Anterior cruciate ligament (ACL) injuries are caused by a sudden twisting of the knee. The anterior cruciate ligament (ACL) is located in the center of the knee. It limits rotation and the forward movement of the tibia, or shin bone. The knee is a joint where four bones join: the tibia; the fibula, the femur, or thigh bone; and the patella, or knee cap. Four ligaments attach to the femur and tibia and give the joint strength and stability. One of these is the anterior cruciate ligament, or ACL. Causes Recent studies estimate that nearly 250,000 ACL injuries occur annually in the United States. The ACL is most often stretched or torn (or both) by a sudden twisting motion—when, for example, your feet are planted one way and your knees are turned another. You can also injure your ACL by quickly changing the direction in which you’re moving, by putting the brakes on too quickly when running, or, if you're a woman, when landing from a jump. A woman's body structure and hormones cause more force on the ligaments, increasing the likelihood of injury. People who ski or who play basketball, volleyball, soccer, or football are most likely to injure their ACLs when they slow down, pivot, or land after a jump. Symptoms If you injure your ACL, you may not feel any pain immediately. You might hear a popping noise and feel your knee give out from under you. Within a few hours, you'll notice swelling at the knee. The knee will hurt when you try to stand on it. It's important to keep weight off the knee until you can see your health care provider, or you may injure the knee cartilage. You should use an ice pack to reduce swelling and keep the leg elevated. If needed, use a pain reliever. If you must walk, use crutches. Be sure to see a doctor right away to have your knee evaluated. Tests and Procedures Your doctor may conduct physical tests and take X-rays to determine the extent of your ACL damage. If the ACL is only partially torn, your doctor may prescribe an exercise program to strengthen surrounding muscles and a brace to protect the knee during activity. You may or may not need surgery. Surgery can reattach the torn ends of the ligament or reconstruct the torn ligament from a piece of strong, healthy tissue taken from another area near the knee or from a cadaver. If the ACL is completely torn, it may need to be replaced surgically. Successful surgery tightens your knee and restores its stability, which helps you avoid further injury. After ACL reconstruction, you'll need to do rehabilitation exercises to gradually return your knee to full flexibility and stability. You also may need a knee brace temporarily and will probably have to avoid sports for about a year.