Valgus Knee Test: Purpose and Interpretation for MCL Assessment
Primary Purpose
The valgus stress test evaluates the integrity of the medial collateral ligament (MCL) by applying a lateral-to-medial force to the knee while stabilizing the femur, which opens the medial joint compartment—increased laxity or pain indicates MCL injury. 1
Test Technique and Grading
The test is performed at two key flexion angles to assess different components of medial stability:
- At 0° (full extension): Tests the entire medial stabilizing complex including the MCL, posterior capsule, and posteromedial corner 2
- At 30° flexion: Isolates the superficial MCL (sMCL) by relaxing the posterior capsule, making this the most sensitive position for detecting isolated MCL injury 2, 3
Interpretation of Laxity
- Grade I injury: Tenderness without increased laxity (microscopic tear) 4
- Grade II injury: Moderate laxity (5-10mm opening) with a firm endpoint (partial tear) 4
- Grade III injury: Severe laxity (>10mm opening) without a firm endpoint (complete tear) 5, 4
Critical Diagnostic Caveat
A positive valgus stress test does NOT definitively indicate isolated MCL injury—it can also occur with posterolateral corner injuries, making additional testing essential. 6 The dial test (external rotation test) can be falsely positive with MCL tears, mimicking posterolateral instability. Therefore, always perform both valgus AND varus stress testing to differentiate medial from lateral pathology 6.
Advanced Diagnostic Correlation
Imaging Confirmation
When clinical examination is equivocal or surgical planning is needed:
- MRI without contrast: Gold standard for MCL assessment, showing significant correlation between percentage of ligament injury on MRI and degree of valgus instability 5, 4
- Stress ultrasound: Achieves 96% sensitivity and 81% specificity for all MCL tears when medial joint gapping is present during valgus stress 1
- Valgus stress radiographs: Medial joint vacuum phenomenon is specific for MCL injury and correlates with injury severity 7, 1
Timing Considerations
The American College of Radiology notes that acute hemarthrosis or joint effusion can limit examination accuracy 8. Consider delayed examination (4-5 days post-injury) when initial acute assessment is equivocal, as this timing optimizes test performance 8.
Associated Injuries to Evaluate
MCL injuries frequently occur with concomitant pathology that affects management:
- ACL tears: Present in combined injuries requiring MRI differentiation 7, 4
- Deep MCL (dMCL) injury: Combined sMCL and dMCL insufficiency produces excess valgus rotation, external tibial rotation, and anteromedial translation 3
- Bone contusions: Visible on MRI and predictive of associated soft-tissue injuries 7, 4
- Meniscal tears: Detected in 5% of MCL injuries on MRI 4
Clinical Decision Algorithm
- Perform valgus stress test at 0° and 30° flexion to quantify medial laxity 2, 6
- Always include varus stress test to exclude lateral pathology that can mimic MCL injury 6
- If Grade II-III laxity or diagnostic uncertainty: Obtain MRI without contrast for definitive diagnosis and surgical planning 1, 5
- If immediate MRI unavailable: Stress ultrasound provides 96% sensitivity for MCL tears and can guide acute management 1
- Consider valgus stress radiographs when medial joint space opening measurement is needed for surgical decision-making 7, 1