How can I diagnose knee pain?

Diagnostic Approach for Knee Pain

For accurate diagnosis of knee pain, radiographs (X-rays) should be the initial imaging modality, followed by MRI as the preferred second-line imaging when radiographs are negative but clinical suspicion remains high. 1

Initial Assessment

History Taking - Key Elements

• Age: Different conditions affect different age groups
  • Under 40: More likely patellofemoral pain (25% lifetime prevalence) 2
  • Over 45: More likely osteoarthritis 2
• Pain characteristics:
  • Location (anterior, medial, lateral, posterior)
  • Onset (sudden vs gradual)
  • Duration (acute vs chronic)
  • Quality (sharp, dull, aching)
• Mechanical symptoms:
  • Locking, popping, giving way
  • Morning stiffness duration (less than 30 minutes suggests osteoarthritis) 2
• Swelling history:
  • Timing, amount, recurrence
• Trauma history:
  • Description of precipitating injury (twisting, direct impact)
• Medical/surgical history:
  • Previous knee injuries or surgeries

Physical Examination

• Inspection:
  • Alignment, swelling, muscle atrophy
• Palpation:
  • Joint line tenderness (83% sensitivity for meniscal tears) 2
  • Patellar tenderness
• Range of motion and strength assessment
• Special tests:
  • McMurray test for meniscal tears (61% sensitivity, 84% specificity) 2
  • Anterior drawer/Lachman for ACL integrity
  • Squat test (91% sensitive for patellofemoral pain) 2

Imaging Algorithm

First-Line Imaging

• Radiographs (minimum two views: anteroposterior and lateral) 1
  • Indicated for:
    • Chronic knee pain (more than six weeks)
    • Acute trauma with:
      • Inability to bear weight
      • Inability to flex knee to 90 degrees
      • Age over 55 years
      • Isolated patellar tenderness
      • Tenderness at fibular head 3
  • Useful for detecting:
    • Fractures
    • Joint effusion
    • Deformities
    • Malalignment 1

Second-Line Imaging

• MRI (without contrast) 1
  • Indicated when:
    • Radiographs are normal/inconclusive but symptoms persist
    • Clinical instability is present
    • Internal derangement is suspected
  • Excellent for detecting:
    • Meniscal tears (96% sensitivity, 97% specificity)
    • Bone marrow contusions
    • Occult fractures
    • Soft tissue injuries 1

Alternative Imaging

• CT (without contrast)

  • Consider when MRI is contraindicated or unavailable
  • Superior for:
    • Bony detail
    • Fracture characterization (80-100% sensitivity)
    • Tibial plateau fractures (100% sensitivity) 1

• Ultrasound

  • Limited but useful for:
    • Superficial soft tissue injuries
    • Patellar tendon injuries
    • Joint effusion or Baker's cyst
    • Real-time assessment of patellar tracking 1

Common Diagnoses and Specific Findings

Osteoarthritis

• Most common in patients over 45 years
• Activity-related pain with minimal morning stiffness (95% sensitivity, 69% specificity) 2
• Radiographic findings may include joint space narrowing, osteophytes

Patellofemoral Pain

• Most common in active individuals under 40 years
• Anterior knee pain during squatting (91% sensitivity) 2
• Pain with prolonged sitting ("theater sign")

Meniscal Tears

• Acute traumatic tears: Common in younger patients after twisting injury
• Degenerative tears: Common in patients over 40 with osteoarthritis
• Positive McMurray test and joint line tenderness 2

Other Conditions Detected on MRI

• Patellar tendinopathy
• Fat pad impingement syndromes
• Hoffa's disease
• Iliotibial band syndrome
• Adhesive capsulitis
• Medial plicae
• Discoid meniscus
• Tumors and cysts 4

Urgent Referral Indications

• Severe pain, swelling, and instability after acute trauma
• Inability to bear weight
• Signs of joint infection (fever, swelling, erythema, limited range of motion) 5

By following this systematic diagnostic approach, you can accurately identify the cause of knee pain and develop an appropriate treatment plan based on the specific diagnosis.