An ACL tear usually occurs as a result of either a twisting force being applied to the knee whilst the foot is firmly planted on the ground or twisting of the knee when landing from a jump.
They can also occur from contact situations as seen in sports such as rugby or football, but these are less common.
There are four main ligaments that give stability to the knee joint. The two collateral ligaments are located on the inside and outside of the knee and are extra-capsular (outside the knee joint capsule) and prevent excessive sideways movement. The other two are the cruciate ligaments which are intra-capsular (within the knee joint) and cross over in the middle of the knee joint and prevent excessive forward and backward movements.
The anterior cruciate ligament runs diagonally from the back of the femur (thigh bone) upwards and forwards to the front of the tibia (shin bone) and prevents the shin bone from moving excessively forward. The ACL comprises of two ligaments that are twisted together, called the anteromedial and posterolateral bundles. One of the bundles is tight when the knee is bent and the other is tight when the knee is straight and each bundle limits different movements. For this reason, reconstructive surgery involves replicating this and the graft (replacement ligament) is made up of 'two bundles' to form one ligament.
The posterior cruciate ligament passes in the opposite direction and crosses the ACL in the middle of the knee, hence the name cruciate which means “cross-shaped”. Together, these two ligaments are vitally important to the stability of the knee joint, especially for contact sports and those that involve fast changes in direction.
In addition to twisting, a direct blow to the outside of the knee causes the knee to buckle inwards and the same excessive movement of the tibia as seen in the non-contact scenario, and tears the ACL. With this type of injury there is often associated damage to other structures within the knee joint such as the medial collateral ligament (MCL) or the meniscus (cartilage - medial meniscus tear) and these need to be accurately diagnosed and treated.
Premiership football physiotherapist Neal Reynolds talks to Sportsinjuryclinic.net about preventing ACL injuries.
It is very difficult to prevent ACL injuries on their own, it is better to look at the whole knee and concentrate on strengthening the surrounding muscles.
It starts off with footwear. There do seem to be more ACL injuries now than there were 10-20 years ago. There are many possible reasons for this, from more coverage in the media, to better diagnosis, to footwear. Neal believes that there is a link between footwear and ACL ruptures. Make sure that the studs are in tightly and are good quality, make sure the boots are also good quality ones.
In terms of the knee itself, loads of balance work, or proprioception, will improve the sensory feedback at the joint.
The other thing is getting the balance between your quads and hamstring muscles right. It is thought that having very strong quads and not strong hamstrings you are more prone to an ACL injury due to the pulling forward force of the quads. So try to get an equal balance in strength and reaction times between the two.
Doing functional exercises like single leg squats as well, really helps with this kind of injury as it is a great proprioceptive exercise and it also starts to get the knee and body into the right alignment and works on getting the quads and hamstrings as efficient as possible.