Anterior cruciate ligament

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The anterior cruciate ligament (or ACL) is one of the four major ligaments of the knee. It connects from a posterio-lateral (back & outside) part of the femur to an anterio-medial (front & inside) part of the tibia.

Damage to the ACL frequently occurs with lateral blows to the knee (as happens with a tackle from the side in American football) and often is accompanied by injuries to the medial collateral ligament (MCL) and the medial meniscus, which is attached to the MCL; physicians are taught "...knee injuries come in threes - anterior cruciate, medial collateral, medial meniscus." A damaged ACL can be confirmed (clinically) by a physician with the anterior drawer test, the Lachman test, or an MRI.

Non-contact tears or ruptures of the ACL often occur when athletes moving quickly in one direction make a sharp or sudden change in direction (cutting). In jump sports, ACL failure has been linked to heavy or stiff landing as well as twisting or turning the knee while landing. Studies indicate that women in jumping and cutting sports such as basketball, volleyball, or football (soccer), are significantly more prone to ACL injuries than men; this is generally believed to be due to differences between the sexes in the angle between the hip and knee, general muscular strength, and possibly training techniques (a new study suggests hormone-induced changes in muscle tension associated with menstrual cycles may be an important factor [1] ( It is one of the most common serious injuries in Australian Rules football.

ACL injuries are also common in alpine skiing, partially because of improvements in boots. Today's boots have been successful in preventing many of the ankle and leg fractures once caused by accidents; however, the tradeoff has been that the stresses have been transferred to the knees, resulting in many ACL tears.

An ACL injury can often be debilitating for far longer than a broken leg.


A partially torn ACL will usually be allowed to heal itself. A completely torn ACL will not grow back. It must be replaced or left unattached. The ACL primarily serves to stabilize the knee in an extended position and when surrounding muscles are relaxed, so if the muscles are strong many people can function without it. However, lack of an ACL generally increases the risk of other knee injuries such as torn meniscus.

There are three options for surgical ACL repair. In the first, two pieces of hamstring tendon are harvested from the back of the injured knee along with a small, attached chip of bone. These are woven together to form a single piece of connective tissue with pieces of bone at each end. In the second, the middle third of the patellar tendon is harvested from the patella (knee cap) to the tibia (shin). In the third, the patellar tendon is harvested from a cadaver.

In all cases the new ligament is threaded through the knee arthroscopically and stapled or screwed into place at each end. Because bone grows much faster than ligaments, the ends of the new ACL becomes attached to the knee in just a few weeks. In about six months, the knee is very close to full strength and after a year or two the knee is generally stronger than before the injury.

Each method has its own pros and cons. Hamstring grafts are not as strong initially, since two tendons are woven together, but there is not significant clinical evidence that hamstring grafts fail more frequently than others. Patellar grafts are often cited as being stronger, but the site of the harvest is often extremely painful for weeks after surgery and some patients develop chronic patellar tendinitis. Replacement via a posthumous donor involves a slightly higher risk of infection. The risk is estimated to be 1 in 3 million. Additionally, donor grafts eliminate tendon harvesting which, due to improved arthroscopic methods, is responsible for most post-operative pain.

All treatment options require extensive physical therapy to build up muscle strength around the knee and restore range of motion.

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