What is the initial approach to managing left knee pain with normal X-rays (radiographs)?

Differential Diagnosis and Management of Left Knee Pain with Normal X-rays

When knee radiographs are normal but pain persists, proceed with MRI without contrast to evaluate for soft tissue pathology, bone marrow lesions, and early cartilage damage that are not visible on plain films. 1

Primary Differential Diagnoses to Consider

Intra-articular Pathology

• Meniscal tears are a leading cause of knee pain with normal radiographs, though note that in patients 45-55 years old, meniscal tears occur with similar frequency in both painful and asymptomatic knees, and most patients over 70 have asymptomatic tears 1
• 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
• 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
• Subchondral insufficiency fractures (previously called spontaneous osteonecrosis) most commonly affect the medial femoral condyle in middle-aged to elderly females and are often radiographically occult initially 1
• Joint effusion may be the only radiographic finding but indicates underlying pathology requiring MRI evaluation 1

Synovial and Bursal Pathology

• Synovitis is associated with knee pain in osteoarthritis and requires MRI with contrast for optimal visualization 2
• Popliteal cyst (Baker's cyst) rupture or presence can be accurately depicted on MRI 1
• Plica syndrome can be evaluated with ultrasound or MRI 2

Referred Pain Sources

• Hip pathology must be considered when knee radiographs are unremarkable; obtain hip radiographs if clinical suspicion exists 1, 2
• Lumbar spine pathology can refer pain to the knee; obtain lumbar spine radiographs if clinically indicated 1, 2

Ligamentous Injuries

• Medial collateral ligament (MCL) injuries are a common cause of medial knee pain not visible on radiographs 2
• Cruciate ligament injuries require MRI for diagnosis 3

Algorithmic Approach to Management

Step 1: Verify Adequate Initial Imaging

• Confirm that knee radiographs included anteroposterior (or Rosenberg/tunnel view), lateral, and tangential patellar views 1, 4
• Critical pitfall: Approximately 20% of patients undergo MRI without recent radiographs (within past year), which is inappropriate 1, 2, 4

Step 2: Clinical Assessment for Referred Pain

• Examine the hip for range of motion limitations, groin pain, or positive impingement signs 1, 2
• Assess for lumbar radiculopathy or neurogenic claudication patterns 1, 2
• If either is suspected, obtain appropriate radiographs before proceeding to knee MRI 1, 2

Step 3: Order MRI Without Contrast

• This is the next indicated examination when radiographs are normal or show only effusion and pain persists 1, 2
• MRI accurately detects meniscal tears, articular cartilage damage, subchondral cysts, BMLs, synovitis, ligament injuries, and stress fractures 1
• MRI can identify subchondral insufficiency fractures much earlier than radiographs 1

Step 4: Consider Alternative Imaging in Specific Scenarios

• CT without contrast is indicated for patellofemoral pain related to repetitive subluxation or maltracking to evaluate trochlear morphology and tibial tubercle-trochlear groove distance 1
• CT arthrography can substitute for MRI when intra-articular abnormality is suspected and MRI is contraindicated 1
• Ultrasound is useful for confirming effusion, guiding aspiration, and evaluating medial plica or popliteal cysts 2

Critical Caveats

Age-Related Considerations

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

Correlation Between Imaging and Symptoms

• Radiographic osteoarthritis and symptoms show weak correlation, with only 15-76% of patients with knee pain having radiographic OA, and only 15-81% of those with radiographic OA having pain 5
• Clinical decision-making should not rely on imaging findings in isolation 5

When to Consider Physical Therapy First

• For anterior knee pain with normal radiographs, exercise therapy and load progression have strong evidence for long-term improvement 6
• Patient education about the condition and pain management strategies are supported by research 6