The ACL is located behind the kneecap (patella). It stabilizes the knee when it rotates. The ACL and PCL connect your thigh bone (femur) to your shin bone (tibia). An ACL injury is the overstretching or tearing of the ACL ligament. A torn ACL is a common knee injury. On average, women are 2 to 8 times more at risk of ACL injuries than men.