Valgus Stress Test in ACL Tear Diagnosis
The valgus stress test is not specifically designed to elicit pain in ACL tears, but it may cause pain in ACL-injured knees due to associated medial compartment injuries or when the test stresses secondary stabilizers of the knee. While primarily used to assess medial collateral ligament (MCL) integrity, the test can produce pain in ACL-injured knees through various mechanisms.
Valgus Stress Test Mechanics and ACL Relationship
- The valgus stress test primarily evaluates the integrity of the MCL by applying a valgus force to the lateral knee while stabilizing the femur, which opens the medial compartment 1
- When performing the valgus stress test, clinicians should test at both full extension (0-5°) to assess all medial tibiofemoral joint structures and at 15-20° of flexion to more specifically assess the MCL 2
- In ACL-injured knees, the valgus stress test may elicit pain due to associated injuries or instability patterns that commonly occur with ACL tears 3
When Valgus Stress May Elicit Pain in ACL Tears
- ACL tears frequently occur with concomitant medial compartment injuries, including MCL tears, which would directly cause pain during valgus stress testing 3, 4
- The ACLISS (ACL Injury Severity Scale) demonstrates that more severe ACL injuries show proportionally increased tissue damage in the medial tibiofemoral compartment, which would be stressed during valgus testing 3
- When the ACL is torn, secondary stabilizers including the MCL may be under increased strain during valgus testing, potentially causing pain even without complete MCL disruption 5
Diagnostic Value and Limitations
- Valgus stress radiographs provide objective measurements of medial compartment gapping but cannot definitively differentiate between specific medial structure injuries 4
- For full-thickness MCL tears, conventional ultrasound has a sensitivity of 79%, specificity of 98%, and accuracy of 95%, which can help differentiate MCL from ACL pathology when both present with pain during valgus stress 1
- The sensitivity and specificity of valgus stress ultrasound for all MCL tears is 96% and 81% respectively, making it a valuable adjunct when valgus stress testing produces pain in suspected ACL injuries 1
Clinical Pearls and Pitfalls
- A common pitfall is attributing pain during valgus stress testing solely to MCL injury when an ACL tear may be the primary pathology 5, 6
- Medial joint vacuum phenomenon on valgus stress radiographs is specific for MCL injury, which may help differentiate from isolated ACL tears 1
- For more accurate assessment of ACL integrity, specific tests like the Lachman test or anterior drawer test should be performed in conjunction with valgus stress testing 6
- The "moving valgus stress test," which applies a constant moderate valgus torque while extending the elbow from full flexion, is highly sensitive for medial elbow pain but should not be confused with knee valgus stress testing 5
Imaging Correlation
- MRI remains the gold standard for diagnosing ACL tears, with valgus stress testing serving as a complementary physical examination technique 1
- Stress ultrasound can accurately detect MCL tears when there is medial joint gapping, which may help differentiate the source of pain during valgus stress in patients with suspected ACL injuries 1
- In acute elbow or forearm pain with normal radiographs, ultrasound or MRI without IV contrast is recommended for suspected ligament injuries, and similar principles apply to knee assessment 1