What is an ACL?

A diagram of the knee, showing the anterior cruciate ligament (ACL).
Injuries to the ACL are common in football.
The ACL is the most commonly injured ligament in the knee.
Damage to the anterior cruciate ligament (ACL) can occur when the knee suffers a sudden, direct blow.
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  • Written By: A Kaminsky
  • Edited By: Niki Foster
  • Last Modified Date: 22 November 2015
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A common sit of sports injury, especially in football, is the knee. The most often injured portion of the knee is the ACL, or anterior cruciate ligament. This ligament is located in the very middle of the knee and helps it rotate properly. It also keeps the tibia in place. It is one of the four ligaments in the knee that help humans walk, and it works with the posterior cruciate ligament to keep the tibia stable.

A healthy ACL is essential for pain-free walking and running. Because of this, athletes are usually very protective of their knees. That's the reason for shock-absorbing sneakers and other technology to keep the knees safe. A badly torn ACL can end an athletic career.

An ACL can be injured from a blow to the side of the leg, but in non-athletes, most frequently occurs when the knee is twisted, from a sudden stop and reverse move, or from simple overextension of the knee joint. Athletes are often taught how to decrease stress on the ACL through changing the way they move in certain circumstances.


If a person suspects he has injured the ACL, he should ice and elevate the joint, and should seek medical evaluation as soon as practical. If, after the injury, the lower leg or foot turns blue and is cool to the touch, the person should seek immediate medical assistance, since circulation to that leg may be severely impaired. In children, an ACL injury may heal by itself, but this is rarely the case with adults. Depending on the severity of the tear, an adult may be able to live with the injury, but surgery is often required.

ACL surgery is usually done arthroscopically, and the surgeon may elect to reconstruct the ligament with a cadaver ligament, or with one fashioned from part of the patient's patellar ligament. Physical therapy is required for the patient to regain full mobility of the knee, although he or she may not be able to participate in athletic activities at the same intensity as before the injury.


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