The usual circumstances of spinal cord injury, in approximate order of frequency in civilian practice are motor vehicle and motorcycle accidents, falls, gunshot or stab wounds, diving accidents, crushing industrial injuries, and birth injury. In the United States, the annual incidence of spinal cord injury has been given as approximately 5 cases per 100,000 population; males predominate (4:1). Each year approximately 3,500 persons die in relation to their spinal injury, and another 5,000 are left with complete or nearly complete loss of spinal cord function. The essential problem in most traumatic spinal cord injuries is that the surrounding vertebral spinal column is disrupted and through fracture, dislocation or disc protrusion, the spinal canal is narrowed, thereby compressing the cord (see review by Ropper and Ropper). Although trauma may involve the spinal cord alone, the vertebral column is almost invariably injured at the same time and often, there is an associated cranial injury as pointed out in Chap. 34.
A useful classification of vertebral column injuries divides them into fracture–dislocations, pure fractures, and pure dislocations. Except for bullet, shrapnel, and stab wounds, a direct blow to the spine is a relatively uncommon cause of serious spinal cord injury. In general medical practice, most fractures and dislocations of the spinal column are the result of force applied at a distance from the site of the disruption of the spinal column. Specifically, all three types of spinal injury are typically produced by a vertical compression of the spinal column, to which either anteroflexion or retroflexion (hyperextension) is added. The most important variables in the mechanics of vertebral injury are the structure of the bones and ligaments at the level of the injury and the intensity, direction, and point of impact of the force. The main elements of the spine are illustrated in Chap. 10.
Approximately 20 percent of spinal column injuries affect more than one level and the cervical spine is most vulnerable because it does not have the support of the thoracic cage. Blows to the head may result in cervical spinal injuries. The cranium itself constitutes a load on the fulcrum of the neck; forceful rotation about the neck can cause transient or persistent dislocation of vertebral bodies or fracture of elements of the vertebrae that destabilize the neck or cause disc protrusion that narrows the spinal canal. This the so-called “whiplash” injury is discussed further on and in Chap. 10. If a hard object at high velocity strikes the cranium, a skull fracture occurs, the force of the injury being absorbed mainly by the elastic quality of the skull. If the traumatizing force is relatively soft yet unyielding, or is applied more slowly, the spine, and particularly its most mobile (cervical) portion, will be the part injured. If the neck happens to be rigid and straight and the force is applied quickly to the head, the atlas and the odontoid process of the axis may fracture.
In the case of cervical flexion injury, the head has usually been bent sharply forward when the force is applied. The cervical vertebrae are forced together at the level of maximum stress, driving the anteroinferior edge of the upper vertebral body into the one below, sometimes splitting it in two. The posterior part of the fractured body is displaced backward and compresses the cord. Concomitantly, there is tearing of the interspinous and posterior longitudinal ligaments. Less severe degrees of anteroflexion injury produce only dislocation of adjacent cervical vertebrae at one of several levels. Vulnerability to the effects of anteroflexion (and to some extent to retroflexion injuries) is increased by the presence of cervical spondylosis or ankylosing spondylitis that narrow the canal or by a congenital narrowness of the spinal canal.