The anterior cruciate ligament (ACL) is one of the four major ligaments in your knee. Ligaments connect your bones and enable you to stretch and flex your joints. The ACL and the posterior cruciate ligament (PCL) work together to allow the forward and backward motion of your knees.The ACL can stretch too far, tear, or completely rupture if you exert too much pressure on the joint. A mild sprain, where the ACL is stretched but still whole, typically requires nonsurgical treatments like activity modification and physical therapy. A severe injury, where the ligament ruptures completely, might need surgery.