Bony anatomy.

The occiput has two paired inferior projections called condyles that articulate with the superior articular surfaces of the lateral masses of C1. At the anterior aspect of the base of these condyles, foramina allow the hypoglossal nerve to exit the skull. The next structure, the atlas, is a ring-shaped vertebra with bilateral paired lateral masses fused by an anterior and a posterior arch. It is the only vertebra that does not have a body or a relationship with an intervertebral disk. The posterior aspect of the anterior arch has an articular surface for the odontoid process of C2 (dens). The lateral masses articulate superiorly with the occipital condyles and inferiorly with C2. The superior aspect of the lateral masses has a groove allowing passage of the vertebral artery before it enters the foramen magnum. The final structure is the axis, a unique vertebra that has a superior projection called the dens. The axis has paired superior articular processes to articulate with the lateral masses of the atlas and inferior articular processes to articulate with C3.

Ligamentous anatomy.

The broad complex of ligaments involved in the CCJ can be divided into intrinsic and extrinsic ligaments ( Fig. 28-2 ).

Ligamentous anatomy. A, Diagram of a cross-sectional view of the craniocervical junction (CCJ) demonstrating the ligamentous anatomy. B, Diagram of a posterior view of the CCJ; the posterior elements have been removed and ligaments are demonstrated. In the center of the image lies the cruciate ligament.

The intrinsic ligaments form three layers anterior to the dura matter and include the tectorial membrane, the cruciate ligament, and the odontoid ligaments. The tectorial membrane is a continuation of the posterior longitudinal ligament that runs posterior to the cruciform ligament. It attaches to the occipital bone and is in intimate contact with the dura matter. The cruciate ligament is composed of the transverse ligament and superior and inferior limbs. The transverse ligament is located in the atlas and traverses the posterior aspect of the dens, fixating it to the posterior aspect of the anterior arch of the atlas. This ligament divides the ring of the atlas into two compartments: an anterior compartment including the dens and a posterior compartment that houses the thecal sac and its contents. The odontoid ligaments comprise the alar and apical ligaments. The alar ligaments are paired structures that attach the base of the skull and the axis at the posterolateral aspect of the odontoid process; they act as stabilizers of the CCJ and limit axial rotation. The apical ligament, also known as the suspensory ligament, joins the tip of the dens with the basion.

The extrinsic ligaments are fibroelastic membranes that replace the anterior longitudinal ligament and the ligamentum flavum. The nuchal ligament is a continuation of the interspinous ligament that attaches to the occipital bone, restricting hyperflexion.

Ligamentous function.

The atlantooccipital and atlantoaxial joints are both necessary for full range of motion. The atlantooccipital permits flexion and extension while limiting rotation, and the atlantoaxial allows rotation while prohibiting other movements. The anatomic morphology of the ligaments in these joints explains the range of motion. Flexion at the atlantooccipital joint is limited by the relationship of the tip of the dens with the basion; extension is limited by the tectal membrane. The alar ligaments at this level minimize axial rotation. Therefore CCJ hyperextension injuries often result in tectal membrane injuries and also can encompass alar ligament injury when rotational force factors into the trauma. The rotational range of motion at the atlantoaxial joint is permitted by the functional relationship of the transverse ligament, alar ligaments, and capsular ligaments.

Special attention has been paid to evaluation of the ligamentous structures of the CCJ using MRI. Thin-slice MRI in three orthogonal planes provides good visualization of clinically relevant ligamentous structures. However, controversy still exists over the value of the imaging findings. Recent studies revealed that MRI signal changes in the alar and transverse ligaments did not correlate with whiplash injuries.

Craniometry.

The normal landmarks of the CCJ are best evaluated in sagittal and coronal cross sections. For accurate craniometrics, the basion, hard palate, and tuberculum sella, which are often excluded from exams, must be visible. However, in the setting of trauma, the usefulness of these measurements is questionable. Interobserver agreement demonstrates that the best correlation between plain film and cross-sectional imaging measurements is the McRae line, extending from the basion to the opisthion. Changes in craniometry secondary to flexion and extension, as well as irregularity of the clivus as seen in cross-sectional imaging, might explain this controversy.

Atlantooccipital dissociation.

Atlantooccipital dissociation (AOD) can be life threatening, typically resulting from high-energy trauma. Although this is an uncommon injury, in fatal traffic accidents the incidence is as high as 8% to 31%. The high fatality rate results from concomitant brainstem or vascular injury. Three types of AOD have been described; the most common is ventral dislocation of the occiput—type I; the most unstable is axial distraction—type II; and the third is dorsal dislocation of the occiput—type III. The most frequent mechanism of injury that produces AOD includes extreme hyperextension of the neck along with some degree of rotation ( Figs. 28-3 and 28-4 ). There is also an association with AOD and the presence of condylar fractures. Clinically these patients present with neurologic symptoms suggesting brainstem injury, and urgent reduction of the dissociation has been shown to improve long-term survival rates. A higher incidence has been reported in the pediatric population.

Classification of atlantooccipital dislocation (AOD). In type I dislocations, the occiput is anteriorly displaced. In type II there is widening of the atlantooccipital space. In type III dislocations, the occipital bone is posteriorly displaced. Close attention to the craniocervical junction relation is crucial for this diagnosis. Occipitocervical fixation is the preferred treatment.

(Adapted from Anderson PA: Injuries to the occipital cervical articulation. In Clark CR, et al, editors: The cervical spine, ed 3, Philadelphia, 1998, Lippincott-Raven, p 393.)

Atlantooccipital dislocation (AOD). Digital scout view ( A ), sagittal CT reconstruction ( B and C ), and coronal reconstruction ( D ) show increase in the space between the basion and tip of the dens (arrowheads). Widening of the atlantooccipital space (black arrows) and atlantoaxial space (white arrows) is seen. Sagittal T2-weighted MRIs ( E and F ) show increased signal and widening of the atlantooccipital space (arrows) and thinning with partial interruption of fibers of the tectorial membrane (arrowhead).

As discussed in previous sections, plain radiographs have limited usefulness in this situation, and MPR CT images are ideally needed for proper visualization of the craniocervical articulation. MRI can be used to identify major ligamentous disruptions and evaluate lesions to the spinal cord. A displacement greater than 10 mm between the basion and the tip of the dens (basion-dens interval) is a reliable method to measure AOD. In a comparison of basion-dens interval, condylar gap, Harris method (basion-atlas index), and the BC : OA Powers’ ratio, Dziurzynki et al. found the basion-dens interval to have the highest sensitivity and specificity for determination of AOD.

Occipital condyle fractures.

Occipital condyle fractures are uncommon lesions in blunt trauma, generally resulting from com­pression mechanisms. They can be classified into three types: comminuted—type I; extension of a linear skull fracture—type II; and avulsion injury—type III. Condylar fractures may be bilateral in up to 50% of cases. Special attention should be paid to type III condylar fractures because they can be associated with injury of the alar ligament, causing instability of the CCJ ( Figs. 28-5 and 28-6 ).

Occipital condyle fracture classification. Type I and II fractures are considered stable and are treated with a collar for 6 to 8 weeks. Type III injuries, if unstable, are treated with a halo vest. If incongruity is present, cervical fusion is considered.

(Adapted from Anderson PA: Injuries to the occipital cervical articulation. In Clark CR, et al, editors: The cervical spine, ed 3, Philadelphia, 1998, Lippincott-Raven, p 391.)

Occipital condyle fracture. Coronal reconstruction ( A ), axial CT without contrast ( B ). Type III fracture; a small bony fragment is avulsed from the right occipital condyle (arrow).

Plain films have a very low sensitivity to detect these lesions; CT is the best diagnostic tool to identify them. MRI can be helpful in identifying ligamentous involvement. With this injury, cranial nerve damage, particularly of the hypoglossal nerve, is often a concern. Close attention should be paid to the other cervical segments when encountering this fracture, because other associated lesions such as type II odontoid, axis, and subaxial fractures may also be encountered. Symptoms tend to improve with cervical traction, the initial treatment of choice.

Fractures of the atlas and transverse ligament rupture.

Fractures of the atlas account for 2% to 13% of acute cervical fractures. Also referred to as Jefferson’s fracture, they typically occur after application of an axial load to the skull, resulting in a burst fracture of the atlas at its weakest points, the anterior and posterior arches. These fractures may be isolated or associated with other lesions such as transverse ligament injury or odontoid fractures ( Figs. 28-7 and 28-8 ). Evaluation of the anterior transverse ligament is essential for the prognosis. Rupture of this structure destabilizes the atlantodental relationship and can result in posterior displacement of the dens in relation to the atlas. If the atlas is considered as a ring, the presence of single fractures is unusual; typically at least two fracture sites will be present. The most common fractures will be (1) posterior arch—associated with hyperextension (2) burst—associated with axial load, and (3) lateral mass fracture—associated with lateral flexion (see Fig. 28-7 ). Isolated C1 fractures tend to not have neurologic compromise, but when displacement of the lateral masses occurs, concomitant rupture of the anterior transverse ligament is possible.

Common types of fractures of the atlas. Arch fractures are stable. In Jefferson or burst fractures the lateral masses are typically laterally displaced. The lateral mass fractures can cause unilateral lateral mass displacement. Atlas fractures can be associated with transverse ligament lesions.

(Adapted from Jackson RS, et al: Upper cervical spine injuries. J Am Acad Orthop Surg 10:271–280, 2002.)

Burst fracture of the atlas. A and B, Axial and coronal reconstruction of a CT without contrast show widening of the space between the lateral masses (arrows) in relation to C2 (arrowheads). Sagittal STIR MRI ( C ) shows prevertebral fluid and fluid signal around the posterior elements of the atlas.

Disruption of the anterior transverse ligament without a fracture of the C1 ring can also occur; these lesions usually are accompanied by disruption of the alar ligaments. They have been classified as type I (lesions of the ligament alone) and type II (presence of avulsed bone fragments).

Evaluation of the atlas typically requires an open-mouth view for a frontal radiograph. A discontinuity of the lateral line that extends from the lateral masses of C1 to C2 would be present. Displacement of more than 7 mm indicates anterior transverse ligamentous injuries. Dynamic (flexion and extension) radiographs can be used as a screening tool. Widening of the atlantoodontoid space would suggest anterior transverse ligament disruption in the trauma setting, but if instability is suspected, dynamic views should be avoided.

CT is helpful for delineating the fracture lines in the atlas. Coronal multiplanar images can demonstrate the discontinuity of C1 and C2 in a similar way to open-mouth radiographs ( Fig. 28-9 ). MRI may be beneficial to study the ligaments, but its usefulness is limited because typical cervical spine MRIs do not cover the atlas in axial views, which hinders evaluation of the anterior transverse ligament. In a normal study the ligaments should be low intensity and homogeneous on gradient recalled echo sequences. Lesions appear as discontinuity of the ligament or significant signal change. MRI has limited use in the initial evaluation of ligamentous involvement of high cervical injuries as well.

Assessment for odontoid alignment. Interruption of the continuity of a vertical line traced from the lateral aspect of the lateral mass to the lateral aspect of the articular process of the axis confirms misalignments of the dens with the lateral masses of the atlas.

(Adapted from Jackson RS, et al: Upper cervical spine injuries. J Am Acad Orthop Surg 10:271–280, 2002.)

Atlantoaxial rotatory deformity.

Atlantoaxial subluxations are rare events that are usually nontraumatic and can be caused by an incomplete odontoid process or an incomplete transverse ligament; they are highly associated with rheumatoid arthritis. The atlantoaxial articulation is the most mobile portion of the spine, with articular facets that are mostly flat, allowing ample movement in multiple directions. Dislocation of the joint can result in compression of the thecal sac and neural structures. Traumatic rotatory subluxation occurs secondary to flexion and rotation force with lesions of the alar ligament; it is more frequent in children than in adults.

Odontoid fractures.

Odontoid fractures are a common injury in the cervical spine, accounting for 5% to 15% of cervical fractures. They can occur in hyperextension or hyperflexion injuries.

The classification of odontoid fractures by Anderson and D’Alonzo links the location of the fracture line with prognosis. Type I fractures compromise the upper part of the dens; type II fractures involve the base of the odontoid process; type III fractures affect the body of the axis ( Figs. 28-10 and 28-11 ). However, a limitation of the Anderson-D’Alonzo classification is the lack of descriptive terms for fracture lines. Roy-Camille et al. proposed an alternate classification that defines the angle of the fracture and the potential of dens displacement: type 1—oblique linear fracture in which its line slopes forward, with dens displacement in an anterior direction; type 2—oblique linear fracture in which its line slopes backward, with dens displacement in a posterior direction; and type 3—horizontal fracture line, with either anterior or posterior dens displacement.

Odontoid fractures. Type I fractures affect the distal dens and are considered stable. Type II fractures course through the neck of the dens and are the most unstable and unlikely to heal. Type III fractures affect the body of the axis, tend to be stable, and have a high likelihood of healing.

(Adapted from Anderson LD, D’Alonzo RT: Fractures of the odontoid process of the axis. J Bone Joint Surg Am 56:1663–1674, 1974.)

Odontoid fracture. Instability of a type II fracture in a 76-year-old patient after trauma. A and B, The fracture is identified at the base of the dens (arrows). Three months after the initial episode the patient returns with neck pain after a new trauma. CT of the cervical spine ( C and D ) shows widening of the previously described fracture (arrows). Sagittal T2- ( E ) and STIR-weighted ( F ) images show the fracture line (arrows). G and H, Another patient with an acute type II odontoid fracture with displacement. Axial and coronal reconstruction of a CT show an anteriorly displaced fracture of the base of the dens (arrows).

Treatment varies with type of injury. Type II fractures have the lowest potential for healing, especially in the elderly, and there is controversy regarding their management. These fractures may be life threatening if secondary spinal cord lesions occur, but this event is extremely rare. Type II fractures have a nonunion rate ranging from 10% to 77%; risk factors for nonunion include displacement greater than 5 mm, inability to maintain reduction, early initial treatment, and advanced patient age.

Evaluation of nondisplaced odontoid fractures with x-rays is limited. High-resolution CT with multiplanar reformats allows proper visualization of the fracture lines and eases classification, which will be critical for treatment decision making. Most odontoid fractures are treated with halo vest immobilization.

Traumatic spondylolisthesis of the axis.

Traumatic spondylolisthesis of the axis, representing around 15% of cervical spine fractures, is caused by hyperflexion of the cervical spine, producing bilateral fractures of the pars articularis of the axis. This fracture typically occurs in the setting of an MVA and tends to decompress the neural axis; patients are typically stable and have a good prognosis. Traumatic spondylolisthesis is similar in appearance to judicial hanging, which occurs after hyperextension of the neck, fracturing the posterior elements of C2 and causing severe spinal cord compression. The term hangman’s fracture is used loosely in the literature to describe traumatic pars articularis defects of the axis.

The classification of C2 spondylolisthesis takes into account the angulation and translation between C2 and C3. It describes three types: type I—fracture through the neural arch with no deviation; type II—fracture with significant displacement over angulation; type IIA—oblique fractures with angulation, with minimal translation; and type III—fracture of the posterior arch of C2, with unilateral or bilateral facet dislocation ( Figs. 28-12 and 28-13 ). These fractures are rare in the pediatric population and if present, child abuse must be suspected. Difficulty evaluating these children arises because of anatomic variation seen as a line connecting the spinous processes of C1 and C3. CT and MRI are recommended if the diagnosis is not clear.

Traumatic spondylolisthesis of C2 is classified as type I when a fracture crosses the neural arch with no angulation or displacement or with minimal angulation and elongation (type IA). Type II fractures have significant angulation and displacement, while type IIA have important angulation with minimal translation. If the fracture shows translation it is considered a type III. These are very unstable fractures and typically need surgical correction to prevent neurologic deficits.

(Redrawn from Torretti JA, Sengupta DK: Cervical spine trauma. Indian J Orthop 41:255–267, 2007.)

Traumatic spondylolisthesis of C2. Axial CT of the cervical spine ( A ) and sagittal reconstruction ( B ) show a traumatic pars interarticularis defect of C2 on the right (arrows). Sagittal T1- ( C ) and T2-weighted ( D ) MRIs demonstrate the pars defect (arrows). The T2-weighted images show increased signal in the posterior elements due to edema.

Pain radiographs have demonstrated great reproducibility in characterizing translation and angulation of hangman’s fractures. MRI is helpful to evaluate spinal cord compromise if the fracture is suspected to be secondary to hyperextension. Vertebral artery lesions tend to be more prominent in comminuted C2 fractures that affect the foramen transversarum.