Documentation:FIB book/Assessing and Comparing Whiplash Injuries Among Male and Female Drivers in Low-speed Rear-Impact Car Crashes

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Introduction

Whiplash injuries have been a concern in the field of injury biomechanics for an extended period due to their high prevalence in low-speed rear-impact crashes. Nearly 85% of neck injuries in vehicle crashes are caused by rear impacts resulting in a whiplash injury[1]. The biomechanics of whiplash injuries stem from an abrupt, forceful motion of the cervical spine and the supporting muscles and soft tissues. Recent findings have proposed that whiplash injury may occur as a result of hyperextension of the lower cervical vertebrae in relation to a relative flexion of the upper cervical vertebrae[2]. This movement results in an S-shaped curve of the cervical spine at the time of impact from a low-speed rear collision (Figure 1)[2]. The damage to soft tissues in the cervical spine can lead to chronic neck pain and stiffness. Watanabe et.al. conducted a study on the long-term impacts of whiplash-associated disorders (WAD) on patients symptoms[3]. Patients with WAD symptoms showcased higher shoulder stiffness (72.0% vs. 45.9%), headache (24.0% vs. 12.2%), and arm pain (13.3% vs. 3.9%) as compared to the control population[3]. After 20 years, whiplash injuries considerably impacted the residual symptoms of shoulder stiffness, headache, and arm pain as compared to asymptomatic volunteers[3]. Long-term suffering accounts for over 60% of the cost of all injuries leading to permanent medical impairment for the insurance companies, for injuries sustained in vehicle crashes[4]. While this is a common injury, similar to concussions, its mechanisms are not completely understood due to the limited ability to diagnose them with medical imaging modalities including X-ray and MRI[1]. On the other hand, there has been very little research conducted to determine if male or female drivers are more likely to suffer from whiplash in low-speed rear impacts. For an easier visualization of the relevant phases that occur in whiplash, refer to Figure 2.

Hyperextension and Hyperflexion of the neck.
Hyperextension of the neck in rear-end impact without a headrest

Figure 1: Hyperextension and Hyperflexion of the neck (top)[5]. Hyperextension of the neck in rear-end impact without a headrest (bottom)[6].

Various stages of whiplash

Figure 2: The mechanism of whiplash can be broken down into three phases including retraction, forward movement, and belt restraint[6].

Retraction (Initial Impact)

When the driver is initially subjected to a low-speed rear-impact collision, the vehicle rapidly accelerates forward. The torso of the driver is initially held in place by the seatbelt however, the lower spine moves forward with the car. The head is free to move and it lags due to inertia. The difference in movement between the head and the lower spine causes the neck to hyperextend.

Forward movement

The movement of the head aligns with the motion of the torso due to rapid breaking or collision with another vehicle. This rapid forward motion causes hyperflexion of the neck as it bends forward past its normal range of motion.

Belt Restraint

After the hyperflexion has occurred, the seatbelt restrains the forward motion of the torso, however, the head may continue to move forward, eventually rebounding back to a slight extension phase.

Criteria and Tolerances

There have been various criteria developed to understand whiplash injuries in low-speed rear impacts and to manufacture protective equipment to minimize injuries[1].  According to statistical data, various factors come into play which can affect the intensity of whiplash injuries including the headrest position, design of the seat, and passenger position[1]. Therefore, the biomechanical analysis can become very intricate due to the multitude of factors that come into play in each scenario[7]. Various tests have been conducted to determine neck injury mechanisms in whiplash injuries. For example, studies conducted by Svenson et al. focused on pressure gradients in the spinal canal during hyperextension of the neck and how this correlated to injuries to the nerve root region in the cervical spine[7]. Animal studies on pigs showcased pressure gradient and nerve cell dysfunction following whiplash motion[7]. On the other hand, mathematical simulations built on the principles of fluid dynamics further validated how pressure fluctuations occur during hyperflexion and hyperextension of the cervical vertebrae, providing a foundation for predicting potential injuries[7].


Relevant criteria developed to determine the probability of whiplash occurring in low-speed rear-impact collisions:

Neck Injury Criterion NIC (generic)[6]

  • Measures the likelihood of a soft tissue neck injury occurring from minor low-speed rear impacts
  • Validated and tested in pigs (Figure 3)
  • The formula was developed by considerations of fluid flow and pressure gradients in the spinal column
  • Only useful in scenarios where the relative velocity and acceleration are both directed rearward (“retraction”).
Experimental setup.png

Figure 3: Experimental setup to determine what magnitude of forces/moment/accelerations has a low probability of causing injury[6].


NIC (Max)[6]

  • It represents the maximum value of NIC(t) between the beginning of the collision and the point where the head, relative to the neck, reverses its direction of motion.
  • The idea for developing this criterion was that it was a better way to determine if injury had occurred by focusing on the maximum retraction of the head relative to the body.


The formula for NIC[6]:

NIC Formula.png

Short-term Symptoms

Following experimental testing, the threshold for a greater than 50% probability of short-term neck injury occurring was determined to be 15 m2/s2.

Long-term Symptoms

The threshold for a greater than 50% probability of long-term neck injury was established at 24 m2/s2.

While these injury criteria provide a quantitative and biomechanical analysis to understand various forces, accelerations, and movements that can result in a whiplash injury during low-speed rear-impact collisions, the severity of whiplash injury in males and females is based on a variety of other factors including anatomical and physiological.

Prevention in Injury Biomechanics

A study was conducted that included rear-end impacts between 1990 and 1999 that resulted in at least one permanent neck injury impairment[8]. Data from 860 occupants was collected of which 302 were female and 142 were male. Two ratios were used to compare the relative risks for impairing whiplash injuries which included:

  1. Driver male (DM)/passenger male (PM) relative risk = 1.4 n = 57[8].
  2. Driver female (DF)/passenger female (PF) relative risk = 2.5 n = 102[8].

Overall, these ratios demonstrate that the drivers whether they are female or male had a higher risk of sustaining whiplash injuries as compared to the passengers.

The results from these studies indicated that females had a relative risk of medical impairment of 3.1 compared to males after adjustment for the average increased risk in the driver position[8]. This meant that after accounting for the additional elevated risk for drivers, females had a 3.1 times higher chance of sustaining medical impairment caused by whiplash injuries as compared to males. Therefore, this study suggests that females are more likely to suffer from whiplash-related injuries compared to males in low-speed rear-impact collisions.

To understand why females and males experience whiplash differently in low-speed rear-impact collisions, the relevant anatomy that's involved in these crashes needs to be examined. Clinical literature has consistently identified females as more susceptible to trauma-related neck pain, commonly resulting from soft tissue cervical spine injury[9]. Structural gender differences in the vertebral bodies in females may explain altered response to dynamic loading leading to increased soft tissue distortion and greater injury susceptibility[9]. Studies conducted by Stemper et al. compared the width and depth of the vertebral bodies, inter-facet width, and disc-facet depth in males and females[9]. The results showcased that the vertebral body dimensions were approximately 7% greater, overall vertebral dimensions were approximately 8% greater, and the segmental support area was 18% greater in male volunteers[9]. Another study by Gilsanz et.al. showcased similar results as the vertebral bodies in females had lower cross-sectional areas 8.22 cm2 +/- 1.09 compared to males 10.98 cm2 +/- 1.25. Volume differences were also observed in the vertebral bodies 22.42 cm3 +/- 2.40 versus 30.86 cm3 +/- 2.6[10]. Overall, these studies highlight a difference in the cervical geometry between females and males showcasing lower dimensions, cross-sectional areas, and volumes in the vertebral bodies which may be a reason why females are more susceptible to whiplash in low-speed rear-impact collisions.

Overarching Concepts

Whiplash injuries pose a significant effect on the healthcare system - namely inducing substantial medical costs and long-term suffering for patients[7]. Many studies attempt to address whiplash injuries through both prevention and treatment and in doing so, multiple recurrent themes are presented: the need for more precise and advanced injury criteria that can consider gender disparities and limitations in current models, the development of more realistic ATDs, and how a diverse understanding of vehicle safety leads to better road safety for all drivers[1] [4] [7].


Head restraints are an important consideration in reducing neck injuries during rear-end collisions[11]. The effectiveness of these restraints is typically evaluated through IV-NIC, NIC, Nkm, Nij, and NDC. However, certain criteria like Nij and Nkm may not be accurate in predicting all types of injuries, which necessitates more advanced and precise injury criteria for rear-end collisions[11]; for example, Nij and Nkm are not wholly accurate in predicting craniocervical junction injuries[12].

Current safety measures in assessing rear-end collisions are also lacking with respect to their applications to multiple groups. For example, research conducted by Linder et al. found that there exists a higher risk of whiplash injuries in females compared to males[4]. Previous studies also show that anti-whiplash measurements are typically more effective for males than females[4]. The particular reasons behind these differences are due to the variation in height, weight, joint stiffness, and geometrical properties between males and females. These factors can influence the protective performance of the seat. In order to address the issue of lack of access to commercial seat models for physical testing, virtual test structures were developed[4]. The development of a virtual female ATD was necessary for conducting correct evaluations of the parameters involved in a rear-end collision. Comparisons between the average-sized male and female virtual dummies to a respective volunteer set displayed similarities in accelerations and displacements in the x direction but also demonstrated differences with T1 angular displacements[4], where the angular displacement in volunteers was larger than in FE models. Furthermore, testing with the FE-BioRID and observing its horizontal displacement showed how a virtual model of the seat is accurate in recreating the loading of the physical seat [2]. Overall, dynamic responses were able to be accurately estimated through virtual ATDs.

The BioRID offers a more biofidelic insight into rear-end collisions through its incorporation of a realistic spinal structure which in turn leads to a more accurate understanding of the low-speed rear-end collision, though limitations exist in the application of the ATD to different populations[7]. Designs like the BioRID P50F allow for improved testing on 50th percentile females, alongside the BioRID II whose design was centred around the 50th percentile males; however, further research and developments are needed in order to capture various size ATDs[13].

A study by Albert et al. (2018), investigated the thoracic responses and injury risks of the Hybrid III (HIII) and THOR-M relative to post-mortem human surrogates (PMHS)[14]. The ATDs demonstrated differences in thoracic biofidelity: the THOR-M displayed expansion at the lower left thorax which was also seen in the PMHS tests; however, the lower right thorax deflected much more than the HIII and PMHSs[14]. The study showed that the HIII had better external thoracic biofidelity than the THOR-M even though changes were made to the THOR’s design explicitly to improve biofidelity[14]. Even after extensive research, the authors concluded that there is a lack of data for a full evaluation of the thoracic biofidelity of ATDs relative to PMHSs[15].

In the broader context, past research emphasizes the complexity of neck injuries in rear-end collisions for drivers. They highlight the need for more precise and advanced injury criteria that consider gender disparities and the limitations of conventional criteria[4] [13]. The development of realistic ATDs is instrumental in enhancing the safety of drivers and reducing the risk of whiplash injuries in rear-end collisions[1] [4] [7]. This research collectively aims to contribute to a more detailed and gender-inclusive approach to vehicle safety, ultimately enhancing road safety for all involved parties.

Strengths and Limitations

The researchers present a collection of notable strengths, particularly the use of actual human cervical spine samples, which bring an exceptional level of detail in both anatomical structure and biomechanical behavior[11]. The employment of these specimens guarantees that the studies reflect true-to-life spinal characteristics. In addition, the integration with a lifelike crash test dummy, such as the BioRID II, enhances the authenticity of the neck motion simulations in rear impact scenarios by taking into account the reaction of the entire body[11].

Gender Base Limitation

While the studies provide valuable insights, they also face significant limitations regarding their sample size and demographic diversity, which could impact the generalizability of its findings[11]. With a cohort comprising only six specimens, the sample is notably small[11]. This size raises concerns about the sturdiness of the data and the strength of the conclusions drawn. In scientific research, especially in studies involving biomechanics and human physiology, a larger sample size is often crucial for ensuring that the results are statistically significant and representative of the broader population. With such a limited group, there's a heightened risk that the findings are coincidental or specific to the particular specimens studied, rather than indicative of general trends.

Moreover, the demographic composition of the sample further compounds these concerns. The inclusion of specimens from only four males and two females introduces potential gender bias in the results[11]. This limited gender representation is problematic as there are known differences in musculoskeletal structure and injury response between males and females[11]. The biomechanical and physiological disparities might not be adequately captured in a sample where one gender is underrepresented. Consequently, the study's applicability to the entire population could be questioned, as the findings may not accurately reflect how different genders would respond under similar circumstances.

Additionally, the reliance on specific dummy models ties the study's findings to the particularities of those figures. These models are designed based on certain anthropometric standards that may not account for the diversity in body shapes, sizes, and weights found in the general population[4]. For instance, if the dummy models are based on average male dimensions, this could lead to results that do not accurately reflect the potential injuries sustained by women, children, or individuals outside the average body type parameters.

Similarly, the use of surrogate models like EvaRID, which is designed to introduce gender differences into crash testing, represents a step towards inclusivity but might not encompass the full spectrum of biomechanical variations between genders[4]. Differences in muscle strength, ligament flexibility, bone density, and overall body morphology between males and females can influence how injuries occur and manifest[16]. Furthermore, there is significant variability within each gender that a single surrogate model cannot encapsulate. As a result, while surrogate models are invaluable for adding a layer of differentiation in crash testing, they are not a complete substitute for a diverse range of test conditions that truly represent the variety found in the global population.

General Limitation

Some of the studies' reliance on older specimens serves as an advantage by providing valuable insights into the older demographic, which is more prone to spinal injuries[11]. However, the advanced age of the specimens simultaneously introduces a significant limitation when it comes to applying the study's findings to the broader population. Younger individuals typically have different cervical spine characteristics, such as greater flexibility, higher bone density, and more resilient intervertebral discs, all of which contribute to a different biomechanical response during vehicular impacts [17]. The elasticity of younger spines allows for a distribution of force that can greatly differ from the response of an older, more brittle spine [18]. The distinct biomechanical properties of a younger person's cervical spine mean that the effects of rear impacts can be considerably different in terms of the types of injuries sustained, their severity, and the recovery process.

One primary area of constraint is the utilization of the Lab Seat model[4]. This model, while valuable for controlled simulations, may not encompass the multifaceted design and material properties of actual car seats. Car seats in the real world come with a range of features such as contoured designs for comfort, adjustable headrests, intricate spring and cushioning systems for shock absorption, and even personalized settings for position adjustments. These characteristics play a critical role in how a seat interacts with the occupant during a crash.

The stationary positioning of crash test dummies in the study is another notable limitation[4]. This static setup is a stark simplification of what happens to real humans in a vehicle collision. In reality, occupants are not perfectly still at the moment of impact; they may be leaning forward, turning their heads, reaching for something, or even adjusting their seating position. These dynamic postures can drastically alter the mechanics of injury during a crash.

Humans also have reflexive responses to imminent collisions, such as bracing for impact, which can change muscle tension and thus affect how forces are transmitted through the body[19]. Static dummies can't replicate these pre-impact muscular changes, which are crucial for understanding the full spectrum of potential injuries.

In summary, while the study is methodologically sound and presents advanced approaches to simulating and understanding cervical spine biomechanics in rear impact scenarios, the inherent limitations in sample representation, modeling fidelity, and the ability to replicate dynamic human behavior should be carefully considered when interpreting its findings.

Controversy in the Field and Its Importance

In the field of automotive safety and biomechanics, controversies often arise from the methodologies employed in research studies, and their implications on safety standards and vehicle design. One significant controversy relates to the representativeness of crash test dummies and human analogues used in experiments. Critics argue that these models may not adequately reflect the diversity of human sizes, shapes, and biomechanical responses, particularly across different genders and age groups[20]. This can lead to safety features that are optimized for a subset of the population, potentially neglecting the unique vulnerabilities of others, such as children, women, or elderly individuals.

Another contentious issue revolves around the validity of simulated crash environments versus the unpredictable nature of real-world collisions. While controlled lab settings offer consistency for tests, they may oversimplify the chaotic variables present in actual crashes, such as seat positioning, occupant awareness and movement, and vehicle interactions[21]. There's an ongoing debate on balancing the need for controlled experimental conditions with the requirement for tests to reflect real-life scenarios more accurately. This includes discussions about the dynamic behavior of occupants prior to a crash, such as bracing or turning, which are challenging to replicate with static dummies but have a significant impact on injury outcomes[21].

Lastly, the application of findings from studies using small sample sizes and specific demographic ranges, like the limited cohort of six specimens mentioned, to the broader population remains controversial[11] [21]. Skeptics of such research question the extrapolation of data from these studies, suggesting that they may not provide a comprehensive understanding of spinal biomechanics in varied populations. This fuels a larger debate on how to design studies that are both scientifically rigorous and broadly applicable, ensuring that safety advancements benefit the entirety of society. The significance of these controversies lies in their potential impact on the development of safety standards and mechanisms that aim to protect all vehicle occupants during collisions.

Future Research

By prioritizing several key areas, future research can work to enhance our understanding and prevention of whiplash injuries in rear-end collisions - specifically in reducing the disparity between males and females. As mentioned previously, limitations with current injury criteria lead to the potential for errors due to gender-specific differences in neck biomechanics[1] [11] [12]. Nij and Nkm are not accurate in predicting craniocervical junction injuries and present an area on which future research can focus[12]. Alongside improved injury criteria, there is a need for the development and use of ATDs accounting for a variety of body shapes, sizes, and ages in order to reduce vulnerabilities[15].

Additionally, to bridge the gap between controlled lab experiments and real-world crash scenarios, future studies should focus on addressing the unpredictability of collisions[4] [11]. Ways to simulate these conditions include accounting for dynamic human behaviour before and during a collision; such as occupant posture, muscle activation, and reflex responses to imminent crashes[7].

Another aspect future research can touch on is the applicability of the research. Ways to improve applicability include expanding sample sizes and the involved demographics. Studies with broader groups, covering a wider range of ages, body types, and genders, would provide a deeper understanding of whiplash injuries. Effective safety mechanisms can also be designed through the collection of diverse real-world crash data[11].

The use of finite element models and other computational methods is greatly useful, but tests conducted in this manner should be validated against empirical data[4] [11]. Future research can expand on this topic by focusing on improving the accuracy of computational models through comparisons of their predictions and parameters with real-world injury data.

Lastly, different age groups play a part in understanding biomechanical responses. Further research is needed to explore how the cervical spine behaves in children while considering factors like bone density, spinal flexibility, and muscle strength[11]. While this knowledge can lead to age-specific safety measures, specifically in reducing the impact of rear-end collisions on children, the methods with which this research goes forward need to be carefully considered given the ethical issues of using younger people as a research topic.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 D. U. Erbulut, "Biomechanics of neck injuries resulting from rear-end vehicle collisions," Turkish Neurosurgery, vol. 24, (4), pp. 466-470, 2014.
  2. ↑ 2.0 2.1 S. Yadla, J. K. Ratliff and J. S. Harrop, "Whiplash: diagnosis, treatment, and associated injuries," Current Reviews in Musculoskeletal Medicine, vol. 1, (1), pp. 65-68, 2008.
  3. ↑ 3.0 3.1 3.2 K. Watanabe et al, "The Long-term Impact of Whiplash Injuries on Patient Symptoms and the Associated Degenerative Changes Detected Using MRI: A Prospective 20-year Follow-up Study Comparing Patients with Whiplash-associated Disorders with Asymptomatic Subjects," Spine (Philadelphia, Pa. 1976), vol. 46, (11), pp. 710-716, 2021.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 A. Linder, K. Holmqvist and M. Y. Svensson, "Average male and female virtual dummy model (BioRID and EvaRID) simulations with two seat concepts in the Euro NCAP low severity rear impact test configuration," Accident Analysis and Prevention, vol. 114, pp. 62-70, 2018.
  5. ↑ C. to W. projects, Wikimedia Commons, https://commons.wikimedia.org/wiki/Main_Page (accessed Dec. 3, 2023).
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 D ’ Entremont, Agnes. “2.3 SPINE INJURY.” MECH 436/536 Fundamentals of Injury Biomechanics.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 M. Y. Svensson et al, "Neck injuries in car collisions — a review covering a possible injury mechanism and the development of a new rear-impact dummy," Accident Analysis and Prevention, vol. 32, (2), pp. 167-175, 2000.
  8. ↑ 8.0 8.1 8.2 8.3 B. Jonsson et al, "The risk of whiplash-induced medical impairment in rear-end impacts for males and females in driver seat compared to front passenger seat," IATSS Research, vol. 37, (1), pp. 8-11, 2013.
  9. ↑ 9.0 9.1 9.2 9.3 B. D. Stemper et al, "Anatomical gender differences in cervical vertebrae of size-matched volunteers," Spine (Philadelphia, Pa. 1976), vol. 33, (2), pp. E44-E49, 2008.
  10. ↑ V. Gilsanz et al, "Gender differences in vertebral sizes in adults: biomechanical implications," Radiology, vol. 190, (3), pp. 678, 1994.
  11. ↑ 11.00 11.01 11.02 11.03 11.04 11.05 11.06 11.07 11.08 11.09 11.10 11.11 11.12 11.13 11.14 Ivancic, P. C., & Sha, D. (2010). Comparison of the whiplash injury criteria. Accident Analysis and Prevention, 42(1), 56-63. https://doi.org/10.1016/j.aap.2009.07.001.
  12. ↑ 12.0 12.1 12.2 Smotrova, E., Morris, L., & McNally, D. (2021). Comparison of standard automotive industry injury predictors and actual injury sustained during significant whiplash events. European Spine Journal, 30(10), 3043-3058. https://doi.org/10.1007/s00586-021-06851-y.
  13. ↑ 13.0 13.1 A. Carlsson et al, "Design and Evaluation of the Initial 50th Percentile Female Prototype Rear Impact Dummy, BioRID P50F – Indications for the Need of an Additional Dummy Size," Frontiers in Bioengineering and Biotechnology, vol. 9, pp. 687058-687058, 2021.
  14. ↑ 14.0 14.1 14.2 D. L. Albert, S. M. Beeman and A. R. Kemper, "Assessment of Thoracic Response and Injury Risk Using the Hybrid III, THOR-M, and Post-Mortem Human Surrogates under Various Restraint Conditions in Full-Scale Frontal Sled Tests," Stapp Car Crash Journal, vol. 62, pp. 1-65, 2018.
  15. ↑ 15.0 15.1 D. L. Albert, S. M. Beeman and A. R. Kemper, "Comparative biofidelity of the Hybrid III and THOR 50th male ATDs under three restraint conditions during frontal sled tests," Traffic Injury Prevention, vol. 24, (S1), pp. S41-S46, 2023.
  16. ↑ T. F. Lang, "The Bone-Muscle Relationship in Men and Women," Journal of Osteoporosis, vol. 2011, pp. 702735-4, 2011.
  17. ↑ K. A. Alahmari et al, "Association of age on cervical joint position error," Journal of Advanced Research, vol. 8, (3), pp. 201-207, 2017.
  18. ↑ Stemper, Brian D et al. “Biomechanical properties of human thoracic spine disc segments.” Journal of craniovertebral junction & spine vol. 1,1 (2010): 18-22. doi:10.4103/0974-8237.65477.
  19. ↑ D. Bose and J. R. Crandall, "Influence of active muscle contribution on the injury response of restrained car occupants," Annals of Advances in Automotive Medicine, vol. 52, pp. 61-72, 2008.
  20. ↑ S. Putka, “Why are there no crash test dummies that represent average women?,” Discover Magazine, https://www.discovermagazine.com/technology/why-are-there-no-crash-test-dummies-that-represent-average-women (accessed Nov. 6, 2023).
  21. ↑ 21.0 21.1 21.2 M. P. Reed, "Occupant dynamics during crash avoidance maneuvers," UMTRI, 2021.

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