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Axis (C2 vertebra)

The axis (C2 vertebra) is the second cervical vertebra and plays a crucial role in cervical spine rotation and stabilization. It is characterized by the odontoid process (dens), a tooth-like projection that extends superiorly into the atlas (C1 vertebra), forming the atlanto-axial joint, which allows for rotation of the head. The vertebra consists of a body, pedicles, laminae, transverse processes, spinous process, and superior/inferior articular facets. It is located inferior to the atlas and superior to the C3 vertebra, at the level of the upper cervical spine, and is palpable only via imaging. The axis is structurally stronger than C1 to bear axial loads and transmit forces from the head to the cervical spine.

Synonyms

  • C2 vertebra

  • Axis vertebra

  • Second cervical vertebra

  • Odontoid vertebra

Function

  • Forms the atlanto-axial joint with C1, allowing rotation of the head

  • Supports axial load from the skull

  • Provides attachment for ligaments stabilizing the upper cervical spine, including the transverse ligament

  • Protects the spinal cord passing through the vertebral foramen

  • Acts as a key reference point in cervical spine imaging and surgery

Location

  • Second cervical vertebra, inferior to C1 (atlas) and superior to C3

  • Lies within the upper cervical spine, posterior to the pharynx, and anterior to the cervical spinal canal

  • Odontoid process projects superiorly into the atlas, forming the pivot for rotation

MRI Appearance

T1-weighted images:

  • Vertebral body and odontoid process appear intermediate signal intensity, slightly darker than surrounding fat

  • Cortical bone is hypointense (dark)

  • Bone marrow within the vertebral body shows intermediate to slightly hyperintense signal, reflecting fatty marrow

  • Ligaments and surrounding soft tissues are low-to-intermediate signal

T1 Post-Contrast (Gadolinium-enhanced):

  • Normally, the axis shows mild homogeneous enhancement of vertebral marrow

  • Pathological lesions (infection, tumor, fracture with marrow edema) appear enhanced and heterogeneous

  • Odontoid fractures may show contrast uptake at marrow edges, highlighting instability or edema

T2-weighted images:

  • Vertebral body shows intermediate signal, while cerebrospinal fluid (CSF) in the canal is bright hyperintense

  • Edema or inflammatory changes appear hyperintense, making fractures, infection, or neoplastic lesions more conspicuous

  • Odontoid process is distinguishable due to cortical hypointensity and marrow signal

STIR (Short Tau Inversion Recovery):

  • Suppresses fat signal to highlight bone marrow edema, ligamentous injury, or soft tissue pathology

  • Normal axis remains intermediate signal, while fractures or inflammation appear bright hyperintense

CT Appearance:

  • Appears as a dense, well-corticated vertebra with clearly defined odontoid process, body, pedicles, laminae, and spinous process

  • Cortical bone is hyperdense, cancellous bone slightly less dense

  • CT is excellent for assessing fractures (odontoid, body, pedicles), degenerative changes, and cervical alignment

  • Surrounding air in the pharynx provides natural hypodense contrast

  • Sagittal, axial, and coronal reconstructions allow detailed evaluation of atlanto-axial joint alignment and spinal canal

MRI images

Axis (C2 Vertebra) mri axial image

MRI images

Axis (C2 Vertebra) mri sag image