Thomas Blaine Hoshizaki

The science and design of head protection in sport.

WE REVIEW THE relationship between science, testing standards, and helmet design to provide an understanding of how helmets protect the brain. Research describing the mechanisms of injury, resulting types of brain injuries, and characteristics of helmet protection are reviewed. The article is designed to describe the state of the relationship between science and helmet performance.

Traumatic brain injuries: the influence of the direction of impact.

BACKGROUND Head impact direction has been identified as an influential risk factor in the risk of traumatic brain injury (TBI) from animal and anatomic research; however, to date, there has been little investigation into this relationship in human subjects. If a susceptibility to certain types of TBI based on impact direction was found to exist in humans, it would aid in clinical diagnoses as well as prevention methods for these types of injuries. OBJECTIVE To examine the influence of impact direction on the presence of TBI lesions, specifically, subdural hematomas, subarachnoid hemorrhage, and parenchymal contusions. METHODS Twenty reconstructions of falls that resulted in a TBI were conducted in a laboratory based on eyewitness, interview, and medical reports. The reconstructions involved impacts to a Hybrid III anthropometric dummy and finite element modeling of the human head to evaluate the brain stresses and strains for each TBI event. RESULTS The results showed that it is likely that increased risk of incurring a subdural hematoma exists from impacts to the frontal or occipital regions, and parenchymal contusions from impacts to the side of the head. There was no definitive link between impact direction and subarachnoid hemorrhage. In addition, the results indicate that there is a continuum of stresses and strain magnitudes between lesion types when impact location is isolated, with subdural hematoma occurring at lower magnitudes for frontal and occipital region impacts, and contusions lower for impacts to the side. CONCLUSION This hospital data set suggests that there is an effect that impact direction has on TBI depending on the anatomy involved for each particular lesion.

Compressive properties of helmet materials subjected to dynamic impact loading of various energies

Abstract Many helmet safety standards require childrens helmets to be tested using adult-weighted headforms of approximately 5 kg and impact velocities representative of adult anatomy. The purpose of this study was to test the individual and combined effect of variable headform mass and inbound headform velocity on helmet test results. Testing was conducted on sample sections of helmet liner materials commonly used in multi- and single-impact helmets. Three densities of expanded polystyrene and expanded polypropylene were moulded into 2.54-cm thick foam blocks and cut into circular samples with a 5-cm diameter. Each sample was impacted once using an EN 960 magnesium K1A headform of variable mass on a monorail apparatus in the crown position. A total of 25 impact conditions were used: 5 headform masses and 5 inbound velocities. A PCB 203B force sensor collected force data at 20 kHz in the y-axis of the impact and a 1000-Hz low-pass Butterworth filter was applied during analysis. A three-way analysis of variance revealed significant main effects for headform mass, inbound velocity, and material density on peak linear acceleration (P<0.01). Inbound velocity and headform mass played a significant role in material performance. It is proposed that the headform mass and inbound velocity used in helmet testing protocols be representative of the intended age group to improve the performance range and safety of sport helmets.

Defining the effective impact mass of elbow and shoulder strikes in ice hockey

Reconstruction of real-life events can be used to investigate the relationship between the mechanical parameters of the impact and concussion risk. Striking mass has typically been approximated as being the mass of the body part coming into contact with the head without accounting for the force applied by the striking athlete. Thus, the purpose of this study was to measure the effective impact mass of three common striking techniques in ice hockey. Fifteen participants were instructed to strike a suspended 50th percentile Hybrid III headform at least three times with their elbow or shoulder. Effective impact mass was calculated by measuring the change in velocity of the player and the headform. Mean effective impact mass for the extended elbow, tucked-in elbow, and shoulder check conditions were 4.8, 3.0, and 12.9 kg, respectively. Peak linear accelerations were lower than the values associated with concussion in American football which could be a reflection of the methodology used in this study as well as inherent differences between both sports.

Dynamic impact response characteristics of a helmeted Hybrid III headform using a centric and non-centric impact protocol

A linear impactor system was used to apply a condensed version of the University of Ottawa Test Protocol, employing five centric and non-centric impact conditions, to a Hybrid III headform fitted with six certified ice hockey helmets. None of the helmeted conditions exceeded linear acceleration thresholds for traumatic or mild traumatic brain injury; however, five of the six helmets had angular acceleration results that were above the 80% risk of mild traumatic brain injury threshold proposed by Zhang et al. High risk of mild traumatic brain injury was associated with non-centric impact conditions and peak angular accelerations, supporting the need for improved three-dimensional helmet certification standards.

Differences in region-specific brain tissue stress and strain due to impact velocity for simulated American football impacts

Concussion has become a prevalent injury in the sport of American football, and its severity can be influenced by the mass of the impactor, velocity, compliance, and direction of impact. As a result, it is important to characterize how American football helmets perform against these impact characteristics. The purpose of this research is to examine how an American football helmet performs across velocities and impact angles which can occur in the sport of American football. The methods used a combination of Hybrid III headform impacts combined with a finite element modeling approach to find the brain deformation variables known to be associated with concussion. At the 9.5 m/s impacts, the brain deformation metrics showed an increase in risk of concussion. Also, the region of the brain with the largest magnitude deformation shifted with differing velocities when analyzed using maximum principal strain but not von Mises stress. The results indicate that impact conditions (location and velocity) can influence the regional brain strains.

The application of brain tissue deformation values in assessing the safety performance of ice hockey helmets

This research was undertaken to examine a new method for assessing the performance of ice hockey helmets. It has been proposed that the current centric impact standards for ice hockey helmets, measuring peak linear acceleration, have effectively eliminated traumatic head injuries in the sport, but that angular acceleration and brain tissue deformation metrics are more sensitive to the conditions associated with concussive injuries, which continue to be a common injury. Ice hockey helmets were impacted using both centric and non-centric impact protocols at 7.5 m/s using a linear impactor. Dynamic impact responses and brain tissue deformations from the helmeted centric and non-centric head form impacts were assessed with respect to proposed concussive injury thresholds from the literature. The results of the helmet impacts showed that the method used was sensitive enough to distinguish differences in performance between helmet models. The results have shown that peak linear acceleration yielded low magnitudes of response to an impact, but peak angular acceleration and brain deformation metrics consistently reported higher magnitudes, reflecting a high risk for incurring a mild traumatic brain injury.

The Influence of Impact Angle on the Dynamic Response of a Hybrid III Headform and Brain Tissue Deformation

ASTM Symposium on the Mechanism of Concussion in Sports, Atlanta, Georgia, USA, 13 November 2012

An examination of the current National Operating Committee on Standards for Athletic Equipment system and a new pneumatic ram method for evaluating American football helmet performance to reduce risk of concussion

Brain injuries are prevalent in the sport of American football. Helmets have been used which effectively have reduced the incidence of traumatic brain injury, but have had a limited effect on concussion rates. In an effort to improve the protective capacity of American football helmets, a standard has been proposed by National Operating Committee on Standards for Athletic Equipment that may better represent helmet-to-helmet impacts common to football concussions. The purpose of this research was to examine the National Operating Committee on Standards for Athletic Equipment standard and a new impact method similar to the proposed National Operating Committee on Standards for Athletic Equipment standard to examine the information these methods provide on helmet performance. Five National Operating Committee on Standards for Athletic Equipment–certified American football helmets were impacted according to the National Operating Committee on Standards for Athletic Equipment standard test and a new method based on the proposed standard test. The results demonstrated that the National Operating Committee on Standards for Athletic Equipment test produced larger linear accelerations than the new method, which were a reflection of the stiffer compliance of the standard meant to replicate traumatic brain injury mechanisms of injury. When the helmets were impacted using a new helmet-to-helmet method, the results reflected significant risk of concussive injury but showed differences in rotational acceleration responses between different helmet models. This suggests that the new system is sensitive enough to detect the effect of different design changes on rotational acceleration, a metric more closely associated with risk of concussion. As only one helmet produced magnitudes of response lower than the National Operating Committee on Standards for Athletic Equipment pass/fail using the new system, and all helmets passed the National Operating Committee on Standards for Athletic Equipment standard, these results suggest that further development of helmet technologies must be undertaken to reduce this risk in the future. Finally, these results show that it would be prudent to use both standards together to address risk of injury from traumatic brain injury and concussion.

Evaluation of Dynamic Response and Brain Deformation Metrics for a Helmeted and Non-Helmeted Hybrid III Headform Using a Monorail Centric/Non-Centric Protocol

Head injuries and concussion in particular has become a source of interest in the sport of ice hockey. This study proposes a monorail test methodology combined with a finite element method to evaluate ice hockey helmets in a centric/non-centric protocol with performance metrics more closely associated with risk of concussion. Two conditions were tested using the protocol a) helmeted vs no helmet, and b) vinyl nitrile lined hockey helmet vs expanded polypropylene lined hockey helmet. Results indicated that the impact velocities and locations produced distinct responses. Also, the protocol distinguished important design characteristics between the two helmet liner types with the vinyl nitrile lined helmet producing lower strain responses in the cerebrum. Furthermore, it was discovered that low risk of injury peak linear and rotational acceleration values can combine to produce much higher risks of injury when using brain deformation metrics. In conclusion, the use of finite element modeling of the human brain along with a centric/non-centric protocol provides an opportunity for researchers and helmet developers to observe how the dynamic response produced from these impacts influence brain tissue deformation and injury risk. This type of centric/non centric physical to finite element modeling methodology could be used to guide innovation for new methods to prevent concussion. Keywords: Ice hockey, Helmets, Standards, Concussion 1.0 Introduction Mandatory protective headgear in impact and contact sports help protect athletes against traumatic brain injuries (TBI) including intracranial bleeds and skull fractures. However, mild traumatic brain injuries (mTBI), such as concussions, are still common with studies reporting helmets not effective in managing the risk of mTBI [1,2]. The National Hockey League (NHL) report an increase in mTBIs’ over the last decade accounting for 18% of all hockey injuries [4]. These statistics suggest that changes in the game including improved helmet technology have had little effect on the incidence of concussion. Present helmet technology is designed to minimize peak linear acceleration during a direct impact [6]. Linear acceleration was chosen as the performance metric for evaluating helmets as this measure has been associated with TBI [6;7;8]. As a result, linear dominant impact conditions have been utilized in standards to evaluate helmets [9;10]. These standards typically use a headform and monorail system for primarily centric (defined as the impact vector passing through the center of gravity of the head) impacts. However, rotational acceleration has also been identified as an important factor in the incidence of concussion and must also be measured. Higher rotational acceleration responses tend to result from non-centric impacts (defined as impacts whose vector does not pass through the centre of gravity of the head) [11;12]. These rotations cause shear stress within the brain which has been proposed as a predictor for mTBI [11;12]. Current helmet standards do not consider rotational acceleration when assessing helmet performance despite several studies associating rotational acceleration to risk of sustaining a concussion [5;12;13]. However, definitive thresholds of injury for concussion using linear and rotational acceleration have yet to be elucidated; this difficulty has been identified by researchers to be due to the kinematics not accounting for the interaction between the impact induced motions and the brain tissue [15;16]. As a result, advanced computational models have been developed to better understand the effect of impact head kinematics on brain tissue damage [13;14;15]. Measuring brain tissue deformation using finite element models of the human brain is considered an effective method in evaluating risk of sustaining an mTBI [16]. Finite element modeling of the brain during impact allows for the examination of the effect of complex loading curves on brain tissue deformations. The characteristics of these linear and rotational acceleration loading curves can then be used as input parameters into complex brain models which can then simulate the deformation of tissue resulting from the kinematics of an impact event [17;18]. Past research has shown how this method can predict the effect of linear and rotational accelerations on the stresses and strains imparted to the brain through car crash analysis as well as hockey and football helmet impacts [13;19;20]. As a result, finite element models for the head and brain provide an opportunity to use brain deformation values to evaluate the ability of a hockey helmet to reduce the risk of brain injury [20]. There is presently no standard which uses a centric/non-centric impact method coupled with finite element analysis to measure brain deformations from helmeted impacts. If such a method was developed it may aid in supplying more information on helmet performance using linear and rotational acceleration as well as brain deformation metrics [13;14;15;21]. Previous research has investigated this type of protocol using a linear impactor system, which was created to replicate player to player collisions [5]. This linear impactor method is different from current drop tower methods used by certification bodies to certify helmets. The development of this type of protocol using the monorail drop system to include centric and non-centric impacts may allow for easier adoption this new protocol using current test equipment. The objective of this study was to use a monorail centric/non-centric impact methodology to compare the dynamic responses of a helmeted and un-helmeted Hybrid III headform. In addition, VN and EPP helmets were tested determine if there is any difference in the management of linear and rotational acceleration between these impact absorbing liners using the proposed protocol. 2.0 Methodology 2.1 Equipment A monorail drop rig was used (Figure 1) to complete the proposed testing protocol for the evaluation of the performance of hockey helmets. For the purpose of this study a 50 percentile male Hybrid III headand neckform (mass 6.08kg ± 0.01kg) was attached to the drop carriage by the base of the neckform with a special jig designed to ensure a 90° angle between the z-axis of the headform and the monorail (Figure 2). A 0.46 ± 0.01m tall anvil extension 0.104 ± 0.05m in diameter was firmly fixed to the monorail base. For non-centric impacts the anvil extension was moved horizontally 6.5cm in line with the x-axis of the headform and secured with C-clamps. Secured on the tip of the impact anvil was a hemispherical nylon pad (diameter 0.126 ± 0.01m) covering a modular elastomer programmer (MEP) 60 Shore Type A (0.025 ± 0.05m thickness) disc (Figure 3). Together the nylon pad and MEP disc weighed 0.908 ± 0.001kg. The MEP was chosen as it is a common material used in helmet standards (CSA; NOCSAE). The nylon and MEP disc combination was not designed to reflect any particular impact scenario on the ice. A 50 percentile adult male Hybrid III headform (mass 4.54 kg ± 0.01kg) (Figure 4) was used in this study. This type of headform is designed to respond in a reproducible and reliable manner and is primarily used in impact reconstructions [22]. The headform was instrumented with nine single-axis Endevco7264C-2KTZ-2-300 accelerometers according to Padgaonkar’s orthogonal 3-2-2-2 linear accelerometer array protocol to measure the three dimensional kinematics of the head from an impact [23]. The headform coordinate system was defined with a left-hand rule. Positive axes were directed toward the anterior, toward the right ear and caudally for x, y and z respectively. The Hybrid III neck with a mass of 1.54 ± 0.05 kg was composed of 4 butyl rubber discs interlocked between five aluminum plates to simulate human vertebrae. The discs were offset towards the front 0.5cm and were slit to elicit a different response in flexion from that in extension [24]. 2.2 Data Collection Inbound velocity was set using the Cadex Impact v5.7a computer program and recorded using a velocimeter (time gate). The nine mounted single-axis Endevco7264C-2KTZ-2-300 accelerometers (Endevco, San Juan Capistrano, CA) were sampled at 20 kHz and the signals were passed through a TDAS Pro Lab system (DTS, Calabasas, CA) prior to being processed by TDAS software. 2.3 Procedure The Hybrid III was dropped at three different inbound velocities (2, 4 & 6 m/s) in order to examine how the dynamic response changes as velocity increased (Marino and Drouin, 2000). Three impact conditions were chosen for preliminary investigation of non-centric impacts using the monorail drop rig and are shown/listed in Figure 5 and Table 1 [25]. Two models of helmets were tested, with three helmets of each model used for a total of 6 helmets impacted. Each model had identical two piece polyethylene (2PE) shells with either VN or EPP liners. Dimensions of the shell and foam liner are described in Table 2. The headform and helmeted headform was impacted using a monorail drop rig and each condition tested three consecutive times, which is standard procedure for testing multiple-impact helmets [9;10]. During testing the average time between impacts was 5 ± 0.50 min, which exceeds requirements by current standards [9;10]. Impact site accuracy was ensured by marking the helmet with a permanent marker when it was in contact with the impact cap prior to the first drop. The helmet was reset after each impact to ensure the mark on the helmet was in line with the mark on the impact cap. A different helmet was used for each impact velocity; therefore a total of 162 total helmeted impacts were performed. For the un-helmeted headform condition there was a total of 81 impacts. 2.4 Finite element model (University College Dublin Brain Trauma Model) In addition, the resulting three-dimensional loading curve responses (x, y and z) were applied to the University College Dublin (UCDBTM) finite element model to pro