The McMurray Test: Mechanism, Procedure, and Interpretation for Meniscal Tears

What is the Mechanism of the McMurray Test?

The McMurray test is a clinical orthopedic examination designed to assess for meniscal tears within the knee joint by physically stressing and entrapping the menisci between the femoral condyles and tibial plateau, eliciting a characteristic click, thud, or pain in the presence of a tear.

Introduction to the McMurray Test

The McMurray test is a foundational orthopedic special test frequently employed in the clinical assessment of knee pain, particularly when a meniscal injury is suspected. Developed by Dr. Thomas P. McMurray, this maneuver aims to provoke symptoms by replicating the forces that can cause or exacerbate a meniscal tear. Understanding its mechanism requires a solid grasp of knee anatomy and biomechanics.

Anatomy Relevant to the McMurray Test

To appreciate the mechanism of the McMurray test, it's crucial to review the key anatomical structures involved:

• Menisci: These are two C-shaped, wedge-like pieces of fibrocartilage located within the knee joint, one on the medial (inner) side and one on the lateral (outer) side.
  • Medial Meniscus: Larger, more C-shaped, and less mobile due to its attachment to the medial collateral ligament (MCL) and joint capsule.
  • Lateral Meniscus: Smaller, more O-shaped, and more mobile.
• Functions of the Menisci: They play vital roles in knee function, including:
  • Shock absorption
  • Load transmission and distribution across the joint surfaces
  • Enhancing joint stability
  • Lubrication and nutrition of articular cartilage
• Femoral Condyles: The rounded ends of the femur (thigh bone) that articulate with the tibia.
• Tibial Plateau: The flattened top surface of the tibia (shin bone) where it meets the femur.

The Core Mechanism: Stressing the Meniscus

The fundamental principle behind the McMurray test is to mechanically trap and stress the meniscus between the articulating surfaces of the femur and tibia. By combining specific movements of flexion, rotation, and extension with valgus (medial stress) or varus (lateral stress) forces, the examiner attempts to "pinch" a potentially torn meniscal fragment.

The test is designed to target different parts of the menisci (anterior, middle, and posterior horns) by varying the degree of knee flexion and rotational forces. A positive test result typically manifests as an audible "click" or "thud," a palpable sensation of movement, or the reproduction of the patient's pain, indicating that a torn or unstable meniscal fragment is being caught or displaced.

Step-by-Step Breakdown of the Test Mechanism

The McMurray test involves specific maneuvers to isolate and stress the medial and lateral menisci:

1. Patient Position: The patient lies supine (on their back) with the knee fully flexed towards their chest.

2. Examiner's Role: The examiner supports the patient's knee with one hand, placing fingers and thumb along the joint line to feel for clicks, and grasps the patient's heel with the other hand to control tibial rotation.

3. Testing the Medial Meniscus:

• Starting Position: Knee fully flexed.
• Tibial Rotation: The examiner externally rotates the tibia (turns the foot outwards). This movement helps to open the lateral compartment and position the medial meniscus for impingement.
• Valgus Stress: The examiner applies a valgus stress to the knee (pushing the knee inwards, creating a force that pushes the lower leg outwards). This further opens the lateral compartment and compresses the medial compartment.
• Extension: While maintaining external rotation and valgus stress, the examiner slowly extends the knee.
• Mechanism: As the knee extends, the femoral condyles glide over the tibial plateau. The combination of full flexion, external rotation, and valgus stress causes the posterior horn of the medial meniscus to be compressed and potentially trapped between the medial femoral condyle and the medial tibial plateau. If a tear exists, the torn fragment may get pinched, leading to a click, thud, or pain. The test targets the posterior and middle segments of the medial meniscus.

4. Testing the Lateral Meniscus:

• Starting Position: Knee fully flexed.
• Tibial Rotation: The examiner internally rotates the tibia (turns the foot inwards). This helps to open the medial compartment and position the lateral meniscus for impingement.
• Varus Stress: The examiner applies a varus stress to the knee (pushing the knee outwards, creating a force that pushes the lower leg inwards). This further opens the medial compartment and compresses the lateral compartment.
• Extension: While maintaining internal rotation and varus stress, the examiner slowly extends the knee.
• Mechanism: Similar to the medial side, the combination of full flexion, internal rotation, and varus stress causes the posterior horn of the lateral meniscus to be compressed and potentially trapped between the lateral femoral condyle and the lateral tibial plateau. A torn fragment getting caught will result in a click, thud, or pain. The test primarily targets the posterior and middle segments of the lateral meniscus.

Interpreting the Results (Positive Test)

A "positive" McMurray test occurs when the maneuver reproduces the patient's familiar knee pain, an audible "click," or a palpable "thud" or "clunk" sensation.

• Click/Thud/Clunk: This sensation often indicates that a torn or unstable meniscal fragment is being caught between the articulating surfaces.
• Pain: Localized pain during the maneuver points to irritation or impingement of the meniscal tear.
• Location of Symptoms: The side of the knee (medial or lateral) where the click, thud, or pain occurs helps to localize the potential tear to the medial or lateral meniscus, respectively.

The mechanism is one of mechanical impingement: a torn piece of cartilage, which is normally smooth and allows for frictionless gliding, becomes displaced or frayed. When the joint is loaded and rotated in specific ways during the McMurray test, this displaced or frayed fragment gets caught, causing the characteristic positive findings.

Limitations and Considerations

While the McMurray test is a widely used clinical tool, it is important to acknowledge its limitations:

• Sensitivity and Specificity: The test is not 100% accurate. It can produce false positives (positive test without a tear) or false negatives (tear present but test is negative). Its accuracy varies depending on the type and location of the meniscal tear.
• Acute vs. Chronic Tears: In acutely injured, highly painful, or swollen knees, the test may be difficult to perform accurately or may not elicit a clear response.
• Clinical Correlation: A positive McMurray test should never be interpreted in isolation. It must be correlated with the patient's history (mechanism of injury, symptoms), other physical examination findings (e.g., joint line tenderness, effusion, range of motion), and potentially diagnostic imaging (e.g., MRI) for a definitive diagnosis.
• Operator Skill: The effectiveness of the test can be influenced by the examiner's skill and experience in performing the specific maneuvers and interpreting the findings.

Conclusion

The McMurray test is a valuable clinical maneuver rooted in the biomechanical principle of physically stressing and entrapping the menisci. By systematically combining knee flexion, tibial rotation, and varus/valgus stresses, the test aims to reproduce the mechanical impingement that occurs with a meniscal tear. While not a definitive diagnostic tool on its own, its ability to elicit a characteristic click, thud, or pain provides crucial information that, when integrated with a comprehensive clinical assessment, guides healthcare professionals toward an accurate diagnosis of meniscal pathology in the knee.