A 28-year-old male footballer presents to the orthopaedic clinic with a 3-week history of left knee pain and swelling following a non-contact injury during a match. He reports a sudden 'popping' sensation in the knee while pivoting on a planted foot. On examination, there is joint effusion, and the anterior drawer test is positive at 90° knee flexion. Lachman test (at 30° flexion) is also positive. McMurray's test is negative. Which structure is most likely injured?

Explanation

## Clinical Presentation & Mechanism **Key Point:** The combination of a non-contact pivoting injury with a 'pop' sensation, positive anterior drawer test, and positive Lachman test is pathognomonic for anterior cruciate ligament (ACL) injury. ## Examination Findings Interpretation | Test | Normal | ACL Tear | PCL Tear | Meniscal Tear | |------|--------|----------|----------|---------------| | Anterior Drawer (90°) | Negative | Positive | Negative | Negative | | Lachman Test (30°) | Negative | Positive (most sensitive) | Negative | Negative | | McMurray Test | Negative | Negative | Negative | Positive | | Pivot Shift | Negative | Positive (most specific) | Negative | Negative | **High-Yield:** The Lachman test at 30° knee flexion is the **most sensitive test** for ACL injury (95% sensitivity), while the pivot shift test is the **most specific** (98% specificity). ## Mechanism of Injury 1. Non-contact pivoting or deceleration injury 2. Sudden change of direction with planted foot 3. Hyperextension or anterior tibial translation 4. Results in anterolateral rotatory instability **Clinical Pearl:** The 'pop' sensation reported by the patient is highly suggestive of ACL rupture. This is often accompanied by immediate effusion (blood-tinged synovial fluid) and functional instability. ## Why ACL and Not Other Structures - **Negative McMurray test** rules out meniscal pathology - **Positive Lachman test** is specific for ACL (not seen in collateral ligament injuries) - **Mechanism** (pivoting on planted foot) is classic for ACL, not PCL (which requires direct posterior tibial force) - **Joint effusion** with negative meniscal signs points to ligamentous injury **Mnemonic:** **ACLPOP** — ACL injury = sudden Pop with Pivoting, positive Anterior drawer and Lachman, negative McMurray. [cite:Moore's Clinically Oriented Anatomy 8e, Ch 6]