Anterior cruciate ligament injury

An anterior cruciate ligament (ACL) injury causes considerable knee pain and restricts the movement and stability of your knee joint.

By Wallace Health I Medically reviewed by Adrian Roberts.
Page last reviewed: October 2018 I Next review due: October 2023

What is anterior cruciate ligament injury?

Your knee joint is where three bones meet; your:

  • Kneecap (patella) — this sits at the front of your knee joint to protect it
  • Shinbone (tibia)
  • Thighbone (femur)

These bones are connected by powerful bands of tissue called ligaments. There are four main ligaments in your knee that hold the bones together and stabilise your knee: 

  • Anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) — these control the back and forth motion of your knee and cross each other diagonally to make an X shape; the ACL sits in the middle of your knee in front of the PCL, which is at the back of the knee
  • Medial collateral ligament (MCL) and lateral collateral ligament (LCL) — these control the sideways motion of your knee and brace against sudden jerks or unusual movements; they are on the sides of your knee, with the MCL on the inside and the LCL on the outside

Damage can occur to any of these ligaments. 

The ACL prevents your shinbone sliding in front of your thighbone and ACL injury is a very common knee injury. There are three types of ACL injury:

  • ACL sprain — the ligament has been stretched and has a few damaged fibres but can still help stabilise the knee joint; this is also called a grade 1 sprain
  • Partial ACL tear — the ligament has been stretched to the point of being loose and its fibres have torn; this is also called a grade 2 sprain
  • Complete ACL tear — the ligament has been ripped apart (ruptured) ie split into two pieces and the knee joint is unstable; this is also called a grade 3 sprain

Partial ACL tears are rare. ACL injuries are usually minor sprains or complete tears. Half of all ACL injuries occur alongside other knee injuries eg damage to articular cartilage or other ligaments, or a meniscus tear.

What causes ACL injuries?

You’re at a higher risk of ACL injury if you:

  • Are a woman — in some sports, women have ACL injuries more than men and it is suggested to be caused by differences in: 
    • Ligament looseness — women tend to have looser ligaments
    • Muscle control, muscle strength and physical conditioning
    • Oestrogen levels — women have higher oestrogen levels and this hormone affects the ligaments
    • Pelvis and lower leg alignment
  • Play certain sports eg basketball, downhill skiing, football, gymnastics and rugby
  • Play sports on artificial turf surfaces
  • Play sports when you aren't in good physical condition for them
  • Use sports equipment incorrectly or equipment that isn't well-maintained eg using ski bindings that aren't properly adjusted
  • Wear shoes that don't fit properly

ACL injury can be the result of:

  • Changing direction suddenly
  • Direct contact eg a blow to the knee or a collision during a rugby tackle
  • Landing awkwardly after jumping or falling
  • Pivoting with your foot firmly planted
  • Stopping abruptly or slowing down when running

How to tell if you have anterior cruciate ligament injury

ACL injury can cause your knee to lose its stability and full range of movement, making it difficult to perform everyday tasks or play certain sports.

The first signs of a torn ligament in your knee include:

  • An audible popping noise
  • Sudden, severe knee pain

Other symptoms include:

  • Discomfort when walking
  • Pain when bearing weight on your knee
  • Restricted movement of your knee
  • Swelling — usually in the first few hours after injury
  • Tenderness and/or bruising along the joint
  • Your knee collapsing unexpectedly

Knee pain and swelling may go away on their own but if you return to playing sports, your knee may still be unstable and you may cause further damage to it eg injure your knee cartilage (meniscus).

Talk to your doctor if you’re concerned about symptoms

You can book an appointment with a Spire private GP today.

Book an appointment

Diagnosis and tests for anterior cruciate ligament injury

You should see your GP if you suspect a knee ligament injury to avoid further damage.

Your GP will ask how you damaged your knee, how it feels and about your medical history. They will examine your injured knee and compare it to your uninjured knee. They may then refer you for tests, such as: 

  • An X-ray to check if your injury involves a broken bone
  • MRI scan to provide detailed images of your soft tissues, such as your ligaments
  • Ultrasound scan

These tests should reveal any damage to your ligaments and, if necessary, your GP will refer you to an orthopaedic consultant.

Common treatments for anterior cruciate ligament injury

Your treatment will depend on your individual needs. Non-surgical treatment may provide your knee with the stability and mobility you need — this is often recommended for less active and/or older individuals. Alternatively, your consultant may recommend reconstruction surgery to rebuild a torn ACL, particularly if you are a young athlete wanting to return to sports. 

You can help reduce the knee pain and swelling of an ACL sprain by:

  • Lying down with your knee elevated
  • Placing ice on your knee for 20 minutes at a time every couple of hours
  • Resting your knee
  • Taking over-the-counter painkillers (eg paracetamol) to relieve the pain
  • Wrapping your knee with a bandage or wrap

If you have a partial or complete ACL tear, your doctor will advise you about non-surgical and surgical treatments.

Non-surgical treatments

If you have a torn ligament in your knee and don’t participate in physically demanding sports, your doctor may suggest non-surgical treatments. These include:

  • Crutches — this will prevent you from putting too much weight on your knee; your doctor may suggest you use crutches while wearing a knee brace
  • Physiotherapy — a physiotherapist will suggest specific exercises to restore knee function and strengthen your leg muscles
  • Wearing a knee brace — this will help keep your knee stable

Surgical treatments

If you have an ACL tear and participate in physically demanding sports, have an active lifestyle and are in good health, your consultant may recommend ACL reconstruction. 

Most ACL tears can’t be stitched back together. Reconstruction surgery therefore involves replacing your torn ligament with a tissue graft. The graft acts as a scaffold onto which new ligament tissue will grow. Graft tissue is usually taken from a tendon, such as your: 

  • Hamstring tendons — these are at the back of the thigh 
  • Patellar tendon — this runs from your kneecap to your shinbone
  • Quadricep tendon — this runs from your kneecap and into your thigh

Graft tissue can also come from a donor (allograft).

Your orthopaedic consultant will discuss the advantages and disadvantages of each type of graft to determine which is most appropriate for you. It takes time for new ligament tissue to grow, which means it may be six months or longer after surgery before you can return to sports. 

Knee arthroscopy (keyhole surgery) is a special type of ACL reconstruction surgery that is less invasive. This usually results in less pain after surgery, less time spent in hospital and a faster recovery. 

After ACL reconstruction and a course of physiotherapy, you should be able to return to your chosen sport within 12 months. However, ACL injury puts you at greater risk of developing osteoarthritis in your knee, even if you have reconstruction surgery. There are many risk factors for developing osteoarthritis, including: 

  • The severity of your ACL injury
  • Whether you have had other knee injuries 
  • Your activity level after treatment

Rehabilitation from anterior cruciate ligament injury

Rehabilitation is important to return to your normal activities, whether you have surgery or not. Physiotherapy will help restore your knee function and range of motion. 

Initially, physiotherapy will focus on the range of motion of your knee and surrounding muscles. Next, it will focus on muscle strengthening to protect your knee ligaments; strengthening gradually increases the stress on the ligaments. Finally, physiotherapy will aim to restore function related to the sport you want to return to playing.

Q & A

Mr Stephen Guy, Consultant Knee Surgeon

Mr Stephen Guy explains what an ACL injury is and how it affects a person's movement. He details that diagnosis of an ACL injury can be made simpler using the LIMP index system and further facilitated by an MRI scanner, and says treatment includes operative and non-operative options.

Preventing anterior cruciate ligament injury 

Appropriate exercise and training can help prevent ACL injury. There are several different specialists who can assess your physical condition and advise you on how to reduce your risk of injury; this includes athletic trainers, physiotherapists, sports medicine doctors and other sports medicine specialists. 

Programmes to reduce your risk of ACL injury include: 

  • Exercises to strengthen your core muscles, including hip, lower abdomen, and pelvic muscles
  • Exercises to strengthen your leg muscles and ensure balanced leg muscle strength — these exercises will focus particularly on your hamstrings
  • Training that focuses on proper technique and knee positioning when: 
    • Jumping and landing from jumps
    • Performing cutting movements
    • Pivoting 

Frequently asked questions

Can an ACL tear heal without surgery?

ACL tears can heal over time without surgery by following a physiotherapy programme and using crutches and knee braces. However, if you take part in physically demanding sports and want to return to them, your doctor may recommend surgery.

Can you walk with a torn ACL?

This depends on the severity of your ACL tear. You may be able to walk on it but may experience discomfort or pain.

Is ACL surgery a major surgery?

Yes, ACL surgery is a major surgery. It involves reconstructing your torn ligament using a tissue graft. Complete restoration of knee function for physically demanding sports can take six months or longer after surgery.

Where is ACL pain located?

Pain caused by an ACL injury is located in the centre of your knee.