Marshall Kendall

Characterization of persistent concussive syndrome using injury reconstruction and finite element modelling.

Concussions occur 1.7 million times a year in North America, and account for approximately 75% of all traumatic brain injuries (TBI). Concussions usually cause transient symptoms but 10 to 20% of patients can have symptoms that persist longer than a month. The purpose of this research was to use reconstructions and finite element modeling to determine the brain tissue stresses and strains that occur in impacts that led to persistent post concussive symptoms (PCS) in hospitalized patients. A total of 21 PCS patients had their head impacts reconstructed using computational, physical and finite element methods. The dependent variables measured were maximum principal strain, von Mises stress (VMS), strain rate, and product of strain and strain rate. For maximum principal strain alone there were large regions of brain tissue incurring 30 to 40% strain. This large field of strain was also evident when using strain rate, product of strain and strain rate. In addition, VMS also showed large magnitudes of stress throughout the cerebrum tissues. The distribution of strains throughout the brain tissues indicated peak responses were always present in the grey matter (0.481), with the white matter showing significantly lower strains (0.380) (p<0.05). The impact conditions of the PCS cases were severe in nature, with impacts against non-compliant surfaces (concrete, steel, ice) resulting in higher brain deformation. PCS biomechanical parameters were shown to fit between those that have been shown to cause transient post concussive symptoms and those that lead to actual pathologic damage like contusion, however, values of all metrics were characterized by large variance and high average responses. This data supports the theory that there exists a progressive continuum of impacts that lead to a progressive continuum of related severity of injury from transient symptoms to pathological damage.

Comparison between Hybrid III and Hodgson–WSU headforms by linear and angular dynamic impact response

In brain injury research, linear and angular resultant acceleration data have been considered important mechanisms contributing to various levels of brain injury. The development of biofidelic headforms with similar dimensions and weight to that of a real human head has allowed for researchers to repeatedly collect data related to the effects of different impacts on the human head. Currently, there are different types of headforms available for impact testing, each with varying degrees of biofidelity and repeatability. Two commonly used headforms were tested: the Hybrid III and the Hodgson–WSU (NOCSAE). The two headforms were outfitted with nine single-axis accelerometers positioned orthogonally following a 3–2–2–2 array. Both headforms show good linearity and correlate well throughout the different velocities and are, therefore, reliable tools. Significant differences are observed in peak linear and peak angular accelerations between Hybrid III and Hodgson–WSU headforms. The shapes of the loading curves are visually different and thus may have significant impact on the output from FE modelling of the brain response.

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.

The Relationship between Head Impact Characteristics and Brain Trauma

Brain injury is complex in nature and extraordinarily challenging when attempting to describe the relationship between the event and the resulting injury. In an effort to reduce its severity and incidence a great deal of research investigating mechanisms of brain injury has involved the areas of anatomical, reconstructive, and finite elements modeling. The anatomical research primarily examines functional and mechanical failure thresholds for different types of brain tissue [1-3]. Approximate strain levels are described for the different tissues that are then used to represent human responses [4]. Anatomical research examines individual brain tissues while reconstructive research simulates how injured individuals were impacted in order to discover relationships between engineering variables such as acceleration, stress, and strain and the resulting brain injury [5-7]. Currently, much of this research has focused on sporting concussions as they are frequent and often documented providing information for accurate reconstructions [5,6]. Finite element modeling provides a tool to obtain brain tissue response values resulting from an impact. Within the term traumatic brain injury (TBI) there are several different types of brain injury lesions, each with their own respective mechanisms and possibly predictive variables [8]. The multiple types of injuries described within TBI may also expound concussion, which has been described to have different levels of severity: sub concussive, transient, and persistent. In addition to examining the nature of the continuum of brain injury associated with the severity of impact, the mechanisms of injury contributing to these outcomes are also examined. The most common mechanisms of brain injury include: falls, collisions, projectiles, and punches. These mechanisms are examined and a synthesis of how they contribute to the outcome of the injury within the continuum of TBI and concussion is discussed. This provides information on how accelerations, resulting from an impact, affect brain tissue response and the location of the highest magnitudes of stress and strain. This review examines the nature of traumatic and concussive brain injury within the context of a continuum based upon impact severity using anatomical, reconstructive and finite element methodologies.

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 development of a threshold curve for the understanding of concussion in sport

Much of what is known concerning human brain injury thresholds is based upon impacts to cadavers and animal models that were used to generate the Wayne State Concussion Tolerance Curve (WSTC) and similar curves. These curves are the foundation for predictive metrics used in standard development as well as helmet design. These curves were based upon a very narrow range of impacts; impacts whose characteristics differ greatly from how the head is impacted in sport. This research examines the uses of time-based curves like the WSTC in the context of understanding mechanisms of brain injury and head protection. Published linear/rotational acceleration magnitude/duration data from Hybrid III laboratory reconstructions of brain injury events were plotted. This research further develops the understanding of injury thresholds in comparison to threshold curves such as the WSTC and Brain Injury Curve Leuven. The data demonstrate the relationships between magnitude and duration of dynamic response on minor traumatic brain injury (mTBI) in sport.

A comparison of the capacity of ice hockey goaltender masks for the protection from puck impacts

Abstract Goaltenders in ice hockey are the only players that are on the ice for the entire game. Their position exposes them to impacts from collisions with other players, falls to the ice, and puck impacts. In competitive ice hockey leagues, head injuries resulting from puck impacts have been reported with some cases resulting in ending the player’s career. Considerable research has been conducted to assess the performance of hockey helmets; however, few have assessed the performance of goaltenders’ masks. The purpose of this study was to compare the capacity of four goaltenders’ masks for the protection from puck impact as measured by head acceleration and peak force. A Hybrid III headform was fitted with four different goaltender masks and impacted with a hockey puck in three locations at 25 m/s. The masks were found to vary in the level of protection they offered as the mask with the thickest liner resulted in lower forces than the thinnest mask for side impacts; however, the thinnest mask resulted in the lowest force for front impacts. Despite performance differences at specific locations, no one mask proved to be superior as peak acceleration and peak force values did not exceed the thresholds necessary for concussion.

Determination of high-risk impact sites on a Hybrid III headform by finite element analysis

The current standards and methods to evaluate helmet performance remain focused on traumatic brain injuries and not concussive injuries. This is reflected in the methodologies currently used and the injury metrics employed to determine the pass/fail criteria for the helmets being tested. To address the problem surrounding concussion and helmets, a method reflecting high risk of concussive injury must be developed. The purpose of this research was to identify high risk of concussive injury impact sites on the Hybrid III headform using the Wayne State head injury finite element model. The Hybrid III headform was impacted using a linear impactor in five different sites with four angles per site at 5.5 m/s. The resulting acceleration loading curves were used as input for brain deformation analyses using the Wayne State University Brain Injury Model. The brain deformation results indicated that there are 12 impact conditions on the Hybrid III headform which reflect a risk of concussion above 80% when compared with the literature. The method used and impact sites discovered by this research differ significantly from the current standard method and may be used to guide future impact sites for the evaluation of helmet performance.

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