Neck Injury Risk in Non-Neutral Position Rear Impact Scenarios Using a Reference Geometry-Based Repositioning

Abstract

Rear-end vehicle collisions may lead to whiplash-associated disorders (WADs), comprising a variety of neck and head pain responses. The source of WADs is an active area of research, but the facet joints, cervical spine ligaments, neck muscles, nerve roots, vertebral arteries, and intervertebral discs have been postulated to be potential injury sources. While most investigations focus on whiplash injuries in neutral position impacts, front-seat vehicle occupants tend to spend a significant amount of time outside the neutral axial head position. Multiple cervical tissues can experience increased mechanical loads and, consequently, higher injury risks during head-rotated impacts. Given the limited experimental data for non-neutral position rear impacts, computational human body models (HBMs) can inform the potential for tissue-level injury. Previous studies have considered external boundary conditions to reposition the head axially but were limited in reproducing a biofidelic movement for a detailed finite element (FE) head and neck model. The objectives of this study were to implement a novel head repositioning method using soft tissue stress initialization to achieve targeted axial head rotations, validate the model intervertebral motion against experimental data, and simulate neutral and non-neutral rear impacts to assess the model tissue-level response and quantify the injury risk. The head and neck model was extracted from the detailed commercial Global Human Body Models Consortium (GHBMC) M50-O FE model, representing a 50th percentile male occupant. The repositioning used a novel method to achieve equilibrium in the target axial rotations of 24.5°, 33.5°, 42.5°, 51.5°, and 60.5°, by pre-stretching the neck soft tissues. The head-rotated models underwent verification tests to ensure the target positions and equilibrium were achieved, and the method was validated by comparing spinal rotations to experimental results. A neutral position and five head-rotated models were evaluated at three rear impact severities (4g, 7g, and 10g) with and without muscle activation. Head motion, ligament distractions, muscle strains, nerve root compressions, and annulus fibrosus (AF) strains were assessed and compared with physiological limits to estimate the injury risks. The repositioning method was able to rotate the head to the five target positions, showing general agreement with reported intervertebral rotations. Under the rear impact scenarios, higher axial head rotations increased the combined three-plane rotations experienced by the head. Increasing head rotations led to higher ligament distractions and muscle strains during rear impacts, increasing their potential for injury. The trends for AF strains varied depending on the muscle activation, with peak strains being observed for initial rotations of 0° and 60.5° without muscle contraction and only for 0° with muscle contraction. Overall, muscle activation decreased ligament displacements, muscle strains, and AF strains, while higher impact severity increased the injury risk for these same tissues. Nerve root compression was minimally affected by head rotation, muscle activation, and impact severity. The models predicted injury potential for the ligaments and muscles starting at a 4g rear impact acceleration and for the AF starting at 7g, particularly under high head rotations. Therefore, the proposed repositioning method introduced in this study enabled the model to achieve steady head rotations with realistic cervical spine movements, increasing the biofidelity of non-neutral rear impact simulations. The tissue-level assessment revealed the effect of varying head rotations, muscle activation, and impact severities on cervical tissues, allowing for a comprehensive estimation of injury thresholds across ligaments, muscles, nerve roots, and AFs. The results of this study can be applied to future assessments and design of head restraints and protection systems.