Anatomy

The anatomy of the lower cervical spine is unique and contributes to the injury patterns that are observed in this region. The cervical spinal canal houses the delicate spinal cord, which fills much of the canal, leaving relatively little room for displacement of the vertebral osteoligamentous structures without the risk of neurologic injury. The subaxial cervical disc spaces curve upward laterally to form the uncovertebral joints. The uncovertebral joints augment the stability of the segment in rotation but allow a large range of motion in flexion, extension, and lateral bending. The facet joints are oriented at 45 to 60 degrees to the coronal plane and also allow significant flexion, extension, and lateral bending motion. Because the intrinsic bony anatomy of the cervical spine provides relatively limited stability, there is a great dependence on the ligamentous structures to stabilize the subaxial cervical region.

Various authors have modeled the cervical spine as either a two-column^4,5 or three-column system.^6,7 Although both systems have merits, the two-column system probably provides the best understanding of the common injury patterns seen in the lower cervical region. The two-column spine consists of an anterior column and a posterior column. The anterior column contains the anterior longitudinal ligament, intervertebral disc, vertebral body, and posterior longitudinal ligament. The posterior column consists of the posterior bony elements, facet capsules, interspinous and supraspinous ligaments, and ligamentum flavum. The most important stabilizer of the anterior column is the anulus fibrosus, whereas the facet joints are the most important stabilizers of the posterior column.⁵

The ligamentous structures provide a check to hypermobility during normal motion. For example, the anterior longitudinal ligament and ventral anulus become taut during extension, whereas the posterior column ligamentous structures act as a tension band during flexion. Compressive loads are resisted by the vertebral bodies, intervertebral discs, and facet joints. Pure tensile loads are resisted by the anulus, interspinous ligament, ligamentum flavum, and facet capsules. Flexion is resisted by the interspinous ligaments, facet capsules and facet joints, anterior longitudinal ligament, and posterior anulus. Extension is also resisted by the bony block of the facet joints. Maximal sagittal plane translation occurring under physiologic loads is 2 to 2.7 mm.⁵ Lower cervical spine injuries can be understood as a failure of the structures designed to resist the forces and moments occurring at the time of the injury. By observing the pattern of bony and ligamentous disruption on imaging studies, the treating physician may generally deduce the force vectors that acted to create the injury pattern and also begin to gain an understanding of the “personality” of the injury, and thus the tendency for displacement under physiologic loads.

Injury Classification

Classification of cervical spine injuries is important for several reasons. First, classification facilitates accurate communication regarding the nature of an injury. Second, a classification system allows the physician to determine the optimal treatment of an injury. Third, classification allows the treating physician to make predictions regarding prognosis of the injury. Fourth, accurate classification is necessary for valid outcomes research to be performed or for data to be compared between centers. Finally, classification may facilitate an improved understanding of the pathomechanics of a particular injury.

Many classification systems have been proposed to describe injuries of the cervical spine. Available classification systems are generally based on specific factors, including mechanisms of injury, radiographic findings, injury severity, and neurologic status.

All classification systems have certain individual strengths and weaknesses. To use a system appropriately, one must understand the rationale of the classification so that an injury can he viewed in the appropriate context.

The simplest method of discussing injuries to the cervical spine is to use radiographic descriptions of the injury. Hence, terms such as compression fracture, burst fracture, teardrop fracture, or facet dislocation remain in common usage, although these descriptive terms provide only a broad overview of the injury and do not provide information on injury severity, neurologic status, or treatment options. Mechanistic classifications are useful in promoting an understanding of injury pathomechanics and also assist the surgeon in designing a rational treatment approach, although biomechanical validation of the presumed injury mechanisms is generally lacking. Despite this limitation, these classification schemes are useful in clinical practice and remain the preferred scheme at most trauma centers managing large volumes of cervical trauma.

The “ideal” classification system would allow all injuries to be placed into a specific category. An ideal system would have perfect interobserver and intraobserver reliability. The system should allow the clinician to better understand the injury and would define both treatment and expected outcome. Finally, the optimal scheme would be simple, reliable, and valid across the spectrum of treating physicians. Needless to say, this “ideal” classification system is not yet available.

Although many classification schemes have been proposed, this chapter reviews several schemes that have strong historical significance or practical utility, or are recent additions to the literature, including those proposed by Whitley and Forsyth (1960),⁸ Allen et al. (1982),⁹ Harris et al. (1986),¹⁰ Anderson et al. (2007),¹¹ and, finally, Vaccaro et al. (2007).³

Whitley and Forsyth⁸ described a mechanistic classification of cervical spine injuries in 1960 based on a review of 159 patients with cervical fractures. In their scheme, fractures were divided into flexion injuries, extension injuries, combined flexion-extension injuries, burst-type injuries, and direct trauma. The authors further divided flexion and extension injuries into those occurring with and without compression. This classification system retains historical importance for promoting a mechanistic thinking about cervical spine injuries.

White and Panjabi⁵ devised an early checklist for instability after cervical trauma. They hypothesized that a similar injury mechanism might produce different injury patterns because of the complex multidirectional forces, moments, and positions of the affected joints at the time of trauma. They devised a point-based system for assessing stability in lower cervical spine injury, summarized in Table 64-1. To use their system, radiographic criteria, physical examination, and a stretch test are required. A score of 5 or more points in this system is said to predict spinal instability.¹²

TABLE 64-1 Diagnosis of Clinical Instability in the Middle and Lower Cervical Spine

The stretch test described by White and Panjabi is performed by securing the patient’s head in halter or tong traction with a roller beneath the head to reduce friction. Initial lateral radiographs of the cervical spine with 10 pounds of traction are carefully analyzed to rule out a disruption of the occipitocervical junction. Serial weight is sequentially added in 10-pound increments, performing neurologic testing with each addition of weight. The end point of the test is reached when “instability” is noted on radiographs or when there is a change in neurologic examination, or when the weight limit (65 pounds or one-third body weight) is reached. Instability on the stretch test is defined as distraction of a vertebral interspace by 1.7 mm or greater or a change in segmental alignment of 7.5 degrees or greater compared with the baseline radiographs.¹² Although advocated by several authors, this type of test has not become a standard method for analyzing stability in most trauma centers because of its cumbersome nature and concerns regarding iatrogenic neurologic injury.

In 1982, Allen et al.⁹ published a mechanistic classification system that has been widely used in recent years. They based their scheme on the clinical review of 165 patients with indirect lower cervical spine trauma. In this system, subaxial cervical injuries were divided into “phylogenies” based on a common proposed injury mechanism, with an orderly progression of severity ranging from mild to severe. They included six injury phylogenies—compressive flexion, vertical compression, distractive flexion, compressive extension, distractive extension, and lateral flexion (Fig. 64-1). The terms used for each category describe the attitude of the cervical spine at the time of injury and the dominant force vector. The authors acknowledged the presence of minor force vectors that may cause separate or combined injuries. “Rotation” in this system was described as localizing an injury asymmetrically to one side rather than acting as an independent force vector. In general, the risk and severity of neurologic injury were noted to increase with increasing severity stages (Box 64-1).

FIGURE 64-1 Distribution of lower cervical spine injuries among 165 patients. CE, compressive extension; CF, compressive flexion; DE, distractive extension; DF, distractive flexion; LF, lateral flexion; VC, vertical compression.

(Data from Allen BL, Ferguson RL, Lehmann TR, et al: A mechanistic classification of closed indirect fractures and dislocations of the lower cervical spine. Spine [Phila Pa 1976] 7:1–27, 1982.)

In 1986, Harris et al.¹⁰ proposed a similar classification system that placed injuries into groups related to a predominant causative force vector or combination of force vectors (Box 64-2). Unlike the Allen scheme, Harris et al. included the rotational vectors combined with flexion or extension but did not emphasize the role of distractive forces.

In 2007, Anderson et al.¹¹ presented the Cervical Spine Injury Severity Score (CSISS), a scheme allowing the observer to grade the degree of osteoligamentous disruption of the injury based on a four-column concept of the cervical spine modified from the work of Louis. The concept is to correlate increasing amounts of osseous separation or ligamentous disruption with a higher degree of instability using an analogue scale, with the goal of identifying patients who would benefit from surgical stabilization. The authors found excellent intraobserver and interobserver reliability when applying their scheme to a cervical trauma population, perhaps owing to the critical analysis of each of the four columns of the cervical spine. When analyzed, the CSISS was noted to perform well for all fracture types and for a spectrum of injuries from relatively minor to grossly unstable.¹¹ Patients with scores greater than 7 points were generally subjected to surgical stabilization, suggesting the system produced consensus regarding the need for operative stabilization.

Vaccaro et al.³ presented a point-based scheme for evaluating subaxial cervical injuries in 2007. The authors reviewed the positive elements of prior classification systems and used the expertise of an experienced group of clinicians in the Spine Trauma Study Group (STSG) to devise a working system known as the Subaxial Injury Classification (SLIC; Table 64-2). The SLIC system applies a severity ranking in three specific areas: (1) a morphologic description of the bony fracture pattern, (2) a rating of the discoligamentous complex, and (3) a rating of the patient’s neurologic status.³ By applying points in each area, the authors were able to produce a severity score that was used to separate treatment into surgical and nonsurgical categories.

TABLE 64-2 Subaxial Cervical Spine Injury Classification System

To use the SLIC system with a high degree of interobserver and intraobserver consistency, it is important that the clinician adhere to a few simple concepts. First, at a given spinal level, the most severe injury pattern should be graded for morphology. Second, if a cervical spine injury demonstrates elements of both burst and translation, then the injury is classified as a translational injury. Third, if both a nerve root and spinal cord injury coexist, then it is the spinal cord injury that should be used to define the SLIC neurologic score.

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