Anterior Cruciate Ligament or the ACL is an important ligament inside the knee. It is present in the middle of the knee, and pairs with another ligament called posterior cruciate ligament, or PCL. Both ligaments help in stability of the knee joint, and prevent the knee from sliding on itself, hence preventing the feeling of buckling, giving way, or falling. In the absence of either one, or both the ligaments, the knee is unstable, and the patient will have difficulty walking, but especially with running, pivoting, and cutting.