Vanderbilt University
Department of
Biomedical Engineering
BME 272-273
Senior Design Project
Designing a Football Helmet System to Reduce Subdural Hemorrhaging by Mitigating Rotational Acceleration
Date Reported: April 26, 2011
Reported by: Group 3:
Doug Browne
Jeffrey Markle
Tyler Severance
School of Engineering
Class of 2011
Abstract
Over the past several years, American Football has garnered significant publicity with regards to the increasing occurrence of traumatic head injuries. Helmet designers race to engineer helmets to further reduce translational acceleration, and thus generate better performance results for the standardized drop test – used to evaluate effectiveness of helmets. Unfortunately, new trends in helmet design fail to account for angular acceleration, which has been proven to cause strain in the blood vessels connected to the brain. Relative strain between the brain and the connecting dura matter leads to vessel deformation and rupture which causes catastrophic brain injuries such as subdural hemorrhaging and to a lesser extent, concussions. Football collisions have been shown to provide enough rotational torque to cause the rupture of these vessels and thus possibly fatal injuries. To address these issues, our team has created a helmet-shoulder pad system that would mitigate this angular acceleration and bring it back down to safer levels. This system involves spring loaded dampers connected to a modified butterfly neck pad. It provides a continuous but variable force to the back of the helmet (without being directly connected) which allows for a normal range of motion, but when subjected to extreme forces, prevents dangerous levels of rotational acceleration. By incorporating this setup in three directions (one on each side, and one in the back), the system helps mitigate collisional forces coming from angles as well as head on. As the head moves rotationally, the force applied to the helmet increases exponentially to resist the head’s movement. Consequentially, this reduces the overall acceleration and thus minimizes the movement discrepancy between the brain and the dura matter. A device utilizing this design premise could be widely used to reduce traumatic brain injuries in American football.
Introduction
Since the inception of the game in 1869, American football has enjoyed ever increasing popularity until becoming the pinnacle of sporting entertainment – a title it achieved in the 21st century (Forbes). However, as the sport continues to grow, more and more athletes are exposing themselves to the inherent risks of the game. One of these, subdural hemorrhaging (SDH), poses a significant risk to athletes of any level and serves as the focus of this design and application process. It has been theorized, but only loosely proven, that subdural hemorrhages, and to a lesser extent, concussions and other similar injuries, are the result of high peak angular accelerations due to violent head collisions (Forbes). This is significant because, based on research performed from 1945 to 1994, subdural hemorrhages have accounted for the majority of football related deaths. To be more precise, it was determined that at least 352 of the 684 fatalities during this timeframe were the result of SDH (Cantu).
This significance of head injuries has dictated the policy of equipment design for several decades. Helmet use has been mandatory for approximately 70 years and facemasks have been required for almost 60. As these new equipment models were integrated, it slowly began to change the manner in which the game was played. New technique, such as attacking a ball carrier with the crown of the helmet, by athletes yielded a significant increase in the use of the head during tackles (Cantu). Thus, the incidence of fatalities as a result of brain injuries began to peak from 1965 to 1969. This spike in traumatic injuries led to the inception of the National Operating Committee on Standards for Athletic Equipment (NOCSAE). By 1973, NOCSAE had created a new set of rules and regulations to enhance player safety, and by 1980, similar standards had been passed down to collegiate and high school levels of football as well. Additionally, NOCSAE began implementing new standards of helmet regulation which further improved the safety of football players. These new regulatory standards required helmets to address the issue of skull fractures by limiting the maximum amount of translational acceleration (TA) incurred by the head. These policies, influenced by studies from the 60’s linking TA and skull fractures, also led to the implementation of the severity index. This score was given based on performance in TA testing and was considered so important that it lasted decades without being challenged.
However, despite the initial indications that helmet related injuries were decreasing, this trend eventually plateaued around 1994, and began to steadily increase once again. Additionally, the cause of a significant majority (94%) of fatalities was directly linked to the incidence of subdural hemorrhaging (Boden). It was obvious that the severity index was no longer an effective tool to determine the safety of helmets and their ability to protect athletes. Modernized testing and improvements in helmet assessment enabled a much better investigation into the precise levels of translational and rotational accelerations (RA) incurred during collisions. Strong mathematical evidence from professional and collegiate collisions as well as theoretical evidence from high school football showed that there existed an overlap between the minimum threshold to induce SDH’s and the maximum RA achieved. Therefore, because TA had been studied thoroughly over the past decade with minimal success, it is reasoned to believe that a much greater cause of SDH is due to the RA from violent collisions. This study investigates both the biomechanics regarding SDH as well as the physics surrounding a football collision. Finally, using the information learned, the team seeks to design a potential football helmet and shoulder pad construct that would limit the peak values of RA and bring it below thresholds for SDH as well as maintain the integrity of the game of football by not altering rules or technique.
Methodology
Mechanics:
The mechanisms behind the causes of rotational acceleration have already been established above; however, this RA is still significant in calculating damage to brain blood vessels. Specifically, SDH occurs when the brain, which is 4% less dense than the surrounding cerebral spinal fluid, moves relative to the dura causing strain in the bridging blood vessels (Figure 1). Cadaveric studies performed at Vanderbilt University verified the findings that blood vessels undergo plastic deformation at 120% strain and rupture at 150% strain (Forbes). These ruptures can cause drastic harm to the brain, and if left undetected, can lead to fatalities.
Design:
The main goal of this project was to design a device that will significantly decrease the angular acceleration a player’s head experiences during a football collision. The largest difficulty in regards to designing a potential solution was finding a balance between reducing the angular acceleration on the head, while at the same time permitting a normal range of motion during game play. Many different design options were considered but it was ultimately decided that a spring-loaded system would accomplish these goals most effectively. There are many advantages to using springs to resist the forces causing rotational acceleration. The first is that the force springs supply is proportional to how far they are stretched. Thus, for harder impacts, the springs will supply greater force to decrease the rotation. The second is that in the “resting” position there is very little force applied to the player wearing the device. This is ideal because it does not restrict the range of motion of a player or inhibit head movement under normal circumstances. This design consists of three coil springs with rotating arms attached to a modified butterfly collar (Figure 2). These rotating arms supply a force to the back of the helmet, thereby increasing the effective mass of the player’s head, making it harder to move, and in turn lowering the levels of rotational acceleration the player would feel. Since the device is located on the back of the shoulder pads, it will only be effective when a player gets hit from the front. It was decided that since most hits in football are head-on collisions this device would be sufficient in mitigating angular acceleration on the majority of collisions.
Simulation:
Since subjecting the finished device to actual physical testing by an accredited testing center would have been unreasonably expensive, it was necessary to construct a mathematical model to gauge the effectiveness of the design. This model used the principles of force and momentum to estimate the amount of angular acceleration the head would experience during collisions of varying severity. This model takes an incoming velocity and converts that to momentum using,
P=m*v (1)
and an average mass for a college or professional football player. Since football collisions are almost entirely inelastic, all of the momentum is transferred to the player being hit. Knowing this, the final velocity of the struck player’s head can be calculated by using the momentum equation,
m1v1+m2v2=m2v2final (2)
where the effective masses of the struck and striking players heads are known. This can be converted to angular velocity by using the equation,
ω=vR (3)
where R is the radius of where the center of mass of the head rotates around the body. This rotation occurs about the C6 and C7 vertebrae (Forbes). Knowing the final angular velocity, and estimating an angle of maximum movement of the head (~70 degrees), it is possible to determine the duration of the collision by using,
Θ=12ωfinal*ωi*t (4)
From there, the average acceleration can be found by taking the total change in angular velocity over time. The device and its effects on rotational acceleration were included in this model as a force with varying magnitude depending on the rotation of the player’s head. This was done by testing the device to see how much force it produced at different angles and fitting those points to a line. This equation was used in the model as a force that opposed the force of the striking player in the form of,
F=m*a (5)
The resulting acceleration is tangential, and must therefore be converted into the angular component, but can then thereby decreased the effective force and angular acceleration on the struck player. As the incoming force was increased, the device would supply greater resisting force and the angular acceleration would decrease at a greater rate.
Results
We developed a shoulder pad-helmet system to reduce the acceleration experienced by a football player undergoing a powerful collision to the head. The device utilized rotational dampers connected to a football neck pad that would maintain constant contact with the helmet during play (Figure 3). These dampers were chosen because the resistive force they applied was relatively low at normal resting positions which allowed a player sufficient range of motion to function within the sport. This is important since one of the major difficulties in increasing the safety of sports equipment is ensuring that players feel the equipment does not detract from their normal playing ability. As the angle of the head increased, as in a dramatic collision where the head is forced to rotate backwards rapidly, the force applied by the dampers increased significantly (Figure 4). The exponential fit equation shows an extremely high correlation (R2=.9936) with the obtained data. This equation was thus valid to use to simulate the resistive force applied by the system at any angle of the head, which was later used in the computer simulation of the accelerations.
Once the quantitative resistance was found, the integral of the force showed the resistance to acceleration added at each instance of the head’s backward rotation. This reduction could be shown quantitatively by depicting the plot of peak accelerations as a function of the approaching velocity of the striking player. Figure 5 shows the effects of collisions at the collegiate level (based on typical masses of college players) and Figure 23 shows similar effects with slightly larger athletes indicative of the professional game. Each graph clearly shows the effectiveness of the construct. Figure 34 shows the expected trend of exponentially increasing peak accelerations as the striking player hits with an increasing velocity as indicated by the blue line. Despite a peak value of 7000 rad/s2, the application of the construct reduces the trend below the red threshold line (4500 rad/s2 (Lowenhielm)) as indicated by the green line on the graph. In fact, the angular acceleration begins to plateau as higher velocities are reached and thus, could theoretically protect athletes from a significant amount of these dangerous acceleration levels. In the second study, the professional football assessment shown in Figure 23, similar effects are seen. With no limits on rotational acceleration, the head begins to experience severe levels of acceleration as the striking player approaches speeds of about 9 m/sec. These speeds are not rare, especially when players lunge into almost every tackle they perform. Thus, it is quite clear how unchecked rotational acceleration can cause catastrophic injury. However, it is also clear, when one examines the green line on the graph, that the values when the construct is used are below the threshold for SDH. This indicates that, with respect to the simulation, the construct could have a significant reduction in these severe injuries and have a lasting positive impact on the health of the athletes who play the game.
One potential criticism of the design is that by utilizing the selected dampeners, the construct creates a spring loaded mechanism which will transfer all of its stored energy during the collision and propel the head forward immediately after the backward movement stops. This is based on the idea that the acceleration can occur in the opposite direction and the peak acceleration will still be present even if it has a different directional component. This logic is flawed however, because the neck itself provides internal resistance and will decrease velocity throughout the duration of the collision. Additionally, the construct attempts to eliminate the peak values of acceleration and although the total amount is the same, the peaks are leveled out. This mathematical difference is crucial in that SDH occurs when peak values are exceeded. As long as the threshold of 4500 rad/s2 is not reached, there should be a dramatic decrease in risk of the player.