Fast facts

  • The anterior cruciate ligament (ACL) is a band of strong connective tissue in the knee, connecting the shinbone to the thighbone.
  • ACL injury can occur with all levels of physical activity, but is commonly experienced by athletes and physically active people.
  • The ACL can be injured in a number of ways, leading to pain, swelling and instability in the knee joint.
  • ACL injuries are treated with rest, icepacks and compression bandages, medication, physiotherapy. In more serious injuries, surgery may be required.
  • With treatment, a mild ACL injury usually heals within six weeks. More severe injuries, such as a tear, may require up to 3-4 months for a full recovery.

Joint

A connecting surface or tissue between two bones.

Physiotherapy

A healthcare profession that treats bodily weaknesses or defects with physical remedies, such as massage or exercise.

What is the anterior cruciate ligament?

Ligaments are a type of strong, connective tissue that connect bones to other bones, providing stability to joints. The knee joint has four major ligaments that stabilise it. Two of those, the anterior and posterior cruciate ligaments, cross one another as they connect the shinbone (tibia) to the thighbone (femur). They are found deep within the knee joint and allow for a wide range of movement within the knee while maintaining the joint's stability.

Injury of the ACL often occurs together with damage to other knee ligaments, or other structures within the knee, such as menisci (see the report on Meniscal tears).

Anatomy of knee ligaments.The knee ligaments. 

 

Joints

A connecting surface or tissue between two bones.

Causes

ACL injury can be caused by a range of activities, such as soccer, football or basketball. A sudden deceleration, twisting of the knee in the opposite direction or a direct blow to the side of the knee can all cause an ACL sprain or tear.

Risk factors

Risk factors for an ACL injury include:

  • Gender - ACL injuries are more common in female athletes than in male athletes performing the same activity, [1] and;
  • Bone anatomy - natural differences in people's knee anatomy can increase their risk of knee ligament injury.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Types

Depending on the degree of damage to your ACL, the injury is described as being:

  • Grade I (mild) sprain - the ligament is stretched but not torn, with minimal pain and swelling;
  • Grade II (moderate) sprain - the ligament is partly torn. There may be swelling, bruising and pain, or;
  • Grade III (severe) sprain - the ligament is completely torn, making the joint unstable and causing swelling and sometimes bruising. Often there is no pain at this grade of injury, as the nerve fibres that transmit pain are also torn. 

Joint

A connecting surface or tissue between two bones.

Nerve

One or more fibres that transmit signals of sensation and motion between the brain or spinal cord and other parts of the body.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Signs and symptoms

Symptoms of ACL injury include:

  • Swelling, pain and tenderness in the knee or around it;
  • Reduced movement of the knee;
  • A popping or snapping sound, which is sometimes heard as a ligament is completely torn;
  • A feeling of instability when walking, and;
  • Bruising, which may develop some time after sustaining a ligament injury.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Methods for diagnosis

During a physical examination, your doctor will check for signs of swelling, instability and reduced movement.

Magnetic resonance imaging (MRI) is commonly used to confirm the diagnosis, assess the grade of ACL injury and check for additional knee injuries.

Magnetic resonance imaging

A type of imaging that uses a magnetic field and low-energy radio waves, instead of X-rays, to obtain images of organs.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Types of treatment

First aid

Following a suspected knee injury, it is important to immediately stop any sporting or physical activity. Rest, elevation, use of icepacks and compression bandages on the affected knee can help reduce discomfort and further injury. Seek medical attention for further treatment.

Medications

Medications for knee ligament injuries include pain-relief medications such as paracetamol. Non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, can be used to treat inflammation.

Surgery

Surgery may be needed to repair a knee ligament injury when the ACL is completly torn, or if there are additional injuries to other knee structures.

During surgery, ligaments can be reconstructed by using a piece of tendon from another part of the leg, a process known as autograft. If a section of tendon is obtained from a donor, the procedure is called an allograft.

Physiotherapy

Physiotherapy can improve knee movement and help the ACL to recover. Physiotherapy may also be useful before and after knee surgery. While your knee heals, crutches could help you when you need to stand or walk.

For ACL injury, open and closed-chain movements are helpful. Open-chain movements include leg curls, bench presses and seated leg extensions. Closed-chain movements include squats and lunges. It is important that you do not perform any exercises unless your physiotherapist or doctor specifically advises it.

NSAIDs

Non-steroidal anti-inflammatory drugs are commonly used to manage arthritis-related pain and inflammation and other musculoskeletal disorders. NSAIDs include aspirin and ibuprofen.

Physiotherapy

A healthcare profession that treats bodily weaknesses or defects with physical remedies, such as massage or exercise.

Tendon

Dense bands of connective tissue that attach muscles to bones.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Potential complications

Potential complications of an ACL injury include:

  • Avulsion fractures, which occur when a fragment of bone breaks away from the attached tendon or ligament;
  • Vascular injury - if the knee dislocates, blood vessels can be damaged, and;
  • Nerve injuries - damage to the nerves travelling through the leg.

Surgery for an ACL injury carries its own potential complications. These include:

  • Infection of the joint;
  • Reduced ability to extend the leg;
  • Knee instability due to graft failure in the knee;
  • Deep vein thrombosis - although a rare complication, deep vein thrombosis can occur as a result of prolonged immobilisation, and;
  • Injury to blood vessels and nerves can sometimes occur during surgery. Injury to the nerves can result in complex regional pain syndrome, which can cause severe pain and delay recovery.

Fractures

A complete or incomplete break in a bone.

Infection

Entry into the body of microorganisms that can reproduce and cause disease.

Joint

A connecting surface or tissue between two bones.

Nerve

One or more fibres that transmit signals of sensation and motion between the brain or spinal cord and other parts of the body.

Tendon

Dense bands of connective tissue that attach muscles to bones.

Vascular

Relating to the blood vessels in the body.

Complex regional pain syndrome

A condition that causes severe, disabling pain in an area of the body, often following an injury. The pain can be so intense that even the slightest touch can be excruciating. It can also be associated with changes in blood flow, sweating, skin changes, muscle wasting and bone loss in the affected region.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Prognosis

With treatment, a mild sprain of the ACL usually heals within six weeks. A moderate sprain can take more than six weeks to heal and a severe injury, such as a tear, could require 3-4 months for a full recovery.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.

Prevention

You can reduce the risk of an ACL injury by following proper technique during sporting activities. A knee brace may prevent further damage to a weakened knee. Physiotherapy can speed up recovery of the injured ligament and strengthen local muscles to prevent repeat injuries. 

Physiotherapy

A healthcare profession that treats bodily weaknesses or defects with physical remedies, such as massage or exercise.

1. Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.