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Anterior Cruciate Ligament (ACL) Injuries


Topic Overview

Picture of the bones and ligaments of the knee

What is an anterior cruciate ligament (ACL) injury?

An anterior cruciate ligament, or ACL, injury is a tear in one of the knee ligaments that joins the upper leg bone with the lower leg boneClick here to see an illustration.. The ACL keeps the knee stable.

Injuries range from mild, such as a small tear, to severe, such as when the ligament tears completely or when the ligament and part of the bone separate from the rest of the bone.

Without treatment, the injured ACL is less able to control knee movement, and the bones are more likely to rub against each other. This is called chronic ACL deficiency. The abnormal bone movement can also damage the tissue (cartilageClick here to see an illustration.) that covers the ends of the bones and can trap and tear the pads (menisciClick here to see an illustration.) that cushion the knee joints. This damage can lead to osteoarthritis.

Sometimes other knee ligaments or parts of the knee are also injured. This includes cartilage such as the menisci, or bones in the knee joint, which can be broken.

What causes an ACL injury?

Your ACL can be injured if your knee joint is bent backward, twisted, or bent side to side. The chance of injury is higher if more than one of these movements occurs at the same time. Contact (being hit by another person or object) also can cause an ACL injury.

An ACL injury often occurs during sports. The injury can happen when your foot is firmly planted on the ground and a sudden force hits your knee while your leg is straight or slightly bent. This can happen when you are changing direction rapidly, slowing down when running, or landing from a jump. This type of injury is common in soccer, skiing, football, and other sports with lots of stop-and-go movements, jumping, or weaving. Falling off a ladder or missing a step on a staircase are other likely causes. Like any other body part, the ACL becomes weaker with age. So a tear happens more easily in people older than age 40.

What are the symptoms?

Symptoms of an acute ACL injury include:

  • Feeling or hearing a pop in the knee at the time of injury.
  • Pain on the outside and back of the knee.
  • The knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the knee joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
  • Limited knee movement because of pain or swelling or both.
  • The knee feeling unstable, buckling, or giving out.

After an acute injury, you will probably have to stop whatever you are doing because of the pain, but you may be able to walk.

The main symptom of chronic ACL deficiency is the knee buckling or giving out, sometimes with pain and swelling. This can happen when an ACL injury is not treated.

How is an ACL injury diagnosed?

Your doctor can tell whether you have an ACL injury by asking questions about your past health and examining your knee. The doctor may ask: How did you injure your knee? Have you had any other knee injuries? Your doctor will check for stability, movement, and tenderness in both the injured and uninjured knee.

You may need X-rays, which can show damage to the knee bones. Or you may need other imaging tests, such as an MRI. An MRI can show damage to ligaments, tendons, muscles, and knee cartilage. Arthroscopy may also be done. During arthroscopy, your doctor inserts surgical tools through one or more small cuts (incisions) in the knee to look at the inside of the knee.

How is it treated?

Start first aid right away. These first-aid tips will reduce swelling and pain. Use the RICE method. The letters stand for Rest the knee, put Ice on it, use an elastic bandage to give gentle Compression to the knee, and Elevate the leg by propping it up above the level of your heart. And at first it's also important to move your leg as little as possible. Take over-the-counter pain medicine.

You may need to walk with crutches and use a knee immobilizer to keep your knee still for the first few days after the injury.

Your knee will need to be checked by your doctor. It's important to get treatment. If you don't, the injury may become a long-lasting problem. There are two ways to treat the injury:

  • Exercises and training, also called rehab. It takes several months of rehab for your knee to get better.
  • Surgery. You and your doctor can decide if rehab is enough or if surgery is right for you.

If you have surgery, you will also have several months of rehab afterward.

Your treatment will depend on how much of the ACL is torn, whether other parts of the knee are injured, how active you are, your age, your overall health, and how long ago the injury occurred.

There are three main treatment goals:

  • Make the knee stable if it is unsteady, or at least make it stable enough to do your daily activities.
  • Make your knee strong enough to do all the activities you used to do.
  • Reduce the chance that your knee will be damaged more.

How can you prevent ACL injuries?

The best way to prevent ACL injuries is to stretch and strengthen the leg muscles, especially the front and back muscles of the thigh (quadriceps and hamstrings).

Here are other things you can do that may help prevent ACL injuries:

  • Avoid wearing shoes with cleats in contact sports.
  • Avoid wearing high-heeled shoes.
  • Avoid sports that involve lots of twisting and contact.

Frequently Asked Questions

Learning about anterior cruciate ligament (ACL) injuries:

Being diagnosed:

Getting treatment:

Living with an ACL injury:

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