Study Design:​ The motion of each cervical vertebra during simulated rear-end car collisions was analyzed.

Objectives. To clarify the mechanism of zygapophysial joint injury during whiplash 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 conducted on the mechanisms of zygapophysial joint injuries.

Methods:​ Ten healthy male volunteers participated in this study. Subjects sat on a sled that glided backward on inclined rails and crashed into a damper at 4 km/hr. The motion of the cervical spine was recorded using cineradiography. Each vertebra’s rotational angle and the instantaneous axes of rotation of the C5–C6 motion segments were quantified. These measurements implemented 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 upper vertebrae in the early phase;​ thus, the cervical spine showed a flexion position (initial flexion). After C6 reached its maximum rotational angle, C5 was induced to extend. As upper motion segments went into flexion, and the lower segments into extension, the cervical spine took an S-shaped position. 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 extension motion and is probably related to the injury mechanism.