Results
This study revealed that direct backward impact applied to the face caused not only an extension motion of the cervical spine but also a flexion motion of the upper segments. Consequently, the cervical spine had an S-shaped curvature with upper cervical spine flexion and lower cervical spine extension. This S-shaped curvature was similar to the curvature in cervical retraction. It was revealed in some studies that the cervical retraction consists of lower cervical extension and upper cervical flexion.
Furthermore, our results indicate that the impact location highly influences the patterns of the cervical retraction-like motion. For the forehead load, the C1-C2 segment was flexed, and the lower cervical spine was extended. For the maxilla load, the segments from occiput-C1 through C4-C5 were flexed. In other words, motion of the head influences the retraction-like motion of the cervical spine during direct face impact. More precisely, if the head is flexed more, the upper cervical spine is flexed more, and the inflection point shifts caudally during the impact.
Regarding the difference in cervical spine alignment, 3 subjects with kyphosis had similar retraction-like motion to those with lordosis. However, for the forehead load, their upper cervical spine flexion was higher than that of the other subjects, while for the maxilla load, their upper and middle cervical spine flexion was less. Because the sample size was small, these conclusions have no statistical significance. However, theses results seem to suggest that curvature of the cervical spine influences the patterns of the cervical spine motion.
Regardless of the impact location and cervical spine alignment, the retraction-like motion was observed in all subjects. In an actual frontal car collision or falling accident, most likely the victim is being forced forward with a flexion motion before he/she receives a blow to the face, causing the upper cervical spine to be flexed more than it was in our experiments. We speculate that retraction-like motions observed in this experiment are the main causes of 2 common types of the cervical spine injuries: odontoid fracture and spinal cord injury without radiographic abnormality.
A previous experimental study using cadaver specimens showed that odontoid fracture was produced with the combined forces of horizontal shear and vertical compression. They could not produce the fracture by hyperflexion, hyperextension, horizontal shear, or vertical compression alone. In our study, most of the applied impact load to the face resulted in a posterior shear force to the occipital condyle, and a flexion motion occurred at C1-C2 for both loading conditions. From these results, we explore the possibility that a flexion motion at the C1-C2 segment adds an element of compression force on this segment, and, therefore, odontoid fracture could occur during retraction-like motion.
In cervical cord injury without radiographic abnormality, rupture of the anterior longitudinal ligament has often been reported in postmortem examinations of patients who had died of this injury and in intraoperative findings of the patients. The injury mechanism is thought to be hyperextension of the cervical spine because most victims have facial injuries. However, direct backward impact applied to the face caused a flexion motion of the upper segments of the cervical spine in our experiments. In fact, a previous biomechanical study using spinal units revealed that rupture of normal spinal ligaments is not produced by hyperextension but is easily produced by shear force. Marar and Orth also reported that the mechanism of the injury is a combination of hyperextension and backward shearing forces in the study of cadavers. Therefore, we consider the possibility that the posterior shear force is concentrated on a segment within a length of the cervical spine with an S-shaped curvature, and that ligamentous rupture and spinal cord contusion occur at this level in an actual accident.
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