What are the tests performed to assess knee instability?

Knee Instability Assessment Tests

The primary clinical tests for assessing knee instability are the Lachman test and anterior drawer test for anterior cruciate ligament (ACL) injury, the posterior drawer test and gravity sign for posterior cruciate ligament (PCL) injury, valgus/varus stress tests for collateral ligament injury, and the pivot shift test for rotatory instability. 1

Anterior Cruciate Ligament (ACL) Instability Tests

Lachman Test

• The Lachman test is the benchmark examination for ACL instability and should be performed with the knee at 25-30 degrees of flexion. 2
• This test assesses anterior translation of the tibia relative to the femur and is more sensitive than the anterior drawer test. 1
• The test can be quantified using instrumented devices like the KT-1000 arthrometer or Rolimeter, which provide objective measurements superior to manual assessment alone. 3

Pivot Shift Test

• The pivot shift test is the most important examination for predicting functional instability and patient outcomes after ACL injury, as it assesses combined rotational and translational instability. 2
• This test reproduces the pathologic rotatory subluxation that occurs during functional activities in ACL-deficient knees. 2
• The pivot shift correlates better with functional instability than any other physical examination test, including the Lachman. 2
• The test is particularly valuable for diagnosing anterolateral rotatory knee instability. 4

Anterior Drawer Test

• Performed with the knee at 90 degrees of flexion to assess anterior tibial translation. 1
• Less sensitive than the Lachman test but still useful as a complementary examination. 1

Posterior Cruciate Ligament (PCL) Instability Tests

Posterior Drawer Test

• The posterior drawer test performed in near extension (not just 90 degrees) is diagnostic in approximately 83% of acute PCL injuries. 5
• This test assesses posterior translation of the tibia relative to the femur. 1

Gravity Sign (Tibial Sag Test)

• The gravity sign is diagnostic in approximately 83% of acute PCL injuries and should be performed with the patient supine, hip and knee flexed to 90 degrees. 5
• Observe for posterior sagging of the tibia due to gravity when both knees are flexed. 1

Active Reduction Test

• Passive reduction of posterior subluxation and active quadriceps contraction to reduce the subluxation are diagnostic in approximately 75% of acute PCL injuries. 5

Reversed Pivot Shift Sign

• Helps diagnose severe posterior and posterolateral subluxations, particularly when combined PCL and posterolateral corner injuries are present. 5

External Rotation Recurvatum Test

• Less sensitive for isolated PCL injuries but may be positive with combined posterolateral corner pathology. 5

Collateral Ligament Instability Tests

Valgus Stress Test

• Assesses medial collateral ligament (MCL) integrity by applying valgus force to the knee. 1
• Perform at both 0 degrees (full extension) and 30 degrees of flexion. 1
• Laxity at 30 degrees suggests isolated MCL injury; laxity at 0 degrees indicates combined ligamentous injury. 1

Varus Stress Test

• Assesses lateral collateral ligament (LCL) integrity by applying varus force to the knee. 1
• Similarly performed at 0 and 30 degrees of flexion with the same interpretation principles. 1

Meniscal Injury Tests (Often Associated with Instability)

McMurray's Test

• Assesses meniscal tears by rotating the tibia while moving the knee from flexion to extension. 1

Apley's Grind Test

• Evaluates meniscal pathology through compression and rotation of the tibia with the patient prone. 1

Bounce Test

• Tests for meniscal tears by assessing terminal extension of the knee. 1

Rotatory Instability Assessment

For complex rotatory knee instability involving multiple ligamentous structures, the pivot shift test remains the primary clinical examination, but may require supplementation with imaging and instrumented measurement. 4

• Rotatory instability involves abnormal three-dimensional motion affecting anteromedial, anterolateral, posteromedial, and posterolateral structures. 4
• Clinical examination should assess all planes of motion, as isolated ACL reconstruction may not fully restore rotatory stability. 4

Instrumented Testing

When objective quantification is needed, the Rolimeter or KT-1000 arthrometer provide significantly more accurate measurements than manual assessment alone. 3

• Both devices show no significant difference in measurement accuracy between each other. 3
• Instrumented testing is particularly valuable for research purposes and monitoring reconstruction outcomes. 3, 2

Critical Examination Principles

• Always examine the uninjured knee first for comparison. 1
• Perform both passive and active range-of-motion testing before stress maneuvers. 1
• Palpate joint line spaces systematically before performing instability tests. 1
• Acute injuries may be difficult to examine due to pain and guarding; consider examination under anesthesia or delayed re-examination after initial swelling subsides. 5

Common Pitfalls

• Relying solely on the anterior drawer test for ACL evaluation, as the Lachman test is more sensitive. 1, 2
• Failing to perform the pivot shift test, which is the best predictor of functional outcomes despite being more difficult to execute. 2
• Testing only at 90 degrees of flexion for PCL injuries, missing the diagnostic value of near-extension testing. 5
• Not comparing findings to the contralateral knee, which can lead to misinterpretation of normal laxity as pathologic. 1