September 14, 2021 Newsletter

📝 Weekly paper summary

A ‘Plane’ Explanation of Anterior Cruciate Ligament Injury Mechanisms

Category

Literature/Narrative Review

Context

ACL injuries are common amongst athletic populations, resulting in missed time from sport and potentially long-term reductions in quality of life. Although the tibiofemoral joint contains six degrees of freedom (3 axes of rotation + 3 axes of translation), it is common for researchers and practitioners to focus on a single degree of freedom ("frontal plane" knee motion rotations, sagittal plane knee flexion, etc.) when interpreting one's risk of sustaining an ACL injury and designing injury prevention or rehabilitation programs. Given the number of researchers and practitioners honing in on single-planar mechanisms, the purpose of this paper was to assess and summarize the literature evaluating single- versus multi-planar ACL injury mechanisms.

Correctness

Given the complexity of the knee joint (e.g., see Hirschmann and Müller, 2015), it is undoubtedly unfair to make any strong inferences about knee injury or pain risk based on single mechanisms. For example, we don't currently have the data to suggest that large knee abduction angles coupled with large knee flexion angles (e.g., knees moving valgus while squatting) meaningfully strain the ACL such that it could sustain a tear. I don't highlight this to downplay the importance of frontal plane knee alignment during gym-based exercises such as a squat (since we also need to be conscious of the general movement behaviours that athletes are carving while training), but rather illustrate that ACL injuries are complex and that we must interpret knee abduction angles in context with knee flexion and internal/external angles, as well as trunk orientation (e.g., Zazulak et al., 2007; Hewett et al., 2009). I'd also argue that the states (i.e., the position and momentum) of these variables, which this paper didn't explicitly highlight, are also relevant as the same movements performed with high versus low velocities or between people with high versus low body masses will nonlinearly impact the loads on both active and passive structures (due to their viscoelastic properties).

Although I like the paper's emphasis on tying together multiple mechanisms of ACL strain, I don't particularly appreciate describing these all as "planar" motions. Yes, we can describe movements in planes, but it creates confusion when researchers compute three-dimensional joint rotations about axes with motion capture systems. For example, researchers usually refer to the second rotation of the decomposed rotation matrix as the knee abduction angle, but this does not mathematically coincide with a rotation in the 2D frontal plane, which is what practitioners are more familiar with (we can correct for this using functional methods or PCA, but it's not a perfect process and is beyond the scope of this summary). This makes it possible to incorrectly compare apples to oranges depending on the computation techniques used by the researcher or practitioner, making it difficult to extrapolate findings to what we may "see" in practice.

I'm actually in the process of publishing a paper right now that validates a relatively simpler frontal plane knee motion measure that absolves both researchers and practitioners of this issue. This method will also help summarise many kinematic variables simultaneously by condensing rotations about the anteroposterior and vertical axis into a single vector. Stay tuned!

Contributions

• Interview and video observation studies indicate that ACL tears occur when the knee is at low (0-30\(^{\circ} \)) flexion angles, the tibia is externally rotated, the foot is planted, and a deceleration followed by an abduction collapse of the knee joint occurs
• The knee joint has the potential to translate further anteriorly (and thus strain the ACL to a greater extent) at shallow knee flexion angles
• Abduction moments lead to high ACL forces, particularly near full knee extension
• Concurrent quadriceps force and knee abduction moments result in ACL loads up to 100% higher in comparison to applying the same moment without any quadriceps force
• Most studies don't assess internal/external rotation of the tibia, and thus its contribution to ACL loading may be underestimated
• The magnitude of passive tibial internal/external rotation range of motion is dependent on the knee flexion angle (if your knee is flexed, you typically have more internal/external rotation range of motion relative to if your knee is extended)
• Injury prediction models and prevention programs should be designed with these multiple mechanisms of injury in mind

🧠 Fun fact of the week

There are only two mammals that like spicy food enough to seak it out. Humans are the obvious choice, but the other is the Tree Shrew! Native to Southeast Asia and China, they are often used in mice or rats in research labs as they are more genetically similar to humans. Finally, they can't get drunk! One study found tree shrews consume the equivalent of 9 beers per night in the form of fermented floral nectar!

🎙 Podcast(s) to check out

I learned a ton about testosterone and other sex hormones specifically in this talk. It was also fantastic to hear Dr. Sapolsky's nuanced take on the context-dependency regarding the benefits or detriments of stress. The discussion on determinism versus free will topped this off as one of my favourite podcasts of all time! I highly recommend giving it a listen.

🗣 Quote of the week

"Men, the philosohper's lecture-hall is a hospital - you shouldn't walk out of it feeling pleasure, but pain, for you aren't well when you enter it."

This quote from Epictetus, Discourses, 3.23.30 is somewhat jarring but greatly represents what learning (and grad school in general) entails. It's uncomfortable to tiptoe into the abyss, and it will still be painful when emerging from it, albeit for different reasons. With any good education, we enter with the unknowns being truly unknown but then emerge with an entirely new set of known unknowns. If you feel entirely comfortable after reading a new paper, taking a new course, or listening to a new podcast, perhaps give it another review to ensure you aren't fooling yourself into a false sense of security.