The cruciate ligaments, ACL and PCL are the important central 'stays' of the knee, contributing significantly to its stability. The word 'cruciate' means 'crossed’ and the two ligaments are indeed crossed over one another, each attaching above to the femur and below to the tibia.

One of the most common problems involving the knee joint is an ACL tear. When it functions normally, the ACL can handle large forces with little or no problem. If, however, the knee receives forces of a high magnitude impact, the muscles cannot help absorb the stress, the ACL may take the entire load, and it may tear. High-magnitude loading can occur during a slip and fall, sudden change in direction, landing off balance while jumping, or hyperextension of the knee. When the ligament tears, it generally ruptures like a rope. Without a healthy ACL the knee is extremely unstable and can “give out” which is to say, the knee joint is sliding too much. This can be a problem because each episode of instability, the “giving out” sensation can cause damage to the meniscus (knee cartilage). Therefore an ACL injury makes the knee more prone to developing meniscus tears and arthritis.

Any sport that requires an athlete to perform common maneuvers such as cutting, pivoting, and sudden turns depends on a functioning ACL. Athletes often have particular difficulty once they have sustained an ACL injury. Since it is the most common knee injury, the presence of an ACL tear is suspected whenever a patient has a serious injury to their knee. ACL reconstruction surgery is commonly, and improperly, referred to as an ACL repair. Unfortunately, a torn ACL cannot be "repaired." Rather, the torn ligament must be entirely removed, and a new ACL must be reconstructed using other, healthy tissue. It is not possible to repair the torn ACL by simply reconnecting the torn ends.

The surgical procedure for an ACL reconstruction is variable, but commonly involves using a segment of another larger ligament or tendon to replace the damaged ACL.

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