A tear may be partial or complete. It prevents the tibia from sliding out in front of the femur, and provides rotational stability to the knee. It prevents the tibia from sliding backwards under the femur. ACL injuries are often associated with other injuries. This type of injury is most often seen in football players and skiers.
Women are much more likely to suffer an ACL tear than men are.
The cause for this is not completely understood. Adults who tear their ACL usually do so in the middle of the ligament or pull the ligament off the bone. These injuries do not heal by themselves. But children are more likely to pull off their ACL with a piece of bone still attached, and these may heal on their own. An MRI is the only imaging study that can demonstrate an ACL tear.