The purposes of this study are to measure the relative linear and angular displacements of each pair of adjacent cervical vertebrae and to compute the deformation of the facet joint capsule during low posterior-anterior (+Gx) acceleration without significant hyperextension of the head. A HYGE type mini-sled has been successfully developed to conduct whole body cadaver tests with a high-speed x-ray imaging system.

A total of twenty-six low speed rear-end impact tests were conducted using six postmortem human specimens. Each cadaver was instrumented with neck targets, which were rigidly embedded in each cervical vertebra (C1 to C7). Sequential x-ray images were collected and analyzed. The coordinates of each moving neck target were obtained as a function of time. Standard coordinate transformation was performed to calculate the relative displacements and rotations of each cervical vertebra relative to its adjacent lower vertebra.

The head and cervical vertebrae were in extension during rear-end impacts. The head showed less extension in comparison with the cervical spine. The rotational motion of the lower vertebral bodies (C4 to C6) increased more rapidly than the upper vertebral segments (Cl to C3). The segmental motion went from relative flexion at the upper cervical levels (C1-C2 and C2-C3) to relative extension at the lower cervical levels (C5- C6), with a transition region at the mid-cervical levels (C3-C4 and C4-C5).

Relative displacements were calculated for each cervical motion segment (C1 to C6) at the neck target locations in a body-fixed coordinate system (three specimens, twelve tests). Upper neck markers moved relative to the adjacent lower vertebrae of up to 7.8 mm. The peak relative displacements occurred before or after head contact with the head restraint. Relative displacements and uni-axial deformations were calculated using facet capsule landmarks for each cervical m otion segment (C1 to C6) in a global coordinate system (three specimens, seven two-dimensional setup tests). Upper facet landmark moved rearward relative to the adjacent low er landmark. Up to 7.8 mm relative displacement and 97% stretch were observed. The peak relative displacements occurred before or after head contact with the head restraint. However, the peak uni-axial deformation happened before head contact with the head restraint.

The cervical spine experienced compression, tension, shear forces, and flexion/extension moment at various stages o f loading and/or at different levels of the neck. If stretching of the facet capsular ligament is the reason for the high incidence of neck pain due to a rear-end impact, the upper cervical spine could be injured by flexion while the lower cervical spine could be injured by extension. And the injury could due to a combination of tension and shear force.

The 20-degree seatback tests resolved in less cervical lordotic curvature, more upward ramping motion of the thoracic spine, and more relative rotations of each cervical motion segment (C1/C2 to C5/C6) in comparison with O-degree seatback tests.