The Occipital Bone, Associated Anomalies, and Injuries

The posterior part of the cranium and most of the skull base are composed of the large, dense occipital bone. The occipital bone is composed of the basiocciput, supraocciput, exocciput, and the occipital squamosa, which are separated by synchondroses. The superior half of the clivus is formed by the basiocciput, and the occipital squamosa encompasses the supraoccipital and interparietal calvarium. The lateral margins of the foramen magnum and the occipital condyles are formed by the exocciput.4–6 The basion and opisthion are terms applied to the anterior and posterior midpoints of the free edge of the foramen magnum. These landmarks are utilized in several measurements that are discussed and illustrated later. Measurements of the foramen magnum in cadaveric skulls have revealed a mean anteroposterior diameter of 34.5 mm (range 29.2 to 40.5 mm) and a mean width of 29.4 mm (range 26.2 to 37.0 mm).7

If the primitive elements of the preatlantal vertebrae persist anterior to the foramen magnum, anomalies of the basiocciput may occur. These anomalies manifest themselves in a variety of bony excrescences along the anterior foramen magnum, including the hypocondylar arch and the paramedian third condyle, which articulates with the dens or the anterior arch of the atlas. Occasionally, anomalous remnants form a pseudoarticulation between the occipital bone and the transverse process of the atlas ( Fig. 4.2 ). The occipital condyles may be hypoplastic and allow a high position of the atlas and the axis. If the segmentation between the cranium and the first cervical vertebra fails, the atlas will be partially or completely assimilated. Frequently, there are accompanying basilar invaginations as well as other abnormalities, such as fusion of the axis to the third cervical vertebra, Klippel-Feil syndrome, and occipital vertebra and condylar hypoplasia.4,8–10

At the posterior lip of the foramen magnum, the remnant of the posterior arch of the atlas may be present as a triangular wedge of bone ( Fig. 4.3 ).11 Atlantoaxial instability is a common associated finding in 50% of patients.12 Atlanto-occipital assimilation may be a harbinger of other generalized anomalies, such as dwarfism, funnel chest, pes cavus, syndactylies, jaw anomalies, hypospadias, and an occasional defect of the genitourinary tract.13 Brainstem and cranial nerve signs may occur from anterior encroachment of the neuraxis at the level of the foramen magnum. If the vertebral artery is constricted, syncope, vertigo, and an unsteady gait may result from ischemia.13

Clival fractures are generally found with high-force, blunt head trauma and several subtypes have been described. These fractures can be associated with cranial nerve deficits and vascular injury, and resultant mortality is high secondary to brainstem trauma or vertebrobasilar occlusion. The diagnosis of clival fractures is technically difficult on radiographs alone because of the dense petrous bones; however, high-resolution CT with three-dimensional (3D) reconstructions may be helpful for diagnosis.14

The Atlas, Associated Anomalies, and Injuries

The first cervical vertebra, the atlas, is ring-shaped and lacks a vertebral and spinous process. Instead, it has two thick lateral masses on the anterolateral aspect of the ring that are connected to the anterior and posterior arches. The inferior surface of each lateral mass articulates with the superior articular facets of the axis.15 Isolated anomalies of the atlas do not have an abnormal craniovertebral relationship and are not associated with basilar invagination.4 Paralleling the atlas ossification centers, these anomalies consist of arch clefts, aplasias, and hypoplasias.4 The majority of these anomalies are of the posterior arch and include complete aplasia, Keller-type aplasia, aplasia with a unilateral or bilateral remnant, and middle rachischisis, hemiaplasia, or partial hemiaplasia of the posterior arch.12,16 It is highly important to note that partial hemiaplasias can mimic fractures on plain radiographic examination.17

The Jefferson fracture results from axial loading injury with disruption of the anterior and posterior arches of C1 that may be associated with injury to the supporting ligaments. The transverse ligament is crucial in maintaining the integrity of the anatomical position of the odontoid and the anterior arch of C1, which can be avulsed secondary to trauma at the osseous insertion site on C1. The clinical presentation of Jefferson fractures are neck pain, dysphagia, and posterior fossa transient ischemia attack (TIA) or stroke. CT is effective at diagnosis of the osseous avulsion of the transverse ligament, but MRI can be more specific in identifying and assessing the degree of ligamentous injury and possible cord injury.18

The Axis, Associated Anomalies, and Injuries

The second cervical vertebra, the axis, forms a pivot for the atlas that allows the head to rotate. The odontoid process projects superiorly from the C2 vertebral body with a facet on its ventral surface that articulates with the dorsal surface on the anterior atlas arch. The axis spinous process is large and is the first bifid vertebra in the cervical spine.15 Clefts of the axis are rare, as are atlantoaxial fusions, which form a block that usually lacks the anterior arch of the atlas and are associated with a hypoplastic or absent odontoid. Partial fusion may occur between the odontoid and the anterior arch of the atlas or may involve unilateral fusion of the hemiarches of the atlas and axis. These fusions can result in uneven joint surfaces with concomitant impairment of function.11 An increased incidence of atlantoaxial fusion is present in patients with the Chiari I malformation.11

Most anomalies of the axis involve the odontoid process and are not associated with basilar invagination. The tip of the odontoid or os terminale usually appears by the age of 3 and fuses by the age of 12.4,13 Failure of the os terminale to fuse, also known as Bergman ossicle, originates from failure of fusion of the odontoid process to the proatlas. This important note can be misconstrued as a type I odontoid fracture, although identification of a well-corticated ossicle should make the diagnosis of fracture less likely.4

Hypoplasia of the odontoid associated, with an oval ossicle widely separated from C2 and located well above the superior facets of the axis, is termed os odontoideum. The os usually does not preserve the normal shape or size of the odontoid, often being half the normal size and rounded or oval, with a smooth uniform cortex.13 The anterior arch of the atlas may be hypertrophied, whereas the posterior arch may be hypoplastic in patients with os odontoideum ( Fig. 4.4 ).13,19 These dysplastic states of the odontoid are uncommon and most often encountered as an isolated phenomenon, but they can be found in several other syndromes involving the bony skeleton, such as Morquio syndrome, mucopolysaccharidoses, spondyloepiphyseal dysplasia congenita, spondylometaphyseal dysplasia, metatrophic dwarfism, and Conradi disease.20–22 Total aplasia of the odontoid is extremely rare but is often associated with an irregularity of the upper margin of the C2 vertebral body.23 Compression at the level of the C1 arch can occur in the setting of both hypoplasia and aplasia secondary to the absence of the attachments for the apical and alar ligaments predisposing to atlantoaxial instability.24

Basilar Invagination and Platybasia

Primary basilar invagination refers to a developmental anomaly involving an abnormally high vertebral column that prolapses into the cranial base.25 The condition has numerous causes, including basioccipital hypoplasia, condylar hypoplasia, hypoplasia of the atlas, atlanto-occipital assimilation, odontoid abnormalities, Klippel-Feil syndrome, Chiari malformations ( Fig. 4.5 ), syringomyelia, and syringobulbia.4,13,20 Secondary basilar invagination, which is often termed basilar impression, is a developmental condition associated with softening of the osseous structures at the cranial base. It may be seen in Paget disease, osteomalacia, osteogenesis imperfecta, rickets, renal osteodystrophy, neurofibromatosis, Hurler disease, rheumatoid arthritis, infections of the cranial base, and fractures of the posterior cranial base.4,13,20 Additionally, platybasia refers to an abnormally flattened cranial base with an increased basal angle. When isolated, platybasia is associated with no signs or symptoms. However, platybasia and basilar invagination or basilar impression often coexist.4,20

Occipitoatlantal Joint

To a large degree, occipitoatlantal stability is provided by the contours of this articulation. The orientation of the articular surfaces varies with growth. In children, the surfaces are oriented horizontally and become more vertical with age. This process is thought to account for the greater instability of this joint in children.26,27 The joint capsules provide additional stability, which increases with age as the capsules become less elastic.

The tectorial membrane and alar ligaments provide ligamentous support to the occipitoatlantal joint. The tectorial membrane represents a cephalad continuation of the posterior longitudinal ligament. It extends from the posterior margin of the body of C2 to the anterior foramen magnum, coursing over the dens and cruciform ligament ( Fig. 4.6 ). Functionally, the tectorial membrane limits extension of the occiput-C1-C2 complex and, to a lesser degree, flexion.28 However, the membrane does not significantly limit axial motion, which is the function of the alar ligaments ( Fig. 4.7 ). These ligaments arise from the lateral apex of the dens and insert on the medial inferior aspect of the occipital condyles.

Atlanto-occipital dislocation occurs with disruption of the ligaments between the occiput and upper cervical spine after a hyperextension injury. Injury to the alar ligaments and tectorial membrane can result in anterior dislocation of the skull base and cervical spine with possible associated odontoid fractures. Dislocation is often accompanied by head injury with high mortality related to brainstem injury. High-resolution CT is effective for accurate diagnosis, and MRI can be utilized to evaluate alar and tectorial ligamentous injury as well as the joint capsule.29