Knee surgery, anterior cruciate ligament

Page last reviewed: 13/07/2011

The anterior cruciate ligament (ACL) is the most commonly injured ligament of the knee. Every year, there are around 30 cases of ACL injuries for every 100,000 people. If the ACL is torn, surgery is commonly used to reconstruct the damaged ligament. 


Ligaments are tough bands of connective tissue (fibres that support other tissues and organs in your body). Ligaments link two bones together at a joint. They strengthen the joint and limit its movement in certain directions.

The knee

There are three bones that meet in the knee:

  • thigh bone (femur),
  • shin bone (tibia), and
  • knee cap (patella).

The three bones are connected by four ligaments. The ligaments hold the bones together and keep the knee stable. There are two collateral ligaments on the sides of the knee, and two cruciate ligaments inside the knee.

The anterior cruciate ligament

The ACL runs diagonally through the inside of the knee. It connects the underside of the femur to the top of the tibia. The ACL prevents the tibia from moving in front of the femur and gives the knee joint stability. It also helps control the back and forth movement of the knee.

Most injuries to the ACL happen when playing sports, such as football, squash, tennis or skiing (see the box, left). ACL injuries account for 40% of all sports injuries.

If the ACL is torn, the knee becomes very unstable and loses its full range of movement. This can make it difficult to perform certain movements, such as turning on the spot, and could make some sports impossible to play.

Reconstructive surgery

It is not possible to repair the torn ACL by stitching it back together. However, it can be reconstructed by grafting (attaching) new tissue onto it. This will encourage new ligament to grow over the grafted tissue. Once new ligament has grown, the knee will be more stable.

Reconstructive surgery is successful in around 90% of cases. A few people may still have knee pain or instability after the operation. Recovery usually takes around six months, although it could be up to a year before athletes can fully return to their sport.  

How the anterior cruciate ligament is torn

The ACL can be torn when the lower leg extends forwards too much. If the knee is straightened more than 10 degrees beyond its normal straight position, this can cause the ACL to tear. The ligament can also be torn when the knee and lower leg are twisted.

Some of the more common causes of an ACL injury include:

  • landing incorrectly from a jump,
  • suddenly stopping,
  • suddenly changing direction, or
  • having a collision, such as during a football tackle.

See the Health A-Z topic on sports injuries for more information about other common sports injuries.

Page last reviewed: 13/07/2011

Depending on how serious the injury is to your anterior cruciate ligament (ACL), surgery may be necessary to restore stability to your knee and enable you to return to sports.


When you tear your ACL, you may experience the following symptoms:

  • You may hear a popping sound or a crack when the ligament tears.
  • You may feel your knee give out from under you.
  • It is likely to be very painful.
  • Your knee will swell up within 24 hours, usually straight away.
  • Your knee will feel unstable and you will be unable to put much weight on it, causing you to limp.
  • You will lose your full range of movement in the knee. In particular, you may not be able to straighten your leg.

In around half of all ACL injuries, other parts of the knee are also damaged. This could be another ligament in the knee or the menisci or articular cartilage. The menisci are cartilage discs that act as shock absorbers. Articular cartilage is a smooth protective layer that allows the bones to glide smoothly over each other.

Partial tears

If you have a partial tear to your ACL, the ligament is stretched so that it becomes loose. Surgery is not always necessary and you may find that physical therapy will help the tear to heal. However, in some cases, you will be left with an unstable knee.

Complete tears

If you have a complete tear of your ACL, the ligament has been split in two and the knee joint is very unstable. Complete tears of the ACL are unlikely to get better without surgery.

You may find that you are unable to take part in certain sports, particularly anything that requires you to change direction or stop suddenly. Some people find that their knee is unstable even during day-to-day activities, such as walking.

When surgery is necessary

Your decision to have knee surgery will depend on how damaged your ligament is and whether or not it affects your quality of life. If you do not feel unstable and you do not have an active lifestyle, you may decide against surgery.

However, delaying surgery could result in further damage to your knee. One study of people with ACL tears found that the chance of having further damage in their injured knee increased by 1% every month between the ACL tear occurring and surgery.

The decision to undergo surgery will be based on some of the following considerations:

  • Your age: an older person who is not very active may be less likely to need surgery.
  • Your lifestyle: for example, whether you will be able to follow the rehabilitation programme after surgery.
  • How often you play sports: surgery may be necessary if you regularly play sports.
  • Your occupation: for example, if you do any form of manual labour.
  • How unstable your knee is: if your knee is very unstable, there is an increased risk of further damage if you do not have surgery.  
  • Any other injuries: for example, if your menisci are also torn, they may heal better when repaired at the same time as ACL reconstruction.

Children can have this reconstructive surgery if necessary. However, as they are still growing, the procedure is likely to be modified to ensure that the growth areas are not affected. It is also a trickier operation and may need to be carried out by a surgeon with a special interest in child injuries.


Ligaments are the fibrous connective tissues that link two bones together at a joint.

Connective tissue
Connective tissues provide support and structure to other tissue and organs.


How is a torn ACL diagnosed?

Your GP should be able to diagnose a torn ACL from a physical examination. They will evaluate the stability of your injured knee and see how much increased movement it has compared to your uninjured knee. Some of the tests they may use are described below.

The Lachman test

During the Lachman test, you will be asked to lie on your back and your injured knee will be held at a 20-30° angle. Your GP will support your thigh while applying pressure to your tibia (shin bone). The amount that your tibia moves and how firm your ligament feels will help determine how much your ACL is torn.

The pivot shift test

The pivot shift test aims to recreate the pressure on your knee that caused your ACL to tear. You start with your leg in a straight position and it is then gradually bent to around 40°. At the same time, the knee joint is twisted inwards. The amount that your tibia moves forwards reveals how much your ACL is torn.  

The KT-1000 test

The KT-1000 is an instrument that looks like a rectangular box. It is strapped to your lower leg, just under your injured knee. It applies force to your leg and the amount of forward and backward movement in your injured knee is measured. 

Imaging tests

Although imaging is not necessary to diagnose a torn ACL, these tests may be used to provide more information about the damage. Two types of imaging test may be used:

  • An X-ray is an imaging technique that uses high-energy radiation to show up abnormalities in bones and certain body tissue. This will reveal if you have any broken bones in your knee.
  • magnetic resonance imaging (MRI) scan uses a strong magnetic field and radio waves to produce detailed images of the inside of your body. An MRI scan may be used to provide an image of your ACL and check for other damage.


Around 50% of people with ACL injuries will develop arthritis in their knee after around 10-15 years. This is usually because of damage to the menisci or articular cartilage. There is no consistent evidence to suggest that ACL surgery will prevent arthritis from developing.

Page last reviewed: 13/07/2011

You cannot have surgery immediately after the injury that caused your anterior cruciate ligament (ACL) to become torn. In most cases, you will need to wait until:

  • any swelling has gone down,
  • the full range of movement has returned to your knee, and
  • the muscles at the front of your thigh (the quadriceps) and back of your thigh (the hamstrings) are as strong as possible.

If you do not have the full range of movement in your knee before having surgery, you will find it harder to recover after the operation. This is likely to be at least three weeks after the injury occurred. To help you prepare for surgery, you may be referred to a physiotherapist. 


Physiotherapists are specially trained healthcare professionals who can help you regain the full range of movement in your knee.

Your physiotherapist may show you some stretches that you can practice at home to keep your leg flexible. They may also recommend low-impact exercises, such as swimming or cycling. These will help improve your muscle strength without placing too much weight on your knee.

Avoid any sport or activity that involves twisting, turning or jumping.

Pre-admission clinic

Before having ACL reconstruction, you will be assessed by a member of the team who will be looking after you in hospital.

At the clinic, you will have a physical examination and be asked for details of your medical history. Any investigations and tests that you need to have will be arranged, such as an X-ray of your knee. This is a good time to ask questions about the procedure, although you can discuss your concerns with your surgeon at any time.

You will be asked if you are taking any tablets or other types of medication. This includes medication prescribed by your GP or bought over-the-counter (OTC) in a pharmacy. It will help if you can give details of any medication that you are taking, for example by bringing the packaging with you.

You will be asked about any previous anaesthetics (painkilling medication) that you have had, and whether you have experienced any problems or side effects, such as nausea. You will be also asked whether you are allergic to anything to avoid an allergic reaction to medication you may need during your treatment.

You will be asked about your teeth, including whether you wear dentures, have caps or a plate. This is because, during the operation, a tube may be put down your throat to help you breathe and loose teeth could be dangerous.

Preparing for hospital

Before you go into hospital for your operation, it is a good idea to be prepared. 

  • Do your homework. Find out as much as you can about what your operation involves. Your hospital may provide written information or have a video of the procedure. You can also ask your surgeon about the operation.  
  • Ask your GP to check that any other medical problems you have are under control, such as high blood pressure (hypertension).
  • Arrange transport. Ask someone, such as a friend or relative, to take you to and from the hospital or book a taxi.
  • Prepare your home. Before going into hospital, put items such as your television remote control, radio, telephone, medications, tissues, address book and a glass on a table next to where you will spend most of your time when you return home.
  • Stock up. Buy food that is easy to prepare, such as frozen ready meals, tinned foods and staples such as rice and pasta. You could also prepare dishes and freeze them so that you can use them during your recovery.
  • Clean up. Before you go into hospital, take a bath or shower, wash your hair and cut your nails. Put on freshly washed clothes. This will prevent you taking unwanted bacteria into hospital with you, which could complicate your care.
  • Arrange help. Ask a friend or relative to help you at home for a week or two after you come out of hospital.
  • Check the hospital's advice about taking the pill or hormone replacement therapy (HRT). If you are advised to carry on taking these, make sure that you have some with you at the hospital and at home.
  • Check whether you can eat anything before your operation. Most anaesthetics are safer if your stomach is empty, so you will normally have to stop eating several hours before your operation. This should be made clear during your pre-admission checks. 


Ligaments are the fibrous connective tissues that link two bones together at a joint.

Connective tissue
Connective tissues provide support and structure to other tissue and organs.

An X-ray is an imaging technique that uses high-energy radiation to show up abnormalities in bones and certain body tissue, such as breast tissue.

What should I take into hospital?

  • A change of nightclothes. 
  • Something comfortable to wear.
  • A pair of slippers.
  • A dressing gown.
  • Personal toiletries, such as your toothbrush and flannel.
  • Any medications that you are taking, including details about any treatment you are currently receiving. 
  • Any equipment that you use, such as a walking stick or hearing aid.
  • Books, iPod, stationery, jigsaws, crossword books and other items to help pass the time during your recovery.
  • Your address book and important phone numbers.
  • Some money so that you can use the telephone (you can take your mobile phone, but you may not be able to use it on the ward).
  • Hospitals can now accommodate most types of diet, including religious requirements, although you may wish to take a few healthy snacks with you for between meals.

Page last reviewed: 13/07/2011

A surgeon can use a number of methods when performing an anterior cruciate ligament (ACL) reconstruction. The most common is to use a tendon, or tendons, from elsewhere in your body to replace the ACL. Ask your surgeon if you want to know more about your procedure.

Initial examination

While you are anaesthetised, your surgeon will carefully examine your whole knee before deciding how to proceed. They will check that your ACL is torn and look for any damage to other parts of your knee.

If there is any other damage, it may be repaired during the surgery. Alternatively, it may be treated after your operation.

Once your surgeon has confirmed that your ACL is torn, the graft tissue will be harvested (removed).

Graft tissue

A number of possible tissues could be used to replace your ACL. Before your operation, your surgeon will discuss the best option with you. Possibilities include: 

  • A strip of your patellar tendon. This is the tendon that runs from the bottom of the kneecap (the patella) to the top of the shin bone (the tibia) at the front of your knee.
  • Part of your hamstring tendons. These tendons run from the back of your knee on the inner side all the way up to the thigh.
  • Part of your quadriceps tendon. This is the tendon that attaches your patella to the quadriceps muscle (the large muscle on the front of your thigh).
  • An allograft (donor tissue). This could be the patellar tendon or the Achilles tendon (the tendon that attaches the back of the heel to the calf muscle) from a donor.

The patellar tendon and hamstring tendons are most commonly used as graft tissues. They have been found to be equally successful. Allograft tissue may be used if you will not be playing any high-demand sports, such as basketball or football, as these tendons are slightly weaker.

Synthetic (manmade) ligaments are not currently used. However, this may change in the future as one recent small trial has shown promising results for their use. 

The graft tissue will be removed and cut to the correct size. This will be placed in the correct position in the knee and fixed to the femur (thigh bone) and tibia (shin bone). This is usually done using a technique called arthroscopy.

Arthroscopic surgery

Arthroscopy is a form of keyhole surgery. It uses an arthroscope, which is a medical instrument shaped like a thin, flexible tube. Inside the arthroscope are bundles of fibre optic cables that act as both a light source and a camera.

Your surgeon will make a small incision (cut) on the front of your knee and insert the arthroscope. The arthroscope will illuminate your knee joint, while at the same time relaying magnified images of your knee to a video screen.

More small incisions (cuts) will be made in your knee so that other medical instruments can be inserted. These will be used to do the repair and reconstruction work.

Usually, the frayed edges of the torn ACL will be trimmed and the graft will be placed in the same position as the torn ACL. The graft will be fixed in place using screws or staples, which will remain in your knee permanently.

Final examination

Once the graft is in place, your surgeon will test that there is enough tension in it (that it is strong enough to hold your knee together). They will also check that your knee has the full range of motion and that the graft keeps your knee stable when it is bent or moved.

Once satisfied, your surgeon will close the incisions and apply dressings. You will be moved to a hospital ward to begin your recovery.


Ligaments are the fibrous connective tissues that link two bones together at a joint.

Connective tissue
Connective tissues provide support and structure to other tissue and organs.

An incision is a cut made in the body with a surgical instrument during an operation.

A tendon is a white fibrous cord that joins bones to muscle. It allows the bone to move when the muscle contracts.

Will I need anaesthetic?

You will be given either:

  • general anaesthetic to put you into a controlled state of unconsciousness, or
  • a spinal anaesthetic, where a regional anaesthetic is injected into your spine so that you are conscious but cannot feel any pain.

How long will the operation take?

The operation usually takes between 1 and 1½ hours.

How long will I be in hospital?

Most people only need to stay in hospital overnight. 

Page last reviewed: 13/07/2011

Most people who have anterior cruciate ligament (ACL) reconstruction have a good outcome with no complications. The surgery is successful in 82-95% of cases.

This does not mean that your knee will be exactly the same as it was before your injury, but you should be able to resume your normal activities after six months.

The surgery is carried out to improve the stability of your knee and to stop it 'giving way'. Following the procedure, you may still have pain and swelling because the ligament can never be completely replaced or repaired. You may have also damaged other structures in your knee that cannot be made as good as new.

Possible complications

As with all types of surgery, there are possible complications.

  • Infection. The risk of infection is less than 1%. In most cases, you will be given a prophylactic (preventative) antibiotic after your operation to prevent an infection occurring.
  • Blood clot. There is a very low risk (approximately 1 in 1,000) of a blood clot forming and causing problems due to embolism (the clot detaching and passing into your blood). If you are considered to be at high risk, you may be given medication to prevent blood clots from forming.
  • Knee pain. This affects up to 18% of people and is more likely to occur when the patellar tendon is used as graft tissue. You may feel pain behind your kneecap or when kneeling down or crouching.
  • Stiffness. Some people experience long-term stiffness in their knee.

Following the operation, there is a 2-8% chance that the graft will fail and your knee will still be unstable. If the first operation is not a success, further surgery may be required. A second operation is a more major procedure and is not as successful in the long term as a first repair.

The risks of surgery should be balanced against the risk that, without surgery, your knee will almost certainly continue to give way.


Ligaments are the fibrous connective tissues that link two bones together at a joint.

Connective tissue
Connective tissues provide support and structure to other tissue and organs.

Page last reviewed: 13/07/2011

Recovery from anterior cruciate ligament (ACL) reconstruction surgery can take up to six months. You will be given a rehabilitation programme to follow that will focus on:

  • reducing the swelling around your knee,
  • restoring movement to your knee, and
  • increasing the strength in your leg muscles.

After surgery

After your surgery, your knee will be bandaged to prevent inflammation (swelling) and you may have to wear a cryocuff around your injured knee. A cryocuff is a waterproof jacket that contains iced water. It helps to reduce swelling. You will also be given painkillers to control any pain.

You may have painful bruising, swelling and redness down the front of your shin and ankle for four to five days after the operation. This is because the fluid that was inside the joint (the synovial fluid) leaks down the shin. This should settle after around 7 to 10 days.

Rehabilitation programme

Your surgeon or physiotherapist (a healthcare professional trained in exercises to restore movement) can advise you about a structured rehabilitation programme. It is very important that you follow it carefully so that your recovery is as successful as possible.

You will be shown exercises that you can start in hospital after your surgery and continue when you get home. These will include movements to bend, straighten and raise your leg. If you are unsure about any of the exercises, ask for assistance.

You will be given crutches to use when you move around. You may need to use them for around two weeks but you should only put as much weight on your injured leg as you feel comfortable with.

Weeks 1-2 of recovery

For a few weeks, your knee is likely to be swollen and stiff, and you may need to take painkillers. Your surgeon or GP will advise about what pain relief is best for you to use. You will be told to elevate your leg as much as possible, for example by putting pillows under your heel when lying in bed.

You may be given a cryocuff to take home with you. This can ease the pain and swelling. Ask your surgeon or physiotherapist how often you should use the cryocuff. If you do not have a cryocuff, you can use a pack of frozen peas wrapped in a towel and place it on your injured knee.

Weeks 2-6 of recovery

Once the pain and swelling have settled, you may be advised to increase or change your exercises. Your physiotherapist will give you advice about what exercises to do. These will be designed to:

  • help you fully extend and bend your knee,
  • strengthen your leg muscles,
  • improve your balance, and
  • begin to walk properly.

As well as specific exercises, other activities that do not put much weight on your knee may be recommended, such as swimming and cycling. After two or three weeks, you should be able to walk without crutches.

Weeks 6-24 of recovery

Between six weeks and six months after your operation, you should gradually be able to return to your normal level of activity.

You will be encouraged to continue exercises, such as cycling and swimming, but you cannot yet take part in sports that involve a lot of twisting, jumping or turning. This is because you need to allow enough time for the grafted tissue to anchor itself in place inside your knee.

After six months

After six months, you should be able to return to playing any type of sport. Some people may take more time to feel confident enough to play sports again, and professional athletes may need longer to return to their previous level of performance. 


Ligaments are the fibrous connective tissues that link two bones together at a joint.

Connective tissue
Connective tissues provide support and structure to other tissue and organs.

Returning to work

How quickly you can return to work depends on what your job involves.

If you work in an office, you may be able to return to work after two or three weeks. If you do any form of manual labour, it could be up to three months before you can return to work, depending on your work activities.


Your GP can advise you about when you can drive again. This will usually be after three to four weeks or whenever you can comfortably put weight on your foot.

Caring for your wound

The wound on your knee will be closed with stitches. These may be dissolvable stitches, which should disappear in around three weeks, or you may need to have the stitches removed by a healthcare professional. Your surgeon will advise you about this.

Your surgeon should also tell you how to care for your wound. Washing it in mild soap and warm water is usually all that is required.

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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