Motion analysis of cervical vertebrae during whiplash loading

Study Design. The motion of each cervical vertebra during simulated rear-en d car collisions was analyzed. Objectives. To clarify the mechanism of zygapophysial joint injury during w hiplash loading, Summary of Background Data. The zygapophysial joint is the suspected origin of neck pain after rear-end car collision. However, no studies have been c onducted on the mechanisms of zygapophysial joint injuries. Methods. Ten healthy male volunteers participated in this study. Subjects s at on a sled that glided backward on inclined rails and crashed into a damp er at 4 km/hr. The motion of the cervical spine was recorded using cineradi ography. Each vertebra's rotational angle and the instantaneous axes of rot ation of the C5-C6 motion segments were quantified. These measurements impl emented the template method. Results. There were three distinct patterns of cervical spine motion after impact, In the flexion-extension group, C6 rotated backward before the uppe r vertebrae in the early phase; thus, the cervical spine showed a flexion p osition (initial flexion), After C6 reached its maximum rotational angle, C 5 was induced to extend. As upper motion segments went into flexion, and th e lower segments into extension, the cervical spine took an S-shaped positi on. In this position, the C5-C6 motion segments showed an open-book motion with an upward-shifted instantaneous axis of rotation. Conclusions. The cervical spine is forced to move from the lower vertebrae during rear-end collisions. This motion completely differs from normal exte nsion motion and is probably related to the injury mechanism.