How to Perform the Lachman Test for ACL Evaluation
The Lachman test is performed with the patient supine, knee flexed 20-30 degrees, examiner stabilizing the distal femur with one hand while applying an anteriorly directed force to the proximal tibia with the other hand, assessing for increased anterior tibial translation and a soft or absent endpoint compared to the contralateral knee.
Test Positioning and Technique
The standard Lachman test requires specific positioning for optimal accuracy 1:
- Patient position: Supine on examination table
- Knee flexion: 15-30 degrees (most commonly 20-25 degrees)
- Examiner hand placement:
- One hand stabilizes the distal femur firmly
- The other hand grasps the proximal tibia and applies anterior force
- The tibial force should be applied medially on the tibia for consistency 1
Key assessment points: Evaluate both the amount of anterior tibial translation AND the quality of the endpoint. A positive test shows increased translation with a soft or absent endpoint compared to the contralateral knee 2, 3.
Biomechanical Rationale
The Lachman test at 15-25 degrees of flexion produces maximal tension in the ACL, making it more specific than the anterior drawer test performed at 90 degrees 4. Arthroscopic studies confirm that baseline ACL tension is greater at 15 degrees than at 90 degrees, and the Lachman maneuver produces maximal tension in the anteromedial and central portions of the ligament 4.
During the test, the medial femoral epicondyle exhibits greater anteroposterior motion in ACL-deficient knees (11.5mm vs 7.7mm in intact knees), while the lateral epicondyle shows less difference 5.
Diagnostic Accuracy
The Lachman test demonstrates 81% sensitivity and 85% specificity overall 3. However, accuracy varies by clinical scenario:
- Complete ACL tears: 68% sensitivity, 79% specificity 3
- Post-acute injuries: 70% sensitivity, 77% specificity 3
- Acute injuries: Higher sensitivity (data limited but generally >85%)
When compared to other clinical tests 2, 3:
- Anterior drawer test: 83% sensitivity, 85% specificity
- Pivot shift test: 55% sensitivity, 94% specificity (best for ruling in injury)
- Lever sign test: 83% sensitivity, 91% specificity (best for ruling out injury)
Clinical Pearls and Modifications
The "drop leg Lachman" modification can increase sensitivity by 1.8-2.4mm of additional translation 6:
- Patient's leg is abducted off the side of the table
- Knee flexed 25 degrees
- Thigh stabilized to table with one hand
- Patient's foot held between examiner's legs
- Free hand applies anterior force
- This technique is physically easier to perform and more sensitive
Important Caveats
Acute knee effusions significantly limit test accuracy. In the acute setting with tense effusion, consider aspiration before examination to improve diagnostic yield 7. The presence of joint effusion on examination shows 91% positive predictive value for internal derangement 8.
The test shows significant inter-examiner variability despite similar technique, with actual tibial movement varying considerably between examiners 1. This underscores the importance of comparing side-to-side rather than relying on absolute measurements.
Previous systematic reviews overestimated Lachman test accuracy by not accounting for the non-independence of sensitivity and specificity in meta-analyses 3. The true diagnostic accuracy, particularly for post-acute presentations and complete tears, is lower than historically reported.
Integration with Other Diagnostic Modalities
When radiographs are negative after acute knee trauma, MRI is the preferred next study with 96% sensitivity and 97% specificity for meniscal tears and high accuracy for ACL injuries 8. MRI shortened diagnostic workup and improved quality of life in the first 6 weeks after injury 8.
Instrumented Lachman testing (KT-1000 arthrometer) provides objective measurements that supplement subjective clinical examination and should be considered for documenting ACL function, particularly in research settings 9. An increase in anterior tibial translation of ≤3mm on instrumented testing has 100% predictive value for a negative pivot-shift test 9.