Lachman Test

Purpose

The Lachman test is the most sensitive clinical test for diagnosing anterior cruciate ligament (ACL) rupture. It assesses the amount and quality of anterior tibial translation relative to the femur.

Why This Position (Biomechanical Rationale)

At 90 degrees of knee flexion (the position used in the anterior drawer test), the medial meniscus abuts the convex medial femoral condyle and creates a "doorstop" effect, impeding anterior tibial translation and potentially masking an ACL tear. At 15-30 degrees of flexion (the Lachman position), the femur's comparatively flat weight-bearing surface does not obstruct forward motion, allowing the ACL to be isolated and tested more accurately. - Campbell's Operative Orthopaedics, 15th Ed

Technique

Lachman test being performed - examiner stabilizing the femur with one hand and applying anterior force to the proximal tibia

Patient position: Supine, involved extremity in slight external rotation

Steps:

Flex the knee to 15-30 degrees
One hand firmly stabilizes the distal femur
The other hand grips the proximal tibia with the thumb resting on the anteromedial joint margin
Apply a rapid, anteriorly-directed lifting force to the posterior proximal tibia
Feel with the thumb for anterior translation of the tibia relative to the femur

Key tip: The hamstrings must be fully relaxed - any tension will prevent anterior tibial translation and falsely suppress the test. Palpate the posterior thigh to verify relaxation before applying force. The tibia should be rapidly accelerated anteriorly so the ACL snaps tight like a short rope - the examiner feels for the hard endpoint rather than just looking for excursion.

Interpreting the Result

Finding	Interpretation
Firm, hard endpoint	Normal ACL
Soft or absent endpoint	ACL rupture (positive test)
Anterior sag silhouette disappears	Further sign of ACL tear

Grading by translation (compared to contralateral knee):

Grade	Translation
Grade 1	1-5 mm
Grade 2	6-10 mm
Grade 3	>10 mm

Each grade is further sub-classified:

A = firm/hard endpoint
B = soft endpoint

So a "Grade 1B" means 1-5 mm of translation with a soft endpoint, which still suggests ACL injury despite minimal excursion. - Campbell's Operative Orthopaedics, 15th Ed, p. 2649

Important Caveat

The test may be negative in chronic ACL ruptures because the ACL stump can scar and adhere to the PCL, creating a false endpoint. Always correlate with history and imaging. - Bailey and Love's Short Practice of Surgery, 28th Ed, p. 514

Comparison With Other ACL Tests

Test	Knee Position	Notes
Lachman test	15-30° flexion	Most sensitive; best for acute injuries
Anterior Drawer test	90° flexion	Less sensitive due to meniscal doorstop effect
Pivot Shift test	Near full extension	Tests anterolateral rotatory instability; high specificity but low sensitivity in acute setting

The Lachman test has been shown in systematic reviews and meta-analyses to have the highest diagnostic accuracy among manual knee examinations for ACL tears. The positive likelihood ratio ranges from approximately 2.19-9.67 and the negative likelihood ratio from 0.13-0.31.

Clinical Pearls

A displacement of only 5 mm can be hard to see visually in a large knee - feel for the endpoint, do not just look
If the patient is in pain or guarding, the test will be less reliable; consider examination under anesthesia
Always compare to the contralateral knee as a baseline
The prone Lachman test is an alternative when supine examination is difficult, though it should not be used as the sole diagnostic test

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