Anterior Cruciate Ligament (ACL) Injury & Tear

The anterior cruciate ligament (ACL) attaches from the anterior tibia (shin bone) to the femoral (thigh bone) intercondylar notch. As seen on the diagram, it lies just anterior to (in front of) the posterior cruciate ligament (PCL). Both ligaments are named based on their insertion site on the tibia. On the femoral side, it attaches to the posteromedial aspect of the lateral femoral condyle. The average length is 33 mm (1.3 inches) and the width is 11 mm (0.43 inches). The anatomy and footprint of the ligament are very important in reconstruction (surgery for an ACL tear).

The primary function of the ACL is to limit anterior translation (forward sliding) of the tibia on the femur in the knee joint. It secondarily aids in limiting the pivot of the knee joint. It has two bundles: an anteromedial (AM) and posterolateral (PL) bundle. The anteromedial bundle is tight in flexion (knee bending) and the posterolateral bundle is tight in extension (knee straightening). Recent evidence has shown that the AM bundle may play a large role in anterior-posterior translation (front to back motion) of the knee, and the PL bundle in rotational stability.

How Common are ACL Injuries?

Anterior cruciate ligament (ACL) injuries are among the most common sports injuries in America. Close to 200,000 reconstructions are performed annually.

What are Some Signs and Symptoms of an ACL Tear?

An ACL injury typically occurs during a twisting injury at the knee. It often occurs in competitive sports, such as from a tackle during football, but it does not always have to include a direct blow to the knee. Sometimes it can occur with a misstep during pivoting.

You may often hear a “pop” and have knee swelling. Most of the time this accompanies a great deal of pain where you may be unable to place weight on the injured leg. This can be followed by the inability to fully flex (bend) or extend (straighten) the knee.

Some other things that could mimic an ACL injury could be a fracture (broken bone), dislocation, meniscus tear, cartilage injury, or another ligament injury.

If this type of injury occurs, you should seek immediate medical attention for appropriate diagnosis and treatment.

How Can You Diagnose an ACL Tear?

ACL tears are often diagnosed based on history and physical exam. During a physical exam, the Lachman or Anterior Drawer test is performed. If your tibia slides anteriorly during these tests more than 1 cm without a solid endpoint, then the test is positive and it’s most likely an ACL tear. Other physical exam tests include a knee arthrometer which can measure the side-to-side difference in the anterior translation of the tibia.

X-rays are not able to diagnose an ACL tear except for a “Segond fracture” which is the avulsion of the anterolateral ligament. This may appear as a fleck of bone on the lateral (outside aspect) of your knee. The anterolateral ligament is an additional injured ligament which often signifies an ACL tear.

Ultimately, an MRI (magnetic resonance imaging) study of your knee can make a definitive diagnosis. This can be very helpful as a surgical plan, as it will also show any other injury that occurred to your knee involving the meniscus, cartilage, or other ligaments. Be sure to disclose to your physician if you have any metal implants, a pacemaker, or any fear of enclosed places, as this can affect your ability to get an MRI.

How Do You Treat an ACL Tear?

Initial treatment should include rest, ice, compression, and elevation. Try to keep it elevated above the level of your heart. Be sure not to put ice directly on your skin – and place it over a protective plastic or cloth. Anti-inflammatory medications such as ibuprofen and naproxen can be initiated if it is safe for you to take these drugs and you do not have any impaired kidney function. Tylenol should be taken as well if your liver function is normal.

An x-ray should be obtained if you are unable to place weight on your knee due to pain. This is to make sure there is no dislocation, and no bones are fractured/ broken. Try to immobilize your knee and keep it protected until you seek medical attention.

When you get to your provider, then you will most likely get more information about your injury and the diagnosis. This will help guide you decide whether or not to have surgery for your injury.

Learn more about ACL reconstruction surgery here.