14 Cervical Facet Dislocation
Introduction
Dislocation of the facets joints of the spine can occur at all levels, but it is most commonly an injury found in the cervical spine. First, the coronal orientation of the joints themselves leaves them susceptible to dislocation with hyperflexion. Second, unlike the substantial size of the lumbar articulating processes, those in the cervical spine are much less robust. 1 Therefore, the articulating processes in the cervical spine are much more prone to fracture and dislocation. Third, the cervical spine is naturally highly mobile in comparison to the thoracic and lumbar spine with the head’s weight serving as a contributing factor. This characteristic leaves the cervical spine vulnerable to sudden changes in movement such as that which occurs in a head-on collision.
Dislocation of the cervical facet joints can be both unilateral and bilateral. In the case of unilateral facet dislocation, there is often a rotatory force experienced along with the hyperflexion. The hyperflexion force vector is enough to raise the inferior articulating processes of both facet joints at the affected level with respect to the superior articulating process. The rotation experienced at the same time causes only one of the two elevated inferior articulating processes to translate forward, locking anterior to the superior articulating process of the vertebra below it. 2 A purely hyperflexion moment without rotation is much more likely to cause bilateral facet dislocation as the force vectors experienced by each facet are theoretically similar. In either scenario, the dislocation is visualized as either a perched facet (one in which the inferior projection of the inferior articulating process of the proximal vertebral body articulates with the superior projection of the superior articulating process of the distal vertebral body) or a locked facet (in which the inferior articulating process of the proximal vertebral body is anterior to the superior articulating process of the distal vertebral body).
All regions of the cervical spine are not created equal. Unlike the subaxial cervical spine, the C1-C2 facet joints are oriented in an axial plane making them less vulnerable to dislocation from hyperflexion. The occipitocervical junction is subject to a number of particular injury patterns that are discussed elsewhere. It is the subaxial cervical spine, specifically C4-C7, that is most prone to hyperflexion injuries. 3 In large part, this is due to the dynamic forces the cervical spine experiences as a collision evolves. At the onset of a head-on collision, the lower cervicothoracic junction of the spine compresses and extends while the subaxial cervical spine flexes with great force. As the forces evolve, the cervical spine is eventually thrown into extension. This evolution of forces, commonly referred to as whiplash, causes the spine to assume an S-shape, a phenomenon referred to as “snaking.” The hyperflexion, if severe enough, can lead to facet dislocation by itself.
In rear end collisions, the damage can be even more severe. Initially, the victim’s neck may hyperextend, forcing the inferior articulating process down into the superior articulating process. If the articular surface fails, fracture of the inferior articulating process can occur, weakening the facet joint as a whole. The inevitable hyperflexion that follows then causes the dislocation, unhindered by the normal ligamentous and joint capsule restraints.
The ultimate result of any facet dislocation in the cervical spine is an unstable spine that requires immediate treatment. Treatment options include nonoperative management with closed reduction followed by immobilization in an external fixation device such as a halo vest or Minerva brace versus operative fixation following either closed or open reduction. The details of the different options are discussed below, but there is a general agreement that the universal presence of ligamentous injury in facet dislocations makes operative fixation a preferred technique for treatment of both unilateral and bilateral facet dislocations of the cervical spine.
Indications
Hyperflexion injury resulting in unilateral or bilateral facet dislocation such as a head-on motor vehicle collision.
Combined hyperextension/hyperflexion injury resulting first in facet fracture due to hyperextension with subsequent facet dislocation due to hyperflexion as is experienced during a severe rear-end collision.
If the examination reveals no neurologic deficit or a complete spinal cord injury, surgical stabilization should occur as soon as the patient is medically stable and an appropriate team is available.
If the examination reveals findings consistent with a partial spinal cord injury, urgent reduction and stabilization is recommended as soon as the patient is hemodynamically stable. Hypotension should be avoided in all patients, especially those with neurologic deficits.
Examination
Any patient that suffers a cervical facet dislocation has sustained forces sufficient to cause a myriad of other life-threatening injuries; therefore, a full trauma workup should be completed with priority given to the ABCs (airway, breathing, circulation). Immobilization of the cervical spine during this evaluation must be a priority.
A full neurologic examination should be performed as this has implications regarding the timing of intervention.
Additionally, evaluation of neurologic status may allow localization of the injury prior to imaging.
Reduction—Closed or Open 4
Class III evidence suggests early reduction of cervical facet fracture/dislocation may be associated with improved neurologic outcome.
If the patient is awake, this can be performed with mild sedation. 5 If the patient is unresponsive or unable to cooperate, magnetic resonance imaging (MRI) is indicated prior to reduction as the neurologic examination cannot be followed and the presence of a large ventral lesion may be a relative indication for an open reduction via an anterior approach.
Closed reduction technique includes halo or tongs traction, which is discussed in Chapter 11. Closed reduction and external bracing is associated with increased morbidity and mortality related to prolonged bedrest.
Success of closed reduction is 80%.
Risk of suffering additional permanent neurologic injury during closed reduction is < 1%.
Risk of suffering additional transient neurologic injury during closed reduction is 2 to 4%.
If reduction fails, the likelihood of other injuries such as facet fracture or herniated disks is increased. This necessitates further imaging studies such as MRI prior to open reduction to determine the initial direction of approach (anterior versus posterior).
Preprocedure Considerations
Radiographic Imaging
Computed tomography (CT) scan: CT is the workhorse of cervical spine trauma evaluation. Identification of osseus abnormality is straightforward while ligamentous injury is not always detectable. Ligamentous injury may be detected due to enlarged spaces between otherwise normal appearing osseus structures.
MRI: This test has, in the past, been advocated as a necessary part of any pre-reduction workup, whether that reduction be in the intensive care unit (ICU) or operating room setting. The rationale for this was to identify any ventral intervertebral disk herniations that may cause neurologic injury during reduction. According to an evidence-based review, there was no relationship between the presence of herniated disks and risk of neurologic injury during closed reduction of facet dislocations in the presence of a ventrally herniated disk. 4 While pre-reduction or preoperative MRI may be useful in terms of defining associated injuries and in some cases dictating surgical approach, as in the obtunded patient, in the absence of a clear indication for MRI, reduction of the dislocation should not be delayed in a patient with a severe neurologic injury.
Cervical X-ray: The role of plain radiographs in the initial assessment of severe trauma has been limited by the advent of aggressive use of CT imaging. Plain films are quite helpful for diagnosing cervical facet dislocations and are employed serially (or with fluoroscopy) during the process of either open or closed reduction.
Medication
Operative Management 11
Approach
If closed reduction has been achieved, anterior fixation and fusion, posterior fixation and fusion, or halo immobilization are treatment options. In general, halo immobilization is associated with a relatively high failure rate and the vast majority of surgeons will offer a direct fixation procedure.
If the dislocation requires open reduction, the surgeon may choose between anterior or posterior approaches depending on the anatomy of the injury and the experience of the surgeon. The presence of a large ventral disk herniation may be a relative indication for an anterior approach as a known unilateral vertebral artery injury. In these cases, the use of MRI is appropriate. If the dislocation is complete enough that the surgeon does not believe an anterior approach feasible for reduction, then a posterior approach is indicated.
Techniques
Options include: anterior fusion with or without plate fixation, posterior fusion and wiring, and posterior fusion with lateral mass plate, rod, clamp, or cable fixation.
Posterior fusion with lateral mass plate, rod, clamp, or cable fixation provides instant stability (allowing early mobilization of the patient). Choice of technique is based on the integrity of the bony structures and the experience of the surgeon.
Posterior fusion with wiring may also be associated with an increased risk of late kyphotic angulation compared to more rigid techniques. In one study, 22 of 165 patients with cervical facet dislocation treated via posterior fusion and wiring developed kyphosis compared to just 1 of 40 patients treated via posterior fusion and lateral mass fixation. 11
Anterior fusion without plating is associated with a higher incidence of graft displacement and late development of kyphosis than posterior fusion with fixation. Six of 101 patients treated in this fashion developed late instability compared to 6 of 237 patients treated via a posterior fusion with lateral mass fixation. 11 The use of anterior fusion with plate fixation is well described and is associated with excellent outcomes. 12 – 16
Operative Field Preparation
Cervical immobilization must be maintained at all times.
With regards to anesthesia, the inherent instability of this type of spinal column injury encourages fiberoptic intubation.
Regardless of the final position (prone or supine), the neck should be kept in a neutral position at all times.
The operative area is cleared of hair using clippers only and cleansed with alcohol.
Povidine iodine or chlorhexidine prep is used to sterilize the operative field widely.
The incisions are marked. Infiltration with 1% lidocaine with 1:100,000 epinephrine is optional.
Operative Procedure
Posterior Approach (Fig. 14.1a, b)
Positioning (Fig. 14.2)
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