On MRI of a knee, the posterior cruciate ligament (PCL) is shown to be intact while the anterior cruciate ligament (ACL) is torn. The clinical test MOST specific for ACL insufficiency — and the anatomical basis for its positivity — is:
- A Posterior drawer test: tibial plateau displaced posteriorly on femur because PCL is lax
- B McMurray's test: clicking over the medial joint line from meniscal impingement
- C Anterior drawer test/Lachman test: abnormal anterior subluxation of tibia due to absent ACL restraint ✓
- D Valgus stress test at 30 degrees: opening of the medial joint line due to medial collateral ligament failure
Correct answer: C. Anterior drawer test/Lachman test: abnormal anterior subluxation of tibia due to absent ACL restraint
Explanation
The ACL runs from the anteromedial aspect of the tibial plateau to the posterolateral femoral condyle and is the primary restraint against anterior tibial subluxation. Its rupture allows the tibia to glide abnormally forward on the femur — demonstrated by the anterior drawer test (90 degrees flexion) and more sensitively by Lachman's test (20–30 degrees flexion, closer to functional range). McMurray's tests the menisci; valgus stress tests the MCL; posterior drawer tests the PCL.
Reference: BD Chaurasia's Human Anatomy, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.