ACL Injuries

By Dr. Gregg R. Foos, MD, FACS


The ACL (anterior cruciate ligament) injury to the knee typically occurs in young athletes. The vast majority of people who have ACL injuries are between the ages of 15 and 25. These injuries usually occur with some type of twisting mechanism to the knee itself. Seventy percent of these injuries occur in noncontact fashion.

ACL injuries are very common and have become even more commonplace as so many of our young athletes are playing at a higher level than in the past. Approximately 80,000 of these types of injuries occur per year. Oftentimes, the injuries result in an extended course of care and treatment, and many of these injuries require surgery.

The ACL is a ligament inside the knee joint that connects the femur (upper leg bone) with the tibia (lower leg bone). The cruciate ligaments are the two main stabilizing ligaments inside the knee joint. The ACL prevents the tibia from coming forward on the femur. It is a soft tissue structure so, even though it is torn, it will not appear on an X-ray. The gold standard test for ACL injuries is an MRI.

As mentioned, ACL injuries commonly occur from a twisting type of mechanism. It is very common that they occur when an athlete is trying to decelerate and change direction at the same time. As all the forces come together and the ligament tears, the athlete will feel a popping sensation, and the knee will buckle or give out on them. The knee then becomes stiff, swollen, and painful and may become difficult to walk on.

The usual course of events is that the swelling resolves as the body reabsorbs the fluid, and the knee starts to feel more normal over the next three to four weeks postinjury. People do not necessarily need their ACL to walk and to perform straight ahead activities. The bigger problem is when people attempt to go back to playing sports, and the knee then gives way or buckles on them. These episodes are called pivot shifts, such that if the athlete attempts to turn or pivot quickly, the knee will give out on them, which will lead to swelling and a decrease in function.