Background. The mechanism of whiplash injury has not been revealed yet. We analyzed in vivo cervical spine motion during simulated rear-end car collisions to clarify the whiplash injury mechanism. Method. The subjects were ten male volunteers. The ethics committee approved the protocol of this study. The subjects sat on a sled that glided backward on inclined rails and crashed into a damper at 8 km/hr. Cineradiography recorded the cervical spine motion during impact and the motion analysis of each vertebra was performed. Findings. During simulated rear-end impacts, torso motion forced the cervical spine to extend from the lowest vertebrae. This early extension motion of lowest cervical vertebrae caused the cervical spine to flex in the early phase. After this initial flexion motion, extension motion started from the lowest motion segment and gradually moved to the upper segments. Approximately 110 ms after impact, while the upper segments were still in the flexion mode, the lower segments have already been extended, creating an S-shaped curvature of the cervical spine. Interpretation. We observed abnormal cervical spine motions during simulated rear-end impacts and consider that this motion may be related to the whiplash injury mechanism.
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