Subaxial Cervical Spine Injury Classification System or SLICS

The Subaxial Cervical Spine Injury Classification (SLIC) system is a classification system for subaxial cervical spine injury.

This is one of the recommended classification systems for lower cervical injury.

Not only it helps to determine the injury pattern and its severity but also it guides the treatment considerations and prognosis.

The classification is similar to TLICS for the thoracic and lumbar spine injuries.

The classification was given by Spine Trauma Study Group [Vaccaro et al, 2007] and is based on the following three characteristics.

• Injury Morphology
• Status of discoligamentous complex
• Neurological Status

Under each characteristic, points are given in order of increasing severity as given below.

Each of the 3 main categories is identified as integrally important to injury classification was assigned a weighted score.

Categories of Injury Morphology

Morphology of sub-axial cervical spine trauma was divided into three main categories referenced to the relationship of the vertebral bodies with each other (anterior support structures):

• Compression
• Distraction
• Translation/rotation.

Classification into each of the three groups can be determined through traditional radiographic imaging studies such as plain X-ray, CT scan and MR images.

Compression

Injury appearances compatible with compression were defined as a visible loss of height through part of or an entire vertebral body, or disruption through an end-plate. This morphology includes

• Compression fractures
• Burst fractures
• Sagittal or coronal plane fractures of the vertebrae
• Tear-drop or flexion compression fractures primarily involving the vertebral body.
• Undisplaced minimally displaced lateral mass and/or facet fractures

Concomitant fractures of the posterior cervical elements may exist with axial loading is more evenly distributed between anterior and posterior support structures.

Distraction

These injuries occur less commonly than compression injuries but signify degree disruption and potential instability.

The distraction injuries are primarily identified by evidence of anatomical dissociation in the vertical axis.

This injury pattern often involves ligamentous disruption propagating through the disk space or through the facet joints, such as that seen in facet subluxation or dislocation (without fracture and translation or rotation).

A hyperextension injury disrupting the anterior longitudinal ligament and widening the anterior disk space is another form of distraction injury.

An extension force may also result in concomitant compression injury to posterior elements or cord compression due to inward buckling of the ligamentum flavum.

MRI may detect subtle distraction injuries.

Facet capsules and bony anatomy of the facet joints are likely the primary posterior determinants of stability and must be considered for distraction injury evaluation.

Translation/Rotation

These injuries are based on radiographic evidence of horizontal displacement of one part of the sub-axial cervical spine with respect to the other exceeding physiological ranges.

This may be evidenced on either static or dynamic imaging.

A suggested threshold of rotation is a relative angulation of 11 degrees or greater.

Translation injuries are

• Facet fracture-dislocations [unilateral, bilateral]
• Fracture separation of the lateral mass (floating lateral mass)
• and bilateral pedicle fractures.

Translational and rotational injuries imply disruption to both anterior and posterior structures as demonstrated in several studies

Disco-ligamentous Complex

The discoligamentous complex includes the following structures

• Intervertebral disc
• Anterior and posterior longitudinal ligaments
• Ligamentum flavum
• Interspinous and supraspinous ligaments
• Facet capsules

Facet joint capsules are the strongest component of the posterior tension band while the anterior longitudinal ligament is the strongest anterior ligamentous structure

The complex provides significant restraint for the spine against deforming forces while allowing movement under normal physiological loads.  If its integrity is broken, spinal instability occurs.

Instability and deformity can lead to long-term impairment, including paralysis.

Disruption is inferred by abnormal bony relationships on imaging and includes

• Widened inter-space between two adjacent spinous processes
• Malalignment or dislocation of facet joints
• Subluxation of the vertebral bodies
• Abnormal widening of a disc space

Distraction and translational injuries are almost always associated with some degree of discoligamentous complex compromise.

Neurological Status

Neurological injury is inherently an important indicator of the severity of spinal column injury. Significant neurologic injury infers a significant force of impact and potential instability to the cervical spine. Moreover, the neurological status may be the single most influential predictor of treatment.

Score Calculation

Morphology

The injury is scored depending on the visible morphology

• No abnormality is given a score of Zero – 0
• Compression – 2
• Distraction – 3
  • Perched facet
  • Hyperextension
• Rotation/translation – 4
  • Facet dislocation
  • Unstable teardrop fracture
  • Advanced-stage flexion compression injury

Thus, the minimum possible score is zero and the maximum 4 under these characteristics.

Discoligamentous Complex

• Intact – 0
• Indeterminate – 1
  • Isolated interspinous widening
  • MRI signal indicating injury
• Disrupted – 2
  • Widening of disk space
  • Perched facet joint
  • Facet dislocation

Neurological Status

• Intact- 0
• Root injury – 1
• Complete cord injury – 2
• Incomplete cord injury – 3
• Continuous cord compression in the setting of the neural deficit (Neuro Modifier)

In this category score of 1 is added in case there is continuous cord compression in the setting of neurological deficit. This is called neuro modifier.

Final Score

The total score is calculated by adding scores from all the categories.

Interpretation and Treatment Guidance

• A score of less than 4 is suitable for nonoperative treatment
• If score is 4, the patient may be treated without or with surgery and other factors might influence treatment decision
• If score is 5 or more, operative treatment should be considered.

The system does not pay attention to the level of injury which can be important in determining prognosis or surgical approaches. This could be a drawback of the system.

References

• Vaccaro AR, Hulbert RJ, Patel AA, Fisher C, Dvorak M, Lehman RA Jr, et al. The subaxial cervical spine injury classification system: a novel approach to recognize the importance of morphology, neurology, and integrity of the disco-ligamentous complex. Spine (Phila Pa 1976). 2007 Oct 1. 32(21):2365-74.

** All the images used have been taken from the referenced article and labeled for easier grasp.