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ACL Injury

Last Updated January 2024 | This article was created by familydoctor.org editorial staff and reviewed by Leisa Bailey, MD

What is an ACL injury?

ACL refers to the anterior cruciate ligament. It is 1 of 4 ligaments in your knee. The other knee ligaments are PCL (posterior cruciate ligament), MCL (medial collateral ligament), and LCL (lateral collateral ligament). The ACL is located behind the kneecap (patella). It stabilizes the knee when it rotates. The ACL and PCL connect your thigh bone (femur) to your shin bone (tibia).

An ACL injury is the overstretching or tearing of the ACL ligament. A torn ACL is a common knee injury. On average, women are 2 to 8 times more at risk of ACL injuries than men. Teenagers also are getting ACL injuries at an increased rate, probably because more kids are involved in organized sports. The increase in ACL injuries is also due to more awareness and advanced testing.

Symptoms of an ACL injury

The primary sign of an ACL injury is a popping noise. This is often combined with pain and swelling. You could experience grinding feelings of your bones or kneecap. Another sign is not being able to put weight on your leg. People with a mild ACL injury might feel like their knee is unstable, or like it might “give out” when they are using it.

What causes an ACL injury?

ACL injuries often occur in sports or active environments. They usually occur without contact rather than with direct contact. Several things can cause the injury, such as:

  • If you are moving and quickly stop or change directions
  • If you are still and make a sudden movement (like a jump or turn)
  • If you overstretch your knee joint
  • If someone hits you in the knee, causing it to move away from the rest of your leg

These things can happen while doing things like skiing, playing soccer or football, or jumping on a trampoline.

When you injure your ACL, it can be a partial or full tear. In a partial tear, only part of the ligament is torn. A full or complete tear happens when the ligament is torn into two pieces.  ACL tears usually happen in the middle of the ligament. Sometimes the ligament is pulled completely off the thigh bone. Other injuries can occur at the same time. These include other torn knee ligaments (primarily the MCL, the ligament that gives your knee stability), a torn meniscus (knee cushion), or bone bruises.

How is an ACL injury diagnosed?

If you think you’ve injured your ACL, seek medical attention. Your doctor will perform a knee exam to check your range of motion and the extent of the injury. They may order an X-ray to look for damage to the bones of your knee. If no bone damage is found, an MRI (magnetic resonance imaging) of your knee could confirm an injury to your ACL.

Can ACL injuries be prevented or avoided?

The leading way to prevent injuries such as an ACL tear is to follow a training routine. A physical therapist, athletic trainer, or other sports medicine specialist can help you develop an exercise program that will help reduce your risk of injury. Such a program will usually include:

  • Exercises that strengthen leg muscles
  • Exercises that strengthen your core (hips, pelvis, and abdomen)
  • Techniques and proper knee position when jumping and landing
  • Techniques for movements that can lead to injury, such as pivoting or cutting (suddenly slowing down and changing direction)

There is no clear evidence that use of a knee brace prevents ACL injuries. There also is no proof that a knee brace helps more during treatment or physical therapy.

ACL injury treatment

When the injury first happens, you can apply first aid to your knee:

  • While lying down, prop up your leg so that it is above the level of your heart.
  • Put ice on it.
  • Use pain relievers, such as ibuprofen (1 brand name: Advil), to reduce pain and swelling.

After this, you should see your family doctor. Your doctor will weigh several factors to decide on a treatment plan. These include the severity of the injury, your age, your physical condition, your medical history, and other injuries or illnesses you may have.

People who are young, active, and healthy typically have surgery. Your family doctor will refer you to an orthopedic surgeon. In surgery, the ACL will be repaired or reconstructed with tissue. This can come from your hamstring or a minor knee tendon. It can also come from a donor. After surgery, you will need intense physical therapy to rebuild strength in your knee and leg. It can take 8 to 12 months of rehabilitation before an athlete who has ACL surgery can return to their sport.

Another treatment option for people who are aren’t as active or healthy is therapy. This is less intense and invasive. Therapy programs can range anywhere from 10-32 weeks. The goal is to increase movement and strength, and allow the ligament to heal naturally.

Living with an ACL injury

Recovering from an ACL injury can be difficult. This is especially true if the injury happened during a daily or regular activity. Keep in mind, your doctor’s goal is to restore your ACL, so treatment is important. Stick to your rehabilitation plan so you heal properly and have the best chance for a full recovery.

There is some evidence that people who have ACL injuries will develop osteoarthritis or degenerative arthritis over time. This outcome is similar regardless of how the injury is treated.

Questions to ask your doctor

  • Why are women and girls more at risk of ACL injuries?
  • How long will I have to do physical therapy?
  • How long after surgery or therapy until I can play sports or be active again?
  • Can I re-injure my ACL? If so, how can I prevent re-injury?
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