The knee is a complex joint which has the ability to bend and to twist. Knee ligaments help to control this movement by connecting the femur (thigh bone) and tibia (shin bone) and bracing the knee against abnormal movements. The Anterior Cruciate Ligament (ACL) connects the front of the shin bone to the back of the thigh bone so acts to control forward and backward movement of the knee. It is also full of nerves which sense the movement of the joint and act as a signalling mechanism to the brain to help with balance and control of the joint and to prevent injury.
The ACL is typically injured during sharp changes in movement direction (especially when decelerating) or if the knee is forced backwards. It is therefore a common injury in sporting activities such as football or skiing. An audible “crack” or “pop”, pain and giving way of the knee, followed by immediate swelling of the knee (due to bleeding in the joint) are common symptoms of an ACL tear.