ACL Knee Injuries

ACL knee injuries occur when the anterior cruciate ligament is overstretched, usually by the knee bending backwards or twisting awkwardly.  This can result in a partial tear to the ligament (a few of the fibres are torn), or in more serious cases, a rupture (when it tears completely).

With a torn ACL, the knee can often feel unstable and may give way.  Women are 5 times more likely to damage their ACL than men.

The Anterior Cruciate Ligament (ACL) is one of 4 ligaments in the knee. It runs from the back edge of the bottom of the thigh bone (femur) to the front edge of the top of the shin bone (tibia).

ACL knee injuries invlolve either partially or fully tearing the ligamen

Its job is to provide stability of the knee by stopping the tibia from sliding too far forwards, or twisting too much. To find out more about the ACL, see the ACL anatomy section

Here we will look at the causes, symptoms, risk factors, treatment options and how to prevent ACL knee injuries. To find out more about other types of injuries, visit the common knee injuries section.

What Causes ACL Injuries?

ACL knee injuries can occur either when there is direct contact with the knee or from an awkward movement

Non-Contact ACL Injuries:

1) Twisting the knee very quickly eg pivoting, changing direction
2) Landing awkwardly from a jump,
3) Hyper extending the knee (when the knee bends the wrong way) by more than 10° causing it to gap at the back
4) Sudden deceleration – coming to an abrupt stop whilst the foot is firmly planted

Tackles in sport is a common cause of ACL knee injuries

Contact ACL Injuries:

1) Usually a blow to the side of the knee when the foot is firmly planted to the ground eg with studs which forces the shin bone too far inwards or backwards

Often thought of as a sporting injury, 20% of sports related ACL knee injuries are due to contact injuries whilst the other 80% are due to non-contact. ACL knee injuries can also occur at other times for example car accidents, falling, or from wear and tear.

Symptoms of ACL Injuries

The most common symptoms experienced with ACL knee injuries are:

1) Audible noise: Approx 50% of ACL tears are accompanied by a popping/cracking noise at the time of injury
2) Swelling: Usually immediate (within 6 hours) and extensive but with a small tear may develop a few days later or not swell much at all. Swelling occurs due to damage to the blood vessels inside the knee (haemarthrosis)
3) Pain: The knee is usually immediately extremely painful especially when you put any weight on the leg or try and bend the knee
4) Giving Way: The knee often feels unstable and may buckle


A doctor can usually accurately diagnose a torn ACL by carrying out a clinical examination. The most common tests used are the Pivot Shift, Anterior Drawer and Lachmans Tests.

Diagnosis of ACL knee injuries is often confirmed with an MRI which will also show if any other structures in the knee have been damaged as well.

Associated Injuries

Usually with ACL knee injuries, the force going through the knee is so great that other structures are damaged at the same time. The most common structure to injure is the meniscus (special cartilage) – usually the lateral (outer) side. Sometimes the medial collateral ligament is also damaged. When all three structures are injured, it is known as the Unhappy Triad / O'Donoghue Triad.

Gender Effect

Studies have shown that women are approximately 5 times more likely to suffer from ACL knee injuries. This is thought to be due to different hormone levels and ligament structure.

The Special Function of the ACL

As well as protecting against abnormal knee movements, the ACL is also the primary structure for providing proprioception in the knee.

Proprioception is where the nerve fibres give the body a sense of the knees position and movement – a normal feature for all joints. For example, close your eyes and touch your finger to your nose. You can do it accurately even though you can’t see what you are doing – proprioception lets you know where the different parts of your body are and what they are doing.

The ACL is responsible for providing proprioceptive input at the knee, sending important information to the muscles that provide the reflex control of knee movement – meaning they constantly make small adjustments to the knee to keep it stable without us being consciously aware of it. Without the ACL, the knee loses it proprioceptive input and therefore is unable to help the muscles make subtle adjustments to keep the knee stable eg when changing direction or when on uneven ground.

With ACL knee injuries, this loss of proprioception can be compensated for, to a certain extent, by specific exercises for the knee muscles. However, for people who have a high demand for proprioception eg those who play sports, this may not be enough.

Treatment Options

Knee arthroscopies can be done follow ACL knee injuries to have a look at the state of the ACL and to repair any damage if necessary

ACL knee injuries can be managed either surgically or conservatively (exercise programme). The decision as to which is most appropriate depends on the individual and what level of knee function they require for their everyday life.

If someone wants to return to sports which involve quick changes of pace, pivoting and sudden changes in direction, ACL knee surgery will almost certainly be necessary. For someone who does not intend to return to sport, or only to activities such as cycling and running, often with rehab they can function perfectly well without an ACL.

Immediate Treatment

PRICE (Protect, Rest, Ice, Compress, Elevate) is the best place to start following ACL knee injuries

1) Protect: You may need crutches for the first few days to take the weight off the knee and/or an ACL knee brace (to prevent the knee from giving way)
2) Rest: You must stop playing sport immediately after an ACL knee injury and wait until your doctor is happy for you to return. See Rest it Right section for more info
3) Ice: Can help to reduce swelling and pain which as a result can speed up healing. Find out how to safely and effectively use Ice and visit the ice wrap section for the best ways to apply ice.
4) Compress: Tubigrip (a special elasticated bandage) can help reduce swelling and provide support after ACL knee injuries.
5) Elevate: Helps to reduce swelling. Keep your leg elevated aiming to have your knee high than you heart to make it most effective

Longer Term Treatment

1) Exercise Rehab Programme

Exercise programmes for a partially or completely torn ACL look to build up the strength of the knee muscles so that they provide enough support and stability for the knee to compensate for the torn ACL. It also helps train the muscles and other ligaments to provide proprioceptive feedback to gain more stability. It usually takes a few months of rehab to fully rehab conservatively from ACL knee injuries. Wearing a knee brace can help to provide support and stop the knee from giving way.

ACL knee braces can really help to reduce instability and pain

2) Knee Brace

Knee Braces are a great at providing extra stability for the knee. Many people find they are able to continue with sports after a partial or full tear of their ACL if they find the right brace. Visit the ACL brace section to find the best brace for you

3) ACL Knee Surgery

ACL Surgery is indicated if the knee keeps giving way/buckling. Any time this happens, there is a risk of damage to the other structures in the knee, particularly the cartilage/meniscus. It is important to avoid repeated giving way of the knee as if the meniscus tears, there is less protection for the knee bones and increases the risk of developing arthritis. ACL Surgery is therefore indicated in individuals who

• Want to return to pivoting type sports eg football, skiing, tennis, rugby, boxing, hockey
• Have problems with the knee giving way during their everyday activities

With a completely torn ACL, it is not possible to repair it by sewing it back together. An ACL reconstruction involves replacing the ligament with other graft tissue (of a ligament type) from around the knee. If other structures have also been damaged, such as the meniscus, they are also sorted out at the same time.

See the ACL Knee Surgery and ACL Reconstruction pages for more information on the operation process, rehab and recovery.

Preventing ACL Knee Injuries

There are a number of things that may help reduce the risk of ACL Knee Injuries:

1) Warming Up: For at least 15 minutes prior to sport
2) Strengthening Exercises: to improve the strength of the quadriceps and hamstring muscles
3) Stretching exercises: to improve the flexibility and therefore performance of the knee
4) Knee Braces: Can often help but will not eliminate the risk of injury – they can actually create a false sense of security. People usually gain most confidence from wearing an ACL brace
5) Proprioception Exercises: to improve the stability of your knee
6) Footwear: where possible, avoid shoes with cleats/studs, particularly in contact sports
7) Neuromuscular Training Programs: Designed to improve dynamic stabilisation of the knee. They also teach individuals how to land from jumps, pivot and change direction without putting too much force through the ACL.

What Next?

To find out more about what surgery for ACL knee injuries involves, visit the ACL Surgery section.

If you are looking for exercises to do to help you recover from any ACL knee injuries, visit the knee strengthening exercises section.

Go to Common Knee Injuries or Homepage

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Updated 16th September 2014
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The material on this website is intended for educational information purposes only.  It should not substitute medical advice, diagnosis or treatment.


See Also

ACL Surgery options

How do I know if I need an ACL reconstruction?

What is the best ACL Brace?

How can I strengthen my leg?

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