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Anterior cruciate ligament (ACL) tears are the most common ligamentous injuries that occur in the knee. (1) People who have an ACL injury usually are involved in high-level cutting and pivoting sports, and many are under 25 years of age. (2) The ACL injury likely occurs when the loading of the ACL exceeds its limits: when a quadriceps femoris muscle force is applied with knee internal rotation, a valgus load is combined with knee internal rotation, or there are excessive valgus loads during rapid deceleration. (3) (Figs. 1, 2).