Concussion in Professional Football: Summary of the Research Conducted by the National Football League's Committee on Mild Traumatic Brain Injury

Elliot J. Pellman, MD; David C. Viano, Dr. Med., PhD

Disclosures

Neurosurg Focus. 2006;21(4) 

In This Article

Protection Against Mild TBI

I: Helmet Standards

Next to impact avoidance, football helmets are the most important factor in protecting a player from mild TBI. In 1973, the NOCSAE established standards for the impact performance of football helmets.[9] The NOCSAE standard limited the SI, which is based on resultant head acceleration. All new football helmets available for use in high school and college football were then certified to the NOCSAE standard, and the wearing of such helmets was made mandatory for college players in 1978 and for high school players in 1980.

The certified helmets cut the SI score by half compared with the headgear worn before the establishment of the standard. By 1980, significant reductions in injuries were observed after the voluntary adoption of the standards by helmet manufacturers. The injury reduction was believed to be the result of the helmet design changes, which targeted serious brain injuries such as brain contusion. Despite the improvement of helmet design for the prevention of serious brain injury, little was known regarding the effectiveness of football helmets in protecting against mild TBI at the time of the initial research conducted by the NFL's Committee on Mild Traumatic Brain Injury. Therefore, the Committee planned a series of research projects aimed at defining the biomechanics of concussive impacts in professional football.

After consideration of various alternatives, the effort focused on analyses of game videos of plays that had resulted in concussions. Experts in biomechanics proposed that, with multiple views of the impact and line markings on the field, the direction and speed of concussive impacts could be determined. Cinematographic analysis methods were developed to determine the actual speed at which players were moving before colliding (Fig. 1). This would allow laboratory reconstructions (reenactments) of the game impacts by using instrumented test dummies to simulate the helmeted players. The reenactments closely matched the situations on the field.

Figure 1.

Still photographs from films showing game action and mathematical calculations of the vector of impacts. The impact velocity of game hits was determined by analysis of two camera views of the collision. Upper: The photos show the impact sequence from two views. Lower: Graph showing the camera locations and the perspective of the two video images of the game impact. The two perspectives are mathematically merged as vectors that change with each time-step of the video. (Reprinted in modified form with permission from Pellman EJ, Viano DC, Tucker AM, Casson IR, Waeckerle JF: Concussion in professional football: reconstruction of game impacts and injuries. Neurosurgery 53: 799-814, 2003.)

Reconstruction of the game impacts involved two Hybrid III anthropometric test devices. Two high-speed videos recorded head kinematics in the reconstruction. The cameras were positioned identically to the views from the game video to allow one-to-one comparison (Fig. 2). With the aid of transducers placed in the head of the dummy, the translational and rotational accelerations of the head could be determined in concussive and noninjurious impacts. Matching the available on-field injury video to clinically confirmed mild TBI made determination of an appropriate "event" possible and verifiable. All mild TBI events in the players were examined, confirmed, and recorded by NFL team physicians.

Figure 2.

Comparison of the laboratory reconstruction (upper) and still photos of the game impact (lower) from a case of concussion sustained in an NFL game. Refinements in the test setup were done until the helmet kinematics matched the game impact sequence. (Reprinted in modified form with permission from Pellman EJ, Viano DC, Tucker AM, Casson IR, Waeckerle JF: Concussion in professional football: reconstruction of game impacts and injuries. Neurosurgery 53: 799-814, 2003.)

When a mild TBI occurred on the field, it was evaluated by a physician and athletic trainer, who completed forms describing the impact and the injury. Mild TBIs were also reported to a biomechanical engineering group contracted to analyze and reconstruct game impacts. Television network tapes of games were obtained from the NFL and analyzed.

The most striking observation in this study is that concussion in professional football involves a mean impact velocity of 9.3 m/second (20.8 mph) and a head velocity change of 7.2 m/second (16.1 mph). These are exceptionally high velocities and accelerations and long durations. Automotive crashes typically involve impact durations of less than 6 msec for head impacts with vehicle rails, pillars, and structures. The NFL results established new information on tolerances in the 15-msec range; there had been a virtual absence of scientific data on human tolerance. The NFL reconstruction data also supported a value of 70 to 75 G for concussion in padded impacts, which is at the high end of earlier tolerance ranges but is consistent with the Wayne State University concussion tolerance curve. Most important, the initial study demonstrated the strong correlation of concussion with translational acceleration, which should therefore be the primary measure for assessment of the performance of helmet protection systems.[10]

One conclusion of the initial biomechanical study was that the current NOCSAE SI and the more widely accepted Head Injury Criterion are adequate performance measures for helmet standards and that the added complexity of measuring rotational acceleration may not be needed for an improved or supplemental NOCSAE helmet standard. The results of this study provided a basis for new helmet evaluation methods, new helmet designs, and the prevention of concussions in football.

II: Biomechanical Testing

It was recognized by the Committee that a greater understanding of the location and direction of helmet impacts was needed to give manufacturers the ability to develop newer, improved mild TBI –resistant helmets. Therefore, NFL game videos were further analyzed for the typical locations of severe helmet impacts in professional football. The magnitude and direction of force causing concussion were determined by the use of selected cases that were reconstructed in laboratory tests.

A request was made to have a biomechanical testing contractor reconstruct the impact in 31 cases by using at least two clear video reviews of the collision. Laboratory tests would then be set up to reenact the game impacts with crash dummies and to measure head responses. The reconstruction emphasized helmet-to-helmet and helmet-to-ground impacts, because the video of other impacts was more obscured from clear view. Helmet contact of the struck player was categorized by the impact quadrant and head level for helmet contacts.

The study demonstrated the importance of face-mask injuries at an oblique angle, with the majority of contacts occurring below the head's center of gravity.[9] Another important aspect was that it described the quadrants on the helmet for which future NOCSAE standards may establish performance requirements (Fig. 3). By defining relevant quadrants, greater performance may be ensured over a segment of the helmet in which risks of concussion are higher in professional football, particularly low on the side and back and oblique to the face mask.

Figure 3.

Upper: Photographs of dummy heads showing location of initial helmet contacts for the struck players. Both concussive and nonconcussive impacts and falls to the ground are shown. Lower: The impact location for the striking players involved no concussions. The impact locations are all shown on the right side of the helmet, although the game impacts occurred on both sides. H-G = helmet-to-ground impact; H-H = helmet-to-helmet impact; MTBI = mild TBI. (Reprinted with permission from Viano DC, Pellman EJ: Concussion in professional football: biomechanics of the striking player-part 8. Neurosurgery 56: 266-280, 2005.)

The laboratory reconstruction of game impacts provided the Committee with data identifying the location and direction of helmet impacts associated with concussion in NFL players. It also provided unique biomechanical data on head responses associated with concussion. The response data also allowed the determination of injury risk functions for concussion.

Using the Logist function, the probability of concussion p(x) was related to various biomechanical parameters (x) measured in the reenactment tests by using the following formula: p(x) = [1 + exp(α–β)]-1 where α and β are parameters fit to the NFL data. The parameters determined for NFL concussion were as follows: α = 2.677 and β = 0.0111 for the Head Injury Criterion; α = 4.678 and β = 0.0573 for translational acceleration; and α = 5.231 and β = 0.000915 for rotational acceleration.10

III: Head-Down Tackling

For decades head-down tackling (or so-called spearing) has been a concern because it can result in catastrophic neck injuries in the striking player. The epidemiological and cinematographic analyses of neck injuries have shown that axial loading with flexion or extension causes the majority of cervical fracture–dislocations. This evidence has resulted in rules changes in high school, college, and professional football banning deliberate spearing and the use of the top of the helmet as an initial point of contact in a tackle. It was observed that players who suffered concussions were sometimes struck by players who were using head-down tackling techniques. The Committee decided to study the biomechanics of this form of injury both in the striking player ("nonconcussed") and the player who was struck ("concussed").

Once again, game film and video were collected from the NFL and correlated with clinical mild TBI data supplied by each club's team physicians. Laboratory reconstruction was performed using Hybrid III male dummies. In the dummy representing the striking player, a six-axis neck transducer was installed between the head and the top of the neck.

In helmet-to-helmet impacts, the striking player lowers the head, neck, and torso to deliver maximum force to the struck player, whose head and neck resist the impact.[15] This is the typical situation when the struck player does not see the tackle and does not prepare for the collision. The key to the concussive blow is the head-down position, which involves a 67% greater mass of the striking player by engaging his torso in the collision. Neck forces couple torso mass into the collision, which contributes to the higher effective mass of the striking player.

The prevention of concussion in the struck player provides another reason, besides preventing neck injuries in the striking one, to enforce rules against head-down tackling or spearing in football. Another means to lower concussion severity may be to reduce the stiffness of the top-crown portion of the helmet and to lower the mass of the helmet, although these changes may be less effective than enforcement of antispearing rules.

IV: Boxing

Because boxing entails considerable risk of closed head trauma, comparisons are often made between this sport and football regarding mechanisms of injury. The risk of concussion is considerably greater in professional boxing compared with professional football. The clinical picture of more severe brain injury is different in football and boxing. Boxers are much more likely to suffer subdural hematomas and deaths from brain injury than are professional football players. A better understanding of the biomechanics of head responses and mechanisms of brain injury would continue to lay the foundation for better protective headgear for sports.

Eleven Olympic boxers were included in this study.[13] These athletes were instructed to strike an instrumented Hybrid III head with their gloved fist two times with four different punches (to the forehead and jaw and with a hook and an uppercut). The height and weight of each boxer were measured and anthropometric data for the dominant hand were collected to allow the effective hand–arm mass to be calculated. Instrumentation was placed in the boxer's clenched hand as well as in the Hybrid III head. A camera recorded the event at a lens speed of 4500 images per second. The punch and head inertial forces were measured.

There were three significant differences noted between the biomechanical forces exerted on the head and brain by boxing punches and the football helmet impacts in the NFL. The boxer's punches resulted in lower translational accelerations in the struck head compared with the football impacts (Fig. 4). The boxer's punch applied a higher moment to the struck head than did the football impacts. This resulted in a higher rotational acceleration in the head that was struck than did the football impacts. Boxers sustain a brain injury by two mechanisms: translational and rotational accelerations of the brain, with a preponderance of the rotational component. Professional football players, on the other hand, sustain mild TBI mostly by translational forces because the shell of the helmet allows the players to slide relative to one another, limiting head rotational accelerations. These differences were further studied using finite element analysis of brain responses.[14] The localized strains in the brain and different biomechanical inputs help explain the clinical differences between head injuries in boxing and professional football.

Figure 4.

Scatterplot showing individual data points for translational and rotational acceleration (Accel) of the Hybrid III head for NFL game impacts and four different Olympic boxing punches. (Reprinted with permission from Viano DC, Casson IR, Pellman EJ, Bir CA, Zhang L, Sherman DC, et al: Concussion in professional football: comparison with boxing head impacts-Part 10. Neurosurgery 57: 1154-1172, 2005.)

Finite element modeling also showed that strains develop late, after the primary impact force, and focus on their response at the midbrain. This study shows a complicated interaction of the head kinematics, detailed geometrical and material properties of the brain, and the role of brain movement and deformation within the skull (Fig. 5).

Figure 5.

Anatomical models showing the deformation pattern of the finite element brain from a frontal and superior view of the hemispheres at 0, 15, and 25 msec for one of the NFL concussion cases. The sequence shows the head kinematics and brain deformations. (Reprinted in modified form with permission from Viano DC, Casson IR, Pellman EJ, Zhang L, King AI, Yang KH: Concussion in professional football: brain responses by finite element analysis-part 9. Neurosurgery 57: 891-916, 2005.)

V: Impact Velocity

In our earlier studies, we found that concussions in NFL players occur at an impact velocity of 9.3 ± 1.9 m/second (20.8 ± 4.2 mph) oblique on the face mask, side, and back of the helmet. There is a need for new testing methods to evaluate helmet performance in protecting against impacts causing concussion.

The NOCSAE certifies the helmets used by professional football players. The impact tests provide confidence that protective helmets are effective in reducing life-threatening head injuries. Data collected from the accelerometers used in the NOCSAE head drop test are used to assess the shock-attenuating properties of the helmet based on the head SI, in which the risk of serious head injury is determined from the SI. This standard does not address helmet performance in reducing the risk of concussion.

It was believed by the members of the committee that the previous NFL mild TBI research findings would allow a recommendation to be made for a new methodology for testing helmets to reduce the risk of concussions.[11] The initial approach involved pendulum impactors that were used to simulate 7.4 and 9.3 m/second impacts causing concussion in NFL players. A helmet was placed on an instrumented Hybrid III head that was supported on the neck, which was fixed to a sliding table for frontal and lateral impacts. The testing evolved to a linear pneumatic impactor, which gives better control and a broader speed range for helmet testing. The NOCSAE has prepared a draft supplemental helmet standard for the 7.4- and 9.3-m/second impacts evaluated using the new impactor. The proposed NOCSAE standard is the first to address helmet performance in reducing the risk of concussion.

VI: Performance of the Newer Helmets

The new understanding of the biomechanics of concussion in NFL players has enabled football helmet manufacturers to make design changes, for the first time, specifically to reduce the risk of mild TBI. The NFL testing techniques addressing concussion were shared previously with the helmet manufacturers and NOCSAE. The Adams USA Pro Elite, Riddell Revolution, and Schutt Sport Air Varsity Commander and DNA helmets are examples of headgear designed using the new information. Using the new mild TBI testing methodology, the Committee believed it would be useful to test the performance of newer helmets in reconstructed game impacts to compare them with a more standard VSR-4 football helmet.

The aim of this most recent study[16] was to investigate the performance of newer football helmets under conditions causing concussion in NFL players. Ten cases of NFL game concussions were selected for reenactment testing with newer helmets to investigate the equipment's effectiveness in reducing the risk of concussion. The range of impact speed was between 7.4 and 11.2 m/second for eight cases of helmet-to-helmet impacts. This was within one standard deviation of the average condition for concussion in the NFL. The two head-to-ground impact cases averaged 7.2 m/second. For each case of helmet-to-helmet impact, the striking and struck dummies were oriented to match the original laboratory reconstructions of NFL players' concussions. Verification tests ensured that the 10 reconstructed impacts from NFL games were set up similarly to the original testing. Identification on all helmets was obscured, and random tests were conducted.

Testing revealed that newer football helmets reduce concussion risks in collisions that were representative of NFL player experiences. Depending on the biomechanical response, the reductions are in the range of a 10 to 20% lower risk of concussion. The newer headgear reduces concussion risk by using thicker and more energy-absorbing padding on the side and back of the helmets and around the ears. This demonstrates an encouraging trend with the newer headgear; and we expect additional progress. The tests should help NOCSAE in its effort to finalize new helmet standards for preventing concussions.

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