A Study of Cervical Spine Kinematics in Rear Impacts and Mitigation of Loading Level to Cervical Soft Tissues

摘要

The purposes of this study are first to understand the kinematics of the human cervical spine in rear impacts, and then to examine the effectiveness of a new seat concept for reducing loading level to the cervical facet joints. A finite element based human model named the Total Human Model for Safety (THUMS) was used to analyze the local and global kinematics of the spine. Precise geometry of the cervical vertebral bodies and related soft tissues were incorporated into the model. Their material properties were carefully defined referring to anatomy textbooks. The model was first validated against human test data in literatures by comparing vertebrae motion as well as head and neck responses. The model was then used to simulate a rear collision at a delta-V of 25 km/h. The motion of the head and torso was characterized by interaction events between the occupant and the seat. To understand the mechanism of cervical joint motion during a rear impact, kinematics of the cervical vertebrae was analyzed in a local coordinate system defined along the joint surface. It was found that the trajectory of the vertebra was characterized by featuring points corresponding to the interaction events. Based on the findings from the analysis, a new seat concept was proposed to reduce the magnitude of joint deformation. For the elevated test speed, the lower seat back frame was reinforced to withstand the severity of the impact. The foam material in the seat back was softened to catch the occupant torso with less shock. The location of the head restraint and the stays were modified to provide a firm support to the occiput. To verify the effectiveness of the new seat concept, another simulation was conducted under equivalent impact conditions. The magnitude of joint deformation calculated using the THUMS suggested that the new concept seat was effective in reducing the loading level to the cervical soft tissues especially stretch. Although shear deformation was increased by the reinforced seat back frame, the magnitude of the increase was thought to be acceptable considering the physiological range of joint motion. The study also revealed that the relative motion between the head and torso was not a transient event but a continuous phenomenon during impact. It was suggested that seat performance should be evaluated by conducting dynamic tests rather than by static measurement.