ACL Knee Injury

Written By: Chloe Wilson, BSc(Hons) Physiotherapy
Reviewed by: KPE Medical Review Board

An ACL knee injury is a common problem in sport but did you know that only 20% of all ACL injuries are a result of direct contact? 

The majority of ACL injuries occur from awkward twisting movements of the knee, particularly when the foot is stuck to the ground. 

Injuries range from a minor sprain to a complete rupture and depending on the severity can take months to fully recover from.  It may be a simple set of rehab exercises or full blown knee surgery.

The anterior cruciate ligament (ACL) is one of four ligaments in the knee which sits deep inside the middle of the joint. 

"Anterior" means situated at the front, "cruciate" means shaped like a cross and a "ligament" is a band of tissue connecting bones.

It is an extremely important knee ligament as it controls the stability of the knee, preventing the lower leg from sliding too far forwards or twisting too much. The ACL is 2cm long, as thick as a pencil and extremely strong. LEARN MORE >

An ACL knee injury occurs when the anterior cruciate ligament is overstretched, usually by the knee bending backwards too far or twisting awkwardly. These can result in:

• Partial ACL Tear: where only some of the ligament fibres are torn or
• ACL Rupture: where the ligament tears completely

The ACL can be stretched just under 2mm before it will tear. In some cases, nearby structures such as the knee meniscus and medial collateral ligament may also be damaged at the same time.

Contact ACL Injuries

Contact injuries account for only 20% of all ACL knee injuries.

In most cases they are caused by a blow to the side of the knee when the foot is firmly planted to the ground e.g. with studs, which forces the shin bone too far inwards or backwards 

An ACL knee injury can also occur at other times, for example car accidents, falling from a height, or from wear and tear.

Gender Effect

Whilst ACL knee injuries most commonly occur in men, this is because they are more likely to play high risk sports. In actual fact, studies have shown that women are at greater risk being approximately 5 times more likely to suffer from an ACL knee injury than men.

This is thought to be due to a combination of factors including hormone levels, ligament structure, pelvis angle, hamstring flexibility and size of the ligament itself. 

• Audible Noise: approximately 50% of the time there is a popping/cracking noise at the time of injury with ACL ruptures.
  
  
• Swelling:there is usually immediate swelling with a completely ruptured ACL injury, certainly within 6 hours, which is often extensive. If however it's only a small tear the swelling may develop a few days later and may be fairly minor. Swelling occurs due to damage to the blood vessels inside the knee - haemarthrosis.
  
  
• Pain: there is usually immediate pain after an ACL knee injury especially when you put any weight on the leg or try and bend the knee.
  
  
• Giving Way: the knee often feels unstable and may buckle and give way with ACL knee injuries. If the ACL has completely ruptured, this instability will often be an ongoing problem.
  
  
• Restricted Movement:knee movement is often limited after an ACL knee injury, particularly knee extension.

Sometimes with an ACL knee injury, the force going through the knee is so great that other nearby structures are damaged at the same time.

The most common associated injury is a meniscus tear, the special cartilage which lines the knee joint. A tear of the lateral meniscus on the outer side of the knee is most common with an ACL knee injury.

Sometimes the medial collateral ligament is also damaged. When all three structures are injured, it is known as the Unhappy Triad / O'Donoghue Triad. This most commonly happens when injury comes from a force through the outer side of the knee when the foot is fixed on the ground.

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. Surgery is therefore indicated in individuals who

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

There are two surgical options depending on the type of ACL knee injury.  In some cases, the ligament can be repaired, but more commonly, it needs to be replaced with a new substitute ligament, known as an ACL reconstruction. 

If other structures have also been damaged, such as the meniscus, they are also sorted out at the same time. LEARN MORE >

As well as protecting against abnormal knee movements, the anterior cruciate ligament 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. 

Prevention is always better than cure and this is particularly the case with an ACL knee injury due to the long recovery time, cost and potential problems in the future. 

There are a number of things you can do to reduce the risk of anterior cruciate ligament injury including special exercise programmes and wearing a specially designed knee brace.

You can find out lots more in the ACL knee injury prevention section.