What diagnostic tests are used to evaluate knee pain?

Diagnostic Testing for Knee Pain

Radiography (X-rays) should be the initial imaging study for evaluating knee pain, followed by MRI if pain persists despite normal radiographs or if there is a joint effusion. 1

Initial Evaluation

Clinical Assessment

• Assess for red flags requiring immediate attention:
  • Inability to bear weight
  • Gross deformity
  • Fever with joint pain
  • Severe pain unresponsive to rest and OTC medications
  • Neurological symptoms
  • History suggesting increased fracture risk 2

Physical Examination

• Compare with uninjured knee
• Evaluate for:
  • Joint line tenderness (83% sensitive, 83% specific for meniscal tears) 3
  • Limited range of motion and crepitus (features of osteoarthritis) 4
  • Varus/valgus alignment changes 4
  • McMurray test for meniscal tears (52% sensitive, 97% specific) 5
  • Lachman test for ACL injuries (more sensitive and specific than drawer sign) 5

Imaging Algorithm

First-Line Imaging: Radiographs

• Standard knee X-rays should include:
  • Anteroposterior (AP) view
  • Lateral view
  • Tangential patellar view 1
  • Weight-bearing views when evaluating for osteoarthritis 2

Second-Line Imaging

• MRI without contrast when:

  • Initial radiographs are normal or show joint effusion but pain persists
  • Clinical suspicion of internal derangement
  • Clinical instability present 1, 2

• CT without contrast when:

  • Suspected radiographically occult fractures
  • Evaluation of bony structures needed 2
  • Patellofemoral anatomy assessment for subluxation/maltracking 1

• CT arthrography when:

  • Intra-articular abnormality is suspected
  • MRI is contraindicated 1

Special Considerations

Age-Based Approach

• For patients >45 years with activity-related knee pain and minimal morning stiffness:

  • Likely osteoarthritis (95% sensitivity, 69% specificity) 3
  • Clinical features (age >50 years, morning stiffness <30 minutes, crepitus, bony enlargement) are 89% sensitive and 88% specific for chronic arthritis 5

• For patients <40 years with anterior knee pain:

  • Consider patellofemoral pain (anterior knee pain during squat: 91% sensitive, 50% specific) 3

Ottawa Knee Rules

Apply these rules to determine need for radiographs in trauma cases:

• Age >55 years
• Tenderness at head of fibula or patella
• Inability to bear weight for 4 steps
• Inability to flex knee to 90 degrees 5

Referred Pain Assessment

• If knee radiographs are unremarkable, consider:
  • Hip radiographs if clinical concern for hip pathology
  • Lumbar spine radiographs if clinical concern for lumbar pathology 1

Common Pitfalls to Avoid

• Ordering MRI without recent radiographs (occurs in ~20% of chronic knee pain cases) 1
• Failing to obtain weight-bearing views when evaluating for osteoarthritis 2
• Over-interpreting meniscal tears on MRI in older patients (majority of people >70 years have asymptomatic meniscal tears) 1
• Neglecting to assess for referred pain from hip or lumbar spine when knee exam is unremarkable 1
• Failing to apply Ottawa knee rules in trauma cases, leading to unnecessary radiographs 5