The knee is the largest joint in the body and good function of this joint is vital to normal movement. Two sets of ligaments in the knee give it stability; the cruciates and the collateral ligaments.

The cruciate ligaments are located within the knee joint and connect the femur (the thighbone in the upper leg) to the tibia (the shinbone in the lower  leg). These ligaments are made up of many strands, a bit like a short length of rope, and hold the bones and knee joint tightly together, and in such a way as to allow it to bend without the bones drifting apart. The anterior cruciate ligament (or ACL) runs from the front of the tibia to the back of the femur, and the posterior cruciate ligament (or PCL) runs from the back of the tibia to the front of the femur, crossing over each other in the middle of the joint. These are the major stabilsing ligaments for the knee joint.

The ACL is most commonly injured during sporting activities, particularly in skiing and football, but can be injured in a number of ways, including changing direction rapidly, landing from a jump, stopping suddenly when running, or by trauma, such as a violent blow.

ACL injuries do not always cause immediate pain, but the patient is usually aware of a popping sound, or the knee gives way. Swelling and pain will occur in the following 12-24 hours.