In vitro low-speed side collisions cause injury to the lower cervical spine but do not damage alar ligaments

Abstract

Whether injuries to the alar ligaments could be responsible for complaints of patients having whiplash injury in the upper cervical spine is still controversially discussed. It is known that these ligaments protect the upper cervical spine against excessive lateral bending and axial rotation movements. The objective of the present in vitro study was therefore to examine whether the alar ligaments or any other structures of the cervical spine are damaged in side collisions. In a specially designed acceleration apparatus, six human osteoligamentous cervical spine specimens were subjected to incremental 90 degrees side collisions from the right (1 g, 2 g, 3 g, etc.) until structural failure occurred. A damped pivot table accounted for the passive movements of the trunk during collision, and a dummy head (4.5 kg) ensured almost physiological loading of the specimens. For quantification of functional injuries, the three-dimensional flexibility of the specimens was tested in a spine tester before and after each acceleration. In all six specimens, structural failure always occurred in the lower cervical spine and always affected the facet joint capsules and the intervertebral discs. In four specimens, this damage occurred during the 2 g collision, while in the other two it occurred during the 3 g and 4 g collision, respectively. The flexibility mainly increased in the lower cervical spine (especially in lateral bending to both sides) and, to a minor extent, in axial rotation. In vitro low-speed side collisions caused functional and structural injury to discoligamentous structures of the lower cervical spine, but did not damage the alar ligaments. Since the effects of muscle forces were not taken into account, the present in vitro study reflects a worst-case scenario. Injury thresholds should therefore not be transferred to reality.

Fig. 1

The acceleration apparatus consisted of three basic components: a pneumatic acceleration unit, a sled, and a rail track. On the sled, the cervical spine specimens were fixed on a damped pivot table, which accounted for the passive movements of the trunk during collision. On the occipital bone of the specimens, a dummy head was mounted which was suspended by a cord until acceleration, since buckling of the specimens would have occurred otherwise. At the beginning of the acceleration, this cord was cut in order to create unconstrained loading conditions during acceleration

Fig. 2

Sled horizontal acceleration and head resultant acceleration during the 1 g and 2 g collisions. Median curves of six specimens. (a_sled sled acceleration; a_{res head} head resultant acceleration)

Fig. 3

ROM of each of the six specimens before acceleration (open symbols) and after acceleration with 1 g (filled symbols) in the segments C0–C1, C1–C2, C2–C3, C3–C4, C4–C5 and in the whole cervical spine (C0–C7) in right and left lateral bending

Fig. 4

ROM of each of the six specimens before acceleration (open symbols) and after acceleration with 1 g (filled symbols) in the segments C0–C1, C1–C2, C2–C3, C3–C4, C4–C5 and in the whole cervical spine (C0–C7) in flexion and extension

Fig. 5

ROM of each of the six specimens before acceleration (open symbols) and after acceleration with 1 g (filled symbols) in the segments C0–C1, C1–C2, C2–C3, C3–C4, C4–C5 and in the whole cervical spine (C0–C7) in left and right axial rotation

Fig. 6

ROM of the one specimen which could not only be tested before acceleration (white) and after acceleration with 1 g (light grey), but also after acceleration with 2 g (dark grey) and 3 g (black). Results for the segments C0–C1, C1–C2, C2–C3, C3–C4, C4–C5 and the whole cervical spine (C0–C7) in right and left lateral bending