Non-contact ACL injury patterns and prevention

Today's guest post comes from a company that I've had the pleasure of partnering with over the past year. EuMotus has been a vital part of my practice in bringing powerful data that was once only available to expensive biomechanical labs. Their passion to provide movement analysis to clients has been an asset to my practice. I cannot say enough about them as a company. With their new series of blogs designed to provide scientific data to injury prevention, I had to share their recent blog on ACL prevention for the soccer community since I do have several soccer players that will benefit from it. If we could identify risk factors for ACL injury in the soccer community and put an emphasis amongst the regional soccer clubs, how will we not be better off? A soccer club that takes safety and sports physicals seriously can strike a serious training and recruiting advantage!

ACL Injury Literature Review In this blog post we will examine recent scientific studies on ACL injury. But why should we care? ACL injury is costly. Replication of injuries is not feasible and measurement is tough. We establish interesting observations and recurring patterns of non-contact ACL injury. In-vivo studies and regressions show that some populations are at higher risk of non-contact ACL injury. Finally, we review literature that shows that we can screen for problematic movement patterns and institute injury prevention programs. Effective injury prevention programs have been found to decrease non-contact lower injury incidence by 52% in females and 82% in males [p]. We will attempt to provide light and understanding towards:

  1. The Cost of an ACL Injury: Why Should We Care?

  2. Measurement of non-contact ACL injury

  3. How non-contact ACL injury happens

  4. At-risk populations for ACL injury

  5. Can we screen for problematic movement patterns and mitigate ACL injury risk?

I. The Cost of an ACL Injury: Why Should We Care? The cost of an ACL injury is high. As many as 250’000 individuals are affected and the cost to the American healthcare system exceeds $2B on annual basis, per the CDC [j]. The injured athlete’s quality of life is impacted. The surgery cost of ACL injury is on the order of $10’000 [u][v]. This cost does not quantify the impact to functional health nor quality of life, nor does it include tens of physical therapy sessions. For athletes with previous ACL injury, rate of return to sport is 81%-82%, previous level of competition is ~64%, and competitive sport is 44%-55% [x][y][z]. Functional health is decreased and return to full health and sport form may be hindered by physical and/or psychological factors (e.g. fear of reinjury). Fear of reinjury is not irrational, as the data show that the probability of a follow-on ACL injury is higher for athletes that have experienced ACL injury than those that have not experienced ACL injury. Return to full lower extremity function can take years, and risk of reinjury during the first year can be higher than risk of reinjury after the first year [g]. Now that we’ve established the costliness of ACL injury, let’s examine the biomechanics in more depth. II. Measurement of Non-Contact ACL Injury and Biomechanics Up to 80% of ACL injury is non-contact, therefore further biomechanics and movement pattern investigation is warranted [c]. While the literature review on the topic if ACL injury may be without a clear causal relation, we will focus on and point to recurring themes and other interesting observations. Biomechanical loading can be performed by in vitro tests of knee and lower extremity load testing. ACL injury analysis in-vivo, however, can only be examined post-injury. Video footage is the main mechanism through which ACL injury mechanisms can be examined. Measurement Error In order to characterize ACL injury, we should be able to quantify certain biomechanical metrics such as knee flexion, hip flexion, tibia rotation, valgus angle etc. ACL injury literature tends review ACL injury post-factum utilizing video review technology. Krosshaug et. al. found a 20° - 30° error in ACL analyst researcher knee flexion observed value from true angle [d] 16.7% of undergraduate physical education majors accurately quantified sagittal plane elbow flexion [q]. Although the studies are limited in scope, physical education and movement science experts in the above two studies demonstrate that despite expert domain-level knowledge and experience, experts should be cautious of assuming accurate quantification of body limb angles such as knee and elbow flexion. Knudson performed a low sample size test in getting college students, kinesiology professors and assistant college basketball coaches to perform visual observation and range of motion scoring in the sagittal plane. 6 out of 10 college students consistently and accurately rated RoM, while only 1 out of 5 kinesiology professors and none (out of 6) basketball coaches were able to do so [r]. Despite the limited sample size and non-randomized control trial, it can be hypothesized that college students are potentially recently trained and exercised in observing and recording ROM values. Conversely, kinesiology professors and assistant collegiate basketball coaches potentially do not actively take part in and/or have never recorded range of motion values in basketball players. In non-contact ACL injury videos, it is possible to identify large expressions of valgus knee, an overextended leg or an eccentric center of gravity. However, one should be wary in the accuracy of lower extremity movement values extracted by relying on unaided visual observation. III. ACL injury: how does it happen? In-vitro tests (i.e. cadaver lower extremity tests) ​In in-vitro experiment, Markolf et. al. found that the combination of internal tibial torque and anterior tibial force produced the greatest resultant loading to the knee ligaments [h]. The maximum knee ligament combined loading is most prominent at full extension or hyperextension. Kiapour et. al. found that a combined loading sequence of knee abduction and internal tibial rotation resulted in higher knee strain than uniplanar loads alone [i]. However, Markolf’s and Kiapour’s studies utilized cadaver knee samples in a quasi-static environment, which does not necessarily replicate in-vivo conditions. DeMorat et. al. found that sudden large force loading of the quadriceps muscle at a low flexion angle (20 degrees) induces knee valgus (2.3 degrees), internal hip rotation (5.5 degrees) and anterior tibial displacement (19.5 mm)[l]. In-vivo video analysis Hewett et. al. investigated a local USA high school sixth through twelfth grade basketball, soccer and volleyball players for nearly a decade [q]. The research team identifies four characteristics of non-contact ACL injury:

  1. Knee buckling (valgus knee) upon landing.

  2. A relatively straight landing leg (overextended knee or low knee flexion).

  3. Majority or all of the weight is placed on the landing leg (rapid and high quadriceps loading).

  4. A laterally tilted trunk (this condition offsets the center of gravity, and potentially causes knee moment)

[Be aware of valgus knee. Created by EuMotus] ​Hewett defines four characteristics that can be used to describe the differences in risk factors when comparing female to male athletes [q].

  1. Ligament dominance. The ligaments absorb most of the force or impulse from situations such as athletes landing from a jump. The quadriceps muscles are weaker and absorb less of the force, thereby increasing ligament loading.

  2. Quadriceps dominance. Females tend to be more prone to quadriceps dominance. In this case, the quadriceps muscle primarily absorb force. Hewett states that the stiffening of the leg caused by quadriceps dominance is associated with lower knee flexion. Quadriceps dominance is also linked with anterior tibial translation. A counteracting mechanism in reducing anterior tibial translation and thereby ligament stress is the posterior kinematic chain in hamstring activation. IOC states similar findings [c].

  3. Leg dominance. Leg dominance is the athlete’s preference of using a certain foot for activities - e.g. jumping, landing or kicking a ball with one of the feet. Hewett finds association in leg asymmetry to greater risk of lower extremity injury [t]. Leg dominance is especially evident in females.

  4. Trunk dominance. Trunk or core dysfunction dominance refers to the athlete’s inability to control their body in space. Hewett makes reference to accelerated growth years, where male’s muscles grow in relation to their bodies, while women’s muscle to body growth remains similar.

Koga et. al. examined 11 ACL injury episodes utilizing video review techniques [a]. Despite anecdotal evidence (as limited by video footage of ACL injury episodes), and a small sample size, the research team found several recurring themes in ACL injury episodes:

  1. “immediate valgus” (the author noted valgus and the other two conditions are apparent within 40ms of ground contact),

  2. “internal rotation motion”, and

  3. forward moving (“anterior tibial translation”).

Based on these observations, Koga et. al. propose an ACL injury mechanism consisting of lateral knee compression due to:

  1. Valgus loading

  2. Anterior loading “caused by quadriceps muscle contraction“ (e.g. muscle contraction caused by single foot landing)

  3. Tibia anterior translation, combined with internal rotation.

Accordingly, the authors make the recommendation that ACL injury prevention programs should contain an element of eliminating problematic movement patterns during knee flexion characterized by valgus knee and internal rotation of the tibia. The author suggests that a preventative mechanism could potentially be found in utilizing knee and hip flexion to distribute loading forces generated by landing on the ground, as well as avoiding internal rotation. Interestingly, while the author mentions knee and hip flexion as central to knee loading, the author does not mention ankle flexion as a potential aid in reducing knee loading. In 10 ACL injury video studies of female handball players, Koga et. al. found a neutral mean valgus angle at initial contact and an increase of 12 degrees of mean valgus angle 40 ms later [b]. In addition, the research found that at initial contact, knees had a mean external rotation of 5 degrees, and had rotated internally by a mean value of 8 degrees during the first 40 ms, followed by an external rotation of 17 degrees. ​In a review of decades of literature, in vitro in vivo and computer simulation studies, Shimokochi and Shultz found that ACL injuries and high ACL loading tend to happen during the following conditions [k]:

  1. with the leg hyperextended,

  2. during deceleration or acceleration,

  3. during quadriceps muscle contraction,

  4. dith a hip internal rotation,

  5. while lacking hamstring co-contraction,

  6. during combined valgus loading and hip rotation.

Manchester United’s Zlatan Ibrahimovic single leg hyperextended knee landing resulted in ACL and PCL tears. In their ACL injury literature review, Shimokochi and Shultz also found that knee stress significantly declined at greater knee flexion angles - generally 40/45 to 60 degrees and at 90 degrees, compared to lesser knee flexion angles - e.g. 0 to 40/45. The authors also found that excessive lower extremity hyperextension increases the ACL load level, and therefore increases the chance of ACL injury. Furthermore, Shimokohchi and Shultz found that an internal knee moment combined with a quadriceps force produced greater ACL strain that an external knee moment combined with a quadriceps force. Kim et. al. hypothesized that tibiofemoral bone bruises were present as a result of non-contact ACL injury [m]. Considering a limited sample size of eight subjects (5 male & 3 female), the researchers utilized magnetic resonance imaging to confirm kinematics hypothesis of key predictors of injury. The kinematics model was constructed by envisioning the potential contact between femur and tibial bones after ACL rupture. The team found three statistically significant characteristics of extended knee ACL injury:

  1. Valgus knee angle of 5 degrees,

  2. Internal tibial rotation of 15 degrees, and

  3. Anterior tibial translation of 22mm.

Kim et. al.’s study was in the range of DeMorat et. al.s’ in vitro study findings on amplitude of lower extremity characteristics of non-contact ACL injury [l][m]. An interesting perspective that Kim brings to light through the review of the literature is that the valgus knee “could be a result of buckling of the knee after ACL rupture.” Kim reinforces that despite observed patterns in vivo, in vitro and during simulation, causality is still not yet clear. ​ IV. At Risk Populations for ACL Injury The IOC’s report investigates the higher rate of female non-contact ACL injur