Anterior Cruciate Ligament (ACL) Injuries


The anterior cruciate ligament is the smallest of the four main ligaments in the knee. Despite its size, it is the most important of the four in keeping your leg stable when you twist your body. It connects the thighbone (femur) to the largest shinbone (tibia) at the center of your knee.

Without the anterior cruciate ligament, your knee would wobble and move around when you twist your body. When the shinbone and thighbone rotate too far in opposite directions - or when the knee is bent in the wrong direction - the anterior cruciate ligament can be torn or sprained.

Often when the anterior cruciate ligament tears, you will have damage to other ligaments - most often the medial collateral ligament, or the cartilage of the knee.


If you tear your anterior cruciate ligament, you may have the sensation of your knee giving out or buckling. You may even hear a popping sound. Your knee joint will start to swell. This happens because the small blood vessels in the ligament also tear and leak blood into the joint.

The pain resulting from a torn anterior cruciate ligament varies widely. The anterior cruciate ligament itself has no pain receptors. But the movement that causes the ligament to tear often causes damage to other parts of the knee that do have pain receptors. Some people are unable to walk. Others may feel they can play through the injury. Your knee often feels as though it will give way or easily bend backward.

Causes and Risk Factors

Quick changes of direction while running cause most anterior cruciate ligament injuries. When a basketball player running down the court plants his foot hard to change direction, his knee buckles as the thighbone and shinbone move in opposite directions, tearing the anterior cruciate ligament.

Basketball, soccer and skiing often cause anterior cruciate ligament injuries. Football players have the greatest risk for multiple knee injuries like combined anterior cruciate ligament, medial cruciate ligament and cartilage damage.


Your doctor will generally ask you how the injury occurred, whether you've had other knee injuries and how your knee has felt since the injury. You may also be asked about your physical and athletic goals. This will help your doctor decide what treatment might be best for you.

Because the anterior cruciate ligament is deep inside the knee, diagnosing the condition can be challenging. Two common physical tests are used to confirm an anterior cruciate ligament injury and its severity:

  • Lachman Test - The injured leg is held slightly bent and the doctor pulls on the lower leg (tibia). If the leg moves significantly more than the other, uninjured, knee, you may have a torn anterior cruciate ligament. A hand-held instrument called an arthrometer may also be used to measure the stability of the knee.
  • Anterior Drawer Test - With the knee bent 90 degrees, the doctor will pull the lower leg (tibia) forward. If the tibia moves excessively forward, that would strongly suggest a torn anterior cruciate ligament.

If more tests are needed to determine whether your anterior cruciate ligament is torn, a magnetic resonance imaging (MRI) scan will be ordered. An MRI is nearly 90% accurate in determining whether an anterior cruciate ligament has been torn and how badly.

MRIs are not good, however, at providing details about a partial tear. The best way to see a partial tear is arthroscopy. With a camera probe into the knee, a doctor can look at the anterior cruciate ligament to see how badly torn it is and whether reconstructive surgery is needed. Though less painful than some procedures, arthroscopy is not pain-free.


If a torn anterior cruciate ligament is not treated, your knee joint will continued to give way, putting you at risk for more injuries to your knee. Additionally, there is usually cartilage damage that can lead to osteoarthritis.

A torn anterior cruciate ligament is usually treated by one or a combination of the following:

  • Arthroscopic reconstruction.
  • Physical therapy and strengthening. This program takes about four months to six months to complete.
  • Surgery to reconstruct the anterior cruciate ligament.

© 2000-2022 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.
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