Understanding ACL injury

The knee joint is biomechanically a simple joint at initial inspection, but delving a little deeper unwraps a biomechanically amazing system. Our knee allows us the ability to run, jump, stand up, squat down and contribute significantly to almost all of our two legged movement patterns that distinguish us from the rest of the animal kingdom.

It is, unfortunately, this marvel of complexity that leaves this joint susceptible to injury. Perhaps not the most common, but certainly the most well known and one that impacts the sufferers life so significantly is the injury of your ACL Ligament. “ACL” stands for Anterior Cruciate Ligament. It has a smaller counterpart is the PCL (Posterior Cruciate Ligament) and two other significant ligaments named the Medial (inner side) Collateral Ligament and Lateral (outer side) Collateral Ligament. All of these ligaments create a restraint of movement between the thigh bone (femur) and your lower leg (tibia and fibula).

(image courtesy: https://commons.wikimedia.org/wiki/File:917_Knee_Joint.jpg)

The ACL: A contributor to Stability

The knee joint is effectively a hinge. Our muscles work to move the bones relative to each other around the joint. The meniscus and articular cartilage absorb the forces we apply through the joint with the meniscus being slightly mobile to allow for shock absorption to occur throughout the range of motion. 

The ACL is a strong band of tissue within the knee that creates somewhat of an X shape with the PCL. The primary function of the ACL is to prevent movement of the tibia (shin) forwards beneath the femur. There is also a rotational restraint provided by a band of the ACL which helps to protect excessive inwards rotation of the knee.

The Injury: A story of excessive force, with a possible twist or overextension

ACL injuries generally occur during sport, in particular Netball, AFL, Soccer/ Football and any team based sport that requires hard change of direction, pivots or deceleration. We often break down the nature of the injury into either “Non-Contact” or “Contact” injury.

  • Non-contact ACL injury: often without anyone within touching distance, this mechanism is seen when rapidly stopping or changing direction while running or landing from a jump. Occasionally it can occur as the second last step before that jump or change of direction. We’re not sure yet whether there is rotation that occurs as part of this mechanism, or perhaps this rotation occurs as a result of the injury. In any case, we often see a jolt, an inwards rotation of the knee and may hear a “pop”. 
  • Contact injuries: are the result of a direct impact, usually to the back of the knee and/ or pushing the knee inwards (like when being tackled). This unfortunate mechanism may not have involved any ‘movement error’ in the mechanism, often lending these injuries to a more confident rehabilitation process. 

Beyond the ACL: Collateral Damage

The mechanism of injury described above can sometimes result in other knee structures being involved. The Meniscus and Collateral Ligaments (LCL/ MCL) are most common here, but occasionally there is also a bruise in one of the bones or even a small fracture. There are also a number of smaller ligaments around the joint that may also be injured and will contribute to the acute swelling and discomfort.

Hopefully, by helping your understanding of the ACL injury, you may appreciate how complex this simple hinge joint truly is. 

As Physiotherapists in sport, our role is often considered to be one of rehabilitation and many of us consider ourselves fantastic resources for those that must journey down the ACL Rehabilitation path. By understanding the nature of these significant injuries, we are also able to guide teams and individuals to prevent these injuries with great effect.

Ensuring you avoid the initial injury, or optimize your rehabilitation from one, will certainly promote long-term knee health.

Disclaimer: This blog provides general information and shouldn’t replace professional medical advice. Always consult a doctor for diagnosis and treatment of a knee injury.

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