When surgeons talk about the stability factor in ACL surgery, what are they referring to exactly?

The anterior cruciate ligament (ACL) is probably the most commonly injured ligament of the knee. It is is located in the center of the knee joint where it runs from the backside of the femur (thighbone) to connect to the front of the tibia (shinbone). The ACL is the main controller of how far forward the tibia moves under the femur. This is called anterior translation of the tibia or tibial translation. If the tibia moves too far, the ACL can rupture. The ACL is also the first ligament that becomes tight when the knee is straightened. If the knee is forced past this point, or hyperextended, the ACL can also be torn. In most cases, the ligament is injured by people participating in athletic activity. As sports have become an increasingly important part of day-to-day life over the past few decades, the number of ACL injuries has steadily increased. This injury has received a great deal of attention from orthopedic surgeons over the past 15 years, and very successful operations to reconstruct the torn ACL have been invented. When combined together, too much of these two motions (tibial translation and internal rotation) are what result in something called the pivot-shift phenomenon. The athlete plants his or foot on the ground and attempts to make a sudden shift or move in another direction. With an unstable knee, the joint buckles or gives way underneath the athlete. Stability means that the ACL (working with the other intact ligaments and soft tissue structures) provides normal knee motion and prevents too much tibiofemoral translation from occurring. No one really expects grafts to restore normal motion and provide perfect check-reins on abnormal motions. The natural, normal ligaments' design and fiber tension behavior are simply too complex for that. But the graft can provide enough tension to protect the knee from instability and restore normal function. This can be done without limiting normal tibial motion too much, a situation called overstraining.

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