Anterior Cruciate Ligament (ACL) Injuries

Condition Basics

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 bone. 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 (cartilage) that covers the ends of the bones. And it can trap and tear the pads (menisci) that cushion the knee joints. This damage can lead to osteoarthritis.

What causes it?

You can tear your ACL when you plant your foot and then push off or change direction. This can happen while playing sports. You can also tear it from getting hit around your knee during contact sports. Injuries like those from a car crash or jumping from a height can also cause a tear.

What are the symptoms?

When you injure your ACL, you feel pain on the outside and back of the knee. You may feel a pop in the knee when you injure it. The knee swells, often within the first few hours after the injury. The knee may feel unstable, and you may have trouble moving it.

How is it diagnosed?

To diagnose an ACL injury, the doctor will do a physical exam and ask questions about how you injured your knee. The doctor will check your knee stability, strength, range of movement, swelling, and tenderness. You may need X-rays or an MRI.

How is an ACL injury treated?

Treatment for an ACL injury includes using first aid right away. For example, put ice on the knee, prop up the leg, and use over-the-counter pain medicines. You may need to use crutches or a knee immobilizer. You will have exercises and training (rehab) or surgery. If you have surgery, you'll need rehab afterward.

How can you help prevent ACL injuries?

One way to help prevent ACL injuries is to stretch and strengthen the muscles in the legs and body. There are also training programs that teach movements that may prevent injury and help with balance. Warming up before training or competing may also help. Some ACL injuries can happen anyway.

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You can tear your ACL when you plant your foot and then push off, change direction, or pivot. This can happen during certain sports, like soccer or basketball.

You can also tear it from getting hit in your leg or knee during a contact sport like football or in high-speed sports like skiing.

Injuries like those from a car crash, stepping in a hole, or jumping or falling from a height can also cause an ACL tear.

What Increases Your Risk

Things that increase your risk of ACL injuries include:

  • Playing sports that involve sudden changes in direction or cutting around other players or obstacles, such as skiing, football, soccer, basketball, baseball, and tennis.
  • Having torn your ACL in the past.
  • Being female. Females have a higher risk of noncontact ACL tears. These are injuries that did not involve a blow to the knee. For example, if you change direction quickly while your foot is planted.
  • Having unbalanced leg muscle strength, such as if the muscles in the front of your thigh (quadriceps) are stronger than the muscles at the back of your thigh (hamstrings).


Many ACL injuries occur during sports. Some can't be avoided, but you may be able to help prevent an ACL injury. These tips can help everyone. But females, who are at a higher risk for noncontact ACL injuries, may find the tips especially useful.

  • Warm up.

    Always warm up before training or competition. Ways to warm up include jogging easily or riding a stationary bicycle for 5 to 10 minutes. Warming up your muscles reduces the risk of injury.

  • Stretch.

    Make stretching part of your warm-up before the activity and your cool-down after the activity. Stretching can help you keep and improve your range of motion and reduce stiffness in your joints. It may also reduce soreness after exercise and reduce the risk of injury. Examples are a quad stretch and hamstring stretch that are done while lying down.

  • Try a neuromuscular training program.

    These programs help strengthen muscles in the legs and the body. They also help teach movements that may prevent injury and also help with balance. Your doctor or physical therapist can recommend a training program.

  • Improve agility.

    Turning and pivoting while in a very straight or erect position may strain the ACL. Learning to crouch and bend at the knees and hips when turning may reduce the stress on the ACL. Agility exercises include running forward and backward and running in diagonals. (You run diagonally to one spot, then cut the other way and run to another.)

  • Work on muscle strength.

    The muscles in the back (hamstrings) and front (quadriceps) of the thighs work together to bend or straighten the leg. Using the quadriceps when changing direction quickly may result in an ACL injury. You can stretch and strengthen the quadriceps and hamstrings to help reduce the risk.


Symptoms of a severe and sudden (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.

What Happens

You'll most likely know it when you have an ACL injury. You may feel or hear a pop. The knee may give out, causing you to fall. The knee swells and often is too painful or unstable for you to keep doing any activity.

An ACL injury can cause partial tears of the ligament or a complete tear (rupture). It can also cause the ligament to separate from the upper or lower leg bone (avulsion). Or it can cause the ligament and part of the bone to separate from the rest of the bone (avulsion fracture). When any of these occur, the lower leg bone moves abnormally forward on the upper bone, with a sense of the knee giving out or buckling.

An ACL injury can lead to long-term knee pain and instability. This is more likely to happen if the injury isn't treated. Treatment to strengthen muscles that support the knee may help.

When to Call a Doctor

Call your doctor now if:

  • You have severe pain in your knee.
  • Your knee appears to be deformed.
  • You have signs of damage to the nerves or blood vessels. Signs include numbness, tingling, a "pins-and-needles" sensation below the injury, an inability to move your leg below the injury, pale or bluish skin, or your leg feels cold.
  • You have severe swelling in your knee right after the injury.

Call your doctor today if:

  • Your knee starts to swell within 2 hours of the injury.
  • You hear or feel a pop in your knee during an injury.
  • Your knee won't bear weight.
  • You can't straighten your leg completely.
  • Your knee is unstable, buckles, or gives out.
  • Your knee "locks" in one position.
  • You've had an anterior cruciate ligament (ACL) injury in the past, and you have reinjured your knee.

Before your appointment, don't put weight on the injured knee. Use crutches if you need to. Apply ice and wrap your knee in an elastic bandage or neoprene (synthetic rubber) sleeve. Rest and elevate the knee. Take a nonsteroidal anti-inflammatory drug, such as ibuprofen (Advil) or naproxen (Aleve), to reduce swelling.

Watchful waiting

Watchful waiting is a wait-and-see approach. It may be okay if you have mild symptoms.

Serious knee injuries need to be checked for possible broken bones as well as ligament or cartilage damage. Whenever immediate swelling follows an injury, there also may be torn blood vessels or damaged nerves in the knee. Your doctor will check your knee to make sure the blood supply to your leg is normal and the nerves are intact.

If you have pain in your knee now and then, or if your knee sometimes gives way or buckles, have your doctor check it. If you have damaged your ACL, it's important to get treatment. This may reduce the chance that you will get osteoarthritis in your knee.

Exams and Tests

To diagnose an ACL injury, your doctor will ask you to describe how you injured your knee and what you felt. The doctor will check your knee for swelling or tenderness. They may gently push and pull on your leg to see if the knee joint moves in an abnormal way. The exam is usually done on both legs so the doctor can compare one leg to the other to see what's normal for you.

You may have an X-ray to help make sure there isn't a different injury, like a broken bone. Ligaments can't be seen on an X-ray. An MRI is an imaging test that can help show the ACL. It can help your doctor see if you have an ACL tear. Often an injury that causes an ACL tear also injures other ligaments or the cartilage called the meniscus. An MRI can help your doctor diagnose these other injuries.

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Treatment Overview

Treatment for an ACL injury includes:

  • First aid right away. This is to help take care of symptoms like pain and swelling.
    • Put ice or a cold pack on your knee for 10 to 20 minutes several times a day. Put a thin cloth between the ice and your skin.
    • Elevate your injured knee when possible.
    • Use crutches and a knee immobilizer.
    • Take over-the-counter pain medicine, such as acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs). Be safe with medicines. Read and follow all instructions on the label.
  • Nonsurgical treatment. This is also called rehab. It can include controlling swelling, increasing knee mobility, strengthening the muscles, and working on balance. If you're returning to a sport or intense activity, you may also work on techniques to reduce the risk of injury. 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.

You don't have to decide about surgery right away. Usually, your doctor would not do surgery for at least several weeks after your injury. This allows time for the swelling in your knee to go down. It also gives you time to strengthen the muscles around the knee and make sure you can move your knee well.

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If you have an acute (sudden) anterior cruciate ligament (ACL) injury, use the following first aid steps to reduce pain and swelling.

  • Rest and reduce your activity level.
  • If it hurts to put weight on your knee, use crutches.

    Use them until you can see your doctor. Crutches can be rented from most drugstores.

  • Ice your knee.

    Put ice or a cold pack on your knee for 10 to 20 minutes at a time. Put a thin cloth between the ice and your skin.

  • Elevate your knee.

    Do this while applying ice or anytime you are sitting or lying down. Try to keep your knee above the level of your heart.

  • Wrap your knee.

    Use an elastic bandage or neoprene sleeve (available at a drugstore).

    Don't wrap your knee too tightly, as this may cause swelling below the bandage. Loosen the bandage if it is too tight. Signs of an overly tight bandage include numbness, tingling, increased pain, and coolness in the foot.

  • Use nonprescription medicine to reduce pain.

    Use acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs). Be safe with medicines. Read and follow all instructions on the label.

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Surgery for an ACL tear can help you stabilize your knee and return to activity. Surgery may include:

  • Reconstruction surgery. The surgeon replaces the ACL with tissue called a graft. Usually an autograft is used. For an autograft, the surgeon uses tendon tissue from your own body. This can be done safely. For an allograft, the tendon tissue is from a deceased donor.
  • When the damaged ACL pulls a piece of bone off, it is called an avulsion fracture. The surgeon may reconnect the bone fragment to the area it was pulled from.

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Current as of: July 17, 2023

Author: Healthwise Staff
Clinical Review Board
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