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Odontoid process

The odontoid process, also known as the dens, is a tooth-like bony projection arising from the superior aspect of the axis (C2 vertebra). It acts as a central pivot around which the atlas (C1) and the skull rotate, forming a critical component of the atlantoaxial joint complex.

The odontoid process is a key structure in craniovertebral junction anatomy and is of major importance in trauma imaging, congenital anomalies, inflammatory conditions, and surgical planning.

Synonyms

  • Dens

  • Odontoid peg

Location

  • Arises from the superior surface of the body of the axis (C2)

  • Projects vertically upward toward the foramen magnum

  • Articulates anteriorly with the anterior arch of the atlas (C1)

  • Stabilized posteriorly by the transverse ligament of the atlas

  • Situated inferior to the clivus and anterior to the spinal cord

  • Forms the central axis of rotation at the atlantoaxial joint

Anatomical components

  • Tip (apex):

    • Superior rounded end

    • Attachment for the apical ligament

  • Neck:

    • Narrow region below the apex

    • Common site of fractures

  • Base:

    • Junction with the body of C2

    • Broadest and strongest portion

  • Articular surfaces:

    • Anterior facet for the anterior arch of C1

    • Posterior facet for the transverse ligament

Relations

Anteriorly:

  • Anterior arch of the atlas (C1)

  • Anterior atlanto-odontoid joint

Posteriorly:

  • Transverse ligament of the atlas

  • Spinal cord and dura mater

Superiorly:

  • Clivus

  • Apical ligament

Inferiorly:

  • Body of the axis (C2)

Laterally:

  • Atlantoaxial joints

  • Alar ligaments attaching to occipital condyles

Developmental anatomy

  • Develops from multiple ossification centers

  • Primary ossification centers:

    • One for the body of C2

    • One or two for the dens

  • Secondary ossification center:

    • Ossiculum terminale at the tip (appears in childhood, fuses by adolescence)

  • Fusion of dens to C2 body:

    • Typically completed by 5–7 years of age

Function

  • Rotation: Serves as the pivot for rotation of the atlas and skull

  • Stability: Maintains alignment of the atlantoaxial joint

  • Load transmission: Transfers forces between skull, atlas, and axis

  • Protection: Preserves spinal cord integrity during head movement

X-ray appearance

Cervical spine radiographs (AP, lateral, open-mouth odontoid view):

  • Odontoid process: Vertical bony projection centered between the lateral masses of C1

  • Margins: Smooth and corticated

  • Alignment: Symmetric relative to lateral masses of the atlas

  • Open-mouth view: Best demonstrates the dens and atlantoaxial relationship

CT appearance

Non-contrast CT:

  • Odontoid process: High-density cortical bone with central trabecular marrow

  • Neck and base: Clearly delineated

  • Atlantoaxial alignment: Well visualized

  • Preferred modality for detailed osseous assessment

Post-contrast CT:

  • Bone: No enhancement

  • Soft tissues: Evaluated for associated ligamentous or prevertebral changes

3D CT VRT appearance

Volume-rendered technique (VRT):

  • Odontoid morphology: Clear visualization of shape, height, and orientation

  • Surface continuity: Excellent assessment of cortical integrity

  • Atlantoaxial relationship: Useful for evaluating alignment and congenital variants

  • Preoperative planning: Valuable in surgical and trauma assessment

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal

  • Marrow within dens: Intermediate to high signal

  • Adjacent ligaments: Low-signal linear structures

T2-weighted images:

  • Bone cortex: Low signal

  • Marrow: Intermediate signal

  • Surrounding CSF: High signal outlining spinal cord

Post-contrast T1-weighted images:

  • Odontoid process: No enhancement

CT VRT 3D image

Odontoid process 3D

CT image

Odontoid process CT coronal

CT image

Odontoid process