Topics

Topic

design image
Dens of axis

The dens, or odontoid process, is a peg-like bony projection arising from the superior aspect of the axis (C2 vertebra). It projects cranially and slightly anteriorly to articulate with the anterior arch of the atlas (C1), forming the median atlantoaxial joint, which enables pivotal rotation of the head. The dens is conical in shape, with a narrow neck at its base and a broader apex superiorly. Its posterior surface is covered by the transverse ligament of the atlas, providing stability, while its tip is capped with articular cartilage. The odontoid process serves as a critical stabilizing structure of the upper cervical spine and a key landmark in spinal imaging.

Synonyms

  • Odontoid process

  • Dens axis

  • C2 odontoid

  • Peg of C2

Function

  • Acts as the pivot for rotational movement of the atlas (C1) and skull

  • Maintains atlantoaxial stability with the transverse and alar ligaments

  • Supports head rotation and neck motion while protecting the spinal cord

  • Serves as a key anatomical landmark in cervical spine imaging and surgery

MRI Appearance
T1-weighted images:

  • Appears as a linear, intermediate-signal bony structure

  • Surrounded by hypointense cortical margins and hyperintense bone marrow

  • The spinal cord posteriorly is intermediate signal; surrounding CSF is hyperintense, providing contrast

  • Fractures or marrow edema appear as focal areas of low signal disruption

T2-weighted images:

  • Cortical bone is hypointense, marrow is intermediate to slightly hyperintense

  • CSF remains hyperintense, creating a sharp outline of the dens and adjacent spinal canal

  • Useful for detecting bone marrow edema, ligamentous injury, or fracture lines

STIR (Short Tau Inversion Recovery):

  • Fat suppression enhances visibility of bone marrow edema or ligamentous injury

  • Normal dens remains hypointense in the cortical region; marrow edema or fracture-related changes appear hyperintense

CT Appearance

  • Dens appears as a hyperdense, well-corticated bony projection from the superior C2 vertebra

  • Cortical continuity, orientation, and morphology are easily visualized in axial, sagittal, and coronal planes

  • Surrounding air in the pharynx appears hypodense, providing natural contrast

  • CT is particularly useful for assessing fractures, congenital anomalies, osteophytes, or post-traumatic malalignment

MRI images

dens of axis MRI axial image

MRI images

dens of axis sagittal image