1 Craniovertebral Junction

Nader S. Dahdaleh

1.1 Key Points

• The craniovertebral junction (CVJ) is composed of the occiput (O), atlas (C1), and axis (C2) along with the atlantoaxial and atlantooccipital joints ( ▶ Fig. 1.1).

  Fig. 1.1 Craniovertebral junction (CVJ) anterior (top) and lateral (bottom) views.

• The CVJ is a very flexible junction accounting for at least 50% of the range of motion of the cervical spine in all planes. The joint orientation determines the direction of motion, and the ligaments determine the biomechanical stability of this junction ( ▶ Fig. 1.2).

  Fig. 1.2 Craniovertebral junction (CVJ) range of motion.

1.2 Bony Anatomy

• The CVJ consists of the base of the occiput, the atlas (C1), and the axis (C2).

• The foramen magnum boundaries consist of the basion anteriorly, the opisthion posteriorly, and the occipital condyles anterolaterally.

• The atlas (C1) is composed of an anterior arch, a posterior arch, and two lateral masses ( ▶ Fig. 1.3).

  Fig. 1.3 The atlas.

• The atlantooccipital joint is cup like in the coronal and sagittal plane allowing for flexion/extension and little axial rotation.

• The C1 anterior tubercle is the attachment site of the anterior longitudinal ligament (ALL) and the longus coli muscle.

• The vertebral artery (VA) and C1 nerve run along the superior lateral groove on C1 (sulcus arteriosus). In less than 15% of the population, the groove is roofed, forming the arcuate foramen.

• The axis (C2) consists of the body, odontoid process (dens), articulating surfaces, pedicles, pars interarticularis lamina, and large, bifid spinous process ( ▶ Fig. 1.4).

  Fig. 1.4 The axis.

• The atlantoaxial joint is convex in orientation allowing for axial rotation about the dens.

1.3 Neural Anatomy

• Cervical nerve roots exit above their corresponding level (e.g., the C2 nerve root exits above the C2 pedicle).

• C1 nerve root: The posterior division (suboccipital nerve) is more prominent than the anterior division. It innervates suboccipital muscles and occasionally branches to the lesser/greater occipital nerve.

• C2 nerve root: Posterior, medial (greater occipital nerve), and lateral divisions innervate suboccipital muscles and scalp from occiput to vertex. It may be sacrificed during atlantoaxial or occipitocervical fusions to enhance the exposure of the lateral mass for lateral mass screw placement and/or to access the atlantoaxial joint for direct arthrodesis.

• The lesser occipital nerve is formed by dorsal divisions of C2 and C3.

1.4 Vascular Anatomy

• The voluminous vertebral venous plexus that surrounds the horizontal portion of the V3 segment of the VA is encased in the suboccipital fascia. If the planes of dissection are respected, the plexus can be preemptively coagulated during exposure of the CVJ.

• The VA leaves the C2 transverse foramen (becoming V3). It takes a 45 degrees lateral projection and ascends (vertical portion of V3) into the C1 transverse foramen.

• The VA then courses medially (horizontal portion of V3) along the C1 sulcus arteriosus and then anteriorly through the atlantooccipital membrane, where it becomes intradural (beginning of V4 segment).

• Blood is supplied to the CVJ primarily through branches of the vertebral and occipital arteries.

• Blood supply to the CVJ emanate from extensions of the VA from the subaxial spine. The anterior and posterior ascending arteries branch from the VA at C2–C3, entering the vertebral column supplying the axis before anastomosing to form the apical odontoid arcade that supplies the atlas and dens.

The occipital artery completes the superior portion of the arcade.

• Lymphatic drainage of the CVJ is through retropharyngeal and deep cervical nodes.

1.5 Muscular Anatomy ( ▶ Table 1.1 )