Dalley, 10th Edition

Chapter 2 ■ Back

82

Anterior tubercle

Anterior arch

Facet for dens Superior articular surface Transverse ligament of atlas Transverse process Foramen transversarium Groove for vertebral artery Posterior tubercle

Foramen for dens of axis

Lateral mass

Vertebral foramen (for spinal cord)

Posterior arch

(B) Superior view,

atlas (C1 vertebra)

Occipital condyles Foramen magnum

Grooves for attachment of alar ligaments

Dens of axis

Posterior articular facet for transverse ligament of atlas

Superior articular facet Foramen transversarium Transverse process Pars interarticularis

(A) Inferior view, cranial base

Body

X-ray beam

Vertebral foramen (for spinal cord)

Inferior articular process

Occipital condyles

Lamina

Cranium

Lateral mass of atlas (C1) Dens of axis (C2) Body of axis (C2) Lower row of teeth

(C) Posterosuperior view, axis (C2 vertebra)

Spinous process (bifid)

Lateral atlantoaxial joint ( red arrows )

Body

Facet for atlas

Superior articular facet

Anteroposterior radiograph of C1 and C2 (taken through open mouth)

Dens of axis

Transverse process

Lateral mass of atlas (C1) Dens of axis (C2)

Vertebral foramen (for spinal cord)

Inferior articular process Lamina

Body of axis (C2)

C3 vertebra

(E) Anterior view, articulating C1 and C2 vertebra as viewed radiographically

(D) Superior view,

Spinous process (bifid)

axis (C2 vertebra)

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FIGURE 2.8. Cranial base and C1 and C2 vertebrae. A. Occipital condyles of cranium. The occipital condyles articulate with the superior articular facets of the atlas (vertebra C1). B. Arches and lateral masses of atlas. The atlas, on which the cranium rests, has neither a spinous process nor a body. It consists of two lateral masses connected by anterior and posterior arches. C and D. Dens (odontoid process) of axis. The tooth-like dens characterizes the axis (vertebra C2) and provides a pivot around which the atlas turns and carries the cranium. It articulates anteriorly with the anterior arch of the atlas (“facet for dens of the axis,” in B ) and posteriorly with the transverse ligament of the atlas (see B ). E. Radiograph and articulated atlas and axis showing the dens projecting superiorly from the body of the axis between the lateral masses of the atlas. Since the atlas and axis lie posterior to the mandible (Fig. 2.7C), anterior radiographs must be taken through the open mouth.

The dens is held in position against the posterior aspect of the anterior arch of the atlas by the transverse ligament of the atlas (Fig. 2.8B). This ligament extends from one lateral mass of the atlas to the other, passing between the dens and spinal cord, forming the posterior wall of the “socket” that

receives the dens. Thus, it prevents posterior (horizontal) displacement of the dens and anterior displacement of the atlas. Either displacement would compromise the portion of the vertebral foramen of C1 that gives passage to the spinal cord. C2 has a large bi d spinous process (Fig. 2.8C, D) that