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Lateral mass of atlas

The lateral mass of the atlas (C1) refers to the paired, thickened bony columns on either side of the atlas that bear the weight of the skull and transmit forces to the axis (C2). Each lateral mass forms key synovial joints superiorly with the occipital condyles and inferiorly with the axis, making them central to craniovertebral stability and motion.

The lateral masses are critical landmarks in cervical spine anatomy, trauma assessment, and surgical planning.

Synonyms

  • Lateral mass of C1

  • Atlas lateral mass

Location

  • Paired structures on either side of the atlas (C1)

  • Situated between the anterior and posterior arches of the atlas

  • Lateral to the spinal canal

  • Inferior to the occipital condyles

  • Superior to the superior articular facets of the axis (C2)

Anatomical components

  • Superior articular facet:

    • Kidney-shaped, concave

    • Articulates with occipital condyle (atlanto-occipital joint)

  • Inferior articular facet:

    • Circular to slightly convex

    • Articulates with axis (atlanto-axial joint)

  • Medial surface:

    • Gives attachment to the transverse ligament of the atlas

  • Lateral surface:

    • Attachment for joint capsule and stabilizing ligaments

Relations

Superiorly:

  • Occipital condyles

  • Atlanto-occipital joint capsules

Inferiorly:

  • Superior articular facets of the axis (C2)

  • Atlanto-axial joints

Medially:

  • Odontoid process (dens) of axis

  • Transverse ligament of the atlas

  • Spinal canal

Laterally:

  • Vertebral artery groove (posterolateral region)

  • Atlanto-occipital joint capsule

Anteriorly:

  • Anterior arch of the atlas

Posteriorly:

  • Posterior arch of the atlas

Function

  • Load transmission: Transfers weight of the skull to the cervical spine

  • Joint articulation: Enables flexion–extension at the atlanto-occipital joints

  • Stability: Provides anchorage for the transverse ligament stabilizing the dens

  • Motion facilitation: Contributes to smooth craniovertebral movement

X-ray appearance

Open-mouth (odontoid) view:

  • Lateral masses: Symmetric bony columns flanking the dens

  • Alignment: Equal spacing between dens and each lateral mass

  • Articular surfaces: Smooth, well-defined cortical margins

CT appearance

Non-contrast CT:

  • Bone detail: Excellent visualization of cortical and trabecular structure

  • Superior and inferior facets: Clearly delineated joint surfaces

  • Symmetry: Bilateral lateral masses appear equal in size and position

Post-contrast CT:

  • Bone: No enhancement

  • Adjacent soft tissues: Expected vascular and ligamentous enhancement patterns

3D CT VRT appearance

Volume-rendered technique (VRT):

  • Lateral masses: Prominent paired pillars between anterior and posterior arches

  • Articular surfaces: Smooth contours forming craniovertebral joints

  • Utility: Helpful for assessing alignment, symmetry, and bony anatomy in multiple planes

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Marrow within lateral masses: Intermediate to high signal

  • Adjacent ligaments: Low-signal linear structures

T2-weighted images:

  • Bone cortex: Low signal

  • Marrow: Intermediate signal

  • Joint spaces: High-signal fluid within atlanto-occipital and atlanto-axial joints

FLAIR:

  • Bone: Low signal

  • Surrounding soft tissues: Normal homogeneous signal

Diffusion-weighted imaging (DWI):

  • No diffusion restriction

  • Bone structures: Typically not conspicuous

Post-contrast T1-weighted images:

  • Bone: No enhancement

MRI images

Lateral mass of atlas mri axial image

MRI images

Lateral mass of atlas mri sag image

CT image

Lateral mass of atlas CT CORONAL

CT 3D VRT image

Lateral mass of atlas