Facet Dislocation Injuries and Surgical Management

Overview

Traumatic injuries to the cervical spine represent a common and potentially devastating collection of injuries with significant clinical impact and a high rate of neurologic injury. The subaxial spine is affected in as many as 65% of all cervical spine fractures and more than 75% of all fracture-dislocations. In fact, the cervical spine is the most vulnerable and most frequently injured portion of the spine following high-velocity trauma. The most common causes of injury to the cervical spine include motor vehicle accidents (MVAs); diving into shallow water; and sporting-related injuries, especially from playing football and rugby. Facet dislocations account for as many as half of all cervical injuries. In particular, this chapter will focus on fracture or dislocation of the cervical facet joint or facet complex.

The three types of subaxial injury morphology described by Vaccaro and colleagues and the Spine Trauma Study Group (STSG) give insight into the complexity and degree of instability created by either a unilateral or bilateral facet disruption or dislocation. For example, distraction injuries, which demonstrate anatomic disruption of the either the bony elements and/or diskoligamentous complex, are often the result of a significant force. The bony and ligamentous components of the facet joint are considered the determinants of posterior stability. Thus disruption of the facet can lead to a tremendous amount of instability of the spinal column and carries the potential for neurologic injury.

A unilateral facet dislocation is the result of the inferior facet being forced anteriorly and thus ventral to the superior facet of the caudal adjacent level; the inferior facet may remain there in a perched or “locked” position ( Fig. 23-1 ). Unilateral facet dislocations are often the result of flexion with some component of distraction and rotation. Typically, the unilateral facet dislocation occurs on the side opposite the rotation. Often, a unilateral facet dislocation is associated with a facet fracture ( Fig. 23-2 ). Unilateral facet dislocation without a fracture is associated with a higher incidence of neurologic injury and may be more difficult to reduce ( Figs. 23-3 and 23-4 ). In fact, unilateral facet dislocation with or without fracture is associated with damage to the facet capsule, interspinous ligament, posterior longitudinal ligament (PLL), and vertebral artery and with disruption or herniation of the intervertebral disk. Bilateral injury to the facet joints or ligamentous facet capsules can lead to translation of the spinal column itself, with even greater potential for neurologic injury. Such an injury also implies disruption of the PLL ( Fig. 23-5 ). Facet injury or disruption can result in spinal instability and neurologic injury that necessitates prompt reduction and stabilization.

Clinical Presentation

Cervical spine injuries should be suspected in those who have sustained head injuries, patients who have been involved in a high-speed MVA, or those who complain of neck pain after a traumatic event. Initial evaluation and treatment should follow the guidelines established by the American College of Surgeons (ACS) through the Advanced Trauma Life Support (ATLS) protocol, whereby the initial priority is given to establishing the airway, breathing, and circulation.

Establishing a competent airway may be complicated in any patient with a suspected cervical spine injury. Cervical spine “in-line” stabilization and immobilization must be maintained throughout the ATLS evaluation. Thus chin-lift and jaw-thrust techniques should be avoided in the setting of potential cervical spine instability, because such maneuvers may reduce the amount of space available for the spinal cord. Obtaining a complete history from the patient and from other in-field medical care providers may also help to guide clinical suspicion for a cervical spine injury. Throughout the primary and secondary survey, a complete physical exam that includes a detailed neurologic exam should be completed, taking note of external signs of trauma such as lacerations, abrasions, and hematomas. In addition, assessment of midline tenderness with palpation, bony step-offs, or movement of the cervical spine should be undertaken. Likewise, the complete neurologic exam should include evaluation of the mentation, manual motor strength, sensation, and deep tendon reflexes. Digital rectal exam is mandatory in all cases of suspected spinal cord injury as is assessment of the bulbocavernosus reflex to assess for spinal shock.

The more severe the injury to the subaxial spine, the greater the likelihood of spinal cord or nerve root injury. Moreover, a more significant neurologic injury suggests a greater force of impact and thus a greater potential for cervical spine instability. Complete versus incomplete spinal cord injuries will present with different neurologic signs. Likewise, injuries to the spinal cord itself portend a much different mechanism and predicted outcome than injury to a spinal nerve root. Spinal cord injuries are classified via the American Spinal Injury Association (ASIA) scale. For example, complete spinal cord injury results in complete loss of motor and sensory function below the level of injury without evidence of sacral sparing. Incomplete spinal cord injury may result in partial weakness with sensory sparing. Likewise, injury to a nerve root will demonstrate loss of motor strength and/or sensation in a particular dermatomal distribution.

In the setting of a unilateral facet dislocation, approximately 25% will present with a nerve root injury, and about 25% will present with an incomplete spinal cord injury; the remaining 50% tend to be neurologically intact. However, in the setting of a bilateral facet injury, two thirds of patients will come to medical attention with a complete spinal cord injury, and one third will be seen with a combination of incomplete spinal cord injury and nerve root injury.

Radiographic Evaluation

Radiographic evaluation in the setting of potential injury to the spinal cord and/or column must accurately depict the spinal axis in a rapid and safe fashion to guide acute management. Radiographic evaluation of suspected injury to the cervical spine begins with radiographs that include anteroposterior (AP), lateral, and open-mouth odontoid views. The radiographs should include the occiput through T1 to completely evaluate the cervical spine ( Fig. 23-6 ). Often the patient’s shoulders can obscure the view of lower cervical spine, thus traction on the patient’s arms, or a swimmer’s view, may be required to fully visualize the lower cervical spine. This is an important point, because most missed cervical dislocations and fractures are in the lower cervical spine. With that in mind, despite the best technique and optimized conditions, plain-film radiographs underestimate the extent of traumatic spine injury. It has been estimated that only 60% to 80% of fractures in the cervical spine can be visualized, even when all three views are obtained. Woodring and Lee’s study that compared radiography (three cervical views) and computed tomography (CT) demonstrated that radiography missed 61% of fractures and 36% of subluxations and dislocations. On radiographs, 23% of the spines were classified as normal, when half of the so-called normal group in fact had an unstable injury.

Given the increased access to CT and the increased efficiency with which CT images and corresponding three-dimensional (3D) reconstructions can be obtained, the use of CT in the setting of cervical trauma evaluation has grown tremendously. CT imaging has been shown to detect as many as 97% to 100% of all fractures in the cervical spine, and it maintains a higher sensitivity and specificity than plain radiographs. Given the ease of use and increased visualization provided by CT, any patient with a suspected cervical spine injury should undergo CT evaluation. In particular, when evaluating for potential facet dislocation, the axial view can be especially helpful. Daffner and colleagues have described the “hamburger bun sign.” When the cervical facets are aligned normally, they resemble the shape of the two parts of a hamburger bun facing each other. However, in the setting of facet dislocation or jumped facets, the two halves of the hamburger bun are reversed ( Fig. 23-7 ).

The biggest limitation of CT imaging is the inability to fully assess the integrity of the ligamentous anatomy. Magnetic resonance imaging (MRI) is the preferred imaging modality for assessing soft tissue such as ligaments, the spinal cord, and/or nerve roots. Goradia and colleagues have demonstrated that the use of MRI in the setting of cervical spine trauma maintains high sensitivity for injury to the intervertebral disk (93%), PLL (93%), and the interspinous soft tissue (100%). Increased signal intensity or short T1 inversion recovery (STIR) sequence signal abnormality on MRI can also demonstrate laxity or movement within a facet capsule, suggesting some degree of movement or potential instability (see Fig. 23-4 ).

Initial Management

Patients with suspected cervical spine injury should remain immobilized in a rigid cervical collar until definitive evidence of injury has been determined. Patients with potential spinal cord injury or cervical spine instability should be monitored in an intensive care setting with full cardiac, hemodynamic, and respiratory capabilities. Furthermore, for those patients who have suffered definite spinal cord injuries, the mean arterial pressure should be maintained at 85 to 90 mm Hg for the first 7 days after the injury to maximize spinal cord perfusion.

Once the patient has been fully evaluated, and a unilateral or bilateral facet dislocation has been diagnosed, reduction of the dislocation and realignment of the column should be undertaken. A variety of methods may be used for performing closed reduction. The basic principle, which was described in 1933, includes the application of weighted in-line axial traction to reduce the facet dislocation. Crutchfield tongs, Gardner-Wells tongs, or a halo-ring orthosis with a traction bar have been described as effective means of applying traction for closed reduction ( Figs. 23-8 and 23-9 ).

A great deal of controversy surrounds the timing of closed reduction in the setting of either unilateral or bilateral facet dislocation. Lee and colleagues concluded that the decision to perform closed reduction is largely dependent on the ability to safely and successfully perform the reduction. The safety profile is determined by the ability to monitor the patient’s neurologic status during application of traction and manipulation and by the risk of further cord injury in the setting of an anterior compressive lesion, such as a disk herniation or hematoma.

A consensus statement of the American Association of Neurological Surgeons (AANS)/Congress of Neurological Surgeons Joint Spine Section states that the overall rate of successful reduction restoring anatomic alignment is about 80%, with a reported rate of neurologic complication less than 1.0%. Although evidence is insufficient to support definitive treatment guidelines or standards, expert opinion stressed the importance of restoration of anatomic alignment and also maintained that for those patients unable to follow the neurologic exam, MRI should be performed before reduction. Likewise, patients with injury rostral to the level of dislocation should not undergo closed reduction, because further injury could result. Closed reduction in awake patients has been shown to be safe in multiple studies. Although prereduction MRI can demonstrate evidence of a disrupted or herniated disk in as many as 33% to 50% of patients with facet subluxation, this has not been shown to influence outcome following closed reduction in awake patients.

Surgical Management

Following evaluation and medical stabilization of a patient with a cervical facet injury, the need for surgical intervention must be determined. The goals of surgery are decompression of the neural elements and stabilization of the spinal column. A great deal of variability exists among surgeons in determining the stability of the spine in the setting of a cervical facet injury and thus the need for surgery.

The STSG conducted a survey of its members in regard to cervical facet dislocation, treatment, and surgical approach. In that study, Nassr and colleagues showed little agreement in the treatment algorithm for neurologically intact patient scenarios (κ = 0.094) and only slightly more agreement on the appropriate treatment for incomplete spinal cord injury scenarios (κ = 0.133) or for complete spinal cord injury scenarios (κ = 0.15). Vaccaro and colleagues devised the Subaxial Cervical Injury Classification System based on the injury morphology, clinical neurologic status, and integrity of the diskoligamentous complex (intervertebral disk, anterior longitudinal ligament [ALL], PLL, ligamentum flavum, interspinous ligament, supraspinous ligament, and facet capsules). Based on the score, the overall severity of the injury can be rated to guide potential intervention. Higher scores are awarded for incomplete neurologic status and greater diskoligamentous disruption, suggesting a greater degree of instability and need for surgical intervention. In fact, all injuries with a score of 5 or more were treated surgically, and all injuries with a score of 3 or less were treated nonsurgically, whereas scores of 4 were considered equivocal.

The AANS/CNS consensus statement regarding management of subaxial cervical facet dislocation injuries states that 26% of cervical facet dislocation injuries cannot be reduced using craniocervical traction. In addition, 28% of patients in whom anatomic reduction is achieved are unable to maintain reduction with external immobilization alone, thus necessitating surgical stabilization. It is recognized that prolonged bed rest with maintained cervical traction for 12 to 16 weeks’ duration is associated with the highest rate of mortality of all treatment strategies. The authors note that certain factors such as vertebral subluxation, ligamentous or bony facet injury, and a locked/perched facet on initial imaging have been associated with failure of nonoperative management. Although facet dislocation with facet fracture may preclude successful closed reduction, for those in whom reduction can be achieved, a high rate of arthrodesis has been shown with external immobilization alone. Injury or disruption of the ligamentous complex in the setting of facet dislocation without facet fracture has been associated with a higher rate of failure when managed with external orthosis alone, and the presence of a laminar fracture in combination with a facet dislocation has led to increased rates of delayed kyphosis following surgical intervention.

Despite successful closed reduction, surgical intervention is often required; options include anterior, posterior, or combined anteroposterior approaches. The choice of approach is dependent on several factors that include surgeon training and preference, but more specifically it often revolves around whether a facet dislocation can be effectively reduced from an anterior-only approach, and whether evidence suggests intervertebral disk herniation. In a survey of the STSG, the anterior approach was preferred in the setting of intact neurologic status, and a combined approach was preferred in the presence of evidence of bilateral facet dislocation. However, a great deal of differing opinion centers around the interpretation of the MRI. Dvorak and colleagues recommend an anterior approach when MRI shows evidence of disk herniation that extends posterior to the posterior vertebral body wall at the caudal level ( Fig. 23-10 ). They argue that the spinal cord can be decompressed by performing the diskectomy; facet dislocation can be reduced by placing the neck in extension and applying axial reduction followed by placement of an interbody graft and anterior plate. When the disk is intact, the surgeon can use either an anterior or posterior approach for unilateral facet dislocations. In the setting of bilateral facet dislocation, an increased rate of delayed kyphosis has been shown with posterior stabilization alone. It is postulated that the mechanism of injury, namely distraction, also disrupts the disk and leads to progressive disk collapse that the posterior instrumentation cannot overcome.