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Medial atlantoaxial joint

The medial atlantoaxial joint is the central articulation between the dens (odontoid process) of the axis (C2) and the anterior arch of the atlas (C1). It is a pivot-type synovial joint, permitting rotation of the head and upper cervical spine. The joint is enclosed by a fibrous capsule and stabilized by several key ligaments, particularly the transverse ligament of the atlas, which holds the dens securely against the anterior arch.

It forms the central component of the atlantoaxial complex, allowing approximately 50% of all cervical rotation and contributing significantly to head-turning mechanics. Due to its anatomical position near the brainstem and spinal cord, its integrity is essential for safe cervical motion and stability.

Synonyms

  • Median atlantoaxial joint

  • Atlanto-odontoid joint

  • C1–C2 pivot joint

Location and Structure

  • Articulation: Between the anterior surface of the dens of C2 and the posterior surface of the anterior arch of C1.

  • Joint type: Synovial pivot joint.

  • Articular surfaces:

    • Dens is covered by hyaline cartilage.

    • Posterior surface of C1 anterior arch also lined with cartilage.

  • Joint capsule: Thin, enclosing the articulation anteriorly.

Relations

  • Anteriorly: Prevertebral muscles and anterior longitudinal ligament

  • Posteriorly: Transverse ligament of atlas separating joint from spinal cord

  • Superiorly: Base of skull via alar ligaments

  • Inferiorly: Body of axis

  • Laterally: Lateral atlantoaxial joints and vertebral artery groove on C1

Ligamentous Stabilization

  • Transverse ligament of atlas: Holds dens against anterior arch

  • Alar ligaments: Connect dens to occipital condyles, limiting rotation

  • Apical ligament: Connects dens tip to basion

  • Tectorial membrane: Continuation of posterior longitudinal ligament over dens

Function

  • Allows axial rotation of head and C1 around dens

  • Maintains stability of craniovertebral junction

  • Supports smooth rotational motion without translation

  • Protects spinal cord by stabilizing dens position

T1-weighted images

  • Cortical bone: Dark (low signal)

  • Bone marrow of dens and C1: Bright signal due to fatty content

  • Articular cartilage: Thin low-signal line on articular surfaces

  • Joint space: Narrow low-signal interval between dens and anterior arch

  • Transverse ligament: Low-signal band posterior to dens

T2-weighted images

  • Cortex: Dark

  • Marrow: Intermediate-to-bright signal

  • Cartilage surfaces: Low-to-intermediate signal

  • Joint space: Appears brighter than T1 due to fluid sensitivity

  • Ligaments: Low-signal structures adjacent to the joint

STIR

  • Bone marrow: Intermediate-to-dark

  • Cartilage surfaces: Low signal

  • Joint fluid: Bright

  • Soft tissues: Intermediate, with enhanced contrast between ligaments and fat-suppressed background

T1 Fat-Sat Post-Contrast

  • Bone marrow: Mild uniform enhancement

  • Joint capsule: Very thin, subtle enhancement

  • Cartilage: Minimal to no enhancement

  • Surrounding soft tissues: Enhanced visibility of ligaments and capsule against a suppressed fat background

CT Appearance

Non-Contrast CT

  • Dens and C1 anterior arch: High-density cortical bone with sharp margins

  • Articular surfaces: Smooth, well-corticated with precise joint spacing

  • Joint space: Thin, uniform lucency

  • Ligaments: Not seen directly but inferred by bony alignment

  • Best for: Fine bony detail, joint congruity, cortical margins, dens morphology

MRI image

Medial atlantoaxial joint MRI axial image