A ligament is a tissue anywhere in the body that connects one bone to another. Four ligaments stabilize the knee joint and connect the femur to the tibia: the Posterior Cruciate Ligament, the Medial and Lateral Collateral Ligaments, and the Anterior Cruciate Ligament, or ACL. This report details what occurs when your ACL tears, either completely or partially. An injury to the ACL or any other ligament is known as a "sprain," or, more commonly, a "tear" when it is most severe.

The ACL runs from the back of the femur, or thighbone, to the front of the tibia, or shinbone, and prevents the tibia from moving too far forward. The most common mechanism of injury to the ACL is a twisting or cutting motion, which stretches or tears the ligament. The injury can happen in one of three "grades":

• Grade 1, in which the ligament is stretched, but not torn.
  
• Grade 2, where the ACL is partially torn.
  
• Grade 3, in which the ACL is completely torn and instability, or looseness of the joint, occurs.

Depending upon the level of activity of the individual, an orthopedic surgeon may recommend operating or pursuing conservative methods to manage the ACL tear. An ACL tear is often accompanied by a tear of the Medial Collateral Ligament or meniscus.