Does spinal alignment influence car occupant responses? -The influence of variation in whole spinal alignment patterns on vertebral kinematics under rear impact conditions-

Abstract: The susceptibility of women to Whiplash Associated Disorders (WADs) has been the focus of numerous epidemiologic studies. Summarising the epidemiologic WAD studies, women were found to be subject to up to three times higher risk of sustaining WADs than men. To improve occupant safety for both men and women, the overall aim of this thesis was to provide fundamental knowledge to understand how gender differences influence the injury mechanisms and risk of sustaining WADs. WADs are generally considered to be derived from cervical soft tissue damage caused by excessive cervical vertebral kinematics. This thesis has focused on the whole spinal alignment in automotive seated postures as one of the gender differences, and investigated its potential impact on cervical vertebral kinematics during a rear impact, clarifying gender specific dynamic characteristics of cervical vertebral kinematics. Previous rear impact sled test series comprising female and male volunteers were reanalysed to determine the dynamic characteristics of inertia-induced cervical vertebral kinematics during rear impacts. For spinal alignment, image data of the spinal column in automotive seated postures, acquired with an upright open Magnetic Resonance Imaging (MRI) system, were analysed. Typical patterns of the whole spinal alignment, including average gender specific spinal alignment patterns, were obtained through Multi-Dimensional Scaling (MDS). Implementing these typical spinal alignment patterns in a whole-body occupant FE model, the potential impact of whole spinal alignment on cervical vertebral kinematicswere investigated in reconstruction simulations of previous rear impact sled tests. In the sled tests, the female subjects were subjected to a more pronounced cervical S-shape than the male subjects, beyond the voluntary muscle-induced cervical kinematics range for female subjects. The average gender specific spinal alignment patterns of the automotive seated posture included a slight kyphotic, or almost straight cervical spine, with a less-kyphotic thoracic spine for the female subjects, and a lordotic cervical spine with a more pronounced kyphotic thoracic spine for the male subjects. In the reconstructed simulations, the average female spinal alignment pattern demonstrated greater intervertebral displacements from the lower cervical spine to the upper thoracic spine with a more pronounced cervical S-shape, compared to the average male spinal alignment pattern. Greater elongation of the cervical ligaments occurred at intervertebral levels where greater intervertebral displacement was found. Rear impact reconstruction simulations performed in this thesis demonstrated a potential impact of gender differences in whole spinal alignment on cervical vertebral kinematics and ligament elongation. The female spinal alignment trend may make women exposed to more significant deformation of the cervical soft tissues due to greater cervical vertebral kinematics during a rear impact. The findings may partially contribute to a greater understanding of the increased injury risk of women sustaining WADs.