There are four main ligaments in the knee - one on either side and two across the middle.
Posterior cruciate ligament (PCL)
The ACL is behind the kneecap (patella) and is in front of the PCL.
It's the second strongest ligament in the knee and stabilises the joint, connecting the thigh bone (femur) and the leg bone (tibia).
The ACL and PCL limit the over straightening, over bending and rotation of the knee.
The average length of the ACL is around 35mm, weighing around 20g.
The medial ligament is one the inside of the knee, with the lateral on the outside.
OPERATE OR NOT?
Surgery is not always essential when the ACL is ruptured.
But in the case of a young sportsman looking to return to sport as soon as possible, it will be.
A middle aged or older person could recover without going under the surgeon's knife.
However, it is believed that repairing the ligament can reduce the risk of getting arthritis in the joint later in life.
Surgeon's will usually graft tissue from either the patella or hamstring tendons to repair the ACL.
Basically the new tendon replaces the ACL and is usually attached to the bones above and below the knee by screws.
The success rate for such operations is high and while the injured person can soon be up and walking, running and twisting are some months further down the line.
ROAD TO RECOVERY
Medical expert Bevan Ellis says: "Many top athletes will go through an accelerated rehabilitation programme to get them back in action within six months.
"It requires intensive physiotherapy which needs to be monitored very closely at every stage.
Simon Jones suffered from a cruciate knee injury in Australia
"For a typical person, an ACL injury would normally take between 8-12 months to get back to full fitness."
Phase one (0-2 weeks after surgery)
The knee will be swollen after the operation, so the first job is to reduce the swelling.
After that, the physio will make the patient do a few light exercises like isometric contractions - keeping the leg still but moving the muscles around the knee.
Phase two (2-6 weeks after surgery)
The swelling should have disappeared, but the graft usually weakens around this time.
The physio may have to back off from the rehabilitation programme until the ligament is up to more exercises.
The patient should be walking normally by then.
Phase three (6-12 weeks after surgery)
By this stage, the knee should be getting stronger and able to take more strain.
The patient should be able to go swimming and use a road bike to get the knee back on track, as well as doing more strength exercises.
Phase three (3-6 months)
The patient will have their full range of movement and strength back, so they can start running properly once more.
They should be able to get back to specific drills and training.
Phase four (6-12 months)
The patient should be able to return to playing sport with their surgeon's approval.