In cervical hyperextension injuries, the mechanism is one of vertical compression with the head in an extended position. Stress is mainly on the posterior elements (the laminae and pedicles) of the midcervical vertebrae (C4 to C6), or sometimes at higher levels (see the named fractures detailed further on), which may be fractured unilaterally or bilaterally, and on the anterior ligaments. This dual disruption in the spinal architecture allows for displacement of one vertebral body upon the adjacent one and compresses the cord between the laminae of the lower vertebra and the body of the one above.
However, spinal cord trauma may also occur from hyperextension injury without apparent damage or misalignment of the vertebrae. In these instances, the spinal cord damage, which can be minor or profound and permanent, is considered to be caused by a sudden inward bulge of the ligamentum flavum or by transient dislocation at the time of injury  which, is permitted because of ligamentous disruption; when viewed with imaging studies, the vertebral bodies are found to have spontaneously realigned. In such cases, rupture of the supporting ligamentous elements and spinal instability can be revealed by gentle flexion and extension of the neck under radiologic observation, which demonstrates movement of the vertebra in relation to an adjacent one. CT and plain lateral spine films are satisfactory means of demonstrating the vertebral injury but the tearing and bulging of ligaments from vertebral dislocation are more dependably demonstrated by MRI.
Another potential mechanism of cord and spinal root injury involving extremes of extension and flexion of the neck is so-called whiplash or recoil injury, most often the result of an automobile accident. When a vehicle is struck sharply from behind, the head of the occupant is flung back uncontrollably, or if a fast-moving vehicle stops abruptly, there is sudden forward flexion of the neck, followed by retroflexion. Under these conditions the occipitonuchal and sternocleidomastoid muscles and other supporting structures of the neck and head are affected much more often than the spinal cord or roots. Nevertheless, in rare instances, quadriparesis, temporary or permanent, results from a violent whiplash injury. The exact mechanism of neural injury in these circumstances is not clear; perhaps there is a transient posterior dislocation of a vertebral body, a momentary buckling of the ligamentum flavum, or retropulsion of the intervertebral disc into the spinal canal. Other ostensible results of whiplash, such as dizziness, are controversial and are discussed in Chap. 10. However, the main comment to be made regarding whiplash is that all manner of neurologic symptoms have been uncritically attributed to it, often with implications for medicolegal and disability determinations.
As mentioned, the presence of a congenitally narrow cervical spinal canal or of acquired spinal diseases such as cervical spondylosis, rheumatoid arthritis, or ankylosing spondylitis adds greatly to the hazard of damage to the cord or roots. Neck trauma of almost any configuration may aggravate preexisting spondylotic symptoms. There are in addition examples of spinal cord compression that result from prolonged static hyperextension of the cervical spine during a protracted period of stupor. This accounts for some cases of quadriplegia following a period of sustained unresponsiveness due to opiate or sedative drug overdose (Ell et al). Arterial hypotension may be an added factor in some instances.
A special type of spinal cord injury, occurring most often in wartime, is one in which a high-velocity missile penetrates the vertebral canal and damages the spinal cord directly. In some cases the missile strikes the vertebral column without entering the spinal canal but due to the shock wave, disrupts and virtually shatters the intradural contents or produces lesser degrees of spinal cord dysfunction. Or, the transmitted shock wave from a bullet passing nearby the vertebral column causes paralysis of spinal cord function that is largely reversible in a day or two (spinal cord concussion, which is described further on).