Knee ligaments Four ligaments support and guide the knee.
All four ligaments work in concert to limit excessive rotational movements. The ACL is the primary restraint that limits how far the tibia slides forward in relations to the femur. When the ACL tears and that restraint disappears, the knee becomes unstable and may unpredictably buckle or give way. An ACL tear is a common knee injury in football and basketball. The ACL may tear suddenly and without warning when an athlete decelerates, cuts to the side, or lands awkwardly.