Craniocervical Injuries: C2 Fractures

Bony Anatomy

The axis is mechanically vulnerable because of its anatomy and corresponding function. The most notable feature unique to the axis is the odontoid process, or dens, which projects from the axis vertebral body to lie behind the anterior arch of the atlas and articulates with an articular facet of C1 at the back of the anterior arch. The odontoid process is the main stabilizer to prevent the atlas from posterior displacement on the axis ( Fig. 31.3 ). Moreover, the odontoid with the transverse and alar ligaments prevents anterior translation. The anatomy of the dens is important for preoperative planning, especially anterior screw osteosynthesis. At the level of 66% of its length, the mean inner diameter is 8.36 and 7.35 mm in the sagittal and coronal plane, respectively; the outer diameter is 12.88 and 11.77 mm in the sagittal and coronal plane, respectively. At the level of 90% of its length, diameters decrease a small amount. The angulation of the dens in the sagittal plane ranges from 2 degrees anterior to 42 degrees posterior without correlation between the space available for the cord and the height of the odontoid. Size and curvature especially vary frequently with the anatomic variability of the dens being high.

The articulating surfaces of the axis serve as a transition point between the more anterior occiput–dens–atlas articulation and the posterior facet articulations of the subaxial spine. The atlantoaxial facet joints are convex and directed slightly laterally. Below C2, the subaxial facet joints are located more posteriorly. The C2 pedicle is the area between the vertebral body and the articular column, whereas the pars interarticularis is the thin, bony bridge between the superior and inferior axis facets. The orientation of the pedicles is approximately 20 degrees superiorly and 33 degrees medially, and the diameter of the pedicles varies between 7 and 8 mm. The pars interarticularis functions as a bridge between the two points and essentially acts as a fulcrum in flexion and extension between the cervicocranium (skull, atlas, dens, and body of the axis) and the relatively fixed lower cervical spine, to which the neural arch of the axis is anchored by its inferior articular facets, stout bifid spinous process, and strong nuchal muscles. Below the dens is the C2 body. Lateral to this structure are the lateral masses of the axis, which support the lateral masses of the atlas. The transverse processes located further lateral to the lateral masses form the transverse foramina that contain the vertebral arteries.

Vertebral Artery

The vertebral arteries are branches from the subclavian artery entering the spine usually at C6–C7 and passing cranially in the foramen transversarium until C2. At the level of C2, the course can vary intra- and interindividually, and many variations have been described, including extracranial origin of the posterior inferior cerebellar artery. Usually, the vertebral artery passes into the C2 foramen transversarium and loops laterally and cranially. At C1, the vertebral artery passes cranially into the foramen transversarium and then turns posterior and medial on the cranial edge of the posterior C1 arch. It then turns medial and rostral and becomes intradural and forms the basilar artery on the ventral surface of the brainstem. The carotid arteries run anterolaterally to the C1 lateral mass.

The blood supply of the odontoid is provided by paired right and left posterior and anterior ascending arteries that arise from the vertebral arteries. They anastomose with branches from the carotid arteries. The ascending arteries penetrate the axis at the base of the dens; moreover, they continue outside the dens and form the apical arcade over the tip ( Fig. 31.4 ). Blood supply is provided both cranially and caudally. Damage to these vessels has been thought to affect bone healing. However, Govender and colleagues performed vertebral angiography in 18 patients, 10 with acute fractures and 8 with nonunion, and found no compromised blood supply associated with odontoid fractures.

Ligamentous Anatomy

The external and internal craniocervical ligaments connect the axis with the occiput and contribute to occipito-atlantoaxial stability. The internal ligaments are most important and include the tectorial membrane, which prolongs the posterior longitudinal ligament (PLL), two alar ligaments that extend from the sides of the dens to the occipital condyles and the foramen magnum, as well as the apical ligament of the dens. The apical dental ligament extends from the odontoid process tip to the ventral surface of the foramen magnum and has only a minor role in stability of the craniocervical junction.

The dens is harnessed to the anterior arch of the atlas by the transverse atlantal ligament. This strong ligament maintains a minimal physiologic space between the atlas and the odontoid, the atlantodens interval (ADI). The ADI is 3 mm or less in adults and 5 mm or less in children. Enlargement indicates loss of integrity of the transverse atlantal ligament with resulting instability ( Fig. 31.5 ). The articular capsular ligaments are thin and do not provide much stability. As shown in a biomechanical cadaveric study, the ligamentous upper cervical spine is stronger in extension than in flexion.

During rotational movement, the atlas rotates around the dens with a range of motion of approximately 40 degrees. Flexion and extension between the atlas and the axis are limited to 20 degrees; lateral bending is limited to 5 degrees. At least 50% of the rotational range of the cervical spine occurs on this level, and the atlantoaxial articulations have little inherent stability. In a recent biomechanical study, stiffness of the cervical spine was shown to increase during maturation, whereas the range of motion decreased.

Mechanism of Injury

The mechanisms of injury for odontoid fractures are most likely a combination of forces including hyperflexion, lateral bending, rotation, and extension. Although multiple injury patterns can result in traumatic spondylolisthesis, hyperextension and axial load are thought to be the major causes in types I and II injuries, whereas flexion forces are more likely to result in types IIA and III injuries. In the elderly who sustain atlas fractures from falls from standing height, the mechanism is usually hyperextension, and the dens may have preexisting bony erosion from degenerative disease that may have predisposed the patient to fracture.

Clinical Assessment

The assessment of the cervical spine is important for triage and initiation of further diagnostics, especially radiologic imaging. However, clinical tests, especially examining the axis, are not available. The cervical spine should be considered as a whole, whereas symptoms at the level of C2 are suspicious for axis injuries. The protocols to evaluate the cervical spine after trauma are reviewed in Chapter 11 .

During inspection, different aspects should be carefully considered. Open fractures of the axis are rare. Soft tissue injuries to the head, face, and occiput should be noted because they indicate the likely direction of injury vectors to the cervical spine. The spine contours should be analyzed for kyphosis or gibbus deformity. After severe injuries, deviation of spinous processes that are usually located in a median sagittal line may be observed. Fractures of the upper cervical spine can be accompanied by rotatory displacement or torticollis best visualized on open-mouth radiographs.

An immediate onset of neck pain after trauma is suspicious for injuries of the cervical spine, especially if associated with tenderness directly on top of the spinous processes. Pain on axial load indicates bony injuries and, if located in upper cervical spine levels, can be an indicator of C2 fractures. The pain is often referred in the distribution of the greater occipital nerve, and thus patients complain of occipital headache. Fractures of the axis often occur in the elderly, where the physical examination is less sensitive due to greater pain thresholds and cognitive impairments and may even be asymptomatic.

Imaging

Plain radiographs, including the lateral and open-mouth views, may be used to evaluate the upper cervical spine but have largely been replaced by CT. Radiologically, odontoid fractures can be assumed if a fracture line is visible or the dens is dislocated, which often results in asymmetry of the dens in between the occipital condyles. Lateral tilting of the dens is suspicious for a type III fracture if no other fracture is visible. In healthy individuals, the odontoid angle is not less than 87 degrees; however, in type III fractures, angulation up to 67 degrees has been noted. The altered odontoid angle may be the only radiologic sign of a fracture.

CT has significantly greater sensitivity and specificity and provides a more complete understanding of the fracture morphology. Thin CT slice thickness should be used to avoid missing transverse fractures. Modern multidetector helical CT obtains volumetric data so that reconstruction can be formatted with equal spatial resolution in all three planes, allowing more precision in determining fracture morphology. Other contiguous and noncontiguous fractures are common with axis fractures and, therefore, complete imaging of the spine is required. MRI is rarely required to evaluate axis fractures. CT angiography may be warranted when there is atlantoaxial displacement or when fractures involve the foramen transversarium. CT also allows estimation of bone quality, which is useful for surgical planning.

Retropharyngeal soft tissue swelling or hematoma formations at the craniocervical region are indicators of probable injury. In geriatric patients, erosions in the dens or remote fractures may be present and can be discriminated by identification of a hematoma as a sign of a fresh fracture and by critical analysis of the fracture lines. In difficult cases, MRI can be used to determine fracture. Moreover, MRI provides information about the integrity of ligaments, especially interspinous ligaments and the anterior longitudinal ligament, that may be helpful in evaluating stability ( Fig. 31.6 ).

Hangman's fractures can usually be diagnosed on the lateral view. Hematoma formation between the axis and the trachea indicates rupture of the anterior longitudinal ligament, which occurs in types IIA and III fractures. CT scans reveal the extent of dislocation, rotational deformity, and intraarticular fractures. MRI is needed infrequently to examine intervertebral discs and ligaments.

Odontoid Fracture

Anderson and D'Alonzo's classification for odontoid fractures depends on the location of the fracture line ( Fig. 31.7 ). Type I fractures are rare and account for approximately 1% of odontoid fractures. They are avulsion fractures of the alar or apical ligaments at the tip of the odontoid process. The most likely mechanism of injury of a type I fracture is traction in the coronal plane of the alar ligaments and may therefore represent an occipitocervical dissociation. Type I odontoid fractures have to be differentiated from a congenital nonunion of the tip of the odontoid, an ossiculum terminale, which is, however, a rare phenomenon that usually does not need any specific therapy. Nevertheless, cases of instability have been reported that required fixation. Moreover, ossifications of the ligaments near the apex of the odontoid can sometimes resemble type I fractures.

Type II fractures are located at the junction of the odontoid base and the C2 vertebral body and have the highest incidence among odontoid fractures, accounting for 65% to 74% of all odontoid fractures. Eysel and Roosen subclassified type II fractures with respect to the orientation of the fracture line to facilitate decision making as to whether ventral or dorsal fixation is more advisable. Type IIA fractures present horizontally, type IIB fractures present with a fracture line extending from superior-anterior to inferior-posterior, and type IIC fracture lines extend from superior-posterior to inferior-anterior. Type IIC fractures are the most unstable. Eysel and Roosen recommended anterior screw fixation for types IIA and IIB fractures, whereas C1/C2 posterior fusion was recommended for type IIC fractures.

Type III odontoid fractures are fractures that pass through the cranial cancellous vertebral body. Although the classification introduced by Anderson and D'Alonzo is commonly used, controversy exists as to the distinction between type II and type III fractures.

Grauer and colleagues modified the classification by Anderson and D'Alonzo to aid in surgical decision making. They distinguished between those fractures that do not involve the superior articular facet of C2 (type II) and those that do involve the superior articular facet (type III). Type II fractures were divided into nondisplaced fractures (type IIA), anterior-superior to posterior-inferior and displaced transverse fractures (type IIB), and anterior-inferior to posterior-superior or comminuted fractures (type IIC). They recommended external immobilization in type IIA fractures, anterior screw fixation in type IIB fractures, and posterior atlantoaxial fusion in type IIC fractures. Hadley and coworkers called a comminuted type II fracture type IIA, which had a much poorer prognosis for healing nonoperatively.

Distinction of type II from type III fractures can be difficult, and interpretation of plain films and CT scans varies between physicians. Barker and colleagues analyzed the interobserver and intraobserver reliability of the Anderson and D'Alonzo classification with special regard to type II and type III fractures, comparing plain films and CT. Both neuroradiologists and spine surgeons participated in the study. Interobserver and intraobserver reliability was better with CT scan than with plain films. Nevertheless, the overall interobserver reliability was moderate. The authors pointed out that the lack in reproducibility and reliability might have affected the classification of odontoid fractures in previously published studies and might therefore have affected study results.

Traumatic Spondylolisthesis (Hangman's Fracture)

Traumatic spondylolisthesis of the axis is classically characterized by bilateral fracture through the C2 pars interarticularis. This injury pattern has been described as the result of judicial hanging since the late 19th century, but in 1965 Schneider et al. first coined the phrase “hangman's fracture” for the radiographic appearance of the injury regardless of its actual cause. In fact, judicial hanging usually results in hyperextension and distraction with complete disruption of the C2–C3 disc space and associated ligaments between them. However, the bilateral fractures through the C2 pars interarticularis observed with hanging can also occur after falls and motor vehicle accidents from various combinations of extension, axial compression, and flexion and cause different degrees of disc disruption. In motor vehicle accidents, traumatic spondylolisthesis of the axis was reported half as commonly (a reported incidence of 27%) as odontoid fracture. In fatal motor vehicle accidents, only occipitoatlantal dislocations were more common. Interestingly, only a minority of judicial hangings actually result in a bilateral fracture through the C2 pars interarticularis. An anatomic study including 34 victims of judicial hanging revealed only three typical hangman's fractures and three other fractures of the axis.

The precise fracture location in hangman's-type injuries varies among patients and even between the left and right side. The pars interarticularis and the pedicle of the axis are, strictly speaking, two distinct anatomic structures, although the two terms have been used interchangeably in the literature. As with most injuries of the upper cervical spine, traumatic spondylolisthesis produces acute widening of the neural canal, and neurologic involvement is relatively uncommon in survivors (seen in 6% to 10%). However, reports of atypical hangman's fractures that involve the posterior C2 vertebral body have shown the potential for spinal canal compromise with displacement of the vertebral body fragment into the spinal canal ( Fig. 31.8 ). Some case reports of these injuries describe closed reduction and halo vest immobilization (HVI) with subsequent neurologic deterioration because of development of a large epidural hematoma. Craniofacial trauma is very common with these injuries; moreover, vertebral artery and cranial nerve injuries have also been reported. Associated cervical spine injuries with C2 traumatic spondylolisthesis almost always involve the upper three cervical vertebrae.

Stability of this injury has been shown to be related to the integrity of the ligaments and disc between the C2–C3 bodies and can be determined radiographically. The most common classification was published first by Effendi et al. and modified by Levine and Edwards ( Fig. 31.9 ). The classification system takes into account both angulation of the dens and displacement of the C2 body in relation to the C3 body. The integrity of the C2–C3 discoligamentous complex (disc, anterior longitudinal ligament, PLL, and C2–C3 facet) is considered based on the radiographic findings.

Type I fractures are nondisplaced, without angulation, and have less than 3 mm of displacement (see Fig. 31.9A ). They usually result from a hyperextension and an axial loading force that fractures the neural arch through the pars, and there is minimal disruption to the C2–C3 disc.

Type II fractures present with significant translation and some angulation (see Fig. 31.9B ). They usually result from a hyperextension and axial load (as seen with type I injuries) followed by flexion and compression. The combined forces result in disruption of the PLL and disc in a posterior-to-anterior direction and may create a compression fracture of the C3 anterior-superior end plate.

Type IIA fractures show slight or no translation but severe angulation of the fracture fragments (see Fig. 31.9C ). This fracture pattern is seen with flexion and distraction and results in loss of integrity of the PLL and the disc.

Type III fractures have a concomitant unilateral or bilateral C2–C3 facet dislocation (see Fig. 31.9D ). The spinal canal is narrowed in this pattern, and spinal cord injury is more likely than other hangman's-type fracture. This pattern is thought to result from flexion and compression resulting in complete disruption of the discoligamentous complex.

Type I fractures are thought to be stable because of an intact C2–C3 discoligamentous complex; types II, IIA, and III are unstable because of disruption at the C2–C3 interspace. In their series of 131 patients, type II fractures were the most common and seen in 56% of patients, followed by type I injuries, type III, and type IIA, which accounted for 29%, 10%, and 6% of patients, respectively.

Josten advocated that type II hangman's fractures represent a very heterogeneous group and divided these fractures into those with intact anterior ligaments (so-called Josten type 2) and those with ruptured anterior ligaments (so-called Josten type 3), whereas a locked dislocation constituted a Josten type 4 fracture.

Recently, Li et al. proposed a novel classification for atypical hangman's fractures based on the feature of fracture patterns, injury mechanism, incidence, and their impact on neurologic deficit. In short, it focuses on the involvement of the posterior cortex of the C2 body.

C2 Corpus Fractures

Corpus or fractures of the body of the axis have diverse patterns and are generally stable and rarely require surgery. Body fractures are commonly associated with other more rostral C2 fractures but can be isolated as well. Benzel et al. classified these “corpus fractures” according to their predominant pattern as type 1 (coronal), type 2 (sagittal), or type 3 (horizontal). However, combinations and variations are common.

Lateral mass fractures of the C2 vertebra are rarely reported and have a mechanism of injury similar to that of those causing lateral mass fractures of the atlas. Axial compression and lateral bending forces combine to compress the C1–C2 articulation and result in a depressed fracture of the articular surface of C2. Patients generally have a history of pain without neurologic deficit. Plain radiographs may be unremarkable, although anteroposterior and open-mouth views sometimes demonstrate lateral tilting of the arch of C1 and asymmetry of the height of the C2 lateral mass. If lateral mass fracture is suspected, CT of the area is helpful to more clearly delineate the injury.

Small avulsion fractures of the anterior-inferior corner of the axis, so-called extension teardrop, tend to be stable injuries, associated with an extension-type mechanism, and without associated neurologic injury. They are distinct from lower cervical spine teardrop injuries resulting from flexion, which are unstable and associated with neurologic injury 75% of the time. A distinguishable radiographic feature of extension-type (C2) teardrop fracture is anterior rotation of the fragment. Conversely, a flexion-type teardrop fracture remains aligned with the anterior margin of the spine. A C2 extension-type teardrop fracture can be associated with traumatic spondylolisthesis of C2 ( Fig. 31.10 ).

Odontoid Fractures

The treatment of odontoid fractures remains controversial with both surgical and nonsurgical methods being recommended. Morbidity is high, especially in the elderly, after either treatment. However, a multicenter study showed that no treatment at all resulted in no healing in all of 18 type II and all of three type III odontoid fractures.