Subaxial Cervical Spine Injuries

Anatomy

The subaxial cervical spine consists of the C3‒C7 vertebrae. The cervical spinal canal houses the spinal cord and is bound by the vertebral body anteriorly, the pedicles laterally, and the laminae posteriorly. The transverse process, which is directed anterolaterally, contains the transverse foramen. The vertebral artery enters the transverse foramen at C6 in 90% of the population and travels up the subaxial cervical spine. Off of the lamina, the inferior and superior articular processes form the facet joints, which are oriented at 45 degrees. The uncovertebral joints are formed by a bony protuberance, known as the uncinate process, on the lateral aspect of the superior vertebral body, which articulates with a convex area in the lateral aspect of the inferior vertebral body. The intervertebral disc is found in the intervertebral disc space and is composed of the gelatinous nucleus pulposus centrally and the fibrocartilaginous annulus fibrosus peripherally. The uncovertebral joint protrudes through an area absent of annulus fibrosus and is believed to be lined by a synovial membrane. The spinous processes project posteriorly and are bifid between C3 and C6. Ligamentous structure is provided on the anterior and posterior aspects of the vertebral bodies by the anterior longitudinal ligament (ALL) and the posterior longitudinal ligament (PLL), respectively. The ligamentum flavum is found connecting adjacent laminae and facet capsules. The interspinous and supraspinous ligaments provide further support between the spinous processes posteriorly. ^,

The principal movements of the subaxial spine are flexion and extension. This is facilitated by the observation that cervical vertebral bodies are not stacked flatly upon one another but are situated with a sagittal slope. The bony and ligamentous anatomy, together with the intervertebral discs, limits excessive motion of the cervical spine. This prevents injury to the cervical spinal cord while allowing functional motion. The PLL, the facet capsules, the ligamentum flavum, and the interspinous ligaments all resist flexion. Extension is limited by the ALL and the annulus fibrosus, as well as the posterior bony anatomy. Excessive movement in these planes of motion can result in injury to these structures. ^,

The cervicothoracic junction is of particular interest because of its transitional and variable anatomy. The potential presence of the vertebral artery, tenuous blood supply, and narrow spinal canal make screw placement in this segment difficult and controversial. The cervical vertebrae enlarge moving caudally and are slightly lordotic up until the cervicothoracic junction, where the alignment becomes kyphotic in the thoracic spine. When this transition of curvature occurs, a transition of weight distribution occurs as well, complicating intervention. For screw placement, three techniques have been advocated and criticized, including pedicle, laminar, and lateral mass screws. Compared with thoracic pedicles, cervical pedicles are smaller, with an increase in height and width and a decrease in angle, with the vertebral body moving caudally toward the thoracic spine. Placement of a pedicle screw risks neurovascular compromise with transverse foramen involvement. Translaminar screw use presents the possibility for penetration into the dorsal spinal canal. At C7 the lateral mass is thin and small compared with higher cervical vertebrae, resulting in screw pullout as a common complication.

Clinical Assessment

In the immediate aftermath of an acute spinal cord injury (SCI), patients frequently develop neurological dysfunction. These neurological problems are likely to manifest with functional deficits, and patients often experience pain. The best medical evidence suggests that patients who experience SCI should undergo serial evaluation and documentation of neurological and functional deficits and pain severity.

Many classification systems have been developed to document and standardize neurological evaluation of the patient with acute SCI. These include the Frankel Scale, the Lucas and Ducker Neurotrauma Motor Index, the Sunnybrook scale, the Botsford scale, the Yale scale, and the National Acute Spinal Cord Injury scales, ^, among others. The ideal scale would have high interrater reliability, reproducibility, and sensitivity to changes in neurological function, and would provide accurate documentation. The scale would then be sufficiently useful for management and research purposes. The American Spinal Injury Association (ASIA) scale is currently the preferred neurological examination tool, as shown in Fig. 36.1 . ^,

This classification system is as follows:

• A = Complete. No sensory or motor function is preserved in the sacral segments S4‒S5.

• B = Sensory incomplete. Sensory, but not motor, function is preserved below the neurological level and includes the sacral segments S4‒S5, and no motor function is preserved more than three levels below the motor level on either side of the body.

• C = Motor incomplete. Motor function is preserved below the neurological level, and more than half of key muscle functions below the single neurological level of injury (NLI) have a muscle grade less than 3 (that is, have grades 0‒2).

• D = Motor incomplete. Motor function is preserved below the neurological level, and at least half (half or more) of key muscle functions below the neurological level of injury have a muscle grade above 3.

• E = Normal. If sensation and motor function as tested using the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) tool are graded as normal in all segments, and the patient had prior deficits, then the ASIA Impairment Scale (AIS) grade is E.

Someone without an SCI does not receive an AIS grade.

The procedure for determining the AIS grade is outlined in the ISNCSCI. Briefly, the neurological examination tests two components: sensory and motor. The sensory examination tests light touch and pinprick sensation at 28 points, corresponding to separate dermatomes on the right and left side of the body. The motor examination consists of testing 10 paired myotomes graded with the standard six-point scale. An NLI is then determined for the most caudal aspect of the right and left side, with antigravity motor function and intact sensation. From these four possible NLIs, the single NLI is the most rostral and is used in the ASIA classification process.

Burns and coworkers noted decreased reliability of the initial examination in the presence of closed head injury, drug effects, mechanical ventilation, and psychological disorders. It has also been suggested that impairment following SCI can be better described and predicted with separate upper and lower extremity ASIA motor scores. ^,

Although the ASIA scale was designed to measure neurological deficits, it does not take into account spasticity, pain, or dysesthesias; therefore, a number of scales have been developed to assess for functional deficits. These scales attempt to document the patient’s deficiencies in daily functioning. These include the Functional Independence Measure Scale, the Barthel Index, the Quadriplegic Index of Function, and the Spinal Cord Independence Measure (SCIM), among others. Currently, the third revision of the SCIM scale, SCIM III, is recommended for functional assessment in patients with acute SCI. This scale was specifically designed for patients with SCI, in that it derives from assessment of the patient’s ability to perform basic tasks, the economic burden of disability, and the impact of the injury on overall comfort. Additionally, SCIM III provides documentation that is sensitive to functional changes, has high interrater reliability, and is reproducible, making it useful for patient care as well as research purposes. ^,

SCIM III consists of three complementary subscales: “self care” (with a score range of 0 to 20) including six tasks, “respiration and sphincter management” (with a score range of 0 to 40) including four tasks, and “mobility” (with a score range of 0 to 40) including nine tasks. The mobility subscale consists of two subscales: one for “room and toilet” and one for “indoors and outdoors, on even surface.” Total score ranges between 0 and 100.

Pain is also a common complication after SCI and can be severely debilitating. Consequently, numerous methods have been developed to measure the pain in both objective and subjective ways. Ideally, these scales would provide a method to document pain following SCI and allow assessment of the efficacy of treatment. The most highly recommended of these to date is the International Spinal Cord Injury Basic Pain Data Set. ^{, ,}

Radiographic Assessment

The Spinal Cord Society recently undertook a literature review and concluded that, in patients who present with cervical spine trauma, computed tomography (CT) is the first-line option for imaging. Magnetic resonance imaging (MRI) is indicated in cases with neurological involvement, in patients with advanced cervical degenerative changes, and in cases where the extent of soft tissue injury needs to be evaluated.

CT is more sensitive than three-view cervical plain films for detecting cervical spine injury. The sensitivity of plain films for diagnosing cervical spine injury is thought to be between 35% to 53%, whereas that of CT can approach 100%. In symptomatic or obtunded patients with negative CT or plain radiographs, investigators have studied the utility of MRI and flexion-extension radiographs to further assess for cervical spine injury and determine the need for continued cervical immobilization. Controversy exists in regard to the utility of both. For the most part, most studies support the use of MRI, although a few do not. ^, In one study, the use of MRI among 10 experienced spine surgeons modified the timing of surgery in 41% of cases, changed the surgical levels in 57% of cases, and showed an operative indication not apparent on CT in 17% of cases. Additionally, MRI is useful for assessing the discoligamentous complex (DLC), which is a consideration for the Subaxial Cervical Spine Injury Classification (SLIC) system and the AOSpine cervical fracture classification system. MRI findings of cord injury typically show edema that peaks at 48 hours, then gradually decreases over 3 weeks, and cord hematoma is a common finding in ASIA-A and -B patients. Dynamic films are thought to be less sensitive for ligamentous injury than MRI and to be of little use in the setting of a normal CT scan and clinical examination. There are also reports of injury to obtunded patients undergoing dynamic imaging, which raises questions as to the safety of the procedure in these patients.

X-rays continue to have a role in emerging countries, but for the most part their use is limited in the developed world. Historically, guidelines were developed to determine whether patients with cervical spine injury required further imaging. These guidelines are still used, albeit for the different purpose of clearing the cervical spine when there is a question of ligamentous injury, without resorting to MRI. The National Emergency X-Radiography Utilization Study Group used five criteria to determine whether or not patients with potential cervical spine injury require imaging. If all five criteria are present, patients require no imaging. These criteria include absence of midline cervical tenderness, absence of focal neurological deficit, normal alertness, absence of intoxication, and absence of painful, distracting injury. The Canadian C-spine rule uses three criteria to determine if a patient requires imaging. These include presence of a high-risk factor that mandates radiography, presence of a low-risk factor allowing safe assessment of range of motion, and ability to actively rotate the neck 45 degrees to the left and right. Anderson and colleagues found that patients who are alert, asymptomatic, and without neurological deficit who can complete a functional range-of-motion examination and are free from other major distracting injury can be released from cervical immobilization without radiographic imaging. The sensitivity of each of these methods for SCI is high, although best medical evidence supports the use of the criteria proposed by Anderson and colleagues.

Injury Classification

Many systems have been proposed to classify traumatic subaxial cervical spine injuries. Conventionally, the old classifications and descriptions that Magerl used for the thoracolumbar spine were adopted to describe cervical spine fractures. The most modern systems are SLIC and AOSpine. Each of these three systems has a different use at the current time. As of the current printing, most spine surgeons still use the conventional Magerl names when describing a fracture in the clinical setting. The SLIC score is often used as confirmation of a surgical decision, indicating whether or not most spine surgeons would operate in a given case. The AOSpine classification is beginning to find use in research, where standard nomenclature and specific descriptions are critical. However, it is still uncommon to find AOSpine classifications used in clinical discussions. The SLIC and AOSpine classifications are described later. In the “Treatment by Fracture Type” section, we use the conventional Magerl descriptions.

Subaxial Injury and Classification

In 2007, publication of the SLIC system and severity scale by the Spine Trauma Study Group posed a more focused and clinically oriented guideline for management of subaxial cervical spine injuries. The three parameters evaluated in the SLIC classification system are injury morphology, spinal stability, and neurological status ( Fig. 36.2 ). According to the SLIC grading system, injury morphology is classified as: 0, no abnormality; 1, compression fracture; 2, burst fracture; 3, distraction injury; and 4, translation injury. The SLIC scoring system grades DLC integrity as: 0, intact; 1, indeterminate; and 2, disrupted. Finally, neurological status is defined as: 0, intact; 1, nerve root injury; 2, complete SCI; 3, incomplete SCI; and +1, persistent cord compression. Patients with a total score of 1 to 3 are recommended to be treated conservatively with a PMT collar. It is recommended that patients with a score of 4 are treated either operatively or nonoperatively based on the patient and surgeon. For example, a patient with a SLIC score of 4 with significant comorbidities may respectably be managed nonoperatively. Lastly, patients with a score of 5 or more are highly recommended for surgical decompression or stabilization.

Studies evaluating the SLIC scoring system demonstrate strong inter- and intraobserver agreement (>90%) in both the overall injury score and treatment plan chosen. These authors also evaluated the validity of SLIC by performing a retrospective review of 185 patients, comparing their management to SLIC guideline recommendations. Of the 66 patients with a SLIC score of 3 or less, 94% were managed nonoperatively; of the 102 patients with a SLIC score of 5 or greater, 95% were managed surgically. Of the 17 that had a SLIC score of 4, 65% were managed conservatively. A prospective study also supports the SLIC guidelines as being effective in preserving neurological status after subaxial cervical spine trauma. Despite evidence and support from proponents of the SLIC system, the guidelines do have certain drawbacks; therefore, there is still controversy over their use. For example, according to SLIC, no abnormality with a score of 0 includes isolated spinous process fractures, laminar fractures, or nondisplaced facet or pedicle fractures. A floating lateral mass without significant displacement, for example, would be difficult to classify.

Spinal stability is assessed primarily by the overall alignment of the spine and the integrity of the DLC. The DLC comprises the intervertebral disc, ALL and PLL, interspinous ligaments, facet capsules, and ligamentum flavum. Although spinal alignment is a reflection of ligamentous stability, isolated bony injuries can result in significant spinal instability with an intact DLC, such as in the case of a three-column bony Chance-type fracture. In the majority of circumstances, a disrupted DLC is established by evidence of distraction or translation to the spinal column or significant disruption of the intervertebral disc space or facet joints. Isolated injuries to the ALL, PLL, ligamentum flavum, or interspinous ligaments may not truly represent disruption of the DLC. Identifying the DLC as indeterminately injured is controversial in this setting and because MRI tends to overestimate true ligamentous injury.

Special attention is warranted in patients with previous cervical spondylosis. For example, a patient suffering from a complete SCI secondary to hyperextension in the setting of cervical spondylosis could be scored a 3 on the SLIC scale, with a score of 0 for normal morphology, 0 for an intact DLC, 2 for complete SCI, and +1 for persistent cord injury; but these patients may actually benefit from surgical decompression.

AOSpine Classification System

AOSpine developed a comprehensive classification system for cervical spine fractures in 2016 ( Fig. 36.3 ). This classification has been shown to have high inter- and intraobserver agreement. The classification is summarized below:

• Type A: Compression injuries: failure of the anterior structures under compression or mechanically insignificant spinous process or lamina fractures.

  • A1—Single end plate compression fractures; no involvement of the posterior wall of the vertebral body.

  • A2—Coronal split or pincer fractures involving both end plates; no involvement of the posterior wall of the vertebral body.

  • A3—Burst fractures involving a single end plate.

  • A4—Burst fracture or sagittal split injury involving both end plates.

• Type B: Tension band injuries: Affect the anterior or posterior tension band. If any translation is seen, they are instead classified as type “C” injuries.

  • B1—Posterior tension band injury involving bony structures only. Anterior structures (such as disk or annulus) may also be involved.

  • B2—Posterior tension band injury involving ligament, or capsuloligaments, possible bony structure involvement as well. Anterior structures (such as vertebral body or disc) may also be involved.

  • B3—Anterior tension band injury involving bone and/or disc, with tethering of the posterior elements that prevents gross displacement.

• Type C: Translational injury in any axis, and can be used in combination with the A and B classification.

• Facet injury: If there are multiple injuries to the same facet, only the highest level of injury is classified. If both facets on the same vertebra are injured, the right-sided facet injury is listed before the left-sided injury if the injuries are of different subcategories. The “bilateral” modifier is used if both facets have the same type of injury.

  • F1—Nondisplaced facet fracture, less than 1 cm, less than 40% lateral mass.

  • F2—Facet fracture with potential for instability, fragment greater than 1 cm, greater than 40% lateral mass or displaced lateral mass.

  • F3—Floating lateral mass; disconnection of superior and inferior articular processes.

  • F4—Subluxation or perched/dislocated facet.

  • BL—Bilateral; used when the same type of facet injury occurs bilaterally on the same vertebra.

Neurological Status:

• • N0—Intact.

  • N1—Transient neurological deficit that resolved by the time of examination.

  • N2—Radiculopathy.

  • N3—Incomplete SCI.

  • N4—Complete SCI.

  • NX—Undetermined; cannot be examined because of head injury or another condition such as intubation/sedation.

• Modifiers:

  • M1—Posterior capsuloligamentous complex injury without complete disruption. Often identified on MRI imaging and associated with very localized posterior tenderness on clinical examination.

  • M2—Critical disc herniation.

  • M3—Diffuse idiopathic skeletal hyperostosis, ankylosing spondylitis, ossification of the posterior longitudinal ligament (OPLL), or ossification of the ligamentum flavum.

  • M4—Vertebral artery injury (VAI).

Management