Anatomy and Biomechanics

The ligaments that maintain the craniocervical articulation can be divided into two groups (Fig. 313-1). The first set attaches the skull to the atlas and includes the articular capsule ligaments, the anterior and posterior atlanto-occipital ligaments, and two lateral atlanto-occipital ligaments. The anterior atlanto-occipital ligament is a continuation of the anterior longitudinal ligament, and the posterior atlanto-occipital ligament courses between the posterior border of the foramen magnum and the posterior arch of the atlas. The cruciate ligament (which includes the longitudinally oriented extensions of the transverse ligament of the atlas) also contributes to the stability of this articulation.

FIGURE 313-1 Lateral (A) and posterior (B) views of the craniovertebral junction. Note that the cruciate ligament is composed of horizontal fibers (i.e., the transverse ligament) and vertical fibers. The tectorial membrane, the rostral extent of the posterior longitudinal ligament, has been reflected in B to allow visualization of more ventral structures.

The primary source of stability across the CVJ is provided by a second set of ligaments that secure the cranium to the axis. These ligaments include the apical dental ligament, the alar ligaments, the tectorial membrane, and the ligamentum nuchae.^5,6 The alar ligaments are paired structures with two components each: the atlantal alar and the occipital alar. These ligaments connect the tip of the odontoid to the occipital condyles and the lateral masses of the atlas, respectively.⁷ The alar ligaments are the main restraints to axial rotation, which occurs mostly across the C1-2 articulation. The alar ligaments also limit lateral flexion and anteroposterior translation.⁵

The tectorial membrane is the continuation of the posterior longitudinal ligament. It begins at the dorsal surface of the odontoid and inserts on the ventral surface of the foramen magnum.^8,9 The tectorial membrane primarily resists hyperextension, although if incompetent, contact between the posterior arch of the atlas and the occiput will limit hyperextension.^5,10 Flexion is restricted by contact of the odontoid process with the anterior foramen magnum.⁵ The apical dental ligament and the ligamentum nuchae contribute only modestly to stability of the CVJ.

Mechanisms of Injury

Craniocervical dissociation is the consequence of complete or nearly complete disruption of the ligamentous structures between the occiput and upper cervical spine. The extreme forces produced by hyperextension, hyperflexion, lateral flexion, or a combination of these forces lead to ligamentous failure.^11–13 The primary force responsible for producing AOD appears to be hyperextension, which leads to rupture of the tectorial membrane.^14,15 Another theory focuses on damage to the alar ligaments by extreme lateral flexion.^16–18 Incompetence of the alar ligaments and tectorial membrane allows anterior dislocation of the cranium with respect to the upper cervical spine.⁵ Other authors have suggested that hyperflexion forces may also be involved in some cases of AOD based on the observation that the posterior elements of C1 and C2 are commonly separated in the setting of craniocervical dissociation.¹²

The increased incidence of craniocervical dissociation in children^3,12,14 versus adults may be related to the relatively high incidence of automobile-pedestrian accidents, the pediatric anatomy of the CVJ, or both.^5,12 The immature shallow, horizontally oriented surfaces at O-C1 are less biomechanically stable than the deep-seated, vertically oriented articulation in adults. Moreover, a child’s ligaments are not as stiff as an adult’s, and they are subjected to greater stress because of the need to support a proportionately larger head than in an adult for a given body size.

Clinical Findings

High-speed motor vehicle accidents and striking of pedestrians by motor vehicles are the two most common causes of craniocervical dissociation.^3,12 Brainstem or upper cervical spinal cord injury at the time of dislocation probably accounts for the high mortality rate seen with AOD.¹⁹ Patients who survive may be neurologically intact or have dysfunction of the brainstem, cranial nerves, spinal cord, or cervical nerve roots.^18–20 The neurological deficits range from minor to severe. Many patients also suffer a head injury, which can confound the clinical examination.

Patients with craniocervical dissociation may have normal results on motor examination, or they may display deficits, including vegetative posturing responses, cruciate paralysis, or quadriparesis. At the level of C1, the normal spinal canal is capacious. As described by Steel and codified as the Steel rule of thirds, a third of the axial spinal canal diameter is filled by the dens, a third is filled by the spinal cord, and a third is space typically occupied by cerebrospinal fluid.²¹ Cruciate paralysis, described by Bell, refers to weakness in the hands and arms with relative sparing of leg strength.²² This mirrors the cervical injury pattern seen clinically as described by Schneider and colleagues.²³ Weakness often recovers, with improvement seen first proximally and then distally in the hands. Long held to be due to injury to the somatotopically organized crossing lateral corticospinal tract, recent evidence refutes this theory.^24,25

Many patients commonly have hemiparesis that may result from unilateral pyramidal injury, although more rostral structures in the brainstem may also be involved. The cranial nerves injured include VI, IX, X, XI, and XII. Compression, dissection, or stretch injuries may damage the anterior spinal, vertebral, or carotid arteries or any combination of these vessels.

Radiology

Although the radiographic findings in patients with craniocervical dissociation can be quite dramatic, they may be subtle or even absent on initial films.^16–19 When alignment of the bony structures of the CVJ appears normal, one must appreciate indirect signs that craniovertebral dislocation may have occurred. Abnormally prominent prevertebral soft tissues should always increase suspicion for serious spinal injury. In the case of AOD, retropharyngeal hematoma is invariably present. Other signs include retropharyngeal emphysema^14,26 or an increase in the interval between the posterior elements of C1 and C2.¹²

Several measurement protocols using plain lateral cervical radiographs to diagnose craniocervical dissociation have been developed. The basilar line of Wackenheim is drawn as a caudal extension of the posterior surface of the clivus in the midsagittal plane (Fig. 313-2A).²⁷ Normally, Wackenheim’s line lies tangential to the posterior tip of the odontoid process and is not altered by flexion or extension. The line will intersect the dens if the occiput is displaced anterior to the atlantoaxial segment. If the skull is displaced posteriorly, the line will not intersect any portion of the odontoid process.

FIGURE 313-2 Schematic diagrams depicting various criteria developed to assess the craniovertebral junction. Wackenheim’s line (A) is drawn as a caudal extension of the clivus and should tangentially intersect the tip of the dens in a normal individual. A normal dens-basion interval (B) should not exceed 5 mm in an adult and 10 mm in infants. C, The distance between the basion (B) and inner surface of the posterior arch of C1 (C) divided by the distance from the opisthion (O) to the inner surface of the anterior arch of C1 (A) defines the Powers ratio. The mean value in normal subjects is 0.77.

The interval between the odontoid tip and the basion remains constant when the CVJ is in the neutral position (Fig. 313-2B).²⁸ A distance of greater than 5 mm in adults and 10 mm in infants has been suggested as abnormal.^24,25 The sensitivity and specificity of the dens-basion interval, however, have been found to be suboptimal. Powers and coworkers reviewed 100 adult and 50 pediatric cervical spine radiographs and reported that 85% of individuals exceeded the proposed limits. With a reported average distance of 9 mm, as many as 50% of patients with known AOD might fall within the range spanned by the normal population.⁴

Subsequently, Powers and colleagues suggested that the ratio of two measurements could be used to define normal craniovertebral relationships (Fig. 313-2C).⁴ The first distance is measured between the basion (B) and the inner aspect of the posterior atlantal arch (C), and the other represents the distance between the opisthion (O) and the inner aspect of the anterior atlantal arch (A). The mean BC/OA ratio in normal subjects is 0.77, and a value greater than 1.0 may indicate AOD. Precise identification of the anatomic landmarks is necessary to properly determine the Powers ratio, and this factor may be inaccurate when congenital anomalies are present or the atlas is fractured. Additionally, in select situations of posterior or pure longitudinal craniovertebral disassociation, a value of less than 1 may be obtained.^29–31

Lee and associates developed the X-line method to detect craniovertebral dissociation.²⁹