The ACL Injury

Jun 19, 2020 | Written by Mike | 0 comments

The ACL Injury

Hey everyone, Mike here

Today we will unpack one of the most notorious ligaments of the human body – the ACL

Whether you’re an anatomist or a sideline expert at the Sunday league, you would have heard these 3 letters thrown around a lot. But what exactly is the ACL and why is it so important?


If you’re confused by the anatomical lingo, have no fear the breakdown is here.


*The ACL is attached towards the front of the shin bone and back of the thigh bone.

Both attachments being within the knee joint itself!


*The ACL crosses it’s close neighbour the PCL to form a cross!


*Thigh bone (femur) → Shin bone (Tibia)

Which of these is the ACL, blue or purple? Leave your comment below


The role of the ACL is to prevent your knee from hyperextending; picture bending your knee the opposite way – not pleasant. Another key role involves preventing your shin bone (tibia) from sliding forward off your thigh bone (femur). Overall, the ACL is crucial in knee stability, preventing it from shifting during quick direction changes such as sidestepping or cutting.


ACL injuries are classified under two subcategories;

1. Contact

Contact ACL injuries account for 30% of cases (Hewett, Myer & Ford, 2006). As the name implies, this type is due to contact, as from a tackle, placing the ACL in an unfavourable position.

Montgomery et al. (2016) analysed the mechanism of injury within 36 professional rugby union games and identified two main scenarios where ACL injuries occurred;

a) Offensive running – Montgomery and colleagues concluded that the ball carrier might be at higher risk of ACL injury as they increase their incident of being tackled.

(A good reason not to be a ball hog)

b) Being tackled – Tackles placing the knee into hyperextension or forcing the tibia to slide back relative to the femur places heavy stress on the ACL, potentially resulting in tears.

Watch below as New England Patriots quarterback Tom Brady sustains a knock to his planted leg during the beginning of a play. As Salem et al. (2018) mentions, contact ACL injuries are associated with higher collateral ligamentous damage. This was the case for Tom Brady, who not only tore his ACL but also his MCL (Medial Collateral Ligament) at the same time.

2 . Non-contact

The more common of the two, non-contact ACL injuries account for 70% of cases (Johnston et al., 2018). You may often hear this described as the knee “giving out from under.” This injury typically occurs after rapid deceleration after a sprint, sudden change in direction or misalignment of the trunk prior to landing such as being bumped mid air (Wetters, Weber, Wuez, Schub and Mandelbaum, 2016; Stuelcken, Mellifont, Gorman & Sayers, 2015).

The image above illustrates the two instances where the ACL is placed in its most vulnerable position.

On the left would be typical during a rapid deceleration movement where the tibia is forced to slide forward off the femur, placing great stress on the ACL whose role is to counter that very movement.

The second instance involves an action where the knee collapses inward such as when sidestepping or rapidly changing direction (Olsen, Mykelbust, Engebretsen & Bahr, 2004). This places a rotational stress on the ACL – picture twirling both ends of a towel before whipping your brother for stealing your lunch. Again, this places enormous stress on the ACL and may result in a tear.

Steer your eyes to a couple of infamous non-contact ACL injuries below.

First is AFL veteran, Chris Judd with an awkward landing after a contest for the ball mid-air. Notice how his knee collapses inward after landing.

Next is previous NBA MVP, Derrick Rose who suffered a complete tear during a playoff game where he stopped rapidly after powering the lane.

I hope this resource has served you well in understanding the anatomy and mechanism of injury of the ACL. If you have suffered injury to this ligament, ensure that you consult your physiotherapist to guide you through the key rehabilitation milestones. For further information, make sure to check out the work of ACL whiz – Mick Hughes.

Till next time!



Hewett, T., Myer, G., & Ford, K. (2006). Anterior Cruciate Ligament Injuries in Female Athletes. The American Journal Of Sports Medicine, 34(2), 299-311. doi: 10.1177/0363546505284183

Montgomery, C., Blackburn, J., Withers, D., Tierney, G., Moran, C., & Simms, C. (2016). Mechanisms of ACL injury in professional rugby union: a systematic video analysis of 36 cases. British Journal Of Sports Medicine, 52(15), 994-1001. doi: 10.1136/bjsports-2016-096425

Olsen, O., Myklebust, G., Engebretsen, L., & Bahr, R. (2004). Injury Mechanisms for Anterior Cruciate Ligament Injuries in Team Handball. The American Journal Of Sports Medicine, 32(4), 1002-1012. doi: 10.1177/0363546503261724

Salem, H., Shi, W., Tucker, B., Dodson, C., Ciccotti, M., Freedman, K., & Cohen, S. (2018). Contact Versus Noncontact Anterior Cruciate Ligament Injuries: Is Mechanism of Injury Predictive of Concomitant Knee Pathology?. Arthroscopy: The Journal Of Arthroscopic & Related Surgery, 34(1), 200-204. doi: 10.1016/j.arthro.2017.07.039

Stuelcken, M., Mellifont, D., Gorman, A., & Sayers, M. (2015). Mechanisms of anterior cruciate ligament injuries in elite women’s netball: a systematic video analysis. Journal Of Sports Sciences, 34(16), 1516-1522. doi: 10.1080/02640414.2015.1121285

Wetters, N., Weber, A., Wuerz, T., Schub, D., & Mandelbaum, B. (2016). Mechanism of Injury and Risk Factors for Anterior Cruciate Ligament Injury. Operative Techniques In Sports Medicine, 24(1), 2-6. doi: 10.1053/j.otsm.2015.09.001