Craniovertebral Junction

Craniocervical junction (CCJ) = C1, C2, and articulation with skull base

Craniocervical junction comprises occiput, atlas, axis, their articulations, ligaments

Bones

• Occipital bone

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    Occipital condyles are paired, oval-shaped, inferior prominences of lateral exoccipital portion of occipital bone

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    Articular facet projects laterally

• C1 (atlas)

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    Composed of anterior and posterior arches, no body

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    Paired lateral masses with their superior and inferior articular facets

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    Large transverse processes with transverse foramen

• C2 (axis)

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    Large body and superiorly projecting odontoid process

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    Superior articulating facet surface is convex & directed laterally

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    Inferior articular process + facet surface is typical of lower cervical vertebrae

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    Superior facet is positioned relatively anteriorly; inferior facet is posterior with elongated pars interarticularis

Joints

• Atlantooccipital joints

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    Inferior articular facet of occipital condyle: Oval, convex surface, projects laterally

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    Superior articular facet of C1: Oval, concave anteroposteriorly, projects medially

• Median atlanto axial joints

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    Pivot-type joint between dens + ring formed by anterior arch + transverse ligament of C1

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    Synovial cavities between transverse ligament/odontoid & atlas/odontoid articulations

• Lateral atlantoaxial joints

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    Inferior articular facet of C1: Concave mediolaterally, projects medially in coronal plane

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    Superior articular facet of C2: Convex surface, projects laterally

Ligaments (from anterior to posterior)

• Anterior atlantooccipital membrane : Connects anterior arch C1 with anterior margin foramen magnum

• Odontoid ligaments

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    Apical ligament: Small fibrous band extending from dens tip to basion

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    Alar ligaments: Thick, horizontally directed ligaments extending from lateral surface of dens tip to anteromedial occipital condyles

• Cruciate ligament

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    Transverse ligament: Strong horizontal component between lateral masses of C1, passes behind dens

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    Craniocaudal component: Fibrous band running from transverse ligament superiorly to foramen magnum and inferiorly to C2

• Tectorial membrane : Continuation of posterior longitudinal ligament; attaches to anterior rim foramen magnum (posterior clivus)

• Posterior atlantooccipital membrane

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    Posterior arch C1 to margin of foramen magnum

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    Deficit laterally where vertebral artery enters on superior surface of C1

Biomechanics

• Atlantooccipital joint: 50% cervical flexion/extension and limited lateral motion

• Atlantoaxial joint: 50% cervical rotation