How is the Lachman test performed on an adult patient with acute knee trauma suspected of an ACL tear?

Lachman Test Performance Technique

The Lachman test is performed with the patient supine, knee flexed to 20-30 degrees, with the examiner stabilizing the distal femur with one hand while applying an anterior force to the proximal tibia with the other hand, assessing for anterior tibial translation and quality of the endpoint.

Standard Technique

The classic Lachman test requires specific positioning and execution 1:

• Patient positioning: Supine with the injured knee flexed to approximately 20-30 degrees 1
• Examiner hand placement: One hand stabilizes the distal femur while the other grasps the proximal tibia 1
• Force application: Apply an anteriorly directed force to the proximal tibia while maintaining femoral stabilization 1
• Assessment parameters: Evaluate both the amount of anterior tibial translation AND the quality of the endpoint (firm vs soft/absent) 1

Diagnostic Performance

The Lachman test demonstrates superior sensitivity compared to other physical examination maneuvers for acute ACL tears 2:

• Office setting sensitivity: 81% with 81% specificity 2
• Under anesthesia: Sensitivity increases to 91% with 78% specificity 2
• Acute injury performance: 99% positive in acute complete ACL tears during initial examination 1

The absence of a firm endpoint is the critical finding in complete ACL tears 1. The Lachman test remains relatively unaffected by associated meniscal or collateral ligament injuries, unlike the anterior drawer test 1.

Alternative Technique for Larger Patients

For examiners with smaller hands or when examining larger patients, the drop leg Lachman test provides a validated alternative 3:

• Patient positioning: Supine with the leg abducted off the side of the examination table and flexed 25 degrees 3
• Stabilization method: The thigh is stabilized to the table with one hand, and the patient's foot is held between the examiner's legs 3
• Force application: The examiner's free hand provides the anteriorly directed force 3
• Advantage: Produces 1.8 mm greater average excursion in conscious patients and 2.4 mm more translation under anesthesia compared to standard Lachman 3

Prone Position Technique

The prone Lachman test offers another alternative with established diagnostic accuracy 4:

• Diagnostic performance: 70% sensitivity and 97% specificity 4
• Positive predictive value: 94% 4
• Clinical utility: Excellent for confirming ACL tear presence (positive likelihood ratio of 20.17), but should not be used alone to rule out injury (negative likelihood ratio of 0.32) 4
• Reliability: 90% agreement between examiners with kappa coefficient of 0.81 4

Critical Examination Findings

When performing the Lachman test in acute knee trauma, assess for 1:

• Endpoint quality: Complete ACL tears demonstrate absent or soft endpoint 1
• Side-to-side comparison: Always compare to the contralateral uninjured knee 1
• Patient guarding: Muscle spasm and pain can limit examination accuracy in the acute setting 1

Clinical Context

Following positive Ottawa knee rules (focal tenderness, effusion, inability to bear weight), radiographs should be obtained first 5. If radiographs are negative but ACL tear is suspected based on positive Lachman test, MRI is the next appropriate imaging study to evaluate for ACL tear and associated meniscal or ligamentous injuries 5. MRI demonstrates 96% sensitivity and 97% specificity for ACL tears 5.

Common Pitfalls

• Inadequate knee flexion: Testing at greater than 30 degrees reduces sensitivity 1
• Insufficient femoral stabilization: Allows the femur to translate posteriorly, creating false-negative results 1
• Ignoring endpoint quality: Focusing only on translation amount without assessing endpoint firmness misses critical diagnostic information 1
• Acute examination limitations: The pivot shift test is positive in only 35% of acute injuries due to pain and guarding, compared to 98% under anesthesia, making the Lachman test more reliable acutely 1