Documentation:FIB book/Spinal Cord Injuries in Motorcycle Impacts

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INTRODUCTION

Motor vehicle accidents are the leading cause of spinal cord injuries for the younger population, accounting for almost 40% of all cases in the USA [1]. The severity of the SCIs will depend on several factors, including the type of collision, the type of vehicle(s) involved, the speed of the vehicle(s) involved, driver attributes, road conditions, and environmental factors. SCIs can result in complete and permanent losses of motor and sensory functions below the injured vertebrae, which, depending on the location of injury, can be accompanied by an inability to breathe independently, speech impairment, and loss of bowel/bladder functions.

There are many safety mechanisms incorporated into four-wheeled vehicles to prevent SCIs, including three-point seat belts, airbags, and anti-rollover mechanisms. Unlike four-wheeled vehicles, however, motorcycles do not have seatbelts and include different safety mechanisms to prevent injury such as anti-lock braking systems, protective equipment such as helmets, and airbags. Still, without the protection of a car’s structure, motorcyclists are more vulnerable to serious injuries and fatalities. In addition, motorcycles are more unstable when compared to four-wheeled vehicles and are more difficult to see due to their smaller size. Because of this, motorcyclists continue to have the highest rates of hospitalizations, serious injuries, and fatalities even though they represent only 2% of the road traffic [2]. The National Highway Traffic Safety Administration reports that, for every mile traveled, motorcycles are 28 times more likely to be involved in a fatal collision when compared to cars [1].

During collisions, motorcyclists are typically thrown from their motorcycle, and they are therefore particularly susceptible to head injuries. Results from two studies suggest that around 12% of motorcycle collisions result in spinal injury, with approximately 20% of those injuries affecting the spinal cord [1][3]. Although spinal cord injuries (SCIs) are less frequently observed, they have been found to be more detrimental, both socially and financially, when compared to other types of injuries [4]. Adults with traumatic SCIs are at an increased risk of developing mental health disorders and secondary chronic diseases such as cardiovascular and pulmonary diseases, diabetes, liver disease, arthritis, and circulatory conditions [5]. The average lifetime cost associated with SCIs is estimated to be $2.5 million for paraplegics and $4.3 million for quadriplegics [6]. The number of disability-adjusted life years (DALY) associated with SCIs surpasses those calculated for other serious conditions such as HIV/AIDs, indicating that the burden of SCIs in the USA is extremely significant [1].

Regardless of their well-established risk, motorcycles have continued to grow in popularity. They are a convenient and less expensive option for transportation when compared to other motor vehicles. Between 2019 and 2020, motorcycle sales have increased by around 67% in the USA, with a total of 780,000 motorcycles being bought in 2020 [7]. The growing popularity of motorcycles has contributed to a higher number of accident-related injuries and fatalities.

In this paper, both retrospective case studies and biomechanical studies will be reviewed. Current protective equipment will be outlined, and their respective effectiveness will be evaluated. Limitations and challenges of current research will be described, and future work for this particular topic will then be outlined.

RETROSPECTIVE CASE STUDIES

Critical Analysis of Data Collection Methods

Retrospective case studies (RCS) use existing data that was previously recorded for an alternative purpose other than research. Because these research papers rely on data collected by others, there is a risk that the data used in retrospective case studies can be incomplete and/or inaccurate, affecting the results and conclusions drawn from these types of studies. In a study where patients reviewed their personal medical records, it was found that 10% contained “very serious” mistakes [8]. This demonstrates that many medical reports may include inaccurate, incorrect, or misleading information. Lack of clarity and organization may result in misinterpreted information, which can have serious implications. The methods of the data collection will have different advantages and disadvantages, and it is therefore important to keep this in mind when reviewing retrospective case studies. Aside from this the quantity of information and data that can be obtained using analysis of records retrospectively is very beneficial as well the fact that these offer a diverse and well preserved  data bank.

The five retrospective case studies that were reviewed for this literature review have been given a paper reference number (PRN). The retrospective case studies will be hereby referred to by these PRNs for the remainder of Section 2.0 for clarity purposes. The data collection methods for each retrospective case study along with a description of the dataset have been outlined in the table below (Table 1).

Table 1. Data Collection Methods for Retrospective Case Studies

PRN Source Method of Collection Dataset Description
1 [1] Case reports from a national rehabilitation center for spinal injuries 398 case reports were used from Rancho Los Amigos National Rehabilitation Center in Los Angeles County, California between 2003 and 2013
2 [2] Questionnaire (survey) Responses of 124 people involved in motorcycle accidents who arrived at the Marseille trauma center in France in 2016. Demographic information, crash characteristics, and injuries were recorded using the abbreviated injury scale (AIS) system.
3 [3] Clinical data collected from trauma centers Clinical data for 1,121 people was collected between 1993 and 2000 from trauma centers and analysed.
4 [9] Police reports Police reports were screened and taken from the Malaysian Institute of Road Safety Research (MIROS) between 2005 and 2007.
5 [10] Medical data and autopsy reports (if applicable) Collected from 28 participating hospitals between 1988 and 1990 for individuals with an AIS score of 2 or higher.

In PRN 1, the data was taken from case reports at a national rehabilitation center for patients with severe spinal or spinal cord injuries that require in-patient rehabilitation. Data about SCIs resulting from all motor vehicle accidents were collected, including automobile, motorcycle and bicycle crashes. Analyzing severe SCIs specifically could help identify the risk factors and crash characteristics of these high injury risk scenarios. However, since SCIs resulting from all motor vehicles were included in the dataset, it may not provide sufficient insight into injury patterns for motorcycle accidents specifically.

In PRN 2, the data was collected from motorcycle riders who were emitted to the Marseille trauma center. There are relatively few data fields for this particular retrospective case study because data was only collected from 1 trauma center. Therefore, a proportion of valuable data might have been lost by not considering multiple trauma centers in the same area. Although it was not specified in the paper, the researchers may not have been capable of collecting data from additional trauma centers due to licensing and protocol differences between various trauma centers. Still, since the data was collected for individuals involved in motorcycle accidents specifically, the information from this RCS will be especially useful to consider.

In PRN 3, clinical data that includes injury severity scores (ISS) was gathered from multiple trauma centers. This standardized score helps assess the trauma severity of the patient and has clinical significance, as it can give insight into the risk of mortality and hospitalization time of the patient. However, these injury severity scores are not as helpful within the context of this particular study because it does not provide specific information on the type of spinal injury.

Police report data is used in PRN 4. This type of database often provides limited and insufficient data fields to allow for complete and thorough accident analysis, as it was not the original purpose of the data collected. The witness statements regarding the incidents that were collected may have introduced additional bias, and the quality of police report data could vary greatly between police officers depending on their levels of training. Screening the police reports may have helped limit these discrepancies, and the remaining data fields would therefore still provide relevant and valuable information.

In PRN 5, medical and autopsy reports from general hospitals and trauma centers for patients with an abbreviated injury scale (AIS) score of 2 or higher were collected and used. Autopsies are not always systematically performed and usually identifies one type of injury as the cause of death. Therefore, occult head injuries or thoracic spinal injuries may be overlooked if a serious cervical injury is present and assumed to be the cause of death. Smaller, yet significant, injuries may not have been recorded, resulting in a loss of valuable data for this RCS. One positive from this paper was the use of AIS. This meant that only more serious injuries were included.

Finally, it is important to note that many of the studies reported missing or incomplete data fields.

Identification of Risk Factors, Crash Characteristics, and Injury Risk

Due to the nature of retrospective case studies, many conclusions have been drawn regarding the risk factors, crash characteristics, and injury risk that lead to motor vehicle accidents. Risk factors associated with spinal cord injury have been identified in the retrospective case studies. Motorcyclists are at a higher risk of thoracic SCIs and are more likely to have complete motor loss when compared to other automobile users (PNR 1). Collision with a fixed object was identified as a significant risk factor in two papers (PNR 4 and PNR 5), with one paper (PNR 4) reporting a 90% increased risk of sustaining a spinal injury. Rear-end collisions and impaired driving were also identified as risk factors for spinal cord injuries. Furthermore, late discovery and diagnosis of thoracic spinal injuries due to visualization issues on imaging techniques will increase the risk and severity of neurological damage [11].

From the five retrospective case study papers we reviewed on motorcycle collisions, one paper (PRN 3) established that 54.8% of all spinal injuries occur in the thoracic region [3]. However, a different paper (PRN 5) found that 47% and 27.5% of spinal injuries occur in the cervical and thoracic areas respectively. The discrepancy between these findings may be due to differences in the dataset caused by different collection methods, analysis methods, and the location and timeframe of data collection. Spinal fractures in the thoracic region are reported to be more difficult to identify and diagnose using imaging techniques when compared to cervical injuries due to the presence of the lungs and ribs [11]. The paper (PRN 5) which reported a higher frequency of cervical spinal injuries used data collected between 1988 and 1990. Since then, imaging technology has improved significantly, resulting in earlier discovery and diagnosis of major comorbidities such as thoracic spinal fractures. In addition, medicine has since adopted a more holistic approach which places an emphasis on quality of life. Medical professionals and researchers have shifted their focus to the entire spinal column instead of focusing only on severe cervical injuries, which may explain why thoracic spinal injuries are more closely investigated in the later studies.

Reviewing RCS helped identify key risk factors, crash characteristics, and injury risk for spinal injuries in motorcycle accidents. Based on these findings, biomechanical studies regarding cervical and thoracic spinal injury mechanisms during impact situations were prioritized. However, since there are many factors which can affect the findings of RCS, biomechanical studies relating to the entire spinal column were considered and reviewed. Reviewing biomechanical studies can provide additional insight into specific injury mechanisms, injury risk, and the corresponding effectiveness of personal protective equipment.

BIOMECHANICAL STUDIES

In order to categorize and analyze the effect of motorcycle accidents on the spinal column and spinal cord, many biomechanical considerations must be taken into account, including the moment experienced by the regional vertebrae, the transverse shear, and axial loading. To further investigate spinal cord injuries resulting from motorcycle accidents, many biomechanical studies were reviewed. Studies on spinal cord injuries during all motor vehicle accidents were considered since they can be linked to motorcycle impacts and give insight into injury mechanisms. Although some studies considered spinal injuries, neurological damage or vertebral foramen breakdown was often reported, and these studies therefore proved useful for this review [12].

SCIs are associated with mechanical deformation of the surrounding tissue, including the vertebrae, and ligaments of the intervertebral disk [13][14]. External forces can cause translocation and cord impingement, resulting in loading that surpasses the tissue threshold levels. The severity of SCIs will depend on the rate and magnitude of these loads. There are specific spinal column injury patterns that often result in spinal cord injuries; the most common ones being dislocation injuries and contusion injuries, which occur with shearing forces and localized compression forces, respectively [13]. More specifically, higher energy dislocation fractures and burst fractures account for about 80% of spinal cord injuries [14]. It is important to note that spinal cord injuries without radiological abnormalities (SCIWORA) occur and are associated with nervous tissue damage, indicating that functional damage has occurred [15].

Dislocation injuries result from lateral or anterior/posterior shearing of the spinal column, which exposes the spinal column to compressive, tensile, and shearing forces. Fracture dislocations occur when one of the vertebrae are subjected to relative translation, resulting in the entrapment of the spinal cord. The highest rates of fracture dislocations have been found to occur within the cervical regions of the spinal column. The severity of the dislocation and the extent of the neurological damage are influenced by the magnitude and direction of the forces applied. In many cases, fracture dislocations are mechanically and neurologically unstable due to soft tissue failure. It was found that individuals experienced significantly lower functional outcomes after high energy impacts such as motorcycle accidents when compared to low energy impact situations [14].

Contusion injuries occur when a bone fragment intrudes into the spinal canal at high speeds during a vertebral burst fracture and are closely associated with burst fractures. Burst fractures account for 15% of all SCIs associated with falls or motor incidents [16]. They typically occur when the vertebral body is deformed at a high rate or is placed under a high magnitude of vertical compression, causing the vertebral body to burst and shatter outwards. Fragments of bone can be launched into the vertebral foramen and impact the spinal cord. The velocity and size of the bone fragments can significantly affect the development and severity of a SCI.

A study by Yoganandan et al. observed the effects of low and high energy vertical loading on the thoracic and lumbar spine of three unembalmed cadaver spinal specimens with a mean age of 50 years old using a drop tower apparatus [11]. Although thorough assessments of the specimens were conducted before testing in order to ensure there were no previous structure deficiencies, the demographic characteristics of each surrogate could have affected these results. Fixing the spinal specimens at the ends helped ensure natural lordosis of the lumbar spine, and including the slight curvature of the thoracic region resulted in a more biofidelic response for this particular study. Although motorcyclists are typically in a seated position with their torso bent forward, the angle of impact will always differ greatly between cases. Therefore, fixing the specimens in a “standing” posture for this biomechanical study will still yield relevant results and will increase the repeatability of the experiment [11].

Different masses and heights were used to distinguish between the high and low energy impacts. As the height and mass increased (i.e., for high energy impacts), the force within the vertebrae increased as well. Multiple compression-related fractures were recorded throughout the vertebrae body, indicating that the load is shared by the entire spinal column during vertical loading. The injuries sustained were considered to be clinically unstable due to the observed disruption of both anterior and posterior regions. The flexion bending moment during loading contributed to the posterior disruptions, and flexion can therefore be attributed as a secondary injury mechanism.

Linear regression analysis was conducted and the thoracic and lumbar spinal columns were found to have a 50% probability of fracture with 3.7 kN of force. Hindered by the unavailability of cadaver specimens, the study was unable to conduct a high number of tests, which would have resulted in a more complete and overriding analysis. Furthermore, each specimen was loaded multiple times, which may have decreased the recorded tolerances and reduced the biofidelity of the experiment [11]. Regardless of its limitations, this study helped identify the biomechanical mechanism associated with spinal injuries resulting from vertical loading (i.e., compression forces), and indicated the associated tolerance limits for the spinal column under compressive loads.

Another study by Khuyagbaatar et al. investigated the effect of bone fragment velocity on spinal cord deformation, strain distribution, and cerebrospinal fluid (CSF) obliteration [16]. A finite element (FE) model of the bovine spine was designed for dynamic impact tests and incorporates the spinal cord, dura mater, and CSF, increasing the biofidelity of the computational model. The FE model was validated by comparing deformation values to those reported in previous experimental studies. Since bovine spines are anatomically and mechanically similar to human spines, the results of this study can be applied to humans. The maximum deformation was found to be at the center of the spinal cord, and the stresses within the spine significantly increased when impact velocity exceeded 4.5m/s.

At this impact velocity, the spinal cord was compressed 43% and the cross-sectional area was reduced by 31%. Neurological issues begin to appear when the cross-sectional area is reduced by approximately 30%, so an impact velocity of 4.5m/s can be considered as a threshold value for possible spinal cord damage. Longitudinal strain, cord compression, and CSF obliteration increased with higher impact velocities while the cross-sectional area of the spinal cord decreased [16]. This relatively well-validated study incorporated the dynamic nature of CSF, which had not been included in previous models. The use of the additional CSF modelling allowed a better understanding of the true mechanism of the spinal column, as even this fluid plays a part in the mechanisms of movement.

A different study by Zhu et al. investigates the effect of constraint conditions on spinal canal occlusion in the thoracic region when placed under tension-compressive loading [17]. A surrogate spinal cord that produces electrical signals was placed within an upper thoracic spine specimen from a human cadaver. The electrical signals produced by the surrogate were proportional to the degree of spinal canal occlusion. Although this allowed for a better understanding of the mechanism of injuries, it failed to take into account the heterogeneity of the spinal cord. Replacing the natural spinal cord with non-degradable materials with similar properties would allow for greater repeatability and would reduce any variation due to degradation of the spinal cord. Certain specimens were constrained, while others were unconstrained and were permitted to translate anteriorly [17].

This experiment found that the decrease in area of the spinal cord was much more significant for the constrained specimen when compared to the unconstrained specimen. Furthermore, the unconstrained specimens resulted in less occlusion of the spinal canal during injury. This indicates the spinal column is at a higher risk of being severely injured for constrained systems. Both common fracture types were observed, with the pure-compression tests resulting in burst fractures and the flexion/extension-compression tests resulting in fracture dislocations. Studying constrained vs unconstrained specimens is especially helpful for the design and evaluation of personal safety equipment. These results suggest that spinal cord injury risk can be mitigated in head-first impact situations by safety equipment which allows the head to move more freely during impact [17].

In all the biomechanical studies reviewed, the forces experienced by specific vertebrae sections during mechanical testing are not completely congruent to the loading applied to the human body at a gross level. This suggests that each vertebra experiences and translates their force slightly differently depending on the loading conditions, adding to the complexity of these injuries [18]. Because of this, it is still unclear what protective equipment is most effective in reducing the incidence and severity of spinal injuries. The effectiveness of personal protective equipment on mitigating spinal cord injury risk will be evaluated further through the reviews of additional biomechanical studies and other research papers in the following section.

PROTECTIVE EQUIPMENT

Motorcycles lack safety mechanisms, and personal protective equipment (PPE) are the primary form of protection for motorcyclists [19]. A study investigating 47 fatal accidents involving helmeted motorcyclists revealed that 21% of helmeted motorcyclists had serious cervical spine fractures, and 25% of those with fractures also had cervical spinal cord damage [20]. Therefore, the effectiveness of PPE for motorcyclists such as back protectors, neck braces, and helmets in providing protection against thoracic and cervical spinal cord injuries must be evaluated.

Back Protectors

Back protectors are becoming an increasingly popular type of protective equipment for motorcyclists as they allegedly protect the posterior torso of the rider against impact-related injuries. They consist of an abrasion-resistant outer hard shell with single or multiple articulated plates covering the spine placed inside of it, extending from the upper thoracic spine to the lower lumbar spine. These back protectors can either be strapped to the body directly or attached to the insides of motorcycle jackets. Often, a foam lining is added to the outer shell which, along with the metal plates, provides shock absorption for the thoracic and lumbar regions of the spine. By reducing the forces experienced due to direct blows to the back and spin, back protectors can aid in the prevention of spinal injuries [2][19]. In addition, some back protector designs limit spinal extension to protect the rider against injuries resulting from hyperextension of the spine [19].

One retrospective case study (PRN 2) which used epidemiological data and biomechanical analysis to evaluate and compare injuries for individuals wearing and not wearing back protectors concluded that the current designs for back protectors are not effective and do not provide sufficient protection against injuries in the craniocaudal direction. As thoracolumbar injuries occur primarily due to craniocaudal axial compressions, no significant difference in injury frequency or severity was observed between individuals wearing back protectors and those who were not [2]. An existing literature review which used several databases to evaluate proactive equipment for motorcyclists found that back protectors also fail to protect motorcyclists against injuries resulting from twisting and bending of the back [19].

When back protectors were worn, lumbar vertebral injuries deflected towards the thoracic vertebrae, resulting in a more serious, higher level spinal cord injury. Although back protectors may limit flexion and extension of the spine, they are not effective at reducing the loads experienced by the spine due to craniocaudal forces. Back protectors would therefore be ineffective at reducing the severity of compression-related spinal injuries. However, in the event of an accident, back protectors could help limit spinal movement of the injured motorcyclist, which could prevent the injury from worsening and limit the extent and severity of spinal cord damage. Still, current designs were reported as being heavy and uncomfortable, often impairing the motorcyclist’s ability to drive safely [19]. Although back protectors reduce soft tissue injuries and can help prevent spinal injuries from being exacerbated after an accident has occurred, there is a lack of evidence surrounding their effectiveness in preventing spinal cord injury during a motorcycle accident.

Neck Braces

Neck braces have been frequently used as a nonsurgical treatment option for those with cervical spine conditions and neck pain. A modified soft cervical collar has been recently introduced as a personal protective equipment for motorcyclists. It is designed to be used in conjunction with motorcycle helmets and aims to limit neck movement in order to decrease the risk and severity of injuries caused by hyperflexion, hyperextension, lateral bending, and other impact forms [20]. Neck loading evaluation standards are still not well established, and the level of protection offered by these devices is therefore still relatively unknown [21].

A biomechanical research paper used a validated human body finite-element model (THUMS V.5) along with a validated computational model of a neck brace and helmet in order to to investigate the effects of neck brace on the hyperextension, hyperflexion, and lateral bending of the cervical spine [20]. Computational simulations were conducted for four different loading conditions to analyze and compare the axial and shear loadings experienced by the neck for the three injury mechanisms mentioned previously. The results from this paper were slightly contradictory. On one hand, results suggest that neck braces slightly decrease the axial and shear loads experienced by the neck during deformation. However, the loads and stress distributions over the cervical vertebrae area increased in simulations which included the neck brace, suggesting that the impact between neck brace and helmet could induce higher loads in the neck. Together, the results demonstrate that neck braces do not effectively reduce the risk and severity of cervical spinal injuries and other neck injuries.

Testing multiple impact scenarios allowed researchers to develop a more comprehensive understanding of the neck’s response under varying, realistic conditions. The validated FEA models are biofidelic and allow for greater experimental repeatability. The four loading conditions that were considered are assumed to provide an accurate and comprehensive representation of various impact situations. However, motorcycle accidents are unpredictable. Therefore, even the realistic impact loading condition used in this simulation is not applicable to all situations, and there are several alternative loading patterns that were not considered could contribute to severe spinal injuries. Furthermore, the FE model of neck brace only models the mechanical behavior of a general neck brace, and is not an accurate representation of any commercial product. It is possible that certain design features of commercial products may increase or decrease the risk and severity of injuries; however, this remains unknown [20].

Another biomechanical study used computational simulations to evaluate the effectiveness of neck braces by simulating 24 impacts with varying impact angles and initial impact velocities for braced and unbraced models. A coupled torso, helmet, and neck-brace FEM of THUMS V.3 with the validated SUFEHN-model was assembled and then used to simulate an impact in the cervical area. The vertical force, extension moment, and  Nij criterion for each simulation was then calculated. Neck braces effectively limited flexion-extension motions and were found to reduce the risk of AIS 3+ injuries by 39% and by 13% for an impact velocity of 5.5 m/s and 6.5 m/s, respectively. This indicates that impact velocity significantly affects the effectiveness of neck braces. Since motorcycle impacts in real life would be at much higher speeds, results from this study indicate that neck braces would not be effective in reducing neck injury risk at impact velocities over 6.5m/s [21].

In this FEM, the neck-brace is modelled as a rigid body to transfer loads from the neck into the upper torso more efficiently for a common neck injury mechanism. However, this scenario is not entirely realistic, as deformation and damage to the neck brace would be observed during impacts. A limited number of loading conditions were investigated, and the results from this study are therefore not applicable to many situations. Despite the limitations, the helmet and SUFEHN model were validated, increasing the biofidelity of the experiment and improving the accuracy of the results. This paper also highlighted the importance of incorporating the seated posture of motorcyclists during experimental studies in order to provide accurate results.

Overall, both biomechanical studies suggest that neck braces are not effective in preventing spinal injuries during motorcycle impacts. Further work is necessary to fully understand the mechanisms of neck injuries and effectiveness of these devices in order to design safety protection equipment that mitigates injuries more effectively. In addition, developing a standard procedure for assessing the performance of protective equipment for the neck would be beneficial, as it would allow for results from various biomechanical studies to be more easily compared [20]. Based on current neck-brace evaluation research, neck braces do not reduce the risks of cervical spine injury, and could potentially increase the severity of cervical injury by increasing the loading and stress level in the neck.

Helmets

Helmets have been widely-established as an effective protective equipment against head injuries and are reported to prevent 37% and 41% of fatal head injuries for motorcycle drivers and passengers, respectively [22]. This positive statistic can overshadow other injury types which may be affected by helmet use. Helmets are often believed to decrease overall injury risk, when this is not the case. Although helmets do protect motorcyclists against head injuries, they are not as effective in preventing other serious injuries as the general public believes them to be. Previously, the research surrounding the effectiveness of helmets has focused on head injuries, and spinal injuries are often overlooked while evaluating their effectiveness.

The knowledge gap surrounding helmet use and spinal cord injuries have resulted in speculation within the motorcycle industry. Researchers have speculated that, without helmet use, the momentum from high-speed objects impacting the skull could be transferred to the spine, causing spinal injuries. Others speculate that the higher mass from the helmet could exert a higher traction force on the head and neck junction, resulting in increased torque on the base of the neck and skull which would increase the risk of neck injury [23][24].

One paper which compared the injuries sustained between 83 helmeted and non-helmeted motorcyclists through statistical analysis concluded that helmet use does not increase the incidence or severity of cervical spine injuries [25]. A second study which analyzed data from 1061 motorcycle crashes found that helmeted riders had a much lower Injury Severity Score (ISS) and did not increase the risk of a cervical injury [26]. These findings were supported by a third study which investigated fatalities specifically [27]. Current research suggests that helmet use does not increase the likelihood or severity of cervical spine injuries. However, due to limited research in this particular field, it is still unknown whether or not helmets are effective in preventing cervical spine injuries.

LIMITATIONS AND CHALLENGES

Overall, there has been significantly less research regarding spinal cord injuries and motorcycle accidents when compared to other four-wheeled vehicle accidents. Researchers therefore face many challenges due to the lack of research and knowledge regarding spinal injury mechanisms and the effectiveness of PPE in preventing spinal injury. Although the limitations of each specific research paper have been outlined in the previous sections, this section will focus on identifying the gaps of knowledge that exist within this field and exploring the significant limitations that are common across research papers.

Limitations in Current Research

Although research papers have identified risk factors and crash characteristics of motorcycle accidents that result in spinal injuries, the effects of loading conditions on injury patterns and injury severity has not been thoroughly investigated. The lack of existing laboratory rigs for motorcycle accidents results in a limited understanding of the overall dynamics as well as the head and neck response in high-speed motorcycle impact situations. Evaluating the biofidelity of an experimental set-up and the resulting data is also a challenge for researchers as there is insufficient experimental data from cadaver studies [21]. Lack of available and reliable data have prevented researchers from creating comprehensive and accurate validation standards for computational models [20]. The absence of a validated model can lead to inconsistent and inaccurate findings, and poses additional challenges in biomechanical research surrounding spinal cord injuries.

The lack of research and information regarding injury mechanisms in the spine during motorcycle impact situations presents a significant challenge for researchers. Without a better understanding of spinal injury mechanisms, researchers and engineers will face additional challenges when evaluating and designing effective personal protective equipment. Since each vertebrae experiences and translates the force differently depending on loading conditions, the type and severity of the injuries sustained is difficult to predict. Realistic impact situations with combined loading are likely to result in several injury mechanisms which could contribute to the same injury. Therefore, injury mechanisms must be investigated individually, and together.

The existing neck injury criterion (Nij) is relevant for cervical spine injuries, but there is no widely-accepted injury criterion that can be used to accurately assess the response of the spinal cord and the associated injury risk. Tolerance levels must be further investigated in order to develop evaluation standards for spinal cord injuries. This is particularly challenging, since spinal cord injuries resulting from motorcycle accidents may not appear immediately. Impacts can cause disk degeneration or disk herniation, which can result in spinal cord impingement and have long-term consequences. Furthermore, much of the current research focuses on the spinal column in general, but fails to consider and investigate the injury mechanism of the cord specifically and the extent of resulting spinal cord damage under different loading conditions.

The complex anatomy of the spine and the heterogeneity of the spinal cord makes it a difficult structure to study and understand, and the current biomechanical studies which investigate the spine’s behaviour under impact situations will therefore have several significant limitations.

Limitations of Biomechanical Studies

Limitations of the biomechanical studies reviewed in this paper include small sample sizes, the neglected dynamic and heterogeneous nature of the spinal cord, and the lack of anthropomorphic test device (ATD) testing that has been completed for motorcycle impacts.

Demographic differences between individuals may result in varying results due to varying anatomy. Larger sample sizes and high number of testing can help ensure the accuracy and precision of results. Biomechanical testing completed with cadaver surrogates may not allow for large sample sizes, and conclusions must be drawn from limited testing. While this provides more reliable results than invalidated computational models, it cannot be assumed that the values and limits extruded from these biomechanical research studies can be applied to the entire population. Using linear regression analysis from cadaver models can be used to average the values, and using validated FEA models can help overcome the issues resulting from small sample sizes and a lack of repeatability.

In addition, it is extremely difficult to replicate the heterogeneity of the spinal cord throughout the spine. Excluding CSF and surrounding soft tissue into biomechanical studies will reduce the biofidelity of the study and affect results. Many approximations and assumptions were made in the biomechanical studies reviewed, and this further reduces the biofidelity of the results. One way to increase biofidelity within computational models is to conduct additional biomechanical studies with PMHS. As live human surrogates cannot be used in injury biomechanical studies, PMHS allows for a sufficiently biofidelic response which provides more realistic results when compared to other surrogate options. However, the procedures for cadaver preparation and storage (i.e., embalming, freezing, and dissection) change the mechanical properties of soft tissue, which reduces the biofidelity of the surrogates [27]. Aside from this, cadaver studies are often limited in terms of sample size and repeatability due to the lack of availability of human cadavers. Cadavers are also often elderly, and their reduced bone density may result in inaccurate tolerance values that are not applicable to the general population. None of the biomechanical studies reviewed in this paper used anthropometric testing devices (ATDs). However, it is important to consider the limitations of the existing motorcycle anthropometric testing device (MATD) that is used to investigate motorcycle accidents specifically. The MATD was initially based on the Hybrid III pedestrian dummy, but has since been modified to improve biofidelity and instrumentation. The pelvis was modified to allow for greater hip articulations and the modified lumbar spine is straight and less stiff. In addition, the unique data acquisition system was added, and frangible femur, tibia, and knee components were included for injury detection purposes. These modifications, along with the increased range of motion of the neck, allow for the MATD to be positioned on the motorcycle [28].

Efforts to further improve the biofidelity of the MATD neck have been based on the standard Hybrid III design. Neck angle adjustment is of particular interest when considering MATD design. Since the position and posture of motorcycle riders will vary greatly between individuals, it is particularly important for the MATD to have a wide neck angle adjustment range. A biomechanical study evaluating the biofidelity of the MATD found that the neck requires further modifications in order to improve the stiffness and the neck’s response to lateral bending [28]. Furthermore, the abdominal instrumentation prevented some chest deformation from occurring, and the thorax’s response was insufficiently biofidelic [29].

Designing effective protective equipment requires an understanding of injury mechanisms, crash characteristics, and risk factors. The lack of research regarding spinal injury mechanisms and the limitations of the existing biomechanical studies poses a significant challenge to researchers and engineers. There is therefore still a lot of work which must be completed in order to fill the existing knowledge gaps.

FUTURE WORK

Additional work in this field is necessary in order to address the limitations of current research and to develop a better understanding of spinal cord injury mechanisms during motorcycle impacts. Previous work often focused on an isolated spine level instead of the entire spinal column. For future work in the biomechanical field, it would be helpful to include the entire spine, as this would help identify any factors contributing to the location and severity of spinal injury [29]. The limitations associated with incomplete or missing data fields in research studies can be overcome by including data from other countries, and implementing additional procedures to ensure that all relevant crash variables are included in police and medical reports. Improving the features and instrumentation of the MATD would be helpful for future biomechanical studies as well. Downsizing the electrical components within the abdomen would improve thorax biofidelity and would allow for realistic chest deformations. The stiffness of the neck requires further improvements, and it must be modified in order to improve its response to lateral bending. The establishment of a widely-accepted injury criterion for the spine would also help improve the biofidelity in ATDs, and would enable researchers to more accurately assess the response of the spinal cord and the injury risks. Although it would be difficult to validate, the implementation of a human FEA model would allow for a better understanding of human biomechanics and would be extremely useful for future work regarding injury and crash mechanisms and the development of tolerance values and injury criteria. Additional medical and biomechanical data must be collected in order to allow for  validation. Furthermore, not all the models used in the biomechanical studies are validated, and additional work is necessary in order to ensure that all models are validated under various loading conditions. Increasing the biodiversity of surrogates in biomechanical studies could be useful in the evaluation and design of personal protective equipment. For helmets specifically, the influence of the helmet’s weight on the risk of cervical spinal injury should be explored further in future studies [17].

The MATD and the validated computational models that are commonly used are based on anthropometric data from a 50th percentile male. Although motorcycles are most frequently used by average males, it could still be beneficial to expand existing ATDs and computational models to include other demographics to ensure that results are more applicable. Additional loading conditions should be explored further, with a particular focus on shear loading. Existing studies focus primarily on vertical loading conditions, and fail to consider the effect of pure shear on the spinal column. Many spinal injuries are fracture dislocations, and since shear forces play a large role in this injury type, it is vital that these areas are covered [16]. Investigating dislocation injury mechanisms further would allow for a more comprehensive and well-rounded understanding of the spinal column's response.

CONCLUSION

In conclusion, the current research that has been completed on this topic is quite minimal. Additional research and biomechanical studies are necessary in order to understand spinal cord injury mechanisms during motorcycle collisions. There are few options for protective equipment that reduce the risk of spinal injury, and additional work on the MATDs as well as computational models is necessary for the development of safer personal protective equipment for motorcycles.

BIBLIOGRAPHY

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