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

The anterior cruciate ligament (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 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 the most common type of knee injury, 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 will 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 doesn't feel unstable and you don't have an active lifestyle, you may decide not to have ACL surgery.

However, when deciding whether to have surgery it's important to be aware that a delay could cause further damage to your knee.

Read more about deciding to have surgery.

Before having surgery

Before having ACL surgery, you may need to wait for any swelling to go down and 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 the back of your thigh (hamstrings) are as strong as possible.

If you don't 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 three weeks after the injury occurred for the full range of movement to return.

You may be referred to a physiotherapist before having surgery. They'll be able to help you regain the full range of movement in your knee.

Your physio may show you some stretches 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 help 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.

Read more about preparing for ACL surgery.

Reconstructive ACL surgery

A torn ACL can't be repaired by stitching it back together. However, it can be reconstructed by grafting (attaching) new tissue onto it.

The ACL can be reconstructed by removing what remains of the torn ligament and replacing it with a tendon from – for example, the hamstring or patellar tendon.

Read more about how ACL reconstructive surgery is performed.

Risks of ACL surgery

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

However, your knee may not be exactly like it was before the injury, and you may still have some pain and swelling in your knee. This may be due to secondary injuries in the knee sustained at the same time as the ACL injury, or afterwards due to the abnormal movement pattern. It usually involves tears or injuries to the cartilage components of the joint.

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

Read more about the risks of ACL surgery.

Recovering from surgery

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

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

Read more about recovering from ACL reconstructive surgery.