtain hip radiographs if clinical suspicion exists before proceeding to knee MRI 2, 4
• Lumbar spine pathology can refer pain to the knee; assess for radiculopathy or neurogenic claudication patterns 2, 4
• Obtain lumbar spine radiographs if clinically indicated 2

Step 3: Proceed to MRI

• MRI without IV contrast is the appropriate next examination when radiographs are normal or show only effusion and pain persists 1, 2, 5
• MRI accurately detects meniscal tears, articular cartilage damage, subchondral cysts, BMLs, synovitis, ligament injuries, and stress fractures 1, 2
• MRI identifies subchondral insufficiency fractures much earlier than radiographs 1, 2

Step 4: Consider Contrast-Enhanced MRI

• MRI with IV contrast may be more accurate for diagnosing Hoffa's disease, deep infrapatellar bursitis, patellofemoral friction syndrome, and adhesive capsulitis 1
• Contrast-enhanced MRI is useful for quantifying synovitis and evaluating pigmented villonodular synovitis 1

Important Caveats

Age-Related Considerations

• In patients over 70 years, the majority have asymptomatic meniscal tears, making MRI findings potentially misleading 1, 2
• In the 45-55 age group, meniscal tears are equally common in painful and asymptomatic knees 1, 2
• The presence of a meniscal tear on MRI does not automatically establish it as the pain source 2

Anterior Knee Pain Specific Etiologies

• Chondral abnormalities, patellar instability and dislocation, femoral trochlear dysplasia, abnormal patellar location, bipartite patella, various tendinopathies, bursal inflammation, and traction apophysitis in pediatric patients all require MRI for diagnosis 6
• Patellofemoral cartilage loss is associated with chronic knee pain, with active pain correlated with BMLs 1

Posterolateral Knee Pain

• Lateral compartment osteoarthritis produces lateral-sided pain but requires radiographic findings for diagnosis 4
• Popliteus tendon pathology is only visible on MRI 4