## Mechanism of ACL Injury **Key Point:** The anterior cruciate ligament (ACL) is the most frequently injured ligament of the knee, particularly during sudden deceleration with external rotation of the tibia on a fixed femur—a classic non-contact injury mechanism. ### Injury Mechanism The ACL is stretched and torn when: 1. The knee is in flexion (usually 20–30°) 2. The tibia externally rotates on a fixed femur 3. Deceleration forces are applied suddenly This occurs commonly in: - Pivoting sports (football, basketball, skiing) - Sudden direction changes - Landing from a jump with the knee in flexion ### Functional Role of ACL The ACL is the primary restraint to anterior tibial translation and provides rotational stability. Loss of ACL function results in: - Positive anterior drawer test - Positive Lachman test (most sensitive) - Positive pivot-shift test (most specific) **High-Yield:** The Lachman test (knee flexed 20–30°, anterior force on tibia) is the most sensitive clinical test for ACL insufficiency and should be performed in all suspected cases. ### Comparison of Knee Ligament Injuries | Ligament | Injury Mechanism | Clinical Test | Frequency | |----------|------------------|---------------|----------| | ACL | Deceleration + external rotation | Lachman, anterior drawer, pivot-shift | Most common | | PCL | Dashboard injury, hyperextension | Posterior drawer | Less common | | MCL | Valgus stress + external rotation | Valgus stress test | Common | | LCL | Varus stress | Varus stress test | Least common | **Clinical Pearl:** ACL injuries are often associated with meniscal tears (especially medial meniscus) and are part of the "unhappy triad" (ACL tear + MCL tear + medial meniscus tear) when all three structures are injured simultaneously. [cite:Gray's Anatomy 42e Ch 6] 
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