What is the purpose of the valgus knee stress test?

Valgus Knee Stress Test: Purpose and Clinical Application

The valgus knee stress test is a clinical examination technique designed to assess the integrity of the medial collateral ligament (MCL) by applying a lateral-to-medial force on the knee to evaluate medial joint stability and detect MCL injuries. 1

Technique and Execution

The valgus stress test is performed in two key positions:

1. Full extension (0°):

   • Tests the integrity of the MCL along with secondary stabilizers (posterior oblique ligament and posteromedial capsule)
   • Higher stiffness is normally observed in this position 2

2. Slight flexion (20-30°):

   • "Isolates" the MCL by relaxing the posterior capsule and cruciate ligaments
   • Shows decreased stiffness compared to full extension 2
   • More specific for MCL injury assessment

Proper Execution:

• Patient positioned supine or seated with the knee at the appropriate angle
• Examiner stabilizes the thigh with one hand
• Other hand applies a valgus (abduction) force at the ankle/distal tibia
• Assessment focuses on medial joint space opening and quality of endpoint

Clinical Significance and Interpretation

Grading System:

• Grade I: Minimal laxity (< 5mm opening), firm endpoint
• Grade II: Moderate laxity (5-10mm opening), soft endpoint but still present
• Grade III: Severe laxity (>10mm opening), no definite endpoint, suggests complete MCL tear

Diagnostic Value:

• Positive test indicates MCL injury when there is:
  • Increased medial joint space opening compared to contralateral side
  • Pain along the medial joint line with stress application
  • Absence of a firm endpoint (in higher-grade injuries)

Clinical Pearls:

• Always compare with the contralateral knee as stiffness is typically consistent bilaterally in healthy individuals 2
• A ratio of medial joint opening ≥2 (injured vs. normal knee) suggests associated cruciate ligament injury 3
• False positive results can occur with isolated MCL tears that mimic posterolateral instability on dial test 4

Advanced Assessment Methods

For more objective evaluation, the valgus stress test can be enhanced with:

1. Stress Radiography:

   • Provides objective documentation of medial joint space opening
   • Can be performed by binding both knees together and applying simultaneous valgus stress 3
   • Useful for documenting instability and treatment planning

2. Stress MRI:

   • Allows functional assessment of ligament integrity
   • Can quantify medial compartment opening under controlled load
   • Helps differentiate partial from complete MCL tears 5
   • Particularly valuable in complex knee injuries

3. Ultrasound:

   • Dynamic capability allows visualization of ligament during stress
   • Can detect laxity or frank separation of injured ligament 1

Clinical Applications

The valgus stress test is essential for:

1. Acute trauma assessment - Particularly in contact sports injuries
2. Surgical decision-making - Determining need for operative vs. non-operative management
3. Evaluation of prophylactic knee braces - Which are designed to protect the MCL during valgus stress 1
4. Monitoring healing - Assessing ligament stability during rehabilitation

Common Pitfalls and Limitations

• Inadequate patient relaxation can lead to false negative results
• Pain may limit proper examination in acute settings
• Isolated assessment may miss associated injuries (cruciate ligaments, menisci)
• Subjective interpretation without objective measurements
• Requires comparison with contralateral side for accurate interpretation

The valgus stress test remains a cornerstone of knee examination, providing critical information about medial knee stability that directly impacts treatment decisions and patient outcomes.