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Telltale signs of an ACL injury

Nov 7, 2023 | Injury

“My life came crashing down.” 

That’s how Matilda’s captain, Sam Kerr, describes the 2014 ACL injury, which almost ruled her out of the 2015 World Cup. 

It’s a sentiment shared by many Australian athletes and weekend warriors who have experienced a similar injury. An ACL injury can significantly affect your ability to engage in sport and can make you prone to reinjury. 

What is your anterior cruciate ligament?

Your anterior cruciate ligament (ACL) stabilises your knee. 

Three bones meet at your knee – your tibia (shin), kneecap (patella) and thigh bone (femur). These are held in place by strong bands of tissue called ligaments. 

The ACL is one of these. It runs diagonally, deep in the middle of your knee. It prevents your shin from sliding out in front of your thigh bone and provides rotational stability to your knee. 

The ACL is formed mainly of collagen fibres with some elastin, giving it high tensile strength (the ability to be stretched and pulled). 

How do you injure the ACL? 

ACL injuries are relatively common, especially if you play a sport that involves pivoting, such as football, netball or skiing. Two-thirds are the result of a non-contact twisting injury, though they can also occur through contact or collisions. 

An ACL injury may be

  • Grade 1 – the ligament is stretched but can still help to stabilise the knee joint
  • Grade 2 – partial tear, the ligament is now loose
  • Grade 3 – complete ACL tear, the ligament has been torn in half or pulled directly off the bone, leaving the knee joint unstable. 

It’s not always easy for you to judge how severely your ACL is injured. Often, these events happen during moments of high adrenaline in the middle of a game. 

Telltale signs of an ACL injury

What are the key ACL injury symptoms? Look for these telltale signs

  • Popping

Ever heard a rubber band snap? You may hear a similar (but louder) sound when your ACL tears. It’s a taut ligament and you can sometimes hear it rupture.

  • Instability

Your ACL plays a key role in stabilising your knee joint. Injuring the ligament can create increased laxity in the knee. If you’ve stretched or torn your ACL, you may experience instability or ‘giving-way’ in your knee joint. 

That’s a strange feeling to describe. It feels like your knee is moving from side to side – that you can’t trust it to support you. 

  • Pain and rapid swelling

An ACL tear is a serious injury – and your body will react accordingly. You may experience pain of varying severity and you will almost certainly see your knee swell rapidly. 

Your knee swells because the joint fills with blood after an ACL injury. This is known as a hemarthrosis. There’s a blood vessel within the ligament that ruptures at the time of injury, filling the joint with blood and making it swell. 

  • Limited movement

An ACL injury can often mean your knee can no longer move through its full range of motion. You may find it hard to straighten your knee. 

What should you do after an ACL injury? 

All acute ACL injuries require assessment by a trained professional, so that initial treatment can be started and a discussion of your treatment options had. Most patients will benefit from an MRI scan to further characterise the injury and assess for other involved injuries to structures within the knee (other ligaments, cartilage and meniscus). The clinician can properly examine your knee, test your range of motion and perform a range of tests to assess the state of your ACL. 

Whilst traditionally thought the ACL can not heal, recent research suggests at times it can recover. Still, many ACL injuries result in longer term instability that can require physical therapy or surgical treatment. While not everyone needs surgery, it often proves the most helpful treatment approach for athletes and others who demand a lot of their knee. 

How can we help

I am an orthopaedic surgeon with a high degree of expertise in all aspects of knee surgery, including ACL reconstruction. 

Read my guide on ACL injury management

If you’d like to explore how I could help you, please ask your GP to complete my online referral form then book an appointment.


All information is general and is not intended to be a substitute for professional medical advice. Any surgical or invasive procedure carries risks. Dr Ross Radic can consult with you to determine if a particular treatment or procedure is right for you. A second opinion may help you decide on your options.