What Is An Anterior Cruciate Ligament (ACL) Injury, How Is It Caused And What Are The Symptoms?

Question: What is an anterior cruciate ligament (ACL) injury, how is it caused and what are the symptoms?

Answer: The anterior cruciate ligament, or ACL, is probably the most important or key ligament in our knee. It really is the crucial ligament of our knee, acting as a main hinge joint and providing stability in our knee for activity, such as changing direction, stopping, starting, landing from a jump, etc.

It's torn or injured most commonly in a traumatic fashion, either with contact or without contact. In other words, if an opponent, a player falls into your knee from the front or the side, you can tear the ligament strictly by the force of the contact or the force of the injury.

It can also be injured in a non-contact fashion with sudden deceleration or changes in direction, such as pivoting while playing basketball or soccer, landing from a jump a little bit off balance.

The muscles and the ligaments are there to prevent our knee from buckling or slipping out of joint. When the muscles are caught off guard or surprised or perhaps fatigued, the ligament is exposed to all that stress and will rupture. What you'll oftentimes hear an athlete tell you is they felt a pop or heard a pop in their knee, sudden pain followed by swelling over the next few minutes, where the knee -- surely by an hour afterwards -- the knee has ballooned up. And most of that is blood from the torn ligament in the knee.

The athlete will subsequently, once the swelling goes down, slowly regain their motion and use of the knee over the next two to three weeks. Then what they'll notice if they have not been diagnosed with an ACL tear but the ligament is torn, they'll notice that the knee is unstable. When they try to suddenly or quickly change directions, when they land from a jump, they'll feel that knee buckle or slip out of joint. We call that the pivot shift phenomenon, because the two joints in their knees will pivot one on the other in an abnormal fashion, and oftentimes the athlete or patient will fall if they're involved in some vigorous activity, indicating that the ACL is likely torn.

With either of those injuries, it's important to make the diagnosis, in other words, see your health care professional, your sports medicine specialist. The best test besides the physical examination is an MRI -- magnetic resonance imaging -- which is very helpful in confirming the diagnosis of a torn ACL. The reason you want to make the diagnosis early on is that with recurrent giving away episodes of the knee, if you try to return to your sport or activity with a torn ACL, you can suffer or sustain other damage in the knee, such as torn or cracks in your cartilage, and some of these can be permanent injuries that can affect your knee throughout your life.

Sherwin Ho, M.D., The University of Chicago Medical Center
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