The ACL is a ligament inside the knee joint that provides forward stability to the knee, in order to prevent forward displacement of the shin bone (tibia) from the thigh bone (femur). This is the most common knee ligament injury and can occur as the result of a sudden twisting movement, such as a fall, in sport or daily life.The ACL is a ligament inside the knee joint that provides forward stability to the knee, in order to prevent forward displacement of the shin bone (tibia) from the thigh bone (femur). This is the most common knee ligament injury and can occur as the result of a sudden twisting movement, such as a fall, in sport or daily life.