Although the management of cervical spine trauma is relatively complex, multiple classification systems have attempted to simplify it through the use of descriptive terms. Most historical classification systems failed to yield sufficient prognostic information to guide clinical treatment until the Subaxial Injury Classification system was developed. This classification system takes into account the injury morphology, discoligamentous complex, and the most important prognostic factor, neurologic status. The early results of this classification system have been encouraging and it is expected to improve spinal trauma care through enhancing more uniform nomenclature and communication for surgeons managing spinal trauma.
Cervical spine trauma remains one of the most common causes of morbidity in the United States with a significant financial burden on our society. For example, the estimated lifetime cost for a low tetraplegic injury (C5-C8) in a 25 year old will be more than $3 million. Attempts to minimize the damage to the cervical spinal cord can result in very important improvements in the quality of life for these devastating injuries. Therefore, the goal of any surgeon is to appropriately identify those injuries that would benefit from surgical stabilization and decompression. Multiple classification systems have been developed by experts to assist others, and the purpose of any classification system is to provide insight into the injury pattern, severity, and prognosis. Unfortunately, traditional classification systems generally sought to describe the injury in great detail but overlooked the more important prognostic value of the neurologic status of patients. The Thoracolumbar Injury Classification System and subsequently the cervical spine with the Subaxial Injury Classification System (SLIC) have been developed to address the deficiencies of other classification schemes. With the introduction of these newer classification systems, the focus in spine trauma has moved to include injury pattern, severity, and neurologic status, thus, providing a better platform for clinicians to define treatment approaches and prognosis. The purpose of this article is to review the traditional and newer classification systems for the subaxial cervical spine and discuss the recent evidence to support the SLIC as a prognostic tool for spine surgeons.
Traditional classification systems
Sir Frank Holdsworth published the first detailed description of subaxial cervical spine trauma in 1970 based on his extensive experience in more than 1000 patients with quadriplegia/paraplegia and many more without spinal cord injury. In his experience, specific fracture patterns were classified as either stable (simple wedge, burst, and extension injuries) or unstable (dislocations, rotational-fracture dislocations, and shear fractures), and patients were treated based on the injury morphology. During his vast experience, he also found that the posterior ligamentous complex was an important structure stabilizing the spine and used it to differentiate between the stable and unstable injuries. Although he dedicated a significant amount of energy into the management of neurologic injuries and how they relate to the patients’ prognoses, he did not incorporate the neurologic status into his classification scheme. He undoubtedly managed patients based on their level of neurologic dysfunction but the classification system failed to reflect this, limiting the value of this scheme when generalized to the spine surgical community.
Allen and Ferguson expanded the descriptive terms initiated by Holdsworth to include several other morphologic variables. In their classification system, the common fracture mechanisms were as follows: compression flexion, vertical compression, distractive flexion, compressive extension, distractive extension, and lateral flexion. They associated neurologic injury with the mechanism of injury and attempted to dictate a treatment plan based on the mechanism of injury. Although the mechanism of injury is associated with the neurologic injury, the mechanism was not predictive of final outcome and, therefore, not always useful in directing treatment. Harris and colleagues modified this descriptive classification system to include the rotational vectors in 1986, but unfortunately the spotlight remained on injury morphology. Additionally, the mechanisms proposed were generally not validated biomechanically but were rather deduced on the bases of radiographic views. Some injuries fail to fit neatly into a specific category, perhaps because of complex or multidirectional force vectors that produced the spinal trauma.
The previous classifications systems focused on descriptive terminology and in doing so became cumbersome and less reliable. When evaluated by members of the Spine Trauma Study Group (STSG), there was only a 65% and 57% agreement among raters for the Ferguson/Allen and Harris classification systems, respectively. The focus was firmly placed on injury morphology, with attempts to fit the fracture pattern into one of these previously determined categories. Frequently, fractures result from a mechanism that does not exactly correlate with one of the groups described by Harris, leaving ambiguity in the assessment of the injury.
Subaxial injury classification system
In 2007, recognizing the difficulty with these traditional classification systems, the STSG sought to create a simple yet useful classification system for subaxial injuries in the cervical spine. The STSG recently derived a novel classification system for thoracolumbar injuries that was well received and easily adopted by the spine community. They took the lessons learned from this previous classification system and incorporated them into the cervical spine, which created a paradigm shift in the thinking of cervical spine trauma. The focus was placed on neurologic injury and discoligamentous complex (DLC) in addition to the injury morphology. There are 6 variables that must be considered when describing a cervical spine injury:
Injury morphology (major category)
Bony injury description (ie, spinous process, lamina, lateral mass, superior facet, inferior facet, pedicle, transverse process, vertebral body)
Discoligamentous complex status (major category)
Neurologic status (major category)
Confounding variables (ie, diffused idiopathic hyperostosis, ankylosing spondylitis, osteoporosis, previous surgery, preexisting myelopathy/stenosis)
In this new classification system, the injury morphology, DLC, and neurologic injury are each evaluated separately and given a point value based on the level of severity ( Table 1 ). Although each of these independent variables correlates with clinical outcome, the sum of the 3 values can be used to dictate care and determine prognosis.
|Complete cord injury||2|
|Incomplete cord injury||3|
|Ongoing cord compression||+1|
Similar to previous classifications systems, the SLIC attempted to categorize the injury morphology but with a simplified approach to make it more useful. Using plain radiographs, computed tomography (CT), and magnetic resonance imaging (MRI), the fracture patterns were divided into 3 major categories in order of severity: compression, distraction, and translation/rotation.
Compressive injuries are judged as those with less vertebral height compared with the adjacent levels. Included in this category are simple compression fractures resulting in loss of height anteriorly and burst fractures. Burst fractures receive an additional point reflecting their increased instability compared with compression fractures. Flexion compression injuries or tear-drop fractures are included in this category. Also included are minimally displaced lateral mass and facet fractures without any evidence of translation/rotation and they are likely the result of a lateral compression-type mechanism.
Distraction injuries are defined as any injury that results in distraction across the fracture, intervertebral disk, or posterior elements. These injuries are inherently unstable and are commonly seen with a distraction-extension–type mechanism. Although a CT scan can frequently determine if there is a distraction, an MRI can discover more subtle distraction injuries across the intervertebral disk. Also included in this category are any injuries that result in perched facets without frank dislocation or translation.
The final and most unstable injury pattern is rotation/translation, which is defined as abnormal translation of one vertebral body relative to the next. In their landmark work on cervical spine stability, White and Panjabi determined that any translation greater than 3.5 mm or angulation greater than 11° relative to the adjacent level is abnormal in the cervical spine. The most common example of a rotation/translation injury is a dislocated facet. Also included in this category are bilateral pedicle fractures resulting in a traumatic spondylolisthesis or ipsilateral pedicle and lamina fractures resulting in a floating lateral mass and rotational deformity of one vertebrae relative to the adjacent vertebrae.
Compressive injuries are assigned 1 point, distraction injuries are assigned 3 points, and fracture/dislocations are assigned 4 points, with a burst fracture adding an additional point.
Holdsworth previously discussed the importance of the posterior ligamentous complex but did not incorporate it into his classification system except to use the ligamentous status to determine stability. The SLIC is the first classification system to place emphasis on the DLC by separating it as a distinct variable with an associated point value. The DLC can be graded as intact, indeterminate, or disrupted. The DLC is defined as the soft tissues supporting the articulation between 2 vertebra: the intervertebral disk, anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, interspinous ligament, supraspinous ligament, and the facet capsules. Any disruption of these structures results in instability between the 2 vertebra. Additionally, a ligamentous injury is less likely to heal compared with a bony injury and, therefore, more likely requires surgery to prevent long-term deformity.
In cases of facet dislocations, the facet capsules and the DLC are obviously disrupted but it is often more subtle. Abnormal facet alignment defined as less than 50% articular apposition or greater than 2 mm diastasis is associated with disruption of the DLC. Additionally, more subtle CT findings that may correlate with DLC disruption include disk space widening, interspinous widening, or vertebral rotation. With improvements in MRI technology, our ability to assess the DLC has improved dramatically, but there are still cases that are difficult to interpret with MRI and it should be judged as indeterminate.
An intact DLC is assigned 0 points, indeterminate is assigned 1 point, and disrupted is assigned 2 points.
The final and possibly the most important predictor of clinical outcome is the presenting neurologic examination. Not only does the initial neurologic examination correlate directly with final outcome but it also correlates with spinal stability. In order for most patients to develop a neurologic deficit, a significant amount of force has to be transmitted through the spine and, in doing so, the supporting osteoligamentous structures become disrupted. This component is a key part of the SLIC, distinguishing it from all previous classification systems. Furthermore, patients with a neurologic deficit are more likely to require and undergo a decompressive procedure. The neurologic examination can be divided into the following categories in order of severity/urgency: intact, root injury, complete spinal cord injury, and incomplete spinal cord injury. Incomplete spinal cord injuries are rated as more severe because they require more emergent attention and are more likely to require a surgical procedure/decompression.
Zero points are assigned if the neurologic status is intact, whereas 1 point is assigned if there is a root injury, 2 points are assigned if there is a complete spinal cord injury, and 3 points are assigned if there is an incomplete spinal cord injury. An additional point is added if there is ongoing spinal cord compression.