91 - Sex-Related Differences in Biomechanical Factors on Brain Deformation Response

Abstract Text: Roughly 1.7 million Americans survive a traumatic brain injury (TBI) each year (Georges & Das, 2022). Of all TBI-related hospitalizations, one-fifth are caused by motor vehicle crashes (MVC) (Peterson et al., 2014). Females have a greater risk of moderate TBI in an MVC when controlling for changes in impact velocity, sex, age, height, Body Mass Index (BMI), and vehicle model year (Forman et al., 2019). Previous work has found significant differences in the anatomy of male and female occupants, such as mass distribution, cervical vertebrae dimensions, and neck strength (Nikolova et al., 2007; Vasavada et al., 2008; Young et al., 1983). Many factors could contribute to sex-based differences in TBI risk during MVC, and neuroanatomy, brain material properties, and head kinematics are the major components associated with the biomechanical factors. Recent research has focused on incorporating more female-based dummies in crash tests, including the development and use of small-statured, 5th percentile female anthropometric test devices (ATDs). Use of these ATDs and other surrogates has enabled the field to get a better understanding of the differences in head exposures between females and males. However, there is still a need to improve our understanding of how the differences in head impact exposure between mid-sized males and small-sized females in the same crash conditions contribute to one sex having a higher or lower risk of TBI in the same crash. The goal of this study was to quantify sex-based differences in head kinematics during automotive crash tests as it relates to brain strain. Finite element (FE) models can be used to simulate head exposures measured by ATDs to predict brain deformation in an MVC, which allows us to simulate multiple head kinematics while controlling features like neuroanatomy and material properties of the model.

We achieved the study objectives by performing head kinematics simulations on an FE model of a single human brain. With this experimental design, we aim to isolate the effect of head exposure on brain strain, while keeping neuroanatomy and material properties constant. The head kinematics were obtained from 52 sets of sex-matched sled tests which involved both post-mortem human surrogates (PMHS) and ATDs representing the 50th percentile male and 5th percentile female anthropometries. This data was acquired from the National Highway Traffic Safety Administration Biomechanics Test Database. Crash conditions involved occupants in simplified sled tests (Gold Standard 2, Gold Standard 3), a 25° recline frontal crash (32 kph), and a 45° recline frontal crash (15/32 kph) condition. Seatbelt load limits were adjusted based on occupant weights. The head kinematics were simulated using the CAB-20MSym FE brain model and the 95th percentile maximum principal strain (MPS-95) was calculated as the metric of brain strain severity (Giudice et al., 2020, 2021). The head kinematics data were also used to calculate multiple brain injury metrics used in the automotive industry: brain injury criteria (BrIC), universal brain injury criteria (UBrIC), and Diffuse Axonal Multi-Axis General Evaluation (DAMAGE) (Gabler et al., 2018, 2019; Takhounts et al., 2013). A Bayesian linear mixed model was used to determine if sex was a statistically significant factor contributing to brain deformation.

The distributions of peak kinematics showed that mean male peak kinematics were greater for peak x- direction and resultant linear accelerations, while the female peak kinematics were greater for the peak x- and z- axis and resultant angular rates. The mean male brain metrics were higher for UBrIC and DAMAGE, and the mean female brain metrics were higher for BrIC. Results from the Bayesian multivariate linear mixed model showed that sex was a significant factor in predicting the peak x-direction and resultant linear accelerations, and for the x- and z- axis angular rates. However, after accounting for head kinematic features, the deformation metrics linear mixed model demonstrated that the impact of sex was statistically insignificant with an estimated coefficient of 0.003 (95% Crl -0.01, 0.01) for MPS-95. This suggests that differences in head exposure alone are not the cause of the disparity in the TBI incidence between males and females in MVC. More work is needed on this topic to identify pertinent factors.

Keywords: Biomechanics, Sex-Related Differences, Brain Deformation, Finite Element Model, Motor Vehicle Crash, Automotive Crash Tests, Anthropometric Test Devices