In the Lachman test for ACL injury, the test is performed with the knee at 20–30° of flexion rather than 90° (anterior drawer) because:

• A The ACL is maximally taut at 20–30° and anterior translation is greatest ✓
• B Hamstrings are maximally relaxed at 20–30° and cannot splint the tibia
• C Posterior capsule is taut at 90° and masks ACL laxity
• D The menisci do not block anterior tibial translation at 20–30°

Explanation

The Lachman test has higher sensitivity (85–98%) than the anterior drawer test (40–75%) for ACL injury. At 20–30° flexion, the ACL is under greater tension (its anterolateral band is taut in extension/near-extension), making it the primary restraint to anterior tibial translation. At 90° flexion (anterior drawer), hamstring muscular splinting and posterior capsule tension partially stabilise the tibia, masking anterior laxity. The Lachman test also avoids pain-inhibited muscle guarding that is more prevalent at 90°. A positive Lachman (>3 mm anterior translation, soft endpoint) strongly indicates complete ACL tear.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.