Foramen Magnum and Occipital Condyles (C0)

The foramen magnum is round to oval shaped. The anteroposterior length measures on average ∼35 mm, whereas the transverse length is ∼30 mm.1 In its anterior half, the foramen magnum is flanked on both sides by the paired occipital condyles, which can be considered an integral part of it. Ovoid in structure, the occipital condyles have a major axis that is oriented anteromedi-ally. Their articular surfaces face downward and slightly lateral and are convex anteroposteriorly. The condyles articulate with the superior facets of the atlas caudally. The average length of the condyles in their major axis is ∼20 mm. Posterior to the condyles, a more pronounced depression represents the condylar fossae. In ∼60% of our specimens, they are the site of a canal that transmits the posterior condylar emissary vein, connecting the vertebral venous plexus with the sigmoid sinus just proximal to the jugular bulb ( Fig. 3.1 ).

The hypoglossal canals course within the occipital condyles, transmitting the respective nerves. They are directed anteriorly and laterally at an angle of ∼45° with respect to the sagittal plane, the intracranial end being located ∼10 mm from the posterior border of the condyle.8

Atlas (C1)

The superior facet of the atlas is oval with a groove on either side. Less frequently, it is kidney-shaped, with the groove present on only one side of the facet. This facet is longer in its anteroposterior dimension (mean 20 mm) than in its transverse dimension (mean 11 mm). Our study showed that none of the superior facets of the atlas were exactly symmetrical to those on the contralateral side.1 Both superior and inferior facets of the atlas face medially toward the spinal canal. The inferior facet of the atlas is almost circular in most of the vertebrae without any signifi-cant difference in the mean anteroposterior and transverse (mean 15 mm) dimensions. The thickness of the inferior facet under the posterior arch of the atlas is on average 3.5 mm (a in Fig. 3.2 ). The thickness of the posterior arch of the atlas separating the vertebral artery groove from the inferior facet is ∼4 mm (b in Fig. 3.2 ). The vertebral artery foramen is in the transverse process lateral to the lateral mass of the C1 vertebra. The groove for the vertebral artery on the superior surface of the posterior arch of the atlas is occasionally converted into a complete bony foramen. The distance from the midline to the medialmost edge of the vertebral artery groove on the outer cortex of the posterior arch is on average 18 mm ( Figs. 3.2 and 3.3 ).

Axis (C2)

The superior facet of the C2 vertebra differs from the facets of all other vertebrae in two important characteristics, making this region prone to vertebral artery injury during screw fixation. First, the superior facet of C2 is closer to the body when compared with facets of the other vertebrae, which are located in proximity to the lamina ( Fig. 3.4 ). Second, the vertebral artery foramen is present partially or completely in the inferior aspect of the superior facet of C2, whereas in the subaxial cervical vertebrae the vertebral artery foramen is located entirely within the transverse process and delimitates the anterior border of the vertebral pedicle ( Figs. 3.5 and 3.6 ). The size of the pedicle of the C2 vertebra varies according to its definition. We have adopted the definition as outlined in Fig. 3.4, making it thus very small in size and essentially being the zone of contact between the vertebal body and the superior articular surface.9,10 The course of the vertebral artery in relation to the inferior surface of the superior articular facet of C2 makes it susceptible to injury during transarticular and interarticular screw implantation techniques ( Figs. 3.4 and 3.6 ).

The axis vertebral body has a thick, conical superior projection—the dens. The dens is flanked by two large superior facets, extending laterally onto the adjoining pars interarticularis and articulating with the inferior atlantal facets. Unlike superior facets of all other vertebrae, these do not form a vertical pillar with the inferior facets, because they are considerably anterior to the inferior facets ( Fig. 3.7 ).

The shape of the superior articular surface of the axis varies from oval to circular. It is convex in the sagittal plane and is directed laterally to articulate with the inferior facet of C1. The average depth of the vertebral artery groove on the inferior surface of the superior facet is 4 mm. In the 20 specimens studied, the vertebral artery groove extended in the superior facet up to its medial third in 5 cases, middle third in 9 cases, and lateral third in 6 cases1 ( Fig. 3.8 ). In only one specimen did the vertebral artery groove extend minimally into the body of the axis vertebra ( Fig. 3.9 ).

In no case did the vertebral artery groove extend into the pars interarticularis. The transverse thickness or the width of the pars interarticularis is 8 mm on average. The total length of the pars interarticularis is ∼15 mm. The angle of its projection toward the superior facet is measured as the angle of the pars interarticularis from the vertical plane and ranges from 38 to 50° (average 45°). The intertransverse process height between C1 and C2 is ∼18 mm.1