Documentation:FIB book/Football-helmets-and-their-impact-on-rotational-brain-injury

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Introduction

In North America, football ranks as one of the most played sports in amateur, collegiate and professional leagues by athletes from a wide range of ages and ability levels. Recently, playing American football has been linked to an increased risk of CTE (Chronic Traumatic Encephalopathy) through the repeated head traumas endured during gameplay with prevalence rates ranging from 50%[1] to 99%[2] amongst retired NFL players. CTE results in devastating effects on retired players including memory loss, impulse control problems, aggression, depression and progressive dementia[3]. During the 2015-2019 period, 61.7 concussions per 100 NFL season games were reported[4]. Another study from 2003 focusing on collegiate athletes reported 5.56 concussions per 1000 athletic exposures (game or practice) [5].

Historically, research has primarily focused on linear kinematics[6], so the injury criteria in this field have been developed to address this specific injury mechanism. However, in the past few years, emphasis has been placed on studying rotational kinematics, as it is believed to be a primary mechanism for brain injuries [7].

Football helmets from the 1950's and 1960's

Helmets play a key role in brain protection while playing football, as this sport exhibits the highest rate of head impact events [8] . The first football helmets used in the 1910s were basic leather headgear and mostly protected the ears. In 1939, the first plastic helmets were introduced and became mandatory in the NFL in 1943 [9]. Since then, large improvements on helmets have been made in terms of energy absorption, in order to reduce skull fracture and brain injuries.

Rotational Brain Injury

Concussions are the most commonly occurring brain injury, with half of all concussions being sport related[7]. Symptoms range from headaches and nausea to blurred vision and impaired memory. Furthermore, the cumulative damage from multiple subconcussive impacts has not been researched in depth. As concussions are prevalent in both American football and ice hockey, these sports have been prime research environments for head acceleration data since the 1970’s. Research into concussions started out qualitatively, but over the years, more quantitative research has been done to measure head impact levels on both helmeted and unhelmeted football players. The former was and is still done by instrumenting helmets with accelerometers to capture impact levels in the moment. However, there is still little research on the specific effects of rotational brain injury, which typically leads to more diffuse brain injury as opposed to the more focal brain injury from linear acceleration, in injurious football impacts[6].

Diffuse Axonal Injury: a type of brain injury commonly caused by rotational accelerations to the head and brain during injurious football impacts

A study performed at Virginia Tech used the method of instrumenting football player helmets over the course of numerous seasons, resulting in a mass of data to analyze. Despite over 27,000 single impact incidents measured between 2003-2006 of Virginia Tech football seasons, the majority of measured impacts are non-concussive and thus could not gain much insight into injury data[10]. This puts the results under question as to whether it can be truly representative of concussion data if there were no injurious concussion data points. It was curious to note that there was no statistical difference in the distribution of head impact between different player positions, affirming the validity of instrumenting any available player helmet.

For the research, the impacts were separated into different regions: front, back, sides, and top, with frontal impact being the most common. Rowson et al. considered top impacts separately from the other impact areas due to the loading case being relatively axial with the spine and was assumed to not lead to any rotational head velocity or acceleration. From Rowson’s study, it was deduced that front and back impacts made up 58% of all concussive impacts, with side impacts making up another 12% [6]. The study yielded the following average rotational acceleration and velocity values for both subconcussive and concussive cases:

Average Rotational Velocity and Acceleration of Subconcussive and Concussive Impacts
Subconcussive Concussive
Rotational Velocity (rad/s) 5.5 22.3
Rotational Acceleration (rad/s2) 1230 5022

It is important to note these are average values, not threshold values. Furthermore, high magnitudes of rotational acceleration can be tolerated for short periods of time but injury risk increases with exposure time. Rowson also found that rotational velocity is a good predictor of the brain’s strain response, and has a very strong correlation to relative brain motion. This could hopefully be used in the future to determine or predict injury.

Experimentation

Data Collection

Throughout multiple research studies in this field, various methods and tools were used to collect data. This included volunteers, Hybrid III heads, and finite element head models. Early studies performed by the NFL also used game footage in attempts to mimic the impacts with Hybrid III dummies[10].

In order to collect data from live volunteers, the football player’s helmets were instrumented with an array of accelerometers called the Head Impact Telemetry System (HITS) which is able to wirelessly transmit data live during the game [6]. This method was used for many studies, however the system was intended for linear impacts, with estimated rotational velocities and accelerations. A study by Rowson et. al. developed a new 6 degree-of-freedom system to more accurately capture both linear and angular accelerations[10]. Unfortunately, due to the size of this system, the helmets could not accommodate a wireless and live telemetry system as well. The data from this new system was downloaded after the game and matched with timestamps from game footage [10].

A study by McIntosh et. al. used a finite element model head in tandem with video footage to find a better method of concussion incidence classification. Video footage of football impacts were screened to find ones suitable for reconstruction using the FE model and kinematic analysis[11].

With new and improving technologies, combined kinematics show promise for being better predictors of injury rather than single direction parameters. This is especially true as there have been recorded cases of extremely high single direction velocities and accelerations without injury [10]. While linear kinematics can be used to determine intracranial pressure [11], angular brain motion may be a better indication of concussion and MTBI. McIntosh’s study found that rotational head kinematics are the most important factor for determining intracranial strain, which is also a strong indicator for MTBI. Furthermore, angular velocity is associated with the brain’s motion pattern and can be used to better classify concussion incidence.

Rotational Brain Injury Prevention and Football Helmets

Up until recently, the majority of helmet studies have focused on absorption and prevention of injury caused by linear acceleration, with very few reporting on rotational impact and accelerations[12]. However, in recent years, there have been multiple studies on helmet design specifically for the prevention and reduction of rotational brain injury, using the following design considerations: reduction of friction, addition of outer shell, and creation of an impact diverting mechanism (IDM).

Reduction of Friction

In the simplest model of a helmeted head impact with a homogenous rigid sphere (head) impacting a rigid surface (helmet), no rotation results if the contact is frictionless, since the moment applied to the head from the normal component of impact is zero, and the overall moment on the head center of gravity depends only on the coefficient of friction of the impacted surface and incident trajectory[12]. Based on this, various models have been considered to reduce friction in football head impacts. In a study done by Finan et al, Fenlon, a Teflon/epoxy composite was placed on the outer surface of a helmet to simulate a low-friction condition in a series of drop tests at various impact angles. Results showed a decrease in peak rotational acceleration by 55% (compared to a helmet without modification) in one oblique impact condition, but an increase in the same variable by 83% at a different angle[12]. This demonstrates that the influence of an outer surface friction modification for football helmets on the rotational acceleration experienced by the head is sensitive to the particular impact scenario. Friction can also be modified within the helmet; a study by Aare et al. adding a low-friction Teflon layer between the helmet shell and liner in motorcycle helmets demonstrated a 56% reduction in rotational acceleration under various angles of impact testing[13].  

Addition of Outer Shell

Another method of designing football helmets for limiting rotational acceleration during impact has been by the addition of a shock-absorbing outer shell around the helmet. In a study done by Zuckerman et al, 70-duro Sorbothane was selected to form an outer shell around a Riddell Revolution football helmet to undergo impact testing, due to the material’s non-Newtonian properties and ability to dissipate energy without undergoing plastic deformation[14]. The modified helmet and a control helmet were impacted 5 times at the front boss, side, and back at 6 m/s via pneumatic ram testing. The modified helmet reduced rotational acceleration at the front boss by 49.8% (9312.8 rad/s2  to 4671.7 rad/s2). Although more studies are needed, these results are still promising as the rotational acceleration of the football helmet with the outer shell decreased to a value below the concussive threshold[6].

Impact Diverting Mechanism

An impact diverting mechanism (IDM) is a design of four layers of polymer films separated by a medium, which provides a low-friction interface upon oblique impact, reducing the amount of friction transferred from the impact surface to the outer helmet shell[15]. The purpose of an IDM is to “attenuate rotation acceleration by decoupling the outer shell from the impacting surface during oblique impacts” [15]. During oblique impact, the top polymer film layer of the IDM stretches and slides, mitigating rotational acceleration during high-speed impact. When the force is removed from the IDM, the system returns to its original shape. In an Abram et al. study, a Riddell Speed football helmet was equipped with IDM and underwent oblique impact drop testing on the helmet’s front, side, and back at various impact angles [15]. Results demonstrated a reduction in rotational acceleration of 77% when compared to a conventional helmet, as well as reduction of rotational velocity, HIC (Head Injury Criterion), RIC (Rotational Injury Criterion), BrIC (Kinematic Rotational Brain Injury Criterion), SI (Gadd Severity Index), and P(AIS 2) [15]. In addition to its ability to potentially reduce the injury severity due to rotational acceleration during oblique impacts, a major advantage of an IDM system is that it can easily take the form of a sports decal or sticker, making it very easy to incorporate into existing football helmets and practices. A visualized application of the IDM system can be seen here.

Further Design for Injury Prevention

All of the aforementioned methods, although demonstrating some promise for reducing rotational acceleration during oblique impact, still require additional studies, testing, and further design for widespread application in football use. This may include consideration of league equipment regulations and feasibility of use without disrupting gameplay.

Problems and Controversies

Although there have been developments in rotational brain injury prevention, there are several limitations and controversies that exist in current research. Especially considering the significantly lower amount of concussive data compared to the amount of sub-concussive injury data, much of the conclusions drawn from experiments can be called into question regarding their validity. However, acquiring more human concussive injury data can be extremely difficult: volunteer testing above injury levels raises clear ethical concerns, using animal studies also has ethical concerns but have additional scaling factors that affect results, and live data does not always yield concussive data. The sections below discuss a few more problems and controversies regarding specific aspects of the experiments.

Biofidelity of the Hybrid III ATD

The Finan et al, Aare et al. and Abram et al. studies utilized Hybrid III dummies in testing[12][13][16]. The Finan et al. study lists the use of the Hybrid III as a primary limitation, and the authors mention the NOCSAE headform - a headform approved for helmet testing - would have been more desirable for their study. However, the Hybrid III was selected as it could be better instrumented to record rotational accelerations. The Aare et al. study used only the neck and head of a Hybrid III dummy and mentions that a more realistic test would have included the entire dummy. The Hybrid III neck is also likely too stiff to be used in this manner, which could affect the biofidelity of these tests. The characteristics of the Hybrid III neck cause a more uniform response to impacts and promote rotation instead of translation in impacts [10]. Additionally, a Rowson et al. study comments on the biofidelity of the Hybrid III neck and states that while the Hybrid III produces accurate results for peak accelerations in concussive impacts, it outputs higher rotational velocities [6].

Test Setup Controversies

Most studies analyzed in this review report limitations with test setups. The Finan et al. study mentions that the number of impact scenarios is infinite, however only seven were analyzed in the study. In this study, tests were conducted using a drop tower and impact plate, which confined impact to the sagittal plane and did not account for other impact trajectories. The Rowson et al. study instrumented the helmets of 335 collegiate football players and found a relationship for concussion probability based on helmet measurements. However, the under-reporting of concussions is indicated as a primary limitation, as it biases data and could lower the estimated injury probability. Additionally, the study does not account for cumulative head impacts, which may have an effect on injury tolerance. The Zuckerman et al. study designed a prototype that utilized external padding, which added 1.606 lbs to the weight of the model. This extra weight of the helmet could have been a large contributor to the reduced acceleration, in addition to being unrealistic in real-life gameplay.

Instrumentation Issues

Several studies comment on the accuracy of instrumentation and measurement devices. The Rowson et al study used the HIT system to measure rotational velocity and acceleration, but this system does not directly measure rotational velocity and acceleration and instead estimates it from linear velocity. Additionally, an error of 1-4% was reported from the HIT system.

Future Research & Limitations

Limitations

A serious limitation in volunteer testing is biased data, since an estimated 53% of the concussions experienced by football players go unreported[17]. This can be explained by the fact that concussions diagnosis is a very difficult process: symptoms can vary tremendously between individuals, and concussions don't appear in imaging[18]. Moreover, the concussion diagnosis varies based on the time (which can be multiple days) after the impact when the symptoms are observed and reported[6]. This also raises the problem of identifying the exact impact that led to the observed concussion.

Current tolerance curves for brain injury are based on animal and cadaver testing and only consider linear accelerations[6]. HIC (Head Injury Criterion) and SI (Severity Index) have been developed from these tolerance curves[6]. Therefore, no injury criteria for rotational acceleration are presently accepted, despite being a primary mechanism for mild traumatic brain injury[19]. A limitation from the tolerance curves themselves is the scaling of injury data from animal testing to humans. The assumptions that are made, as well as those present in cadaver data make the determination of these injury criteria a speculative processes.

A limitation of the existing studies on football helmets is that they account for a small sample size of actual concussive cases, all with very different head impact exposures. Individual differences in human volunteers can lead to different tolerances to concussion, perception of injury, and symptoms. Therefore, it is difficult to draw clear and defined conclusions about injury when considering these various limitations in testing.

Future Research

Future research in rotational brain injury of football players is needed to overcome the limitations highlighted in this literature review.

A proper injury threshold for rotational brain injury should be developed and validated, requiring helmet manufacturers to meet specific safety standards. The development of such injury criteria has shown to be very difficult, since the mechanisms behind rotational brain injury are still not clearly defined. It will require an effective combination of FE models and animal testing, combined with available human data from instrumented helmets. This source of data is still scarce[10] and further measurements should be taken in order to build a complete database from reported and recorded injurious impacts.

Future research should also address the biofidelity limitations brought upon by the usage of the Hybrid III ATD. The use of alternative ATDs that may be biofidelic in impact situations specific to football (i.e. BioRID) should also be considered and applied in these tests. There also needs to be testing on more impact scenarios and work is required to understand the consequences of cumulative head impacts on brain injuries.

Considering existing research and initial designs, more effort should be put into developing helmets that protect against rotational brain injury. This will require helmet manufacturers to collaborate with independent researchers in order to facilitate the development of new technologies[14]. A key point of such improvement will be testing a wide range of materials in order to find one that is able to dissipate as much energy as possible, while still deforming only elastically, since football helmets will sustain repetitive impacts. One promising class of material are non-Newtonian compounds, since they exhibit different properties depending on the impact characteristics; Zuckerman proposed the study of oxide and metallic glass-forming materials and crystal compounds as promising candidates for helmet applications[14].

References

  1. ↑ McKee AC, Cairns NJ, Dickson DW, Folkerth RD (2016). "The first NINDS/NIBIB consensus meeting to define neuropathological criteria for the diagnosis of chronic traumatic encephalopathy". Acta Neuropathol. 131(1):75-86. doi: 10.1007/s00401-015-1515-z.
  2. ↑ Mez J, Daneshvar DH, Kiernan PT, Abdolmohammadi B, Alvarez VE (2017). "Clinicopathological Evaluation of Chronic Traumatic Encephalopathy in Players of American Football". JAMA. 318(4):360-370. doi: 10.1001/jama.2017.8334
  3. ↑ https://www.bu.edu/cte/about/frequently-asked-questions/. "What is CTE". Boston University CTE Research Center. Accessed December 3, 2021
  4. ↑ Mack, Christina D., et al. (2021). “Epidemiology of Concussion in the National Football League, 2015-2019.” Sports Health, vol. 13, no. 5, Sept. 2021, pp. 423–430, doi:10.1177/19417381211011446.
  5. ↑ Booher, M. A., J. Wisniewski, B. W. Smith, and A. Sigurdsson (2003). "Comparison of reporting systems to determine concussion incidence in NCAA division I collegiate football". Clin. J. Sport Med. 13:93–95, 2003.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 S. Rowson, S. M. Duma, J. G. Beckwith, J. J. Chu, R. M. Greenwald, J. J. Crisco, P. G. Brolinson, A.-C. Duhaime, T. W. McAllister, and A. C. Maerlender (2011). “Rotational head kinematics in football impacts: An injury risk function for concussion,” Annals of Biomedical Engineering, vol. 40, no. 1, pp. 1–13, 2011.
  7. ↑ 7.0 7.1 King, A. I., K. H. Yang, L. Zhang, W. Hardy, and D. C.Viano (2003). "Is head injury caused by linear or angular acceleration?". Proceedings of the International Research Conference on the Biomechanics of Impact (IRCOBI).
  8. ↑ Hootman, J. M., R. Dick, and J. Agel (2007). "Epidemiology of collegiate injuries for 15 sports: summary and recommendations for injury prevention initiatives". J. Athl. Train. 42:311–319.
  9. ↑ Riffenburgh, Beau (1986). The Official NFL Encyclopedia. New American Library. pp. 34–38. ISBN 9780453005241.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 Rowson, S., Brolinson, G., Goforth, M., Dietter, D., & Duma, S. (2009). "Linear and Angular Head Acceleration Measurements in Collegiate Football". Journal of Biomechanical Engineering. 131(6). doi:10.1115/1.3130454
  11. ↑ 11.0 11.1 McIntosh, A (2014). "The biomechanics of concussion in unhelmeted football players in Australia: A case–control study". BMJ Open. 4.
  12. ↑ 12.0 12.1 12.2 12.3 J. D. Finan, R. W. Nightingale, and B. S. Myers (2008). “The Influence of Reduced Friction on Head Injury Metrics in Helmeted Head Impacts”. Traffic Inj Prev. 2008 Oct;9(5):483-8. doi: 10.1080/15389580802272427.
  13. ↑ 13.0 13.1 Magnus Aare & Peter Halldin (2003). "A New Laboratory Rig for Evaluating Helmets Subject to Oblique Impacts". Traffic Injury Prevention, 4:3, 240-248, DOI: 10.1080/15389580309879
  14. ↑ 14.0 14.1 14.2 S. L. Zuckerman et al. (2018). “A football helmet prototype that reduces linear and rotational acceleration with the addition of an outer shell”. Journal of neurosurgery, vol. 130, no. 5, pp. 1634–1641, May 2018, doi: 10.3171/2018.1.JNS172733.
  15. ↑ 15.0 15.1 15.2 15.3 D.E. Abram, A. Wikarna, F. Golnaraghi, and G. G. Wang (2020). “A modular impact diverting mechanism for football helmets”. Journal of biomechanics, vol. 99, Jan. 2020, doi: 10.1016/J.JBIOMECH.2019.109502.
  16. ↑ Abram, Daniel E.; Wikarna, Adrian (Jan. 2020). "A modular impact diverting mechanism for football helmets". Journal of biomechanics. 99. Check date values in: |date= (help)
  17. ↑ McCrea, M., T. Hammeke, G. Olsen, P. Leo, and K. Guskiewicz (2014). "Unreported concussion in high school football players: implications for prevention". Clin. J. Sport Med. 14:13–17
  18. ↑ K.J. Schneider (2019). "Concussion - Part I: The need for a multifaceted assessment", Musculoskelet. Sci. Pract., 42 (2019), pp. 140-150, 10.1016/j.msksp.2019.05.007
  19. ↑ Gennarelli T. A. (1983). "Head injury in man and experimental animals: clinical aspects". Acta Neurochir. Suppl. (Wien) 32:1–13


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