How to diagnose the cause of knee pain?

Diagnostic Approach to Knee Pain

Initial radiographs (including AP, lateral, sunrise/Merchant, and tunnel views) should be the first imaging step in evaluating patients with knee pain, followed by MRI when specific pathologies are suspected or radiographs are inconclusive. 1

Initial Evaluation Algorithm

Step 1: Clinical Assessment with Focused History

• Age considerations:

  • <40 years: More likely patellofemoral pain (25% lifetime prevalence) or traumatic meniscal tears 2

  • 45 years: More likely osteoarthritis (OA) or degenerative meniscal tears 2

• Key symptoms to identify:

  • Pain location (anterior, medial, lateral, posterior)
  • Mechanical symptoms (locking, catching, popping, giving way)
  • Swelling pattern (acute vs. chronic)
  • Pain triggers (activity-related, rest pain, night pain)

Step 2: Physical Examination

• Essential tests:
  • Joint line tenderness (83% sensitivity for meniscal tears) 2
  • McMurray test for meniscal pathology (61% sensitivity, 84% specificity) 2
  • Anterior knee pain during squat (91% sensitive for patellofemoral pain) 2
  • Assessment for effusion, range of motion limitations, and crepitus

Step 3: Initial Imaging - Radiographs

• Standard radiographic views: AP, lateral, sunrise/Merchant, and tunnel views 1
• These views help identify:
  • Fractures
  • Osteoarthritis (joint space narrowing, osteophytes)
  • Osteochondritis dissecans (OCD)
  • Loose bodies
  • Joint effusion

Step 4: Advanced Imaging Based on Radiographic Findings

If radiographs are normal or show only effusion:

• MRI without IV contrast is the next appropriate test 1
  • Highly sensitive for detecting:
    • Meniscal tears
    • Ligament injuries
    • Bone marrow lesions (BMLs)
    • Cartilage damage
    • Synovitis

If radiographs show OCD or loose bodies:

• MRI without IV contrast to characterize the OCD lesion or when concomitant pathology is suspected 1
• CT without contrast may be indicated to confirm loose bodies when MRI is not definitive 1

If radiographs show osteoarthritis:

• MRI is generally not indicated unless symptoms are not explained by radiographic findings 1
• Consider MRI for more accurate cartilage assessment when needed 1

If radiographs show signs of prior injury:

• MRI without IV contrast to assess for associated injuries 1
• CT without contrast may be helpful for evaluating patellofemoral anatomy in chronic subluxation 1

Specific Diagnostic Pathways

For Suspected Patellofemoral Pain

• Typically affects physically active people <40 years 2
• MRI can evaluate patellar tracking, cartilage damage, and medial patellofemoral ligament integrity 1

For Suspected Meniscal Tears

• Acute traumatic tears: Common in younger patients (<40 years) with twisting injuries 2
• Degenerative tears: Common in patients >40 years, often coexisting with OA 2
• MRI has high sensitivity and specificity for meniscal pathology 1

For Suspected Osteochondritis Dissecans (OCD)

• Initial radiographs may identify the lesion
• MRI helps determine stability of the OCD fragment 1
• Hyperintense rim or cysts at fragment periphery on MRI suggest instability 1

Common Pitfalls to Avoid

1. Overreliance on imaging: Remember that meniscal tears are often incidental findings in older patients and may not be the source of pain 1

2. Premature MRI: Approximately 20% of patients with chronic knee pain have MRI without recent radiographs, which is often unnecessary 1

3. Misattribution of pain: Referred pain from hip or lumbar spine pathology can present as knee pain, especially when knee imaging is unremarkable 1

4. Overlooking bone marrow lesions: BMLs on MRI are associated with increased knee pain and may indicate the origin of pain in patients with osteoarthritis 1

5. Failure to correlate clinical and imaging findings: Imaging findings must be interpreted in the context of the patient's symptoms and examination

By following this systematic diagnostic approach, the cause of knee pain can be accurately identified, leading to appropriate treatment decisions that will improve patient outcomes in terms of morbidity, mortality, and quality of life.