Documentation:FIB book/Prevention of Skull Fractures in Snowboarding

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Introduction

In 2010, an estimated 8.2 million people in the United States participated in snowboarding[1].  Head injury is the leading cause of death and critical injury in snowboarding accidents[2]. Head injuries only constitute 3% to 15% of all injuries in snowboarders, but account for 50% to 88% of the fatalities[3].  Currently, there are no major institutional recommendations regarding mandatory helmet use for snowboarders, unlike cycling or hockey[4]. However, helmet-use is associated with a lower incidence of skull fractures in snowboarders[5],[6],[7],[8].

In this literature review, the prevention of skull fractures in snowboarding through the use of helmets will be discussed. The injury mechanism of skull fractures in snowboarding and the types of experimental and theoretical techniques being used in studies will be presented to provide the reader with a better understanding of the biomechanical relevance of the topic. Additionally, the limitations of the current research and areas for future work will be identified.  

Background

Types of Skull Fractures

The two common types of skull fractures associated with snowboarding accidents are linear fractures and depressed fractures. Typically, skull fractures occur from direct impact to the head by a rigid convex object or rigid flat object, causing a depressed or linear fracture, respectively. These types of objects are found on ski hills and mountains, such as the hill itself, trees, rails and boxes, and more. Convex objects tend to penetrate the skull while flat objects cause linear fractures[9].

CT Scan of a Linear Skull Fracture

Linear Fractures

The most common fractures are linear fractures and usually do not require any medical intervention for the fracture itself. Linear fractures affect the entire thickness of the skull and do not typically affect the brain because there is no bone displacement. Widely distributed forces such as impacts to a flat surface are often the cause of linear fractures. In snowboarding, a common cause of linear fractures is falling and hitting your head on the snow. Linear skull fractures usually have little clinical significance unless they are close to or transverse a suture of the skull or involve a venous sinus groove. The resulting complications may be suture diastasis, venous sinus thrombosis, and epidural hematoma[10].

Depressed Fractures

Depressed fractures affect a localized area of the skull and can penetrate or infringe on the brain. Broken pieces of the skull may be displaced inwards and require surgery to repair underlying tissue damage or to lift the bones off the brain to reduce the pressure[11]. Penetration of the brain can be fatal. A force concentrated on a small area, such as impact from a snowboarding rail, can cause a depressed fracture.  

Risk Factors

Snow stiffness, speed, skill level, and snowboarder morphology are factors that can increase the risk of a skull fracture from snowboarding[12].  Studies have found that younger and less experienced snowboarders are more likely to suffer skull fractures during snow sports[13],[14].

Injury Mechanisms of Skull Fractures in Snowboarding

Fractures occur when the dynamic input exceeds the tolerance of the skull [15]. A 2018 study investigated injury patterns and risk factors among snowboarders by collecting and analyzing data from the National Trauma Data Bank database. Of the 2704 snowboarders identified, 5% had skull fractures[16]. The majority of the related severe head injuries involved a simple backward fall on a ski slope, with the back of the skull bearing the brunt of the impact. Researchers believe that the tendency for snowboarders to fall backwards is due to the opposite-edge phenomenon. At any given moment in time, only one edge of the board should be in contact with the snow. Not doing so violently shifts the center of the snowboarder to the opposite side, inducing acceleration at the edge of the board leading the snowboarder to be thrown down onto the ground resulting in a hard impact on the head[17].

Multibody modelling can be used to estimate head impact velocities and finite element modelling can be used to investigate the effect of the fall conditions and of biological variability on skull fracture. The mechanism of skull fractures is influenced but not limited to the following four factors: impact velocity, impact surface, cortical thickness and cortical density [18]. A 2011 paper aimed to numerically model accidental falls from standing height in order to reveal the mechanism of skull fracture within the occipital region [18]. The knees were bent slightly during the fall with the buttocks impacting the ground first and the back of the head impacting the ground last. It was revealed that height did not have an influence on either the behavior of the body or impact velocity but the weight did. Risk of skull fracture increased if: velocities are greater than 6.5 m /s , impact occurred on hard surfaces, there is a decrease in cortical thickness and if the subject has a high cortical density [18].

In order to investigate the mechanism of snowboarding injuries, Japan based researchers collected the information of 38 individuals who had sustained snowboarding-related major head injuries. Sixty-six percent of serious head injuries affected the occipital region, 16% involved the frontal region, 13% involved the temporal region, and only 3% involved the parietal region. Six individuals sustained cranial fractures, 2 in the frontal region, 2 in the temporal region and 2 in the occipital region [17]. Frontal and temporal impacts among snowboarders are rare as people typically stretch out their arms in the directions of these falls typically resulting in distal radius fractures when impact velocities and forces are high. The biomechanics of skull fracture under dynamic loads has previously been investigated using 12 unembalmed human head cadavers. Impact loading tests were conducted at a rate of 7.1 to 8.0 m/s on the vertex, parietal, temporal, frontal, and occiput regions of the skull.  The overall mean values of the failure parameters were 11.9 kN (±0.9) for the load, 5.8 mm (±1.0) for deflection, 4023 N/mm (±541) for stiffness, and 28.0 J (±5.1) energy[17].

A study attempting to synthesize and investigate the head injury criteria utilized real world accident data from 27 pedestrians who had sustained skull fractures from direct impact with car windscreen found that HIC for 50% skull fracture was approximately 667. The HIC was derived by numerically accident replications using finite element models of the Hybrid III head and the windscreen[19]. There is a limited number of studies that have been conducted to specifically investigate the injury mechanism of skull fracture in snowboarding. However, knowledge of which types of falls and the conditions which lead to skull fracture can help researchers extract relevant information from other studies conducted that have examined skull fractures.  

Experimental and Theoretical Techniques  

The following studies are largely based on evaluating the effectiveness of helmets when exposed to head impacts experienced by snowboarders during falls. Although these tests are not explicitly testing or modelling the thresholds of skull fracture, they have been used to calculate Head Injury Criterion (HIC), allowing researchers, scientists, and engineers to evaluate the likelihood of such skull fracture to occur.

ATD Testing

A study conducted by Scher et al. examined the head and neck accelerations experienced by snowboarders in opposite-edge-phenomenon impacts. In this study a custom-built cable system and a snow-covered slope were utilized to trip a 50th percentile Hybrid III ATD, recreating the fall mechanism and impact in various types of snow[20]. This ATD was fitted in a jacket, snow-pants, snowboard boots, and a snowboard to accurately replicate the exposure of a typical snowboarder. The ATD contained a triaxial accelerometer mounted at the center-of-mass of the head and six load cells to measure the forces and torques experienced in the neck by the ATD. The arms of the ATD were removed at the shoulder joint and replaced with heavy-duty custom-designed rollers, allowing the ATD to slide down and off the cables on the ramp once released by a pneumatic system. This process was documented using stationary, panning, and high-speed video cameras. From each trial the resultant linear acceleration, upper neck force, and upper neck torque were recorded and used to calculate the Head Injury Criterion (HIC) as well as the Neck Injury Criterion (NIC) of each head impact[20].  

Another test done by Richards et al.[21] utilized the same set-up as described by Scher et al. with a custom-built cable system, a 50th percentile Hybrid III ATD fitted with snowboard gear, and various high-speed cameras[20][21]. This was done to analyze and determine the fall kinematics and head velocity of a snowboarder during the same opposite phenomenon impact.  

Utilizing ATDs in these studies allow for real-world, repeatable, safe tests allowing for analysis of human-body kinetics during high velocity snowboarding impacts. These studies have been very useful for determining the types of forces and kinetics that result in the skull fracture of snowboarders. However, utilizing ATDs for impact tests do have limitations. In both of these studies the arms of the ATDs were removed to attach it to the cable system[20][21]. Although convenient for the study design, removing such parts may change the anthropometry of the test subject altering the accuracy of human kinematics during these backward-falls.  Another major limitation that accompanies using ATDs to model human kinetics is the lack of active muscle contractions and their contributions to fall kinematics[21]. During the fall, muscles in the neck may contract absorbing some of the impact force or changing the overall orientation of the head during the skull-snow contact, aspects that are not accounted for in these studies. These studies do not consider the skill level of the snowboarder: more skilled snowboarders may modify their fall kinematics through experience with falls, allowing them to brace, protect, and mitigate impact severity[21]. Lastly, the snow in both of these experiments were transported to an indoor facility to cover the indoor slope. Temperature change and movement can alter the properties of the snow resulting in different damping properties than what is experienced in real-word impacts[12].

A study conducted by Dressler et al. investigated the biomechanics of the head and neck during head-first snow impacts[22]. Although head-first snow impacts are not the most common injury mechanisms experienced by snowboarders, it is a very prevalent injury for terrain park snowboarders. These tests utilized Hybrid III 50th percentile male ATDs attached to a custom-built drop tower designed to simulate torso interaction during head first impacts. Head-first impacts were simulated on hard and soft snow with and without a helmet. Such snow consistencies were maintained in the lab often overnight to allow snow freezing for hard snow conditions. Once the desired snow conditions were achieved, the snow was placed into aluminum-shell boxes of identical sizes for the ATD head to be dropped into. The head was instrumented with three uniaxial accelerometers located at the center of gravity and the upper neck was fitted with six-axis load cells. Data was collected via analog devices and high-speed cameras. To evaluate the risk of injury achieved by these impact mechanisms, head accelerations and neck loads were compared to the injury assessment reference value (IARV). HIC15 was also calculated for all drop tests[22].  

Dressler et al.’s experiment found that there is a substantial risk for cervical spine injury from head-first impacts on both soft and hard snow at relatively low drop heights[22]. However, this experimental set-up only tested relatively low drop heights at where there is little likelihood of sustaining a skull fracture, or even a concussion. A significant limitation that arose in this experimental setup came from the quality and consistency of the snow. Because the snow was manipulated by hand, frozen, and altered to obtain the desired shape and hardness, it may not be a good representation of the conditions found in real-world snowboarding injuries[22].

Multibody Modelling

Multibody modelling has been developed to numerically reconstruct the mechanisms and impact conditions involved in snowboarding head injuries. The system developed by Bailly et al. modified a model originally developed and validated to explore vulnerable road user safety, such as pedestrians, cyclists, and motor cyclists. This was used to measure the impact velocity, location, and linear and rotational acceleration of the head experienced by snowboarders during opposite-edge-phenomenon impacts[12]. To produce biofidelic human body kinematics, the properties of the model were based on biomechanical data modelled by a set of rigid bodies connected with mathematical joints fitted with a snowboard, boots, bindings, and associated weights. Because snow stiffness, speed, and snowboarder morphology were identified as the main factors influencing head impact metrics, the damping properties of soft and hard snow were evaluated and implemented in the model by performing head drop tests on ski slopes using standardized magnesium heads instrumented with tri-axis accelerometers. Linear acceleration of the head was recorded and the properties of numerical snow were calibrated based on such acceleration at the center of gravity of the head[12].

A second study done by Bailly et al. expanded on the previous multibody model by analyzing different fall mechanisms experienced in snowboarding head injuries, such as collisions with obstacles and collisions between users. Again, this study looked at the influence of crash conditions on head impact in the form of location, speed, and linear and rotational impact.  This was done by varying parameters such as the initial velocity, user height, position, approach angle, slope steepness, obstacles, and snow stiffness[23].  

Both of these models were evaluated on its reproducibility of previous experiments done with Hybrid III ATDs fitted with snowboarding equipment[20]. The results were then compared to the HIC36, the criterion used by the Federal Motor Vehicle Safety Standard (FMVSS), for a 50th percentile Hybrid III ATD to determine the risks of head injury[12]. Theoretical techniques such as this are very beneficial for the testing and evaluation of protective gear such as helmets when considering skull fracture as many different test scenarios and parameters can be easily adapted. However, there are some limitations with multibody modelling. One major limitation of these studies is that they did not account for the muscle flexion that may occur during these impacts. Flexion of the muscles may alter the kinematics of the fall[23] and therefore change the loading of the head and neck in these scenarios. To account for muscle forces, the multibody modelling could be supplemented with a finite element analysis (FEA) to create more dynamic and realistic impact mechanics due to the analysis of muscle attachments, contractions, and support. Unfortunately, from the literature research conducted, no experimental or theoretical papers were found analyzing FEA of the head in snowboarding impacts.

Injury Prevention - Helmets

Testing Criteria

In testing for the efficiency of helmets in reducing skull fractures in snowboarding accidents, there are three main values which can be evaluated: HIC36 in accordance to the FMVSS, and the linear acceleration in accordance to the EN 1077 and ASTM 2040. The FMVSS outlines a HIC36 value of 1,000 as the threshold for 50% probability of skull fracture[24]. When conducting studies about the probability of skull fractures in snowboarding, researchers measure the HIC36 values experienced by the test volunteers, with or without a helmet, to analyze if a skull fracture is likely to occur. The European Committee for Standardization and the American Society for Testing and Materials both have standards in place for recreational helmets, EN 1077 and ASTM 2040, respectively. The testing of helmets is done by projecting head-forms in helmets onto a rigid surface. For the EN 1077, these head-forms must have an initial speed of 19.4 km/h and a linear acceleration below 250g, while for the ASTM 2040 the initial speed must be 22.3 km/h with a linear acceleration below 300g[23].  

Effects of Helmet Use

To see the effects of helmets in snowboarding accidents, snowboard falls with impacts to the head need to be analyzed for the scenario in which helmets are not used. The study done by Bailly et al. described in the previous section Multibody Modelling, performed tests without the use of a helmet. It was found that in 42% of the simulated scenarios a HIC value of 1,000 was exceeded. The study shows that the probability of head injury increases with increased speed and hardness of the snow. At a speed of 45 km/h in hard snow, a HIC value greater than 1,000 is experienced. In comparison to the European and American helmet standards, in 91% and 72% of tests, the 250g and 300g thresholds were exceeded, respectively[24]. Due to the high forces placed on the head during a snowboard fall and the increase of risk in hard snow, helmet wearing has been recommended by multiple studies.  

Helmets are worn to protect the head from direct impact, thus having the capabilities to decrease the probability of skull fracture. In snowboarding, depressed or linear skull fractures can occur from direct impact to the head by a rigid convex object or rigid flat object. Helmets are able to protect against both of these types of impacts by distributing the impact force into a larger area and absorbing some of the force through deformation. The absorption of force through deformation is attained through the usage of foams in the helmet. The consideration of material properties of the foam is crucial as the foam must be able to deform enough to absorb some of the impact energy, but not deform so much as to “bottom out” and transmit a large amount of energy to the head[23]. As the deformation of the foam is dependent on the impact speed, one type of helmet may not be appropriate for all types of snowboarding.

The use of helmets greatly reduces the possibility of skull fracture when in icy, or hard snow, conditions. On soft snow, it was found that there is not a large difference in the probability of seeing a skull fracture whether a helmet is worn or not, averaging at 0.4% and 0.6%, respectively[25]. However, on icy snow for a fall occurring from the opposite-edge-phenomenon, helmets showed a great reduction in the likelihood of skull fracture. For the no helmet condition, HIC averaged 2235 (±863) with a probability of skull fracture of 78.2%, while for the helmeted condition the average HIC was 965 (±322) with a probability of skull fracture of 17.6%. In all tests the conditions were kept the same; the ATD was released at a speed of 28.5 (±1.2) km/h, 50 ms before the fall, and the fall duration was about 400 ms[25].

Helmet Limitations

The standard tests for helmets done by the European Committee for Standardization and the American Society for Testing and Materials do not take into account the different impact conditions, thus as per Bailly et al.’s recommendation, the standard requirements should be broadened[24]. The area of the head covered by helmets protects the head against 60% of impacts, but to be able to protect against more impacts the helmet coverage area should be extended downwards. As also recommended by Bailly et al., devices for the reduction of rotational acceleration head in helmets should be evaluated[24].  

In snowboarding accidents where the head impacts the hill, helmets can greatly reduce the probability of skull fracture. However, it was found that when the head is impacted by a fixed object, such as a tree, the probability of skull fracture remains at 99.99%[25]. The testing conditions remained the same as outlined in the above section Effects of Helmet Use, except the impact with a stationary object occurred midway through the fall at 200 ms. Head acceleration and HIC was measured for both the helmeted and non-helmeted conditions. Without a helmet, the head acceleration was 696 Gs with a HIC value of 12,185, and with a helmet these values decreased to 333 Gs and 3,299, respectively[25].

Other Limitations and Future Work

Compared to skiing, snowboarding is a very new sport. There is a wide range of studies related to skiing and much fewer studies done on snowboarding. During our research, many of the statistics we came across were for “skiing and snowboarding” so it was difficult to find epidemiologic information specific to snowboarding topics.  

Some studies have found that certain fall mechanisms, such as the opposite-edge phenomenon, have been studied widely while many other mechanisms of falls, such as impacts with stationary objects have yet to be explored[24]. This presents a limitation as there is a gap in research regarding the many other mechanisms resulting in skull fracture during recreational snowboarding. There is still lots of research that has yet to be done in this field which may contribute to better engineering solutions for protective equipment such as helmets and stationary object padding for the future.

Many of the datasets collected and tests conducted examining the conditions leading to skull fracture did not take into account impact on surfaces with snow or the different consistencies of snow. Therefore, parameters of HIC, impact velocity and such may not apply to real world falls on snow surfaces.

References

  1. ↑ Kim S, Endres N, Johnson R, Ettlinger C, Shealy J. Snowboarding Injuries. The American Journal of Sports Medicine. 2012;40(4):770-776.
  2. ↑ Levy A, Hawkes A, Hemminger L, Knight S. An Analysis of Head Injuries among Skiers and Snowboarders. The Journal of Trauma: Injury, Infection, and Critical Care. 2002;53(4):695-704.
  3. ↑ Ackery A, Hagel B, Provvidenza C, Tator C. An international review of head and spinal cord injuries in alpine skiing and snowboarding. Injury Prevention. 2007;13(6):368-375.
  4. ↑ Haider A, Saleem T, Bilaniuk J, Barraco R. Efficacy of Safety Helmets in Reduction of Head Injuries in Recreational Skiers and Snowboarders. Journal of Trauma and Acute Care Surgery. 2012;73(5):1340-1347.
  5. ↑ Rughani A, Lin C, Ares W, Cushing D, Horgan M, Tranmer B et al. Helmet use and reduction in skull fractures in skiers and snowboarders admitted to the hospital. Journal of Neurosurgery: Pediatrics. 2011;7(3):268-271.
  6. ↑ Rughani A, Lin C, Ares W, Cushing D, Horgan M, Tranmer B et al. Helmet use and reduction in skull fractures in skiers and snowboarders admitted to the hospital. Journal of Neurosurgery: Pediatrics. 2011;7(3):268-271.
  7. ↑ Srinivas S, McLoughlin R, Hazeltine M, Green J, Hirsh M, Cleary M et al. Pediatric snow sport injuries differ by age. Journal of Pediatric Surgery. 2020;.
  8. ↑ Nakaguchi H, Fujimaki T, Ueki K, Takahashi M, Yoshida H, Kirino T. Snowboard Head Injury. The Journal of Trauma: Injury, Infection, and Critical Care. 1999;46(6):1066-1069.
  9. ↑ Mills N. Foam protection in sport. Materials in Sports Equipment. 2003;:9-46.
  10. ↑ Dunn I, Kim A, Gormley W. Brain Trauma. Encyclopedia of Neuroscience. 2009;:407-416.
  11. ↑ Simon L, Newton E. Basilar Skull Fractures [Internet]. NCBI. 2020 [cited 9 November 2020]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470175/
  12. ↑ 12.0 12.1 12.2 12.3 12.4 Bailly N, Llari M, Donnadieu T, Masson C, Arnoux P. Head impact in a snowboarding accident. Scandinavian Journal of Medicine & Science in Sports. 2016;27(9):964-974.
  13. ↑ Srinivas S, McLoughlin R, Hazeltine M, Green J, Hirsh M, Cleary M et al. Pediatric snow sport injuries differ by age. Journal of Pediatric Surgery. 2020;.
  14. ↑ Nakaguchi H, Fujimaki T, Ueki K, Takahashi M, Yoshida H, Kirino T. Snowboard Head Injury. The Journal of Trauma: Injury, Infection, and Critical Care. 1999;46(6):1066-1069.
  15. ↑ Yoganandan N, Pintar F, Sances A, Walsh P, Ewing C, Thomas D et al. Biomechanics of Skull Fracture. Journal of Neurotrauma. 1995;12(4):659-668.
  16. ↑ Basques B, Gardner E, Samuel A, Webb M, Lukasiewicz A, Bohl D et al. Injury patterns and risk factors for orthopaedic trauma from snowboarding and skiing: a national perspective. Knee Surgery, Sports Traumatology, Arthroscopy. 2016;26(7):1916-1926.
  17. ↑ 17.0 17.1 17.2 Nakaguchi H, Tsutsumi K. Mechanisms of snowboarding-related severe head injury: shear strain induced by the opposite-edge phenomenon. Journal of Neurosurgery. 2002;97(3):542-548.
  18. ↑ 18.0 18.1 18.2 Hamel A, Llari M, Piercecchi-Marti M, Adalian P, Leonetti G, Thollon L. Effects of fall conditions and biological variability on the mechanism of skull fractures caused by falls. International Journal of Legal Medicine. 2011;127(1):111-118.
  19. ↑ Marjoux, D., Baumgartner, D., Deck, C., & Willinger, R. (2008). Head injury prediction capability of the HIC, HIP, SIMon and ULP criteria. Accident Analysis and Prevention, 40(3), 1135-1148. doi:10.1016/j.aap.2007.12.006
  20. ↑ 20.0 20.1 20.2 20.3 20.4 Scher I, Richards D, Carhart M. Head Injury in Snowboarding: Evaluating the Protective Role of Helmets. Journal of ASTM International. 2006;3(4):14203.
  21. ↑ 21.0 21.1 21.2 21.3 21.4 Richards D, Carhart M, Scher I, Thomas R, Hurlen N. Head Kinematics During Experimental Snowboard Falls: Implications for Snow Helmet Standards. Journal of ASTM International. 2008;5.  
  22. ↑ 22.0 22.1 22.2 22.3 Dressler D, Richards D, Bates E, Van Toen C, Cripton P. Head and Neck Injury Potential With and Without Helmets During Head-First Impacts on Snow. Skiing Trauma and Safety. 2012;:235–49.  
  23. ↑ 23.0 23.1 23.2 23.3 Bailly N, Llari M, Dannadieu T, Masson C. Numerical Reconstruction of Traumatic Brain Injury in Skiing and Snowboarding. American College of Sports Medicine. 2018;:2322–9.  
  24. ↑ 24.0 24.1 24.2 24.3 24.4 Bailly N, Llari M, Donnadieu T, Masson C, Arnoux P. Head impact in a snowboarding accident. Scandinavian Journal of Medicine & Science in Sports. 2016;27(9):964-974.
  25. ↑ 25.0 25.1 25.2 25.3 Scher I, Richards D, Carhart M. Head Injury in Snowboarding: Evaluating the Protective Role of Helmets. Journal of ASTM International. 2006;3(4):14203.


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