Torn ACL (anterior cruciate ligament) in the knee

At a glance

  • Most anterior cruciate ligament (ACL) tears are sports injuries, for instance caused by sudden twists with too much force on the knee joint.
  • The knee usually swells up, hurts and feels unstable.
  • Immediately after the accident, it's important to rest, cool and raise the knee.
  • Surgery is an option, but not always needed – depending on the extent of injury and your personal situation.
  • The "rest and rehabilitation" phase after surgery takes four to twelve months. After conservative treatment, this phase is about two to three months shorter.


Photo of women playing volleyball

The anterior cruciate ligament (ACL) is one of the four large ligaments that stabilize the knee. It is the ligament that is most likely to tear. Tears (also called ruptures) are usually the result of sports injuries – for instance, if you fall when skiing, or twist your knee while playing soccer.

A torn ACL can be treated surgically, but surgery isn’t always needed. This will depend on things like how bad the injury is, how stable your knee is, and your age. It is also important to consider your personal situation when choosing the treatment. This includes factors like what your job involves and the sports you want to continue to practice.

Another ligament in the knee, called the posterior cruciate ligament, can tear too. But that’s much less common and isn’t covered in this article.


When the ACL tears, you often hear a popping sound. The knee usually swells up soon after and hurts, especially when you move it. Sometimes you can still stand on your leg normally after the ACL has torn. But your knee will feel unstable – as if the thigh bone and shinbone have shifted slightly out of place. You might have this feeling when you crouch down, step sideways, or turn on the spot. It often feels like that when you put your entire weight on the leg, too – for example, when climbing stairs.

In about 90% of torn ACL cases, other knee ligaments or cartilage are damaged as well:

  • in 50% to 75%, a meniscus or both of them,
  • in about 50%, the joint cartilage, and
  • in about 15%, the medial or lateral collateral ligament of the knee.

Bones are usually bruised too. These injuries often cause more intense pain and other symptoms.


The thigh bone and shinbone are joined at the knee by four main ligaments: the anterior (front) and posterior (back) cruciate ligaments, and the lateral (outer) and medial (inner) collateral ligaments. The anterior cruciate ligament (ACL) has the following functions:

  • It supports the knee by making sure that the shinbone doesn’t move forward.
  • Together with the posterior cruciate ligament, it prevents the knee from twisting: The cruciate ligaments wrap around each other and stabilize the joint.

The ACL also has special (receptors) that trigger muscle reflexes to stabilize the knee.

Figure: View of the right knee from the front and from the side
Illustration: Cross-section of the right knee, seen from above

The ACL might tear if you put a lot of strain on it. For example, if you

  • twist your knee under a lot of force – perhaps by stopping suddenly and changing direction while running, or bending your leg inwards with a lot of weight on it, or
  • land awkwardly from a jump while your leg is straight.

Risk factors

Most torn ACL injuries happen during sports like soccer, handball, volleyball, basketball and skiing. In ball sports, women are more likely to tear their ACL than men are. There are various possible reasons for this: For instance, women usually have less strength in the muscles at the back of the thigh.


Most ACL tears happen in ball sports. This injury is most common in athletes between the ages of 15 and 45. The longer and more intensely you do the sport, the greater the overall risk of an ACL tear. A review of over 50 studies involving ballplayers showed that

  • 3% to 4% of the women and
  • 2% of the men

had an ACL injury at some point.


If a torn ACL is treated successfully, the knee later functions normally (or almost normally) in about 80% to 90% of people. When the injuries have healed, most people continue to do sports. For fear of another injury, though, about 20% to 30% decide to switch to a different kind of sport – or reduce the intensity of their sport – although that’s often not necessary.

Over the long term, a torn ACL increases the risk of developing osteoarthritis of the knee at a younger age – especially if a meniscus was injured too. A review of studies done over a period of more than 10 years found that osteoarthritis could clearly be seen in x-ray images in about

  • 20% of knees after a previous torn ACL injury, and
  • 5% of knees with no previous torn ACL injury.

The risk of osteoarthritis of the knee doesn’t depend on how the tear was treated.

Visible changes on an x-ray don’t mean that you have symptoms, though. Other studies have shown that x-ray images often don't say much about what symptoms the person has. Plus, even after a torn ACL injury, you can make sure that your knee works well by doing suitable exercises for the knee.


To diagnose a torn ACL, the doctor first asks about the symptoms and how the accident happened. They then examine the knee with their hands and do various examinations. The thigh and/or calf are moved to check how stable the knee is.

If needed, (MRI) provides exact images of the knee, showing the ligaments and meniscus tissue. An x-ray can show whether any bones are broken – but not whether ligaments are injured or torn.


Doing specific exercises before sports activities can help prevent torn ACL injuries. There are special injury-prevention programs involving exercises to warm up, strengthen the core and leg muscles, and improve your balance, coordination and power. Medical societies recommend various prevention programs, including the FIFA 11+ program available through the German Football Association (DFB) and Stop-X developed by the German Knee Society.

These programs were developed for sports with a high risk of torn ACL injuries, like soccer or handball. They are especially worthwhile for very active athletes because the risk of injury in ball sports depends on the amount of training you do and how many competitions you take part in.


If you have a torn ACL, the first thing you should do is rest the knee. Immediately after the accident, it should be elevated (raised) and cooled until the acute pain and swelling improve. A compression bandage and anti-inflammatory painkillers like ibuprofen can be a good idea too. In the first few days after the injury, crutches can take weight off the knee – especially if it’s very unstable.

A torn ACL can be treated in two main ways:

  • Conservative treatment aims to strengthen the knee muscles so they can compensate for the torn ACL. Whether this is an option depends on whether other parts of the knee are injured too – and if so, which ones. After successful conservative treatment, you can usually put a normal amount of strain on the knee again within two to three months. How long it takes to return to sports depends on various factors, including the type of sport.
  • Surgical treatment involves replacing the torn ACL. This procedure is known as ACL reconstruction. The doctor removes a piece of tissue from a tendon in your body and places it in the knee. This "graft" is usually taken from a tendon on the inner side of your thigh muscles, or sometimes from the patellar tendon or the quadriceps tendon. First, the doctor drills a tunnel or socket into the thigh bone and shinbone. The graft tissue is then anchored in place in the tunnel or socket using screws, buttons, or pins. It is attached in a position where it can take over the function of the original ACL as well as possible. Surgery is done using arthroscopy, where small surgical instruments are inserted through several cuts around the knee.

If you have surgery, it is usually done two to four weeks after the accident. Waiting this long allows the swelling and knee to calm down. If it’s still stiff, swollen, or inflamed when surgery is done, the tissue in the knee may harden and later reduce the knee's range of motion. Because of this, certain exercises are done before surgery, to improve the knee’s range of motion and strengthen the muscles.

After surgery, a rest and rehabilitation phase is needed. This takes four to twelve months, depending on the type of sport.

So far, there’s only one larger study that has compared conservative and surgical treatment. It showed that conservative treatment is successful in more than 50% of people over the long term – compared to 80% to 90% of people who have surgery. As far as we know, having conservative treatment first and then surgery later if needed doesn't have any knee function disadvantages. But the risk of further injuries, for example of a meniscus, is slightly higher after conservative treatment.

Medical or personal reasons to have surgery include the following:

  • Wanting to continue doing intense sports after the treatment, such as competitive sports or sports that involve sudden changes in direction, turning movements, or rapid stopping.
  • A more major injury to a meniscus, especially if it causes problems like a locked knee.
  • If one of the collateral ligaments in the knee (medial or lateral) is torn too. Then the knee is usually so unstable that conservative treatment is not an option.
  • Doing a job that puts a lot of strain on the knees – for example, a job that involves climbing a lot of stairs while carrying heavy objects, perhaps when delivering parcels or moving furniture.

Deciding whether or not to have surgery also comes down to personal preference. Some people very much want to avoid surgery and try to do without it. Others want to avoid the risk of needing surgery later, after conservative treatment, and would rather have surgery right away.


After surgery, it takes months for the new ACL to fully heal and be able to handle its new job. To reduce the strain on the new ACL and stabilize the knee, it is important to have strong thigh muscles. The goal of rehabilitation is to strengthen these muscles and help the knee get used to the new ACL. It takes a while to get a feel for your knee again and be able to move it as naturally as before the injury.

If the ACL is not treated surgically, special rehabilitation exercises help to strengthen the muscles and make sure they stabilize the knee. Physical therapy is recommended in all treatment approaches.

It is generally possible to start doing gentle sports four to six months after surgery. But most people have to wait for nine to twelve months before doing things like intensive ball sports or martial arts again. The recovery time for conservative treatment is two to three months shorter.

What exactly the rehabilitation involves and how long it takes depends on various factors, including

  • your physical condition (for example, how fit you are),
  • the rehabilitation goals (for example, if you want to keep doing competitive sports),
  • the extent of injury, and
  • the type of graft tissue used (if you had surgery).

It is sometimes hard to accept that it will take so much time and effort to get back to your previous level of performance. Because of this, the success of rehabilitation not only depends on doing your exercises regularly: It is important to be patient too.

Everyday life

If you're planning to have surgery, it helps to already make preparations for the rehabilitation phase after surgery. For example, you could

  • practice using crutches,
  • get rid of obstacles and tripping hazards in your home – for instance, you might consider sleeping in another room to avoid climbing stairs,
  • figure out how you will get to work, school, or university because you probably won't be able to drive a car for several weeks,
  • organize help in everyday life – for example, with shopping or other errands,
  • get a shower stool so you can sit down while showering.

Using crutches a lot can cause other problems, like muscle ache and tension in the back, neck, or shoulders. This can also be prevented by practicing before surgery.

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IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Created on February 21, 2023

Next planned update: 2026


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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