The Anterior Cruciate Ligament (ACL) 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 and is most frequently torn during athletic activities.  A large majority of ACL injuries occur in noncontact situations and often result from rapid movements such as changing direction or stopping quickly.  The ACL is important not only to provide stability to the knee while running, cutting, and jumping, but it also provides stability during daily activities as well.  Many simple daily tasks do not require an ACL, and many people can play recreational sports without an ACL, however an ACL-deficient knee is at higher risk for meniscus tears and cartilage injury in the future. If torn, the ACL can be replaced during arthroscopic surgery in what is known as an ACL reconstruction.