The anterior cruciate ligament (ACL) is one of the main ligaments in the center of your knee. It runs from the back of the femur (thigh bone) to the front of the tibia (shin bone), and assists in proper knee joint movement. In particular, the ACL prevents the tibia from shifting out from underneath the femur.
An ACL tear can create an unstable knee that gives way during activities, especially during cutting and pivoting sports. It is most commonly injured when the knee twists with the foot planted on the ground. However, it can also occur due to a direct blow to the knee or knee hyperflexion or extension. This damage can result in a partial ACL tear or, more commonly, a complete tear.