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C1 Vertebra (Atlas)

The C1 vertebra, also known as the atlas, is the first cervical vertebra and supports the weight of the skull. It is a ring-shaped vertebra that lacks a vertebral body and spinous process and forms the atlanto-occipital and atlanto-axial joints, permitting flexion–extension and rotation of the head.

The atlas is a critical structural and biomechanical element of the craniovertebral junction and is a key focus in trauma imaging, congenital anomalies, and instability assessment.

Synonyms

  • Atlas

  • First cervical vertebra

Location

  • Situated at the uppermost level of the cervical spine

  • Between the occipital bone superiorly and the C2 vertebra inferiorly

  • Forms the atlanto-occipital joints with the occipital condyles

  • Forms the atlanto-axial joints with the dens of C2

  • Surrounds the cervicomedullary junction

Anatomical components

  • Anterior arch:

    • Short curved segment forming the anterior part of the ring

    • Anterior tubercle on its anterior surface

  • Posterior arch:

    • Longer curved segment forming the posterior part of the ring

    • Posterior tubercle replacing a spinous process

  • Lateral masses:

    • Thickened weight-bearing portions on either side

    • Superior articular facets for occipital condyles

    • Inferior articular facets for C2

  • Transverse processes:

    • Project laterally from lateral masses

    • Contain transverse foramina for vertebral arteries

  • Facet for dens:

    • Articular surface on the posterior aspect of the anterior arch

Relations

Superiorly:

  • Occipital condyles

  • Atlanto-occipital joint capsules

Inferiorly:

  • Axis (C2)

  • Atlanto-axial joints

Anteriorly:

  • Anterior longitudinal ligament

  • Prevertebral muscles

Posteriorly:

  • Posterior longitudinal ligament

  • Spinal cord and posterior arch groove for vertebral artery

Laterally:

  • Vertebral arteries within transverse foramina

  • Upper cervical nerve roots

Ligamentous attachments

  • Transverse ligament of the atlas

  • Alar ligaments

  • Apical ligament of the dens

  • Anterior and posterior atlanto-occipital membranes

Function

  • Load bearing: Supports the weight of the skull

  • Nodding movement: Permits flexion and extension at the atlanto-occipital joints

  • Stability: Maintains alignment between skull and cervical spine

  • Protection: Encloses the upper spinal cord

X-ray appearance

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

  • Ring configuration: Anterior and posterior arches forming a complete ring

  • Lateral masses: Symmetric alignment with C2 on open-mouth view

  • Atlantodental interval: Normally less than 3 mm in adults

  • Posterior arch: Smooth curved contour

CT appearance

Pre-contrast CT:

  • Cortical bone: Hyperdense outer cortex of anterior and posterior arches

  • Lateral masses: Well-defined dense weight-bearing structures

  • Articular facets: Smooth joint surfaces with occipital condyles and C2

  • Transverse foramina: Round or oval osseous canals laterally

  • Canal: Wide spinal canal surrounding the dens and spinal cord

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Bone marrow of lateral masses: Intermediate signal

  • Ligaments: Low signal linear structures

T2-weighted images:

  • Bone cortex: Low signal

  • Marrow: Intermediate signal

  • CSF around spinal cord: High signal within the wide atlantal canal

STIR:

  • Bone marrow: Intermediate-to-low signal

  • Ligaments and soft tissues: Low signal

  • Useful for highlighting marrow and ligamentous signal changes

CT image

C1 Vertebra (Atlas) CT coronal Anatomy, Location and Imaging Appearance -img-00000-00000

CT image

C1 Vertebra (Atlas) CT AXIAL Anatomy, Location and Imaging Appearance -img-00000-00000

X-Ray image

X Ray cervical spine lateral anatomy labelled -img-00000-00000 (1) - Copy