## Anterior Cruciate Ligament (ACL) Injury **Key Point:** The ACL is the most frequently injured ligament of the knee, accounting for approximately 40% of all knee ligament injuries. It is particularly vulnerable to non-contact injuries involving sudden deceleration, pivoting, or twisting with a planted foot. **High-Yield:** ACL injuries typically occur in: - Sudden deceleration with foot fixed - Pivoting or cutting movements - Direct blow to the lateral knee (valgus stress) - Hyperextension injuries **Clinical Pearl:** The classic mechanism is an athlete planting their foot and twisting the knee inward (internal rotation of tibia on femur), which is why ACL injuries are common in sports like football, basketball, and skiing. ### Mechanism of Injury The ACL prevents anterior translation of the tibia on the femur. When the foot is planted and the body rotates internally, the ACL experiences maximal tensile stress and is prone to rupture. ### Other Knee Ligaments — Comparative Injury Patterns | Ligament | Injury Mechanism | Frequency | | --- | --- | --- | | ACL | Non-contact twisting, deceleration | Most common (40%) | | PCL | Direct blow to anterior tibia, hyperflexion | Less common (10%) | | MCL | Valgus stress (blow to lateral knee) | Common (25%) | | LCL | Varus stress (blow to medial knee) | Uncommon (5%) | **Mnemonic:** **ACL = Athlete's Common Loss** — remember ACL injuries dominate sports medicine presentations. 
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.