The anterior cruciate ligament, or ACL, is one of four ligaments that hold your knee together. If the ACL is torn, then you usually feel as if your knee is about to give out at you at any time, because there isn't enough support.
There are three bones that make up your knee, and two major ligaments: the anterior cruciate ligament and the posterior cruciate ligament. These two ligaments stabilize the bones: the femur, which is the bone in the upper part of the leg, the knee cap, and the tibia, which is the leg in the lower part of the leg. The two ligaments make a cross shape and control the knee's motion back and forth. The ACL provides up to 90 percent of the knee's stability.