The anterior cruciate ligament (ACL) is one of the main restraining ligaments in the knee. It runs through the centre of the knee, from the back of the femur (thigh bone) to the front of the tibia (shin bone), and it acts as a link mechanism between the thigh and lower leg.
The main function of the ACL is to stabilise the knee, especially during rotation, sidestepping, and pivoting movements.
This means that when the ACL is ruptured or torn, the tibia moves abnormally on the femur and comes out of joint, and the knee buckles. The main feeling is a sense of the knee giving way during twisting or pivoting movements, and a sense of not trusting the knee when turning.
It is usual to be able to return to walking and straight line running following a torn ACL but to not trust the knee on rough ground or twisting movements. All too often the diagnosis can be delayed because the knee may recover for straight line activities, giving a false sense of security, but there is no control when returning to pivoting sports.
And when the knee gives way, there is a risk of further damaging the other key structures in the knee – hence the importance of preventing re-injury by making an early diagnosis.