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Of the more than 1 million acute spine injuries that occur annually in the United States, 50,000 are fractures of the bony spinal column and 11,000 include injury to the spinal cord. Trauma to the cervical spine accounts for nearly 50% of all spine injuries and results in almost half of the estimated spinal cord injuries. The subaxial spine accounts for the majority of the injuries: 65% of fractures and 75% of all dislocations. However, despite the prevalence of these injuries, their classification, management, and treatment can vary widely among surgeons.
From a functional standpoint, the cervical spine can be divided into two distinct regions: the craniocervical junction (occiput to C2) and the subaxial cervical spine (C3 to C7). The craniocervical junction consists of unique anatomic structures. It obtains stability primarily through ligamentous attachments, and the joints have a lower degree of intrinsic stability to provide a more substantial range of motion. The subaxial spine has a higher degree of intersegmental stability. Secondary to variations in anatomy of the cervical spine are injury patterns characteristic to each region. Recognition of the specific injury patterns facilitates diagnosis and treatment.
Injury Classification and Pathophysiology
In 1970, Sir Frank Holdsworth created the first comprehensive classification system for spinal column injuries based on his experience with more than 2000 patients; he recognized the importance of the posterior ligamentous complex in determining stability. In 1982, Allen and colleagues proposed a classification system of subaxial cervical spine injuries based on plain radiographs in which mechanism of injury was inferred from the recoil position of the spine. Six categories were originally described: compressive flexion, vertical compression, distractive flexion, compressive extension, distractive extension, and lateral flexion. Harris then modified the system to incorporate rotational vectors in flexion and extension. The six mechanisms described were flexion, flexion and rotation, hyperextension and rotation, vertical compression, extension, and lateral flexion. Each mechanism was further subdivided to determine the severity of the injury with respect to the primary vector; the goal was to indicate the degree of tissue damage and instability. Both systems are comprehensive, but they are rarely used because of their complexity and the difficulty in correlating the subtypes and injury severity.
A novel classification system, the Subaxial Injury Classification (SLIC), was developed in an effort to convey information about injury pattern, treatment considerations, and prognosis. The system consists of three components: (1) injury morphology, (2) integrity of the diskoligamentous complex (DLC), and (3) neurologic status of the patient. Injury morphology is determined by the pattern of spinal column disruption on imaging studies.
Injury Morphology
Within the SLIC system, injury morphology is divided into three categories: compression, distraction, and translation or rotation. Compression is a visible loss of height through part of or the entire vertebral body or by disruption through the end plate. This morphology includes traditional compression fractures and burst fractures, sagittal and coronal plane fractures, and teardrop or flexion compression fractures. Undisplaced or minimally displaced lateral mass and facet fractures are categorized as compression-type injuries, and they are likely the result of a lateral compression mechanism.
Distraction is defined as an anatomic dissociation in the vertical axis, and it signifies a greater degree of destruction and potential instability. This injury pattern commonly involves ligamentous disruption that propagates through the disk space or the facet joints. A hyperextension injury disrupting the anterior longitudinal ligament and widening the anterior disk space also represents a distractive pattern of injury.
Translation or rotation injuries have radiographic evidence of horizontal displacement of one part of the subaxial spine relative to another. By definition, both anterior and posterior structures are disrupted, and a high degree of instability is present. A relative angulation of greater than 11 degrees has been the suggested threshold for rotation. Unilateral and bilateral facet fracture-dislocations, fracture separation of the lateral mass or a “floating” lateral mass, and bilateral pedicle fractures are also examples of translational injuries.
Diskoligamentous Complex
The DLC is represented by the anterior and posterior longitudinal ligaments and by the posterior ligaments (ligamentum flavum, interspinous ligament, supraspinous ligament, and facet capsule). The integrity of the DLC is postulated to be directly proportional to spinal stability.
The facet joint capsules are the strongest anatomic structure posteriorly, whereas anteriorly the anterior longitudinal ligament is strongest. Therefore, abnormal facet alignment, characterized by articular apposition of less than 50% or diastasis of more than 2 mm through the facet joint, can be considered an absolute indication of DLC disruption. Another absolute indication of DLC disruption is abnormal widening of the anterior disk space on neutral or extension radiographs.
The interspinous ligament is the weakest ligament in the subaxial cervical spine. Consequently, radiographic evidence of interspinous process widening indicates incompetence of the DLC only if lateral flexion radiographs demonstrate abnormal facet alignment or angulation of greater than 11 degrees at the involved interspace.
Neurologic Status
The neurologic status of the patient can be an indicator of the degree of spinal column injury and can influence the decision whether the patient should undergo surgical intervention, in particular if new neurologic deficits are present. In the SLIC system, neurologic status is defined as intact, root injury, complete cord injury, or incomplete cord injury. Continued cord compression exists as a modifier in those patients with either complete or incomplete spinal cord injuries. In patients with translational or rotational injuries, assessment of spinal cord compression is made after attempted reduction of the injury.
Application of the Subaxial Injury Classification System
Within the SLIC system are three injury axes: morphology, DLC, and neurologic status. A numeric value is generated from each axis, and the tally of the three numbers is the SLIC score; the larger the number, the more severe the injury. The score can be used to guide treatment, but it should not be used in isolation. Both the descriptive information on which the scoring is based and the final score itself are necessary to understand the injury fully and to make treatment decisions ( Table 18-1 ).
| Points | |
|---|---|
| Morphology | |
| No abnormality | 0 |
| Compression | 1 |
| Burst | 2 |
| Distraction | 3 |
| Rotation or translation | 4 |
| Diskoligamentous Complex | |
| Intact | 0 |
| Indeterminate | 1 |
| Disrupted | 2 |
| Neurologic Status | |
| Intact | 0 |
| Root injury | 1 |
| Complete cord injury | 2 |
| Incomplete cord injury | 3 |
| Presence of continued cord compression in the setting of spinal cord injury | +1 |
Radiographic Evaluation
Determining mechanical stability of the cervical spine is of paramount importance to prevent further displacement and late deformity, either of which can cause pain or neurologic dysfunction. In most cases, mechanical instability manifests as abnormal alignment or static deformity on routine radiographic studies. Based on cadaveric studies, White and Punjabi and their colleagues defined cervical spine instability as follows: (1) destruction of either all the anterior or all the posterior elements, thereby rendering them unable to function; (2) more than 3.5 mm of displacement of one vertebra in relation to another anteriorly or posteriorly; and (3) greater than 11 degrees of rotational difference between adjacent vertebra.
Anteroposterior, lateral, and open-mouth odontoid views are the traditional radiographic views for symptomatic patients after traumatic cervical spine injury. The reported sensitivity of the three-view series for detecting injury to the subaxial spine has been reported to range from 53% to 83%. In selected patients, upright radiographs may help identify more subtle instability patterns. The role of dynamic flexion-extension radiographs to determine instability is limited in the acute setting because of pain, muscle spasm, and limited cervical excursion. Thin-cut computed tomography with sagittal and coronal reconstructions has been more widely used in the acute trauma setting to identify fractures and dislocations, and this may be a more sensitive tool for detecting acute injuries.
MRI may be more beneficial for evaluating for more subtle forms of instability, in particular for assessing ligamentous integrity. However, although MRI is sensitive in identifying ligamentous injury, the clinical significance of these findings is controversial, and decisions should therefore be based on the patient’s entire clinical picture. Because MRI has a moderate positive predictive value of detecting ligamentous injury, it can lead to a high number of false-positive results. Thus, direct evidence of a posterior ligamentous complex injury shown by MRI should not be used in isolation to assess mechanical stability or determine a treatment plan. MRI may be most useful in the evaluation of an obtunded patient or uncooperative patient in whom physical examination is not possible. Finally, when surgery is contemplated, MRI can provide valuable information on the location and degree of stenosis, the presence of associated epidural hematoma, and evidence of spinal cord signal change and intraspinal edema or hemorrhaging.
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