A review of ‘Video Analysis of Anterior Crucial Ligament (ACL) Injuries: A Systematic Review’ by Carlson, et al. (2016).

To understand how we can prevent ACL tears or re-ruptures, we first need to understand the specific mechanisms in which they occur. If we can understand these mechanisms, we should be able to minimize risk factors contributing to ACL ruptures. There have been many intrinsic factors known to increase the risk of ACL ruptures but I won’t that discuss today. Attached is the article explaining these factors (Ref: Here)

Currently, there isn’t a consensus on the exact mechanisms of how the ACL is torn. There are many studies on increased risk factors that we commonly hear about, but the key is to really understand the kinematics +/- kinetics. However, since we don't have force plates on the ground when ruptures occur, we can only draw on the kinematics to design injury prevention programs.

In this systematic review, all types of video study designs were included along with qualitative analyses from experts in biomechanics and sports medicine. There were:

• 7 qualitative studies: Descriptions of what was occurring during the tear.
• 13 quantitative studies: Identifying joint angles at the time of landing, estimated ACL rupture timing, etc.

Results and Findings

Qualitative Analysis:

Common features present during non-contact ACL injuries include:
-Close proximity to opposing players or minor perturbation
-Increased shoe-surface friction
-High-risk maneuvers (decelerating, sidestepping)
-Landing on the heel or the flat portion of the foot.
-Landing with the center of mass located posterior to the base of support

Quantitative Analysis:

-Joint angles at the time of landing increases the risk of rupture
-Sports involved in jumping and cutting, non-contact ACL injuries appears to occur with the knee flexed < 30 deg in neutral varus/valgus angulation at initial contact with an average of 16 deg +/- 8.5
-Landed with a less plantar-flexed ankle (flatfooted or on heel)
-Landed with a more flexed hip relative to controls
-Estimated non contact ACL Injuries occurring between 20-50msec after initial contact

How can we implement these findings into practice?

1) The likelihood of an ACL injury increase when someone has their knee flexed less than 30 deg.

This means that the player is landing or decelerating with a straighter knee when a non-contact ACL injury occurs.

Why is this the case and what do I do about it?
One explanation is weak quadriceps strength, power and motor control. If we go back to our anatomy and look at the quadriceps function, it serves two purposes: 
1) Concentrically, it extends our knee
2) Eccentrically, it serves to slow down our leg in swing phase, essentially acting as “breaks.”

Imagine, you have a player decelerating from a run or absorbing shock from landing, the quadriceps has to turn on eccentrically to “slow down the movement”. If the quadriceps are weak, the player will land in a less knee flexed position and likely use a hip strategy. 
(Note: Think about stroke patients. They LACK knee flexion and hyperextends their leg because their quadriceps are weak. They do not want to bend their knee because it requires more force for the quadriceps to contract to “act as a break” or else the patient will buckle and fall.)

Looking back at this study, it shows that non contact ACL tears are occurring when the knee is less than 30 deg of flexion. This is the reason why the quadriceps is an extremely important muscle to consider when implementing a strengthening, power and motor control program.

2) Landed with a less plantar-flexed ankle.

From the picture below, we see that when the ankle’s initial contact is flat or with a heel strike, it changes the alignment of the knee. We can once again see a decrease in knee flexion, which increases the likelihood of a non-contact ACL injury. There are studies that show previous ankle sprains being a risk factor for tearing your ACL.

Take home: We need to assess if this a motor control issue that needs to be addressed with exercises or is this an ankle ROM issue.

3) Estimated non-contact ACL injuries occurring between 20-50m/sec after initial contact.

This puts into perspective how quickly the ACL tears after initial contact. From the first key point, we understand that quadriceps strengthening is important, but this point here shows that we need to think about quadriceps power (How quickly can we generate a force?).

Take home: We not only need to work on Quadriceps strength but also the rate of development of the muscle.

Conclusion

There is still no definitive answer as to what an optimal injury prevention program looks like but this study gives us an understanding of some risk factors we commonly see. If we can reverse these weaknesses, it may allow us to take a proactive approach when it comes to ACL ruptures.