Musculoskeletal SystemDifficulty 3/5
A positive Lachman test in an acutely injured knee most directly implicates which structure?
- AThe posterior cruciate ligament resisting posterior tibial translation
- BThe anterior cruciate ligament resisting anterior tibial translation
- CThe medial meniscus, which produces clicking on rotation
- DThe medial collateral ligament resisting valgus stress at the knee
Show answer and rationale
Correct: B. Lachman assesses anterior tibial translation on a fixed femur at 20-30 degrees of flexion, testing the anterior cruciate ligament. It is more sensitive than the anterior drawer in acute injury because hamstring guarding interferes less.
- A — Posterior translation is tested by the posterior drawer and posterior sag sign.
- C — Meniscal pathology is assessed by McMurray and joint-line tenderness.
- D — The MCL is tested with valgus stress at 0 and 30 degrees.