Knee Assessment: Lachman, Posterior Drawer, and McMurray Tests

What are the three special tests for the knee?

Special tests of the knee are specific physical examination maneuvers used by healthcare professionals to assess the integrity of various knee structures, including ligaments and menisci, aiding in the diagnosis of common knee injuries. Three foundational tests include the Lachman Test for the Anterior Cruciate Ligament (ACL), the Posterior Drawer Test for the Posterior Cruciate Ligament (PCL), and the McMurray Test for meniscal tears.

Understanding Special Tests in Knee Assessment

Special tests are a cornerstone of orthopedic and sports medicine examinations. They are designed to place specific stress on particular anatomical structures of the knee, eliciting a response (such as pain, instability, or a characteristic "clunk") that can indicate injury or dysfunction. While these tests provide valuable diagnostic information, they are always interpreted in conjunction with a thorough patient history, symptom presentation, and other clinical findings.

1. The Lachman Test: Assessing ACL Integrity

The Lachman Test is widely considered the most sensitive and specific clinical test for evaluating the integrity of the Anterior Cruciate Ligament (ACL), especially in acute injuries.

• Purpose: To detect laxity or rupture of the ACL.
• Mechanism: The test assesses the anterior translation of the tibia relative to the femur, which is primarily restrained by the ACL. A torn ACL will allow excessive anterior movement.
• Procedure:
  • The patient lies supine with the knee flexed to approximately 20-30 degrees.
  • The examiner stabilizes the distal femur with one hand.
  • With the other hand, the examiner grasps the proximal tibia, just below the knee joint.
  • A firm, swift anterior force is applied to the tibia, attempting to pull it forward relative to the femur.
  • The examiner assesses the amount of anterior translation and the quality of the "end-feel" (the sensation at the end of the movement).
• Clinical Significance: A positive Lachman test is indicated by excessive anterior tibial translation compared to the uninjured knee, often accompanied by a "soft" or absent end-feel, indicating an ACL tear.

2. The Posterior Drawer Test: Evaluating PCL Stability

The Posterior Drawer Test is a key examination technique used to assess the integrity of the Posterior Cruciate Ligament (PCL), which prevents posterior displacement of the tibia on the femur.

• Purpose: To identify laxity or rupture of the PCL.
• Mechanism: This test directly assesses the posterior translation of the tibia. When the PCL is torn, there will be excessive posterior movement.
• Procedure:
  • The patient lies supine with the hip flexed to 45 degrees and the knee flexed to 90 degrees, with the foot flat on the examination table.
  • The examiner sits on the patient's foot to stabilize it.
  • With both hands, the examiner grasps the proximal tibia, just below the knee joint, ensuring the hamstring muscles are relaxed.
  • A firm, swift posterior force is applied to the tibia, attempting to push it backward relative to the femur.
  • The examiner observes and feels for the amount of posterior translation and the end-feel.
• Clinical Significance: A positive Posterior Drawer test is indicated by excessive posterior tibial translation compared to the uninjured knee, often with a "soft" or absent end-feel, suggesting a PCL tear. A "sag sign" (posterior displacement of the tibia when the knee is flexed to 90 degrees) may also be evident before the test is even performed.

3. The McMurray Test: Detecting Meniscal Tears

The McMurray Test is a classic maneuver used to screen for tears in the menisci, the C-shaped cartilage pads that cushion the knee joint.

• Purpose: To detect tears in the medial or lateral meniscus.
• Mechanism: This test involves a combination of knee flexion, extension, rotation, and valgus/varus stress to trap and compress a potentially torn meniscus between the femoral condyles and the tibial plateau, eliciting pain or a palpable "clunk."
• Procedure:
  • The patient lies supine.
  • To test the medial meniscus:
    • The examiner fully flexes the patient's knee and hip.
    • The examiner externally rotates the tibia while applying a valgus (outward) stress to the knee.
    • The knee is then slowly extended while maintaining rotation and stress.
  • To test the lateral meniscus:
    • The examiner fully flexes the patient's knee and hip.
    • The examiner internally rotates the tibia while applying a varus (inward) stress to the knee.
    • The knee is then slowly extended while maintaining rotation and stress.
• Clinical Significance: A positive McMurray test is indicated by a palpable or audible "click" or "clunk," or the reproduction of the patient's pain, during the maneuver. The location of the click/pain (medial or lateral joint line) helps localize the torn meniscus.

Important Considerations for Special Tests

• Professional Application: These tests should only be performed by trained healthcare professionals (e.g., physicians, physical therapists, certified athletic trainers) who understand the nuances of technique, interpretation, and potential contraindications.
• Holistic Assessment: No single special test is 100% accurate. Their results must always be considered within the context of a comprehensive patient history, observation, palpation, range of motion assessment, and potentially imaging studies (MRI, X-ray) to arrive at an accurate diagnosis.
• Patient Comfort and Safety: Proper technique is crucial to avoid causing undue pain or further injury. Communication with the patient throughout the examination is vital.

These three special tests represent fundamental components of a thorough knee examination, providing critical insights into the structural integrity of the knee and guiding appropriate management strategies for knee injuries.