Anterior cruciate ligament (ACL) injury

Expert reviewer, Mr Damian McClelland, Trauma and Orthopaedic Consultant, and Clinical Director for Musculoskeletal Services at Bupa
Next review due October 2023

The anterior cruciate ligament (ACL) is one of the ligaments inside your knee – it joins your thigh bone (femur) to the front of your shin bone (tibia). An ACL injury is a partial or complete tear, a stretched ligament, or a detachment of the ligament from your bone.

An image showing the different parts of the knee

About anterior cruciate ligament (ACL) injury

The ACL passes in front of another ligament, the posterior cruciate ligament (PCL). The cruciate ligaments get their name from the fact they form a cross within the knee as they run in different directions from the thigh to the shin bone. Along with the other ligaments in your knee, your ACL keeps your knee stable and prevents your thigh and shin bones moving out of place.

When your knee ligaments are stretched but not torn, they’re called a sprain. Knee ligament injuries are given different grades depending on how severe they are. Around half of all people with an ACL injury will have injuries to other parts of their knee as well, such as a meniscus tear.

Causes of anterior cruciate ligament (ACL) injury

Most anterior cruciate ligament (ACL) injuries happen during sports, especially football, basketball and skiing. Women are more likely to injure their ACL than men. An ACL injury may happen if you:

  • twist your knee, particularly when your foot is on the ground – for example, you land from a jump and twist in the opposite direction
  • quickly change direction when running or walking
  • slow down or stop suddenly when running
  • over extend your knee joint

If you’re playing contact sports, you can also injure your ACL if you have a direct blow to your knee from a collision with another player.

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Symptoms of anterior cruciate ligament (ACL) injury

If you injure your ACL, you may hear a popping sound and feel pain in your knee. Your knee may feel unstable and give way (or feel like it will), and you may not be able to bear weight on it. If you’re playing sport, you’re unlikely to be able to carry on – you won’t be able to walk on it. You’ll quickly develop swelling around your knee joint within an hour or two of injuring it. And you may lose the full range of movement in your knee.

Self-help for anterior cruciate ligament (ACL) injury

If you injure your knee, you should follow the POLICE procedure to manage any type of soft tissue injury to your knee. POLICE stands for protect, optimal loading, ice, compression and elevation.

Additionally, there are certain things you should avoid in the first three days after your injury so you don’t damage your knee further. These can be remembered as HARM, which stands for heat, alcohol, running and massage.

To find out more about POLICE and HARM, you can visit our page on what to do if you injure your knee.

If you’re having difficulty bearing weight on your knee, you may need to use crutches or wear a brace to support you for a while. Your doctor or physio will explain how long you’ll need to use these for.

Treatment of anterior cruciate ligament (ACL) injury

You’ll usually need to go to A&E with this kind of injury, where they’ll examine you and take some X-rays of your knee. They may then refer you to an acute knee clinic, which will organise any investigations and treatment you need.

The treatment you’re offered for your ACL injury will depend on how severe the damage is and how active you normally are. The initial treatment will be to control your pain and swelling using the POLICE and HARM self-help measures (see above). Further treatments may include physiotherapy, medicines and occasionally, surgery.

You may see an orthopaedic surgeon (a doctor who specialises in bone surgery) or a sports medicine professional, such as a sports doctor or physiotherapist. There are different treatments that your doctor or physiotherapist may suggest, and a lot that you can do yourself to help you recover.


You can take over-the-counter painkillers, such as paracetamol or ibuprofen, if you’re in pain. Your doctor may be able to prescribe you stronger painkillers if your knee is really painful. As well as easing your pain, non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen may help to reduce inflammation and swelling. Always read the patient information that comes with your medicine, and if you have questions ask your pharmacist or GP for advice.


Your physiotherapist will assess your knee and then plan a programme of rehabilitation exercises to suit your individual needs. The physiotherapy programme will be designed to give you the best chance of returning to your usual activity. Make sure you do the exercises as this is an important part of your recovery. The aim of physiotherapy is to help your knee recover its full range of movement, its strength and stability, and return to working normally.


Your surgeon will advise you if you need surgery; for example, if your knee is unstable and gives way when you play sport or sometimes, just walk. Surgery may be the best option to repair your injury if you exercise a lot or are in a physically demanding job. For instance, you:

  • play sports that include a lot of twisting and turning – such as football or basketball, to a high level – and you want to get back to it
  • have a very physical or manual job, for example you're in the military, a firefighter, police officer or work in construction

You may prefer to give rehabilitation with physiotherapy a try first before you have an operation. Either way, your doctor probably won’t recommend that you have surgery right away after your injury. It’s usually best to wait at least a week or two, to allow the swelling in your knee to go down first.

It usually takes between six and 12 months to return to full activity after surgery. And you’ll need to be prepared to undergo an intense and lengthy period of physiotherapy after your operation. Ask your doctor about the pros and cons of surgery, and how it might help you.

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Related information

    • Acute knee injuries. Brukner & Khan’s Clinical Sports Medicine: Injuries, Volume 1, 5th ed. (online). McGraw-Hill Medical., published 2017
    • Knee ligament injuries. PatientPro., last edited 15 February 2017
    • Anterior cruciate ligament injury. BMJ Best Practice., last reviewed August 2020
    • Anterior cruciate ligament injury. Medscape., updated 16 June 2016
    • Knee assessment. PatientPro., last edited 26 February 2016
    • Meniscus tears. American Academy of Orthopaedic Surgeons., last reviewed March 2014
    • Sprains and strains. NICE Clinical Knowledge Summaries., last revised April 2020
    • Knee pain. NICE Clinical Knowledge Summaries., last revised July 2017
    • Treatment of sports injuries. Brukner & Khan’s Clinical Sports Medicine: Injuries, Volume 1, 5th ed. (online). McGraw-Hill Medical., published 2017
    • Treatment for knee pain. Chartered Society of Physiotherapy., last reviewed 26 March 2020
    • Personal communication, Damian McClelland, Clinical Director for Musculoskeletal Services, Bupa, 12 October 2020
    • Coffey R, Bordoni B. Lachman Test. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021

  • Reviewed by Rachael Mayfield-Blake, Freelance Health Editor, October 2020
    Expert reviewer, Mr Damian McClelland, Trauma and Orthopaedic Consultant, and Clinical Director for Musculoskeletal Services at Bupa
    Next review due October 2023