An anterior cruciate ligament (ACL) injury is a tear in a knee ligament that connects the thighbone (femur) to the shinbone (tibia). The ACL helps keep your knee stable.

You can tear your ACL when the upper leg bone is moving in a different direction from the lower leg bone. Sometimes the ACL is completely torn. Other times it is only partially torn. It can happen during sports or from trauma when you fall or are hit around the knee.