Craniocervical Injuries: Atlas Fractures, Atlanto-Occipital Injuries, and Atlantoaxial Injuries

Occiput

The occiput forms the major portion of the foramen magnum ( Fig. 30.2A ). From the anterolateral aspect of the foramen magnum, the occipital condyles project caudally on each side to form convex bony surfaces that articulate with the matched concave superior articular facet of the atlas, forming a synovial articulation (see Fig. 30.2B ). From the anterior view, they are wedge-shaped, with increased extension medially tapering off more laterally. The geometry of the occiput as it articulates with C1 allows the condyles to move like a rocking chair within the C1 lateral masses. Posteriorly, on the inside of the skull along the midline, the internal occipital crest extends toward the transverse sulcus and is the key location for occipital fixation. Just lateral and ventral to the occipital condyles is the hypoglossal foramen, through which cranial nerve XII descends. Given the close proximity of cranial nerve XII, it is prone to injury with fractures of the occipital condyle, CCDs, and occiput–C1 transarticular screw fixation. The ventral aspect of the foramen magnum is bounded by the basion, which is the caudal extent of the clival plate. The dorsal boundary of the foramen magnum is the opisthion.

Atlas

The atlas is a complicated ring-shaped structure allowing for a critical link between the occiput and the axis. The ring is formed from large lateral masses connected together by thin ventral and dorsal arches (see Fig. 30.2C ). The lateral masses viewed anteriorly are trapezoidal in shape to match the occipital condyles, essentially narrow medially and projecting out to a thickened lateral aspect, similar to a bowtie. Lateral to the lateral mass is a foramen through which the vertebral arteries pass as they course rostrally from C2 to wrap posteriorly over the C1 arch and enter the foramen magnum to form the basilar artery. Just anterior to the C1 ring lies the internal carotid artery (ICA) and the hypoglossal nerve, which are at risk of injury when placing C1 lateral mass screws. Internally within the ring, the transverse atlantal ligament (TAL) connects the lateral masses together while passing posterior to the dens. The TAL is an important stabilizer in flexion and extension at the C1–C2 junction. The atlas is critical in providing flexion and extension proximally as it articulates with the occipital condyles while providing about 50% of rotation as the C1 lateral masses rotate over the lateral masses of C2.

Axis

The axis is unique in its anatomy. The dens extends behind the ventral arch of the atlas and is maintained in position by the TAL. The axis is an essential component of the craniocervical articulation because all the major restraining ligaments attach between the axis and occiput and not directly to the atlas. The lateral masses of C2 are directly under those of C1, providing support in axial loading. The vertebral arteries extend along the inferior lateral border of the C2 body and then exit laterally through the transverse foramina.

Ligamentous Anatomy

The CCJ is not connected through the ligamentum flavum or intervertebral disks (see Fig. 30.2B ) as seen in the subaxial cervical spine. The ligamentous structures of the CCJ can primarily be divided into the external ligaments outside the canal and the internal ligaments within the canal.

The external ligaments include the ligamentum nuchae, the anterior and dorsal occipitoatlantal membrane, and the occipitoatlantal and atlantoaxial facet joint capsules. The anterior and dorsal occipitoatlantal membranes are thin structures that offer minimal protection to the dura. The facet joint capsules are thin and redundant to facilitate a wide range of motion and are key stabilizers of the occipital cervical junction.

The internal ligaments are the key stabilizers of the CCJ. The alar ligaments are thick, paired, cordlike structures that project laterally and ventrally from the tip of the dens to the ventral medial aspect of the occipital condyles. They are the primary rotational restraints of the upper cervical spine. With an average in vitro load to failure of 210 N, however, these vitally important ligaments tolerate less than 50% load to failure than the cruciate ligaments of the knee. These complex ligaments serve a variety of functions. With the head in midposition, they are slack. By turning the head in one direction, the alar ligament contralateral to the direction of rotation tightens, and the ipsilateral ligament slackens. Together with the tectorial membrane, the alar ligaments limit flexion. However, they play no role in limiting extension. The contralateral alar ligament limits lateral bending. The broad tectorial membrane, which constitutes the rostral extension of the posterior longitudinal ligament, effectively limits axial distraction and atlanto-occipital flexion and is considered, along with the alar ligaments, to be one of the major stabilizing ligaments of the CCJ. Anteriorly, the well-developed atlanto-occipital membrane, an extension of the anterior longitudinal ligament (ALL), limits extension, with the thinner anterior atlantoaxial membrane contributing to a less significant degree. The apical ligament is a rudimentary structure extending from the dens to the ventral midpoint of the foramen magnum.

The primary stabilizing ligament of the atlantoaxial motion unit is the cruciate ligament complex. The cruciate ligament lying behind the dens consists of the TAL and fibers that extend upward to attach to the basion and caudally to the axis. By crossing the odontoid at its waist, atlantoaxial flexion, translation, and distraction are minimized, yet rotation is allowed. In flexion, the cruciate ligament is placed under tension, thereby preventing the odontoid from compressing the spinal cord.

Supplemental ligamentous support of the CCJ is provided by a number of smaller ligaments, such as the apical and cruciate ligaments, the obliquely aligned accessory atlantoaxial ligaments, the anterior atlanto-dental ligament, and the facet joint capsules. Although the facet joint capsules have long been considered secondary stabilizers of the CCJ, a recent biomechanical, cadaveric study has indicated that they share equal importance with the tectorial membrane and alar ligaments in stabilizing the CCJ.

Kinematics

The CCJ accounts for approximately 60% of axial-plane cervical spine rotation, 40% of sagittal-plane flexion–extension motion, and 45% of overall neck motion. The occipital-atlas joint has roughly 15, 8, and 0 degrees of motion in flexion–extension, lateral bending, and axial rotation, respectively. The normal C1–C2 rotational excursion ranges from 80 to 88 degrees, and flexion–extension excursion ranges from 20 to 30 degrees. Total left-to-right lateral bending amounts to about 20 degrees at C1–C2.

Plain Radiographs

Although now largely supplanted by CT imaging except in children, plain radiographs with or without tomography were historically considered the initial workup for cervical trauma. With regard to the upper cervical spine, lateral radiographs have several shortcomings. Because the standard lateral cervical spine radiograph is centered in the midneck region, interpretation of the CCJ and atlantoaxial joint can be impaired by parallax or the obliquity of the C1 superior articular surfaces and the occipital condyles. Even minor malrotation of the head further distorts the occipital condyles and the neural arch of the axis, thus reducing the diagnostic value of such radiographs. However, a single cross-table lateral of the cervical spine is still an acceptable radiographic screening tool to evaluate for any obvious malalignment, fracture dislocation, or any prevertebral soft tissue swelling or large soft tissue shadows, which are indicative of upper cervical spine injury, warranting the need for advance imaging.

Computed Tomography

CT is the imaging modality of choice in high-risk trauma patients. Helical images with sagittal and coronal reformats have been shown to be timely, cost-effective, and more sensitive and specific in high-risk patients, particularly at the craniocervical and atlantoaxial joint. Three-dimensional image reformations, obtained from fine-cut CT scans, are rarely clinically useful but may assist with the interpretation of more unusual upper cervical injury patterns.

The anatomy of the upper cervical spine is unique; therefore one must be careful when looking for various fracture patterns. The CT axial plane images are usually oriented perpendicular in the midportion of the subaxial spine, which makes them oblique and distorted in the upper cervical spine. Similarly, the coronal reconstructions are usually aligned with the midportion of the subaxial spine, making it difficult to visualize the relationship between the occiput, C1, and C2 in the coronal plane. If the axial or coronal cuts are too oblique to fully evaluate the upper cervical region, then one should request that the axials be reconstructed to run in line with the C1 ring and the coronals to run parallel to the odontoid. Additionally, axial, sagittal, and coronal cuts can be reformatted to better assess the CCJ. Furthermore, positioning any patient with the head rotated will result in the physiologic rotational displacement of the atlantoaxial articulation, which may be difficult to distinguish from injury. It is important to emphasize that the patient needs to be supine with the head in the neutral position.

Additionally, upper cervical spine injuries are commonly associated with vascular injury, commonly the vertebral artery and internal carotid artery. Vilela and colleagues looked at 29 patients with unstable CCD injuries who were screened for vascular injuries: 15 of the 29 patients had positive findings for vascular injuries, which included 14 internal carotid injuries and 16 vertebral artery injuries. Fractures through the foramen transversarium or involving the posterior arch are at high risk for vertebral artery injury. Therefore computed tomography angiography (CTA) or magnetic resonance angiography (MRA) is indicated in the presence of injuries in the craniocervical and atlantoaxial junction. CTA and MRA aid in the diagnosis of patients with symptoms or signs of vertebral artery stroke and may prove valuable in surgical planning for screw placement.

Magnetic Resonance Imaging

If magnetic resonance imaging (MRI) is indicated, it is imperative to communicate to the radiologists the goal of the MRI so that it is protocoled appropriately. An MRI examination geared specifically toward the CCJ will be more sensitive than a standard cervical spine MRI. This may be described as a skull base protocol. An MRI is indicated either in the presence of neurologic injury to assess spinal cord injury or to aid in the diagnosis ligamentous injuries resulting in instability of the craniocervical and atlantoaxial joint. It can provide information regarding the extent, location, and compressive pathology in those patients with neurologic injury, especially if the fracture pattern does not match the neurologic examination. Indicators of a highly unstable injury include significant prevertebral soft tissue swelling, increased joint edema or widening at the occipitocervical joints or C1–C2, tectorial membrane disruption, subarachnoid hemorrhage, and ligamentous injury to the alar ligaments. MRI may be overly sensitive and must be interpreted in light of the mechanism and CT findings.

Dynamic Fluoroscopy

The traction test is a unique dynamic examination used to determine whether a CCD actually exists when CT, MRI, and all other tests are equivocal. There is no other spine trauma situation in which it is used, and even in the realm of CCD, its utility is quite limited because most injuries are delineated with CT imaging, either alone or combined with MRI. In the few cases in which CT and MRI are not diagnostic, a radiographic traction test can be performed. Greater than 2 mm of distraction between the occiput and C1 or between C1 and C2 is indicative of an unstable injury ( Fig. 30.3 ). The amount of weight required for traction testing has not been well defined, although cadaveric studies suggest that the craniocervical traction test reliably demonstrates instability and requires no more than 5 to 10 lb of traction to yield a positive result when the alar ligaments, the tectorial membrane, and the joint capsules are disrupted. In the authors’ experience, the primary role of traction testing has been to confirm that there remains sufficient ligamentous integrity of the CCJ to proceed with nonoperative treatment in situations in which imaging studies have shown some worrisome features for craniocervical instability (e.g., degree of joint subluxation) but other findings (e.g., extent of soft tissue swelling) have been less convincing. Our experience has been that traction testing has served to decrease our diagnosis of CCD in situations when the diagnosis would otherwise have been made if based on strict interpretation of static imaging parameters, thus saving patients from the morbidity of an unnecessary occipitocervical fusion.

Mechanism of Injury

Injuries to the occipital-cervical spine largely fall into two broad categories: (1) atlanto-occipital dissociations (AODs) or CCDs and (2) OCFs. AOD and CCD both involve the failure of ligamentous attachment and stability between the cranium and the cervical spine. Historically, the term AOD has been used; however, because these injuries can involve the occipital–C1 articulation, the C1–C2 junction, or a combination of the two, the term CCD is probably better because it is more accurate and encompassing. By pure terminology, AOD is a type of CCD, yet there are other patterns of CCD that really would not fall into the pure definition of AOD.

The CCJ is the most mobile segment of the human spine due to the osteoligamentous components providing the main structural support. This mobility in the upper cervical spine accounts for almost one-quarter of all cervical spine injuries. The age of patients with craniocervical trauma can range from young children and teens injured during high-speed trauma to elderly patients with low-impact injuries. CCD occurs after complete or near complete disruption of the osteoligamentous structures. This can occur through extreme hyperextension, hyperflexion, lateral flexion, or a combination of forces; however, the most common injury responsible for CCD is often hyperextension or distraction, resulting in rupture of the tectorial membrane and alar ligaments. Injury to these two structures allows for anterior translation of the cranium with respect to the upper cervical spine.

Alker and colleagues found that among 146 traffic fatalities, there was a 5% incidence of CCD. Historically, disruption of the CCJ was often fatal due to low clinical suspicion, lack of familiarity with upper cervical radiographic anatomy, and multiple life-threatening injuries leading to delay in diagnosis. However, with advancements in the training of emergency responders and the implementation of clinical and radiographic screening parameters, there is a lower incidence of delay in the diagnosis of CCD. Expedited diagnosis has decreased preoperative neurologic deterioration.

Classification

Traynelis and colleagues identified three CCD patterns based on the direction of displacement: occiput anterior to atlas (type I), longitudinal distraction (type II), and occiput posterior to atlas (type III; Fig. 30.4 ). This system is limited because the extreme instability of CCD injuries renders the position of the occiput relative to the neck arbitrary and more dependent on external positioning forces. There is also an absence of a severity component to the injury.

A useful classification system must quantifiably assess the stability of the CCJ. Signs of instability are translation or distraction of more than 2 mm in any plane, neurologic injury, or concomitant cerebrovascular trauma. The problem lies in segregating patients with minimally displaced (≤2 mm) craniocervical injuries who can be treated nonoperatively versus those with highly unstable but partially reduced injuries who require operative stabilization in spite of misleading well-aligned static images.

The Harborview CCDs attempts to identify the severity of the traumatic disruption in a three-tier system analogous to that of basic ligamentous extremity injury ( Table 30.1 ). Type 1 injuries are isolated structural injuries that are stable and can be treated nonoperatively; these include unilateral type III occipital condyle injuries or isolated alar ligament tears. The type 2 injury, which is a craniocervical disruption with borderline radiographic screening values, is inherently unstable but may be missed on cursory evaluation or even be difficult to categorize as unstable based on careful review of the imaging. Clinical evaluation of these patients is often unhelpful because the lesser degree of displacement usually equates to the absence of neurologic deficits. Incomplete, type 2 stable injuries of the CCJ can have similar degrees of displacement as partially reduced yet highly unstable injuries. The differentiation between the two is a primary challenge by timely recognition of CCD. The authors have found dynamic traction testing to be a useful diagnostic aid in the accurate categorization of patients with type 2 injuries (see Fig. 30.3 ). A type 3 injury is a complete disruption of all interconnecting ligaments with obvious unacceptable instability and craniocervical distraction of more than 2 mm on static radiographs. It is frequently accompanied by severe neurologic injury. This injury is further subdivided into type 3a, which include patients who survive the injury, and type 3b, which includes patients who do not survive within 24 hours of injury.

Imaging

As mentioned earlier, the atlanto-occipital joint is difficult to evaluate on plain radiographs. The diagnosis of CCD is often missed on plain radiographs, with sensitivity ranging from 57% to 76%. Prevertebral soft tissue swelling on the lateral cervical plain radiograph may be a key finding ( Fig. 30.5 ). The Powers BC/AO ratio, which compares the distance between the basion and posterior arch of the atlas (BC) with the distance between the anterior arch of atlas and the opisthion (AO), has poor reliability, as does the atlanto-odontoid-basion distance, initially described by Wholey and colleagues. Harris and colleagues refined the basion–dens interval (BDI) and added the basion–axis interval (BAI). In uninjured normal patients, both the BAI and the BDI are 12 mm or less in 95% of adults (“Rule of 12s”; Fig. 30.6 ). However, Bellabarba and colleagues noted that 35% of patients may have normal BAIs and BDIs and still have a CCD. Thus the Harris lines are not completely reliable and seem to have much higher specificity than sensitivity in detecting CCD.

One of the primary advantages of CT imaging over plain radiographs is that parasagittal and coronal CT images can be used to directly assess the congruency of the CCJ. Additionally, inclusion of the upper cervical spine in routine head CTs obtained for the assessment of the obtunded patient can reveal the presence of a suboccipital hematoma, which may be indicative of CCD. The inclusion of the foramen magnum in routine cranial CT scans has also increased the rate of detection of occipital condyle and type III odontoid fractures, which may be an indicator for CCD as well. Definitive CT scan of the cervical spine should include reformatted views, including sagittal and coronal views, especially of the craniocervical junction ( Fig. 30.7 ). Subluxation or distraction can therefore be directly identified rather than relying on indirect measurement via radiographic lines, especially because there is currently no specific measurement that strongly correlates with craniocervical injury.

Pang and colleagues reported an average craniocervical interval (CCI) of 1.28 mm in normal children 0 to 18 years of age, with a high degree of conformity between left- and right-sided measurements. None of the CCIs exceeded 1.95 mm. In adults, gapping of more than 2 mm between the occipital condyles and C1 lateral masses suggests craniocervical instability. Recent studies have supported the use of the revised CCI (>2.5 mm abnormal) and the condylar sum (sum of the left and right atlanto-occipital interval ≥5 mm abnormal) as highly sensitive and reliable for the detection of CCD on CT compared with the BAI and Powers' ratio. The coronal reformats are also useful to assess widening either between the occiput and C1 ( Fig. 30.8 ) or between C1 and C2 ( Fig. 30.9 ). Certain fracture patterns may also be suggestive of distraction, such as type III OCFs caused by alar ligament avulsions. A horizontal cleavage fracture of the anterior C1 ring has been implicated as having a distractive injury pattern that is associated with CCDs ( Fig. 30.10 ).

Studies have suggested that a delay in diagnosis may result in secondary neurologic deterioration in patients with these potentially life-threatening injuries. Although the advent of a systematic head and neck CT protocol has likely contributed to the reduction of missed craniocervical injuries, improved education and awareness of these injury types among survivors of high-energy trauma has probably played a greater role.

If there are questionable findings on CT but the diagnosis is still in question, MRI may be warranted. MRI can aid in identifying ligament injuries, intramedullary changes, hematoma formation in the epidural or paravertebral spaces, and cranial nerve injuries. In the rare cases where cord disruption is suspected, MRI may play an important role in the discussion of life-prolonging interventions. Despite the availability of definitive diagnostic testing, patients with CCD continue to be subject to critical delays in timely diagnosis, with diagnostic delays as long as 2 years having been reported.