The Anterior Cruciate Ligament (ACL) as indicated above is one of the four main ligaments in the knee.  It prevents back to front movement of the tibia (shin bone) with respect to the femur (thigh bone).  It also provides rotational stability of the knee.  It is the most commonly torn ligament in the knee, yet not all people need surgery to treat an ACL rupture.  An ACL is only needed during high-level athletics that involve cutting-type movements.  Simple daily tasks do not require an ACL, and many people can play recreational sports without an ACL.