What causes knee pain when straightening it?

Knee Pain with Straightening: Differential Diagnosis and Evaluation

Pain when straightening the knee most commonly indicates patellofemoral disorders, meniscal pathology, or terminal extension-related osteoarthritis, and requires systematic evaluation starting with radiographs before considering advanced imaging. 1, 2

Primary Diagnostic Considerations

Patellofemoral Pathology

• Patellofemoral disorders are the leading cause of anterior knee pain and pain with extension, including cartilage loss, subluxation/dislocation, and friction syndrome 2
• Patellar tendinopathy commonly causes anterior knee pain, particularly during terminal extension movements 2
• Fat pad impingement (Hoffa's disease) correlates with peripatellar pain and may worsen with full extension 2
• Medial plicae can cause anterior knee pain that worsens with knee straightening 2

Meniscal Tears

• Meniscal tears affect approximately 12% of adults and can cause pain during extension, particularly when the tear creates mechanical obstruction 3
• The McMurray test (concurrent knee rotation and extension) has 61% sensitivity and 84% specificity for meniscal tears 3
• Joint line tenderness demonstrates 83% sensitivity and 83% specificity for meniscal pathology 3
• Critical pitfall: In patients over 45 years, meniscal tears are often incidental findings, with the majority of people over 70 having asymptomatic tears 1, 4

Osteoarthritis-Related Pain

• Knee OA is the most likely diagnosis in patients ≥45 years with activity-related pain (95% sensitivity, 69% specificity) 3
• Bone marrow lesions (BMLs) and synovitis/effusion are associated with increased knee pain in OA patients 1, 4
• Some OA braces incorporate extension stops specifically to prevent painful full extension 1

Structural Pathologies Requiring Urgent Attention

• Subchondral insufficiency fractures most commonly involve the medial femoral condyle in middle-aged to elderly females, with radiographs often initially normal 2
• Osteochondritis dissecans with loose bodies can cause mechanical symptoms during extension 1, 2
• Adhesive capsulitis can cause both pain and restriction with knee straightening 2

Critical Differential: Referred Pain

Before attributing symptoms solely to knee pathology, evaluate for referred pain sources:

• Lumbar spine pathology must be considered when knee radiographs are unremarkable, particularly if clinical examination suggests spinal origin 2, 5
• Hip pathology can refer pain to the knee and should be evaluated if knee imaging is normal 2, 5
• A thorough clinical examination of the lumbar spine and hip should precede knee-focused imaging 5

Diagnostic Algorithm

Initial Evaluation

• Start with weight-bearing radiographs including frontal projection, tangential patellar view, and lateral view 1, 5, 4
• Assess for joint space narrowing, osteophytes, subchondral changes, and alignment abnormalities 1

When to Advance to MRI

• MRI without IV contrast is indicated when radiographs are normal or show only effusion but pain persists 1, 5, 4
• MRI accurately depicts meniscal tears, articular cartilage abnormalities, bone marrow lesions, synovitis, and subchondral insufficiency fractures 1, 4
• Avoid premature MRI: approximately 20% of chronic knee pain patients undergo MRI without recent radiographs 1, 5

Age-Specific Considerations

• In patients <40 years with anterior knee pain during squats, consider patellofemoral pain (91% sensitivity, 50% specificity) 3
• Teenage boys and young men are more likely to have knee extensor mechanism problems like patellar tendonitis 6
• In patients >70 years, bilateral structural abnormalities can exist with primarily unilateral symptoms, limiting diagnostic discrimination 5, 4

Common Pitfalls to Avoid

• Do not overlook referred pain from hip or lumbar spine before attributing all symptoms to knee pathology 2, 5
• Not all meniscal tears seen on imaging are symptomatic, particularly in patients over 45 years 1, 5, 4
• Radiographs may be initially normal in subchondral insufficiency fractures, which later show articular surface fragmentation and subchondral collapse 2
• In patients with established OA on radiographs, MRI is not usually indicated unless symptoms are unexplained by radiographic findings 4

Specific Physical Examination Findings

• Assess for anterior knee pain during squatting (suggests patellofemoral pathology) 3
• Perform McMurray test with knee rotation during extension (meniscal pathology) 3
• Palpate joint lines for tenderness (meniscal tears) 3
• Evaluate for effusion and range of motion limitations 1
• Test hip range of motion and lumbar spine if knee examination is unremarkable 2, 5