Atlanto-Occipital Anatomy

3 Atlanto-Occipital Anatomy

Suzanne Labelle, Fady Y. Hijji, Ankur S. Narain, Philip K. Louie, Daniel D. Bohl, and Kern Singh

3.1 Bony Anatomy

3.1.1 Occipital Bone (Fig. 3.1)

• External occipital protuberance.

• Superior nuchal line.

• Inferior nuchal line.

• Occipital condyle:

– Articulate with superior facets of the atlas vertebra.

Fig. 3.1 Bony anatomy of occipital bone. (Reproduced with permission from Bambakidis NC, Dickman CA, Spetzler RF, Sonntag VKH, eds. Surgery of the Craniovertebral Junction. 2nd edition. New York, NY: Thieme; 2012)

3.1.2 Atlas: First Cervical Vertebra (C1; Fig. 3.2)

• Foramen magnum:

– Allows passage of spinal cord.

• Anterior arch:

– Anterior tubercle.

– Articular facet for dens on posterior surface.

Fig. 3.2 Bony anatomy of atlas and axis. (Reproduced with permission from Bambakidis NC, Dickman CA, Spetzler RF, Sonntag VKH, eds. Surgery of the Craniovertebral Junction. 2nd edition. New York, NY: Thieme; 2012)

• Posterior arch:

– Equivalent to lamina of other cervical vertebrae.

– Posterior tubercle:

∘ Equivalent to spinous process.

• Transverse process:

– Contains transverse foramen:

∘ Allows for passage of vertebral artery.

∘ Vertically oriented.

• Lateral masses:

– Equivalent to vertebral body (no formal vertebral body).

– Contains tubercle for attachment of cruciate ligament.

– Superior articular surface:

∘ Horizontally oriented.

∘ Articulates with occipital condyle forming atlanto-occipital joint:

▪ Biaxial condyloid synovial joint: fluid-filled articulations involving occipital condyles that allow movement in two planes.

▪ Permits flexion and extension; minimal lateral bending and rotation.

– Inferior articular surface:

∘ Horizontally oriented.

∘ Articulates with lateral mass of axis forming atlanto-axial joint.

3.1.3 Axis: Second Cervical Vertebra (C2; Fig. 3.2)

• Dens:

– Unique to only C2.

– Anterior articular facet:

∘ Articulates with anterior arch of atlas.

∘ Forms median pivot joint (Fig. 3.3).

– Posterior articular facet:

∘ Serves as articulating surface for the transverse ligament of the cruciate ligament:

▪ Prevents subluxation between C1 and C2.

Fig. 3.3 Atlantoaxial joint side-to-side rotation. (Reproduced with permission from Bambakidis NC, Dickman CA, Spetzler RF, Sonntag VKH, eds. Surgery of the Craniovertebral Junction. 2nd edition. New York, NY: Thieme; 2012)

– Small vertebral body beneath dens:

∘ Small pedicles attaching to lateral masses.

• Transverse process:

– Contains transverse foramen:

∘ Vertebral artery exits here:

▪ Makes a 45-degree turn and re-enters cervical spine at transverse foramen of C1.

• Spinous process:

– Small and bifid.

• Lateral masses:

– Superior articular facet:

∘ Horizontally oriented.

∘ Articulates with atlas forming atlantoaxial joint:

▪ Uniaxial synovial joint: fluid-filled joint that allows movement in one plane.

▪ Plane joints between both superior articular facets of axis and corresponding inferior articular facets of atlas.

▪ Median pivot joint between dens and anterior arch of atlas.

▪ Permits side-to-side head rotation (50% of cervical rotation).

▪ Permits small degree of flexion/extension (10 of 110 degrees of the cervical spine).

– Inferior articular facet:

∘ Horizontally oriented.

∘ Articulates with C3 (Table 3.1).

Table 3.1 Comparison to other cervical vertebrae

Vertebra	Similarities	Differences
Atlas (C1)		Posterior arch in place of lamina Posterior tubercle in place of spinous process Anterior arch with anterior process
Axis (C2)	Spinous process Vertebral body	Articular processes in place of lamina Dens/odontoid process
Both	Transverse processes with foramina for vertebral artery Articular surfaces on lateral masses	No uncinate processes Lateral masses in place of body