Knee Anterior Cruciate Ligament (ACL) Injury

Knee Anterior Cruciate Ligament (ACL) Injury

PHYSIOTHERAPY Body parts: Knee,

Knee anterior cruciate ligament injuries are fairly common injuries in sports and exercises but do you know that actually, only 20% of anterior cruciate ligament injuries are due to direct contact?

What is the anterior cruciate ligament of the knee?


The anterior cruciate ligament of the knee is one of the four ligaments of the knee, and this particular one sits deep inside the middle portion of the knee.

It is extremely important as it

  • controls the stability of the knee
  • prevents the lower leg from sliding too far forwards
  • prevents over-twisting of the knee

The ACL is

  • 2 cm long
  • as thick as a pencil and
  • extremely strong and durable

What is an anterior cruciate ligament Tear?

An ACL knee injury happens when the anterior cruciate ligament is stretched beyond its ability to stretch (typically due to the knee bending backwards too much or twisting awkwardly).

These cause:

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

The ligament can be stretched just below 2 mm before it will tear. In some cases, patients may also damage nearby other structures such as knee cartilage or the knee medial collateral ligament as well at the same time they injure their anterior cruciate ligament.

But what causes an ACL knee injury?

An ACL knee injury can happen due to direct contact with the knee or from an awkward movement, usually with the foot is fixed to the ground e.g. eg when wearing studs or cleats

Non-Contact Injuries

As mentioned earlier, non-contact injuries account for 80% of all knee anterior cruciate ligament injuries and are caused by:

  1. Twisting the Knee: such as very quick pivoting or changing direction
  2. Landing Awkwardly: from a jump or a fall
  3. Hyper-Extending the Knee: when the knee bends the wrong way by more than 10° causing it to gap and over-stretch at the back of the knee
  4. Sudden Deceleration: coming to an sudden stop/standstill whilst the foot is firmly planted

Contact Injuries

Contact injuries only cause 20% of all ACL knee injuries and are typically due to a force to/at the side of the knee with the foot is firmly planted to the ground e.g. with studs which then forces the shin bone to move either too far inwards or backwards

An ACL knee injury can also occur at other times by

  • motor vehicle accidents
  • falling or
  • even from wear and tear

Gender Effect

So while knee anterior cruciate ligament injuries seem to occur in many men, this is simply because more men simply play higher risk sports (this will change as more and more women play higher risk sports) BUT more important to note is:

Studies are showing that women are at greater risk of up to 500% chance to suffer from a knee anterior cruciate ligament injury as compared to men.

The supporting thoughts and studies point factors such as

  • hormone levels
  • ligament structure
  • pelvis Q angle
  • hamstring flexibility
  • size of the ligament itself

What are the common symptoms of anterior cruciate ligament injuries?

The most common ACL symptoms experienced with injury are:

  • Audible Pop: approximately 50% (1 in 2 incidences) of the time there is a popping or cracking noise at the time of injury when / with ACL ruptures
  • Knee Swelling: usually immediately, in less than 2-6 hours, and extensive swelling with full ruptures. But if it's only a small anterior cruciate ligament tear the knee swelling may develop a few days later or it may not even swell much if any at all. Swelling occurs due to damage to the blood vessels inside the knee - and this is termed as haemarthrosis.
  • Knee Pain: typically there is immediate knee pain after an ACL knee injury especially when you put any load/weight on the leg/through the affected knee or when you try and bend the knee.
  • Instability / Buckling: the injured knee will often seem or feel like it's unstable and can give way or buckle anytime. If the ACL has completely 100% torn, this knee instability will unfortunately be an ongoing problem until/unless repaired surgically.
  • Decreased Range of Movement:knee movement is often decreased after an ACL knee injury, particularly knee extension. This is termed as extension lag, where the knee heals with a slight flexion.

Diagnosing a knee anterior cruciate (ACL) ligament Injury

A sports or orthopedic doctor can usually accurately diagnose an ACL knee injury by doing a couple of clinical examination.

The most common tests used are the

  1. Pivot Shift
  2. Anterior Drawer and
  3. Lachmans Tests

Doctors will confirm their diagnosis of ACL tears by ordering an MRI imaging which will also show if any other structures in the knee have been damaged as well.

Associated knee Injuries


Typically with knee ACL injuries, the force that injures the ACL will often be strong enough to also damage other nearby knee structures as well.

The most commonly injured structures together with ACL are:

  • meniscal / meniscus tear, which is 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.
  • medial collateral ligament can also be damaged.

In some unfortunate circumstances, it is possibly to have ALL THREE structures injured, known as the Unhappy Triad / O'Donoghue Triad. It's most common when the knee ACL injury comes from a force through the outer side of the knee when the foot is fixed on the ground.

Anterior cruciate ligament Treatment Options

Treatment of an ACL knee injury will depend on a combination of things:

  1. severity / degree of injury
  2. level of knee instability
  3. lifestyle of the individual

ACL injuries are managed either surgically or conservatively (without surgery) with good knee physiotherapy program.

Immediately after an ACL knee injury, the best thing that patients need to do is follow PRICE principles -

  1. Protect
  2. Rest
  3. Ice
  4. Compression and
  5. Elevation

Then, there are two treatment options:

1) Knee Physiotherapy & Rehab (Recommended)

This approach is typically recommended especially

  • if there is little to no knee instability
  • if there is no severe knee pain
  • if patient is not highly active / sporty (just do stuff like running, cycling etc)

and what our knee physiotherapists in Phoenix Rehab Singapore will do for patients with partially or completely torn anterior cruciate ligament injuries is 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.

We will also train the muscles and other ligaments to provide proprioceptive feedback to gain more stability.

Typically it takes a few months of knee ACL physiotherapy to fully recover conservatively from an ACL knee injury.

2) ACL Knee Surgery

Corrective knee ACL surgery is recommended if the knee is very unstable and keeps unbuckling. This is important as each time the knee buckles, it'd damage other structures in the knees, such as the knee cartilages and knee meniscus.

It is important to avoid repeated buckling of the knee because if the meniscus tears, there is less protection for the knee bones and increases the risk of developing knee arthritis.

Of course, falls can cause other issues such as concussions and fractures such as wrist fractures, hip fractures, back and neck (spine) pain and more

Corrective ACL surgeries is recommended / indicated in patients who:

  1. Want to go back to pivoting type sports such as football, skiing, tennis, rugby, hockey - any sports with a lot of knee pivoting
  2. Have problems with the knee giving way during their everyday activities

In some patients the ACL cannot be repaired, and instead the ligament is replaced with a surgery procedure called knee anterior cruciate ligament reconstruction.

Why Is our knee anterior cruciate ligament So Important?

Our ACL ligament in the knee has two (2) core functions:

  1. protects against abnormal knee movements
  2. provides knee proprioception

Proprioception is a special feedback system that allows you and your body to know where the knee is in space and in movement (this is common in all healthy joints).

An example of joint proprioception is simply closing your eyes and trying to locate your knees with your hands or trying to move your knees - it's easy when your proprioception is normal. Proprioception lets you know where your joints are and what they are doing and how they are moving.

So our anterior cruciate ligament is responsible for knee proprioception, which helps to control and influence reflex control of the knee and its movements. This allows our knee to stay and keep stable without having us to keep actively being aware of it (ie autopilot knee stability).

Without knee proprioception, your body will not know how to adjust the knee position especiall with movement, in changing direction or when on uneven ground...leading to even more knee sprains.

With an ACL knee injury, this loss of proprioception can be compensated to a certain degree, with specific knee proprioception physiotherapy exercises for the knee muscles. BUT for people who have a high demand / need for proprioception e.g. those who play sports, this may not be enough.

Preventing knee anterior cruciate ligament Injuries

Prevention is always better than cure, especially in the case of knee ACL injuries due to the possibility of long term knee pain, knee instability, recovery time etc.

We can help provide specialized knee physiotherapy for your ACL injury.


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