Unilateral facet dislocations result in 0% to 25% subluxation and may be pure dislocations without fractures or more commonly associated with a facet fracture.
Bilateral facet dislocations result in 25% to 50% or even greater subluxation, and in some cases there may be significant distraction across the intervertebral disc space (Figure 1). These may be associated with facet fractures and often bilateral lamina or spinous process fractures. In all cases, the disc anulus is significantly disrupted and there may be associated disc herniation.
Posterior Column Injuries. Isolated posterior column injuries include spinous process fractures, lamina fractures, and posterior ligamentous injuries. Complex injuries of the posterior column include posterior ligamentous injuries with or without fracture of the spinous processes or lamina. As noted by Nicoll5 and Holdsworth,6 one of the major factors critical to stability is the posterior ligamentous complex; therefore, this structure is injured in many of the other injury types described above. We believe that the use of other common names, such as bilateral facet dislocation, more reliably describes the injury and would be used by the majority of physicians even though a significant posterior ligamentous injury is present.
Special Cases. Several patterns of injuries are complex and do not fit well into the previously described sections or are associated with preexisting disease. Special cases include the bilateral pedicle fracture with traumatic spondylolisthesis, fractures in the ankylosed spine, and spinal cord injury without radiographic abnormality.
Quantification of Stability: Cervical Spine Injury Severity Score.
The Cervical Spine Injury Severity Score was developed to measure stability after cervical spine trauma. The system is based on bony and ligamentous disruptions and does not include neurologic function. Neurologic function has been adequately addressed with standardized measurements published by the American Spinal Injury Association.7
The Cervical Spine Injury Severity Score applies to all areas of the subaxial spine from the caudal aspect of C2 to T1. It is easy to learn, reliable, and applies to all fracture types. It is a continuous variable so that discriminate statistical analysis can be performed. The score correlates to increasing instability and, we hypothesize, will relate to treatment decisions and ultimate prognosis. The score is based on skeletal injury and does not take into account neurologic function or deficits. Preexisting diseases such as ankylosis or congenital stenosis can modify the score based on decisions made by the observer.
Four-Column Model. The injury severity score is based on evaluation of the four columns of the cervical spine independently using a standard visual analog scale. The four columns are modification of the three pillars described by Louis.8 In his model, the spine is structurally supported by three legs: the anterior column consisting of the disc, body, and ligaments; and by the paired lateral pillars, including the lateral masses with facet articulations and capsules. In addition, the posterior osteoligamentous complex is added creating the fourth column (Figure 2A).
The four columns are the anterior, right pillar, left pillar, and posterior osseous ligamentous complex. The anterior column includes the body, intervertebral disc and anulus, and anterior-posterior longitudinal ligaments. The pillars include the pedicles starting at their junction with the vertebral body, the superior and inferior facets, the lateral mass, and facet capsules. Each pillar is scored separately. The posterior column includes the lamina, spinous processes, nuchal ligaments (supraspinous, infraspinous, the ligament of nuchae), and ligamentum flavum.
Analog Score. A visual analog score from 0 to 5 is applied to each column and summed. Thus, the injury severity score ranges from 0 to 20, with 0 being no injury and 20 the most severe. Fractional scores may be used.
The analog score is based on the degree of bony displacement and ligamentous disruption (Figure 2B). Standard computed tomography sagittal and axial reconstructions are used to evaluate the injuries. For example, a score of 1 is given for nondisplaced fractures, whereas for complete ligamentous disruption or displacement of greater than 5 mm, a score of 5 is given. A general concept is that add score of 5 is given to the most severe injury that can occur to that particular column. In patients with multiple levels of injury, only the most severe level is scored. Figure 2B outlines generally the analog scores based on degree of bony and ligamentous displacement. These are only general guidelines as each observer is tasked to rate the severity of injury based on his own criteria. Although this may increase variations among individuals, it has the advantage of allowing other factors not usually included in systems, such as results of ancillary studies, to be factored in as new information becomes available.
To test reliability, 35 consecutive cases of cervical spine injuries were scored by 10 reviewers. The reviewers’ clinical experience ranged from second year orthopedic surgery residents to an attending spine surgeon with 20 years of clinical experience. These reviewers were from different academic trauma centers. The study was performed with IRB approval. Additionally, to examine intraobserver reliability, 5 cases were repeated.
Plain radiographs and CT were downloaded to CDs as DICCOM images. Efilm lite software was used to open and scan images. Enhancement tools included brightness/contrast, zoom, measuring rulers, and synchronization. CT data included scout views and axial, coronal, and sagittal reconstructions. CTs were obtained in 1.5-mm slice thickness and included the entire cervical spine from the occiput to T1.
The cases were redacted of patient-identifying information and given a case number. The cases were scored in random order by each examiner without any clinical information. Interclass correlation coefficients (ICCs) were measured to determine both intra and interobserver reliability. A score of ≤0.4 was poor, 0.4 to 0.75 fair or moderate, and >0.75 excellent.9 A power analysis was done before initiation of the study. Using 10 observers and 35 cases was predicted to give greater than 85% power of showing reliability if present.10 All analyses were performed with SAS (SAS Institute, Inc., Cary, NC).
There was a broad range of stability in the case series with mean 8.2 and standard deviation of 6.6. Thus, the cases were distributed from very stable to highly unstable. Examples of the use of the system are given in Figures 1, 3, 4, and 5.
The interobserver ICCs ranged from 0.75 to 0.98 and averaged 0.88. Intraobserver reliability for the five duplicate cases ranged from 0.97 to 0.99. These data show excellent reliability in using the quantification injury severity score. Table 2 gives scores of observers and primary author for cases examples.
No differences in reliability were observed based on fracture morphology, between columns, or by experience of the observer.
Despite a long history and numerous approaches, no classification systems for subaxial cervical spine injuries have ever been universally accepted or clinically validated. All such systems are based on one of three factors: mechanism of injury, morphologic features, or degree of instability. In some cases, combinations of these factors are used.
Mechanistic classification systems attempt to predict the primary injury vector from patterns seen on radiographs or computed tomography scans.11 These injury vectors can be simulated in cadaveric models to allow confirmation and assist in injury prevention research. Many of the mechanisms accepted in describing injury mechanisms have entered the accepted nomenclature describing injuries. In some cases, the mechanistic terms imply a specific morphologic pattern, e.g., compression fracture, flexion-distraction injury.
Unfortunately, mechanistic systems may not always accurately describe injury patterns or the actual mechanism of injury. From cadaveric studies, it has been shown that application of the same injury vector can produce different fracture patterns. Shono et al created cervical spine injuries in cadavers using impulses applied to the skull vertex.12 Injuries created from the same magnitude and direction of forces included bursting fractures of the vertebral body and bilateral facet dislocations. Other limitations of mechanistic systems include the lack of knowledge of head position at the time of impact, the influence of musculature, and the effect of disease states such as age, ankylosis, and osteoporosis on injury patterns. Furthermore, the spine under acute loads has been shown to buckle, thereby producing variable injury vectors at different levels.13,14 For example, a flexion moment may be present at one level with an extension moment with rotation at a lower level (Figure 1). Once the spine is rendered unstable, its position can be in almost any location, making identification of the mechanism difficult. Mechanistic classification systems do not account for secondary vectors that act on the injured spine.
Morphologic classification systems describe what is observed on plain radiographs, CT, and in some cases MRI. Common terms such as burst fracture, avulsion fracture, and fracture dislocation are well accepted and are used to communicate injury patterns among surgeons. Too often, morphologic terms tend to describe mechanism of injuries but do not characterize instability. Other morphologic descriptors imply a mechanism but are accepted as a pathoanatomic entity. Terms such as hyperflexion, extension, and compression describe presumed direction of forces but also give a morphologic image of a specific injury pattern. Thus, when attempting to use pure morphologic systems, mechanistic terms are difficult to avoid.
The concept of stability was initially described in 1949 by Nicoll who reported 166 patients with thoracolumbar injuries.5 He identified radiographic factors that prevented return to work in Welsh injured miners. The factors that were associated with an “unstable” spine were disruption of the posterior osseous ligamentous complex and complete dislocations. Thus, Nicoll characterized stability and its prognosis on outcome; he concluded that an injured miner would function according to three characteristics: pain, mobility, and power and endurance.5
Holdsworth furthered Nicoll’s concepts in 1970.6 He specifically described the five types of “violence” or mechanisms that act on the spine during injury: pure flexion, flexion and rotation, extension, vertical compression, and direct shearing force. He was the first to describe the importance of the posterior osteoligamentous complex in terms of stability of the motion segment.
White and Panjabi have provided the most widely accepted definition of stability as “ability of the spine under physiologic loads to maintain an association between vertebral segments in such a way that there is neither damage nor subsequent irritation of the spinal cord or nerve roots, and, in addition, there is no development of incapacitating deformity or pain due to structural changes.”15 Using this definition and based on classic biomechanical experiments, they created a checklist of parameters that, when scored, produced an index of stability (Table 1). A score of 5 or greater results in clinical instability. This is an effective checklist to evaluate cervical injuries but has never been validated clinically and has poor reproducibility among observers.
Important characteristics of useful classification systems are that it is clinically relevant, easy to use, applicable in a variety of situations, teachable, reliable, and valid. The system must have the ability to be used readily in multiple settings. When dealing with traumatic cervical spine injuries, the most important setting is the initial patient contact. Once an injury is identified, the system must be applicable to determine treatment direction. The system should have relevance in other environments such as during various periods of healing and in the research laboratory.
The currently proposed system of morphologic descriptions is based on commonly used and conceptually recognized patterns. It does not divide into many subgroups such as Allen-Ferguson or the AO system since the quantification scale can provide the distinctive patterns that differentiate injuries even with similar patterns to one another.11,16 The proposed morphologic description is currently undergoing testing for reliability.
A classification system must be reliable and valid. Reliability is the measure of variability among observers and by repeated observations. Most commonly, this is measured by interobserver and intraobserver kappa values or ICC.9,17 These statistical values range from −1 to 1, with acceptable values being greater that 0.6. Validity is the accuracy of the system to measure the condition under assessment. The Cervical Spine Injury Severity Score had excellent reliability with intraobservor ICC >0.97 and interobservor ICC >0.88. Furthermore, the results were equal between experienced and inexperienced observers.
A limitation of this study is that MRI was not used. This could affect results as occult ligamentous injuries may be better appreciated and therefore scored appropriately. Aversely minor injuries of questionable significance may be identified which could inappropriately increase the quantifiable score. MRI was purposefully not used in this investigation as they were only available in the more severe cases. Further investigations testing the reliability using MRI are ongoing. Another important component of stability and treatment decisions is neurologic function. This was not included in the system as the authors believed that it can be scored separately using an accepted systems such as recommended by the American Spinal Injury Association or by the method described by Vaccaro et al.18,19 Further studies evaluating the reliabilty of the morphologic descriptions and validation of the quantification system are planned.
A name or description allows communication, distinguishes injuries from one another, may imply or directly state important biomechanical forces, and allow categorization into phylogenies. However, the name or description may not relate to prognosis, guide treatment, assess severity of skeletal injury, or predict natural history. Quantification of the severity of instability will overcome many of these descriptive limitations. Such a severity score will incorporate both bony and ligamentous pathologic changes and can be adapted to a variety of pathoanatomic conditions, provide numerical values so that discriminate statistical methods can be used, and hopefully direct treatment decisions. The results demonstrate excellent reliability of the injury severity score system to quantify stability in subaxial cervical spinal injuries. Quantifying stability based on fracture morphology along with accurate assessment of the neurologic injury will allow surgeons to better characterize these injuries and ultimately produce treatment algorithms that can be tested in clinical trials.
- Classification of subaxial cervical spine injuries is essential to allow communication, determine prognosis, and direct treatment.
- Current systems based on mechanism and morphology are limited and do not reliably measure degree of instability.
- The development of common morphologic names may aid in the management of these injuries.
- The cervical spine injury severity score based on summations of analog scores for each of four spinal columns is reliable and easy to use. The development of common morphologic names may aid in the management of these injuries.
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Keywords:© 2006 Lippincott Williams & Wilkins, Inc.
cervical spine injuries; spinal cord injuries; classification; reliability; intraclass correlation coefficient; mechanism of injury