After a rear end impact, various clinical symptoms are often seen in car occupants (e.g. neck stiffness, strain, headache). Although many different injury mechanisms of the cervical spine have been identified thus far, the extent to which a single mechanism of injury is responsible remains uncertain. Apart from hyperextension or excessive shearing, a compression of the cervical spine can also be seen in the first phase of the impact due to ramping or other mechanical interactions between the seat back and the spine.
It is hypothesized that this axial compression, together with the shear force, are responsible for the higher observed frequency of neck injuries in rear end impacts versus frontal impacts of comparable severity. The axial compression first causes loosening of cervical ligaments making it easier for shear type soft tissue injuries to occur.
To test this hypothesis, an in vitro experiment was designed to investigate the theory that axial compression reduces the shear stiffness when the cervical spine is moved due to a rear-end impact. Specimens from C1-T1 were tested. Results showed that shear stiffness values were reduced significantly with increased axial compressions. More dynamic tests are needed to further test this hypothesis for the rear-end impact neck injury mechanism. Also, investigations of soft tissue injuries in rear-end crash victims are necessary to guarantee the merit of this hypothesis.