The anterior cruciate ligament (ACL) is the major stabilizing ligament of the knee. The ACL is located in the center of the knee joint and runs from the femur (thigh bone) to the tibia (shin bone), through the center of the knee. In this position, it functions to prevent a buckling type of instability of the knee.

Usually the tearing of the ACL occurs with a sudden direction change or when a deceleration force crosses the knee. The patient often feels or hears a popping sensation, has the rapid onset of swelling, and develops a buckling sensation in the knee when attempting to change direction.