Knee surgery, anterior cruciate ligament


Knee surgery, anterior cruciate ligament
Knee surgery, anterior cruciate ligament

If you tear the anterior cruciate ligament (ACL) in your knee, you may need to have reconstructive surgery.

The ACL is a tough band of tissue joining the thigh bone to the shin bone at the knee joint.

It runs diagonally through the inside of the knee and gives the knee joint stability. It also helps to control the back-and-forth movement of the lower leg.

ACL injuries

Knee injuries can occur during sports such as skiing, tennis, squash, football and rugby. ACL injuries are one of the most common types of knee injuries, accounting for around 40% of all sports injuries.

You can tear your ACL if your lower leg extends forwards too much. It can also be torn if your knee and lower leg are twisted.

Common causes of an ACL injury include:

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

If the ACL is torn, your knee may become very unstable and lose its full range of movement.

This can make it difficult to perform certain movements, such as turning on the spot. Some sports may be impossible to play.

Deciding to have surgery

The decision to have knee surgery will depend on the extent of damage to your ACL and whether it's affecting your quality of life.

If your knee does not feel unstable and you do not have an active lifestyle, you may decide not to have ACL surgery.

But it's important to be aware that delaying surgery could cause further damage to your knee.

Before having surgery

Before having ACL surgery, you may need to wait for any swelling to go down and for the full range of movement to return to your knee.

You may also need to wait until the muscles at the front of your thigh (quadriceps) and back of your thigh (hamstrings) are as strong as possible.

If you do not have the full range of movement in your knee before having surgery, your recovery will be more difficult.

It's likely to take at least 3 weeks after the injury occurred for the full range of movement to return.

Before having surgery, you may be referred for physiotherapy to help you regain the full range of movement in your knee.

Your physiotherapist may show you some stretches that you can do at home to help keep your leg flexible. They may also recommend low-impact exercise, such as swimming or cycling.

These types of activities will improve your muscle strength without placing too much weight on your knee. You should avoid any sports or activities that involve twisting, turning or jumping.

Reconstructive ACL surgery

A torn ACL cannot be repaired by stitching it back together, but it can be reconstructed by attaching (grafting) new tissue on to it.

The ACL can be reconstructed by removing what remains of the torn ligament and replacing it with a tendon from another area of the leg, such as the hamstring or patellar tendon.

The patellar tendon attaches the bottom of the kneecap (patella) to the top of the shinbone (tibia).

Risks of ACL surgery

ACL surgery fully restores the functioning of the knee in more than 80% of cases.

But your knee may not be exactly like it was before the injury, and you may still have some pain and swelling.

This may be because of other injuries to the knee, such as tears or injuries to the cartilage, which happened at the same time as or after the ACL injury.

As with all types of surgery, there are some small risks associated with knee surgery, including infection, a blood clot, knee pain, and knee weakness and stiffness.

Recovering from surgery

After having reconstructive ACL surgery, a few people may still experience knee pain or instability.

Recovering from surgery usually takes around 6 months, but it could be up to a year before you're able to return to full training for your sport.

The knee

The 3 bones that meet in the knee are the:

  • thigh bone (femur)
  • shin bone (tibia)
  • kneecap (patella)

These bones are connected by 4 ligaments – 2 collateral ligaments on the sides of the knee and 2 cruciate ligaments inside the knee. 

Ligaments are tough bands of connective tissue. The ligaments in the knee hold the bones together and help keep the knee stable.

Why is it necessary?

The decision to have knee surgery will depend on the extent of damage to your anterior cruciate ligament (ACL) and whether it affects your quality of life.

If your knee does not feel unstable and you do not have an active lifestyle, you may decide not to have ACL surgery.

But you should be aware that delaying surgery could result in further damage to your knee.

One study of people with ACL tears found their risk of damaging the injured knee increased by 1% for every month between the injury occurring and surgery.

Things to consider

When deciding whether to have ACL surgery, the following factors should be taken into consideration:

  • your age – older people who aren't very active may be less likely to need surgery
  • your lifestyle – for example, whether you will be able to follow the rehabilitation programme after having surgery
  • how often you play sports – you may need to have surgery if you play sports regularly
  • your occupation – for example, whether you do any form of manual labour
  • how unstable your knee is – if your knee is very unstable, you are at increased risk of doing further damage if you do not have surgery 
  • whether you have any other injuries – for example, your menisci (small discs of cartilage that act as shock absorbers) may also be torn and may heal better when repaired at the same time as ACL reconstruction


If necessary, children can also have ACL reconstructive surgery. But as they're still growing, the procedure is likely to be modified to ensure that the growth areas are not affected.

It's a trickier operation and may need to be carried out by a surgeon with a special interest in childhood injuries.

If surgery is not possible, a brace and refraining from sports until the child is fully grown may be an alternative.


Before having knee surgery, you may need to wait for any swelling to go down and for the full range of movement to return to your knee.

You may also need to wait until the muscles at the front of your thigh (quadriceps) and back of your thigh (hamstrings) are as strong as possible.

If you do not have the full range of movement in your knee before having surgery, your recovery will be more difficult.

It's likely to take at least 3 weeks after the injury occurred for the full range of movement to return. Your GP may refer you to a physiotherapist to help you prepare for surgery.


Physiotherapists, or physios, are healthcare professionals who use physical methods, such as massage and manipulation, to encourage healing. A physio will be able to help you regain the full range of movement in your knee.

Optimising the muscle function in the knee will help to ensure a more successful outcome after surgery. It may also provide the knee with sufficient stability so that surgery may not always be necessary.

Your physio may show you some stretches you can do at home to help keep your leg flexible.

They may also recommend low-impact exercises, such as swimming or cycling. These types of activities will improve your muscle strength without placing too much weight on your knee.

You should avoid any sports or activities that involve twisting, turning or jumping.

Pre-admission clinic 

Before having anterior cruciate ligament (ACL) surgery, you'll be asked to attend a pre-admission clinic. You'll be seen by a member of the team who will look after you while you're in hospital.

A physical examination will be carried out and you'll be asked about your medical history. You may also need to have some investigations and tests, such as a knee X-ray.

You'll be asked about any tablets or other types of medication you're taking, both prescribed and bought over the counter from a pharmacy.

A member of your care team will also ask about any anaesthetic you've had in the past and whether you experienced any problems or side effects, such as nausea.

They'll also ask you some questions about your teeth, including whether you wear dentures, caps or a plate.

This is because a tube may be put down your throat to help you breathe during the operation, and any loose teeth could be dangerous.

The pre-admission clinic is a good time to ask any questions you have about the procedure. But you can discuss any concerns with your surgeon at any time.

Read more about preparing for surgery.

Preparing for hospital

It's a good idea to be fully prepared before going into hospital for surgery. Below is a list of things to consider if you're about to have an operation.

Do your homework 

Find out as much as you can about your operation and what it involves. Information or a video about the procedure may be available at your hospital.

Ask your surgeon if you're unsure about anything.  

Other medical problems

Ask your GP to check that any other medical problems you have, such as high blood pressure (hypertension), are under control.

Keep clean

Have a bath or shower before going into hospital, and put on clean clothes. This will reduce the chances of taking unwanted bacteria into hospital.

Eating before your operation

Anaesthetics are often safer if your stomach is empty, so you'll usually have to stop eating several hours before your operation.

You should be given more advice about this during your pre-admission clinic.

Prepare for returning home

Stock up on food that's easy to prepare, such as tinned foods and staples like rice and pasta. You could also prepare meals and put them in the freezer.

Put things you'll need, such as books and magazines, where you can easily reach them.

Arrange help and transport

Ask a friend or relative to take you to and from hospital. You'll also need to arrange for someone to help you at home for a week or two after you come back.

Read more about having an operation.

How is it performed?

A number of methods can be used to reconstruct an anterior cruciate ligament (ACL). The most common method is to use a tendon from elsewhere in your body to replace the ACL.

You'll either have a general anaesthetic, which means you'll be totally unconscious during the procedure, or a spinal anaesthetic, where anaesthetic is injected into your spine so you're conscious but unable to feel pain.

Your anaesthetist will discuss the procedure with you and can recommend which type of anaesthetic to use.

The operation will take between 1 and 1.5 hours, and will usually require an overnight stay in hospital.

Examining your knee

After you've been anaesthetised, the surgeon will carefully examine the inside of your knee, usually with a medical instrument called an arthroscope.

Your surgeon will check that your ACL is torn and look for damage to other parts of your knee. Any other damage found might be repaired during the surgery to your ACL or after your operation.

After confirming that your ACL is torn, your surgeon will remove the graft tissue, ready for relocation.

Graft tissue

A number of different tissues can be used to replace your ACL. Tissue taken from your own body is called an autograft. Tissue taken from a donor is called an allograft.

A donor is someone who has given permission for parts of their body to be used after they die by someone who needs them. 

Before your operation, your surgeon will discuss the best option with you.

Tissues that could be used to replace your ACL are listed below. 

  • a strip of your patellar tendon – this is the tendon running from the bottom of the kneecap (patella) to the top of the shin bone (tibia) at the front of your knee
  • part of your hamstring tendons – these run from the back of your knee on the inner side, all the way up to your thigh
  • part of your quadriceps tendon – this is the tendon that attaches the patella to the quadriceps muscle, which is the large muscle on the front of your thigh
  • an allograft (donor tissue) – this could be the patellar tendon or Achilles tendon (the tendon that attaches the back of the heel to the calf muscle) from a donor
  • a synthetic graft – this is a tubular structure designed to replace a torn ligament

The most commonly used autograft tissues are the patellar tendon and the hamstring tendons. Both have been found to be equally successful.

Allograft tissue may be the preferred option for people who are not going to be playing high-demand sports, such as basketball or football, as these tendons are slightly weaker.

Synthetic (man-made) tissues are currently used in certain situations, such as revision surgery and multi-ligament injuries.

The graft tissue will be removed and cut to the correct size. It will then be positioned in the knee and fixed to the thigh bone (femur) and shin bone (tibia).

This is usually carried out using a technique called knee arthroscopy.


An arthroscopy is a type of keyhole surgery. It uses a medical instrument called an arthroscope, which is a thin, flexible tube with bundles of fibre-optic cables inside that act as both a light source and camera.

Your surgeon will make a small incision on the front of your knee and insert the arthroscope.

The arthroscope will illuminate your knee joint and relay images of your knee to a television monitor. This will allow the surgeon to see the inside of your knee clearly.

Additional small incisions will be made in your knee so that other medical instruments can be inserted. The surgeon will use these instruments to remove the torn ligament and reconstruct your ACL.

Your surgeon will make a tunnel in your bone to pass the new tissue through.

The graft tissue will be positioned in the same place as the old ACL and held in place with screws or staples that will remain in your knee permanently.

Final examination

After the graft tissue has been secured, your surgeon will test that it's strong enough to hold your knee together.

They'll also check your knee has the full range of motion and that the graft keeps your knee stable when it's bent or moved. 

When the surgeon is satisfied everything is working properly, they'll stitch the incisions closed and apply dressings.

After the procedure, you'll be moved to a hospital ward to begin your recovery.

Read more about recovering from knee surgery.

Risks and results

In more than 80% of cases, surgery to repair an anterior cruciate ligament (ACL) fully restores the functioning of the knee.

ACL surgery will improve the stability of your knee and stop it giving way. You should be able to resume normal activities after 6 months.

But your knee may not be exactly like it was before the injury. You may still experience some pain and swelling in the replacement ligament.

If other structures in your knee are also damaged, it may not be possible to fully repair them.

As with all types of surgery, there are some risks associated with knee surgery.

They include:

  • infection – the risk of infection is small (less than 1 in 100); you may be given an antibiotic after your operation to prevent infection developing
  • blood clot – the risk of a blood clot forming and causing problems is very low (about 1 in 1,000); if you're thought to be at risk, you may be given medication to prevent blood clots forming
  • knee pain – affects almost 2 in 10 of people who have ACL surgery and is more likely to occur when the patellar tendon is used as graft tissue; you may have pain behind your kneecap or when kneeling down or crouching
  • knee weakness and stiffness – some people experience long-term weakness or stiffness in their knee

After ACL surgery, there's also a small chance (less than 1 in 10) that the newly grafted ligament will fail and your knee will still be unstable.

If the first operation is unsuccessful, further surgery may be recommended. But subsequent operations are often more difficult and do not usually have the same long-term success rate as a first tendon repair.


Recovering from anterior cruciate ligament (ACL) knee surgery can take up to a year.

After knee surgery, the wound will be closed with stitches or surgical clips. If the stitches are dissolvable, they should disappear after about 3 weeks.

If your stitches are not dissolvable, they'll need to be removed by a healthcare professional. Your surgeon will advise you about this.

They'll also tell you how to care for your wound. Washing it with mild soap and warm water is usually all that's required.

Your knee will be bandaged and you may also be given a Cryo/Cuff to wear. This is a waterproof bandage that contains iced water to help reduce swelling. You may also be given painkilling medication.

You may have painful bruising, swelling and redness down the front of your shin and ankle, caused by the fluid inside your knee joint (synovial fluid and blood) leaking down your shin.

These symptoms are temporary and should start to improve after about a week.


Your surgeon or physiotherapist can advise you about a structured rehabilitation programme. It's very important that you follow the programme, so your recovery is as successful as possible.

You'll be given exercises you can start in hospital after your surgery and continue when you get home.

The exercises will include movements to bend, straighten and raise your leg. Ask if you're unsure about how to do any of the exercises.

You'll also be given crutches to help you move around. You may need to use them for about 2 weeks, but you should only put as much weight on your injured leg as you feel comfortable with.

Weeks 1 to 2 of your 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 the type of pain relief that's best for you. You'll be advised to raise your leg as much as possible – for example, by putting pillows under your heel when you're lying in bed.

You may be given a Cryo/Cuff to take home with you to help ease the pain and swelling. Ask your surgeon or physiotherapist how often you should use the Cryo/Cuff.

If you don't have a Cryo/Cuff, you could place a pack of frozen peas wrapped in a towel on your injured knee.

Weeks 2 to 6 of your recovery

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

The exercises will help you to:

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

After 2 to 3 weeks, you should be able to walk without crutches.

As well as specific exercises, activities that do not put much weight on your knee may also be recommended, such as swimming and cycling. 

Weeks 6 to 24 of your recovery

You should gradually be able to return to your normal level of activity between 6 weeks and 6 months after your knee operation.

You'll be encouraged to continue with activities such as cycling and swimming but should avoid 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 6 months

After 6 months, you may be able to return to playing sport.

Some people may need to take more time before feeling confident enough to play sports again, and elite athletes may need longer to return to their previous level of performance.

Returning to work

How quickly you can return to work after having knee surgery will depend on what your job involves.

If you work in an office, you may be able to return to work after 2 to 3 weeks.

If you do any form of manual labour, it could be up to 3 months before you can return to work, depending on your work activities.


Your GP can advise you on when you can drive again. This will usually be after 3 to 4 weeks, or whenever you can comfortably put weight on your foot.

The information on this page has been adapted by NHS Wales from original content supplied by NHS UK NHS website
Last Updated: 10/03/2022 12:51:03