Bony Anatomy

The vertebral bodies are relatively large weight-bearing cuboid structures that are connected to each other by the intervertebral disc. The uncinate process or uncus is a cranial extension at the posterolateral corner of the vertebral body. The uncus is responsible for resistance to lateral bending, lateral listhesis, and rotation. With aging or degenerative processes the uncus can hypertrophy, narrowing the neuroforamina, which are located directly behind. Preexisting foraminal stenosis may predispose to neural injury, especially in hyperextension injuries. Short transverse processes extend lateral from the body about midlevel in anteroposterior direction and are confluent with vestigial ribs forming the foramen transversarium. Within this structure lies the vertebral artery and venous plexus except at C7, which is void.

The pedicles extend posteriorly and outward from the cranial aspect of the posterior wall of the body to the lateral masses. These short tubular structures may be cannulated with screws but, because of the close proximity of the vertebral arteries, this technique is often considered dangerous and supplanted by the use of the lateral masses as screw anchorage sites. The lateral masses or pillars when viewed laterally are parallelogram in shape and appear square when viewed ventrally or dorsally. They have cranial and caudal projections, the superior and inferior articular facets, respectively. These have cartilage surfaces that form the facet articulations. The superior facet is located behind the neuroforamen and, when fractured, may cause root injury. The laminae are thin plates of bone that extend from the lateral mass obliquely posterior and meet each other at the base of the spinous process. The spinous processes project posteriorly, angling downward, and are progressively larger from cranial to caudal direction. The spinous process of C3 to C5 is always bifid, whereas C6 may be and C7 is never bifid.

Ligamentous Anatomy

The ligaments of the cervical spine are essential to maintain both stability and range of motion. The anterior and posterior longitudinal ligaments lie on the respective surfaces of the vertebral body. The latter extends laterally at each disc and narrows behind the body. The annulus joins the corresponding vertebral bodies and is the primary restraint to all directions of movements. All cases of traumatic subluxation or dislocation indicate injury to this structure.

The posterior ligamentous complex consists of the nuchal ligaments, ligamentum flava, facet joint capsules, and all bony attachments. The nuchal ligaments include the ligamentum nuchae, which is a thick band strongly attached to the spinous processes of C2 and C7 and which extends fibers downward attaching to the tips of the C3-C6 spinous processes. The ligamentum nuchae blends with the supraspinous ligament, which bands together each spinous process at their tips. Between the spinous processes are the interspinous ligaments. The ligamentum flava attaches to the underside of the cranial lamina and top-most aspect of the caudal vertebrae. These elastic structures afford flexion and, when disrupted, indicate a significant failure from that direction. Each facet articulation has relatively redundant facet capsules that offer only a small amount of stability but when disrupted, like the ligamentum flava, indicate significant injury.

Neurovascular Anatomy

The neural anatomy is important to explain neurologic injury and to plan treatment. Advanced imaging affords an excellent means of visualizing the pertinent neuroanatomy and must be judiciously analyzed before placement of any fixation devices. The spinal cord is an elastic structure that lies inside the spinal canal. In adults its anteroposterior diameter measures about 8 mm and cross-section is slightly oval. At each disc level ventral and dorsal roots sprout and join laterally, forming the spinal nerve in the neuroforamina. The neuroforamen is a truncated cone angling outward. It is bordered anteriorly by the posterior lateral corner of the disc and uncus, above and below by pedicles and posteriorly by the superior articular facet and lateral mass. After exiting from the neuroforamen, the spinal nerve divides into ventral and dorsal primary rami.

The vertebral artery ascends from the subclavian artery to pass within the foramen transversarium at C6, although in 1% of cases this can occur aberrantly at C7 or higher levels. It exits the foramen transversarium of C2 turning anteriorly and medially in C2 and then again laterally into C1. The vertebral artery being enclosed within the spinal column may be injured due to intervertebral distraction, fractures about the foramen transversarium, and translational injuries such as facet dislocations. Further, the vertebral arteries may be damaged intraoperatively by lateral mass or pedicle screw placement.

Biomechanics and Kinematics

The subaxial cervical spine has complex motions that are controlled by the disc annulus complex, various ligaments, and bony projections including the facets and uncus. The motions are usually coupled combining angulations along more than one axis or angulations with translation. For example, during flexion-extension, 1 to 2 mm of obligatory translation occurs. Similarly, in rotation, both lateral bending and rotation occurs simultaneously.

Kinematically each segment of the subaxial spine normally has approximately 11, 5, and 5 degrees of movement in flexion-extension, lateral bending, and rotation.⁶ Small amounts (1 to 2 mm) of translation occur in both anterior-posterior and lateral directions. Normal passive motion requires little application of load, which is measured by the neutral zone. Injury will increase the dimension of the neutral zone and when this exceeds certain thresholds such that the neural structures are at risk, the spine is deemed unstable.

Anatomic Considerations for Surgery

Understanding anatomy is essential for the avoidance of adverse events during surgery. Anomalies are not uncommon in the cervical spine and should be carefully interpreted. An important axiom is that congenital bony malformations are associated with vascular malformations. Therefore when anomalies are present, a preoperative computed tomography (CT) angiogram or magnetic resonance angiography should be considered.

Anterior decompression is well described using the standard Smith-Robinson approach. If required, corpectomies can be performed after removal of the disc. If possible, the posterior longitudinal ligament should be left intact. Distraction either by pins placed into adjacent vertebral bodies or with cranial tongs can restore axial length. Decompression should be oriented to the midline and be at least 16 mm wide depending on the transverse distance between the foramen transversarium. Common errors are biasing the decompression usually to the side opposite that from which the surgeons stands or too narrow a decompression. A plate with unicortical screws may be placed after insertion of a graft or a cage. The plate should be oriented so that screws have purchased into the body and not adjacent discs or laterally into the foramen transversarium.

The posterior cervical spine is dissected carefully as disruption of ligamentum flava exposes the spinal canal to risk of iatrogenic damage. Further, laminar fractures may be displaced downward onto the spinal cord. The joint capsules should be preserved to avoid any further destabilizing injury. In younger patients, simple exposure of noninvolved levels may result in spontaneous fusion.

Screw fixation into the lateral masses requires identification of a proper starting point and screw direction. The lateral mass is first examined and its borders defined. The cranial and caudal borders are the corresponding facet joints, the lateral border is the edge of the lateral mass, and the medial border is the valley at the junction of the lamina and lateral mass. Several starting points have been described.^7–9 However, with modern fixation most authors use a modified technique where the starting point is 1 to 2 mm medial to center of the lateral mass. The author marks this point with a small burr so that when a pilot hole is started it is located at the selected point. The pilot hole is oriented upward attempting to be parallel to the facet articulations and outward about 15 to 30 degrees. This outward orientation is essential to avoid injury to the vertebral artery, which lies directly anterior to the starting point as seen on axial images. The outward angulation is often limited by the next caudal spinous process.

Clinical Examination

The clinical examination is an essential component of the evaluation process. It depends on cooperation between examiner and patient, requiring normal mentation. Sensitivity is reduced by the patient’s inability to report symptoms due to conditions such as distracting pain, mild head injuries, or intoxication.¹⁰ The clinical examination is performed by reviewing the history of injury and symptoms of cervical pain, numbness, weakness, or paresthesias. The examination assesses spinal tenderness, which is best achieved by carefully logrolling the patient and palpating from the occiput to sacrum. Tenderness, swelling, hematoma, and gaps between spinous processes are indications for further evaluation.

The neurologic examination is performed using standards established by the American Spinal Injury Association (Fig. 77–1).¹¹ These should be recorded in the medical record in a timely manner. The neurologic examination includes testing of cranial nerves, motor and sensory function, and perineal function. Motor function of key muscle groups in the arms and legs are assessed from grades 0 to 5 (Table 77–1). Several sensory functions including pinprick, light touch, and proprioception or vibratory function are tested in both the upper and lower extremities. Sensory function is graded 0, 1, or 2 (absent, hypoesthesia, and normal, respectively). Deep tendon and pathologic reflexes are determined. Hyperreflexia and findings of clonus, Babinski signs, or Hoffmann signs are indications of spinal cord compression, although in cases of acute spinal cord injury areflexia is initially common. The level of spinal cord injury is defined as the lowest-functioning root level with at least grade 3 motor function. Complete spinal cord injuries are defined as the absence of any motor or sensory function including sacral roots distal to the zone of injury. Incomplete cord syndromes have some retained motor or sensory function and have a significantly better prognosis than complete lesions. The overall impairment is graded by the modified Frankel (ASIA) scale: A—motor and sensory complete; B—motor complete with sensory sparing; C—motor and sensory sparing but not functional strength; D—motor and sensory sparing with functional strength; E—normal motor and sensory.¹¹

FIGURE 77–1 Chart illustrating (A) Frankel (ASIA) grading of spinal cord injury severity and (B) the sensorimotor examination used in determining the level of spinal cord injury and ASIA motor score.

TABLE 77–1 Key Muscle Groups

Examination of the perineum is an important component of the clinical examination. The perianal skin is tested for pinprick sensation. Digital rectal examination is performed assessing tone and the presence of voluntary contraction. Finally, sacral reflexes are assessed. The anal wink is obtained by gently scratching the perianal skin and observing a contracture. The bulbocavernosis reflex is tested during digital rectal examination by compression of the glans penis or clitoris or alternatively by pulling on a Foley catheter. During this maneuver an anal contracture should occur.

Interpretation of the perineal examination depends on the level of injury and time from injury. In cervical cord injuries the perineal function may be initially absent, but the bulbocavernosis reflex may return in several days, indicating resolution of spinal shock, which is an initial depolarization of axonal tissue after injury. If the patient has not made any distal recovery at this time, the prognosis is grave. In some cases only sacral function is spared, changing the patient from complete to incomplete with a dramatic change in prognosis. Injuries at the conus or cauda equina that have retained perineal function or bulbocavernosis reflex have excellent prognosis for bladder and bowel function.

Patient Presentation

After the clinical examination, a decision is made whether radiologic studies are indicated. To determine the proper course, patients can be divided into one of four groups: asymptomatic; temporarily nonevaluable but asymptomatic; symptomatic; and obtunded. Asymptomatic patients are those who have no pain, no cervical tenderness, neurologic signs and symptoms and who are awake, alert, and nonintoxicated.¹ Additionally, they do not have any distracting injuries that may preclude pain assessment of a potential cervical spine injury. Distracting injuries include long bone fractures, burns, visceral injury, dislocations, and craniofacial or thoracic trauma. The temporarily nonevaluable patient is otherwise asymptomatic but is either intoxicated where sobriety is expected within 24 hours or will have resolution of other painful injuries through reduction or fixation. It is hypothesized that these patients can be evaluated similar to the asymptomatic patients in a delayed fashion.¹² Symptomatic patients have cervical pain, tenderness, or neurologic symptoms such as paresthesias, weakness, or numbness. The obtunded patient is one who will not be able to fully participate in the clearance process and at the same time is at highest risk for injury. Included in this group are disabled and infant patients.

Cervical Spine Injury Severity Score

The Cervical Spine Injury Severity Score (CSISS) is based on independent analysis of four columns (anterior, posterior, right column, and left lateral column) (Table 77–2).²⁵ The anterior column includes the body, disc including the annulus, anterior and posterior longitudinal ligaments, and transverse processes. Each lateral column is scored separately and includes the facet projections, lateral mass, pedicles, and facet joint capsules. The posterior column includes the lamina, spinous process, ligamentum flavum, and nuchal ligaments (Fig. 77–2A).

TABLE 77–2 Cervical Spine Injury Severity Score (CSISS)

^* Scores < 5 are generally treated nonoperatively. Scores > 7 are usually treated surgically.

FIGURE 77–2 A, The Cervical Spine Injury Severity Score (CSISS) is based on analysis of four anatomic columns: anterior, each lateral column, and posterior. B, Each column is scored independently using the analog scale from 0 to 5. A nondisplaced fracture is valued at “1,” while increasing scores are given proportionally to the amount of displacement. A “5” is given for the worst injury to a given column that is possible. Fractional values may be used. The CSISS is the sum from each column ranging from 0 to 20.

Each column is scored using a 0-5 analog scale (Fig. 77–2B). Fractional scores can be used. Scores increase proportional to either displacement of fracture fragments or separation as a result of soft tissue injury. For example, a nondisplaced fracture is scored 1 while the worst injury possible for that column (e.g., facet fracture dislocation with 10-mm displacement) is a 5. Each column is scored independently and summed, giving the CSISS ranging 0-20.

Anderson and colleagues²⁵ assessed reliability in 34 cases with 15 examiners. Both intraobserver and interobserver reliability were excellent. Construct validity was also good as all patients with scores equal to 7 had surgery, whereas only 15% less than 5 had surgery (Fig. 77–3). Caput and colleagues²⁶ retrospectively correlated CSISS score to surgical approach in 70 patients. They found a significant correlation with high CSISS scores (>11) to a posterior or combined anteroposterior approach. The anterior approach had lower scores with a mean 6.3, indicating that anterior surgery was used for less severe injuries, in accordance with well-known biomechanical studies.

FIGURE 77–3 A, Sagittal reconstruction showing C5 flexion-axial loading injury. There is a small teardrop fragment and the body is retropulsed into the spinal canal. The patient was a Frankel C quadriplegia. B, The next sagittal image shows wide separation of the spinous processes, indicating disruption of the posterior osseous-ligamentous complex. The CSISS anterior and posterior columns are scored 4 and 5, respectively. C and D, Subluxation of the left and right sagittal computed tomography (CT) in the plane of the lateral masses is present. The CSISS is 2 for each lateral column. E, Axial CT. Note the displaced laminar fracture. F, Traction lateral radiograph after applying 50 pounds. Alignment is significantly improved and retropulsion of C5 is reduced. G, The CSISS score totaled 13, indicating a highly unstable fracture and the need for surgery. The patient was treated by anterior corpectomy and fusion with fibular allograft and translational plate. Neurologically the patient improved to Frankel D and healed the fusion.

Subaxial Cervical Spine Injury Classification

The Subaxial Cervical Spine Injury Classification system (SLIC) evaluates fracture morphology, the discoligamentous complex, and neurologic function, creating a comprehensive system to aid treatment decision making (Table 77–3).²⁷ The system assigns points for each domain and if the score exceeds a threshold, surgery would be indicated.

TABLE 77–3 Subaxial Cervical Spine Injury Classification (SLIC)

Overall Subaxial Cervical Spine Injury Classification Score

The SLIC score is the sum of the individual components. A patient with an SLIC score equal to 3 is treated nonoperatively, whereas surgery is recommended in a patient with a score equal to 5 (Fig. 77–4). Scores of 4 can be treated either operatively or nonoperatively.

FIGURE 77–4 A, Sagittal computed tomography (CT) showing C7 burst fracture. The patient was neurologically intact. The discoligamentous complex is intact. The SLIC score is 2 (2 for burst fracture, 0 for DLC, and 0 for neurologic). The Cervical Spine Injury Severity Score is 6 (4 for anterior, 1 for each facet diastasis, and 0 for posterior). The patient was treated successfully nonoperatively in halo vest. B, Diastasis (arrow) of both facet joints (left shown) is present. C, Retropulsion of bone from C7 burst fracture is seen on axial CT. D, Traction of 25 pounds reduced the fracture. E, The patient was treated in a halo vest. The lateral radiograph at 12 months shows excellent alignment and healing of the burst fracture.

Vaccaro and colleagues²⁷ tested the reliability of the SLIC system. Interobserver reliability was only moderate, and both injury morphology and discoligamentous complex scores were only fair. Intraobserver reliability was excellent for overall and individual components. Similar to the CSISS, the system was valid when tested against treatment decisions. The SLIC system predicted accurately what the individual observers would recommend in 91% of cases.

These quantitative systems do not “name” or describe the injuries. Both authors of the aforementioned studies recommend using common colloquial terms that have a long history of use. Examples of these are facet dislocations, lateral mass fractures, and flexion/axial loading injuries. Unfortunately, injuries may be combined or similar so that distinguishing between any two fractures may be difficult.

Anterior Column Injuries

Anterior column injuries include compression fractures, burst fractures, flexion axial loading injury, and disc distraction injuries (Table 77–4). Transverse process fractures are included in this group, although they have no effect on spinal stability but may be associated with vertebral artery injury. Most anterior column injuries are easily recognizable on plain radiographs or CT. When present, they suggest a hyperflexion mechanism and a search for a concomitant injury to the posterior ligamentous complex should ensue. The notable exception is the disc-distraction injury resulting from opposite forces (i.e., extension and posterior shear). These injuries can be subtle, especially in patients with preexisting spondylosis.

TABLE 77–4 Anterior Column Fracture Types and Severity of Injury

Anterior Compression Fractures

Burst Fractures

Flexion Axial Loading Injury

Transverse Process Fractures

Transverse process fractures are quite common, occurring in isolation or in association with other more severe injuries. These fractures are not involved with spinal stability and therefore are not significant in treatment decision making for the spine. However, transverse process fractures at C6 and above may warn of possible vertebral artery injury.²⁹ No consensus has been reached regarding whether routine vascular evaluation and subsequent treatment with anticoagulants are required in patients with these injuries. This topic is discussed in detail in Chapter 82.

Disc-Distraction Injury

Radiographic Analysis

A variety of fracture patterns are encompassed with this moniker. Typically the disc space is wider than normal or is hyperlordotic. This may be difficult to discern, especially in spondylotic spines. Retrolisthesis is usually present, although its magnitude, usually around 1 to 2 mm, is often underestimated in terms of its significance. Large degrees of displacement, up to 50% translation, can occur but are rare. Because of the injury to the anterior longitudinal ligament, soft tissue swelling will usually be present. Small avulsions of the anterior endplate can be confused with fractured osteophytes. Impaction of the spinous process can cause their fracture, also involving the lamina, which may be displaced into the spinal canal. Similarly, lateral mass fractures from impaction can occur.

MRI is useful to identify questionable cases. Classic findings are increased signal in anterior soft tissues both rostral and caudal to the injured disc. Increased signal intensity across the disc space transversing the anterior annulus and anterior longitudinal ligament is pathognomonic (Fig. 77–5). Rarely the posterior longitudinal ligament will be involved. Disc herniation and spinal cord compression from stenosis or malalignment may be present as well. Posterior increased signal in the facet joints and ligamentum flavum are indicative of more extensive injury and greater instability.

FIGURE 77–5 A, Lateral sagittal computed tomography (CT) of 57-year-old neurologically intact male with an ankylosed spine from diffuse idiopathic spinal hyperostosis who sustained a hyperextension disc distraction injury at C7-T1. The disc is hyperlordotic and laminar fractures are visualized. Widening between spinous processes indicates probable injury of the posterior ligamentous complex. The patient is neurologically intact. The Cervical Spine Injury Severity Score is 11 (anterior—4, left—2, right—3, posterior—2). The Subaxial Cervical Spine Injury Classification is 5 (distraction—3, DLC—2, neurologic—0). B, Left sagittal reconstruction. Comminuted facet fracture is seen. C, Right sagittal reconstruction demonstrates subluxation of the C6-7 facet and C7 pedicle fracture (arrow). D, Axial CT shows displaced laminar fracture of C7 (arrow). E, Postoperative lateral radiograph following posterior instrumentation of C4-T3. F, Postoperative anteroposterior radiograph.

Treatment of Anterior Column Injuries

Anterior column injuries with low SLIC or CSISS scores can be treated non-operatively (Table 77–5). These include compression injuries, stable forms of burst fractures and flexion/distraction injuries, and most disc-distraction injuries (see Fig. 77–4). Less significant injuries can be treated with a cervical collar, while burst fractures and flexion/axial loading injuries can be treated with a cervical thoracic orthosis (CTO) or halo vest.

TABLE 77–5 Treatment of Anterior Column Injuries

Surgical indications for anterior column injuries are those with evidence of disruption of the posterior ligamentous complex as demonstrated by high CSISS and SLIC scores (see Table 77–5). In the case of compression fractures when surgery is warranted, the author recommends posterior fusion because a single-level anterior fusion may fail in the face of a vertebral fracture at the location of caudal screw fixation. Burst fractures and flexion/axial loading injuries can be reduced or significantly improved with tong traction. Although both anterior and posterior approaches may be used, the authors recommend addressing pathology at its major location (i.e., anteriorly) (see Fig. 77–3). After strut grafting or insertion of a cage, an anterior plate is applied. Although a rigid locked plate has theoretical advantages over a translational plate, the author has not observed any clinical differences in outcomes between the two plates. In rare cases of extensive comminution, associated facet dislocations, or those having residual displacement of lamina fractures, combined anterior and posterior approaches are indicated.

Surgical indications for disc distraction injuries are cases with any significant displacement, ongoing neurologic symptoms, or chronic pain. Anterior discectomy and fusion are indicated in neurologically intact patients and those with radiculopathy. Patients with cord injuries require individualized care. If stenosis is limited to the injured level, then anterior cervical discectomy and fusion are recommended. Not uncommonly, patients will have developmental narrowing of the spinal canal and multilevel stenosis. Several options are available including single-level anterior cervical discectomy and fusion combined with posterior decompression (laminoplasty) or posterior decompression (laminoplasty or laminectomy) with posterior fusion.