Shown here is the anterior cruciate ligament (ACL) of the knee joint in an anatomic dissection. This ligament helps in stabilization of the knee joint. The ACL can be injured with excessive force applied to the knee joint. Sporting events account for most injuries. This can occur with a contact injury resulting in valgus collapse of the knee. Non-contact ACL injuries are more common and most often occur with the knee close to full extension when there is sudden deceleration priot to a change of direction or landing motion (jumping or pivoting). When the ACL tears, then the joint becomes unstable, with abnormal anterior displacement of the tibia on the femur. Such injuries typically require surgical reconstruction.