Atlanto-Occipital Joint
Two paired synovial joints between occipital bone (above) and atlas/C1 (below).
Type
Synovial ellipsoidal (condyloid) joint — movement around 2 axes (transverse + anteroposterior).
Articular Surfaces
| Bone | Surface |
|---|---|
| Occipital bone (above) | Convex articular surfaces of occipital condyles |
| Atlas/C1 (below) | Concave superior articular facets of lateral masses; elongated, kidney-shaped, constriction in middle; directed medially + forwards |
Reciprocally curved — convex above fits into concave below
Ligaments
Fibrous capsule — surrounds joint; thick posterolaterally, thin anteromedially
Anterior atlanto-occipital membrane — anterior margin foramen magnum → upper border anterior arch of atlas; strengthened anteriorly by anterior longitudinal ligament
Posterior atlanto-occipital membrane — posterior margin foramen magnum → upper border posterior arch of atlas; arches over groove on posterior arch → canal for vertebral artery + 1st cervical nerve; laterally continuous with posterior capsule
Movements
| Movement | Axis | Prime Movers |
|---|---|---|
| Flexion + extension (“Yes” movement) | Transverse | Flexion: longus capitis, rectus capitis anterior; Extension: rectus capitis posterior major + minor, obliquus capitis superior, semispinalis capitis, splenius capitis, upper trapezius |
| Lateral flexion (slight) | Anteroposterior | Rectus capitis lateralis; + semispinalis, splenius, SCM, trapezius |
Rotation NOT possible at atlanto-occipital joint — rotation occurs at atlantoaxial joint
Line of gravity passes in front of atlanto-occipital joints → head tends to fall forwards → maintained by constant traction of extensor muscles, especially semispinalis capitis
Blood and Nerve Supply
- Arterial: Vertebral artery
- Nerve: 1st cervical nerve (suboccipital nerve)
Applied Anatomy
- Fracture-dislocation — membrana tectoria, alar ligaments + apical ligament stabilise joint from above; severe trauma → dislocation → compresses medulla oblongata → invariably fatal
- Hanging — death from dislocation of dens + rupture of transverse ligament of atlas → displaced dens crushes spinal cord + medulla; or Hangman’s fracture (fracture through axis); or axis-C3 separation
- Vertebral artery compression — posterior atlanto-occipital membrane arches over vertebral artery + 1st cervical nerve in groove on posterior arch; ossification of membrane → compresses vertebral artery → posterior fossa ischaemia
The atlanto-occipital joints are two paired synovial joints, one on each side, formed between the occipital bone above and the atlas vertebra (C1) below.
Type
Synovial joint of the ellipsoidal (condyloid) variety.
Since these are ellipsoidal joints, they permit movement around two axes — a transverse axis and an anteroposterior axis.
Articular Surfaces
| Bone | Surface |
|---|---|
| Above — occipital bone | Convex articular surfaces of the occipital condyles (two) |
| Below — atlas (C1) | Concave superior articular facets of the lateral masses of the atlas, which are elongated and kidney-shaped with a constriction in the middle; directed medially and forwards |
The articular surfaces are reciprocally curved — convex above fitting into concave below.
Ligaments
1. Fibrous Capsule (Capsular Ligament)
Surrounds the joint on all sides, attached to the margins of the articular surfaces. It is thick posterolaterally and thin anteromedially.
2. Anterior Atlanto-Occipital Membrane
- Extends from the anterior margin of the foramen magnum (above) to the upper border of the anterior arch of the atlas (below)
- Laterally continuous with the anterior part of the capsular ligament
- Anteriorly strengthened by the cord-like anterior longitudinal ligament
3. Posterior Atlanto-Occipital Membrane
- Extends from the posterior margin of the foramen magnum (above) to the upper border of the posterior arch of the atlas (below)
- Inferolaterally arches over a groove on the posterior arch of the atlas, forming a canal for the vertebral artery and the 1st cervical nerve
- Laterally continuous with the posterior part of the capsular ligament
Movements
Since these are ellipsoidal joints, movement occurs around two axes:
| Movement | Axis | Muscles Responsible |
|---|---|---|
| Flexion and extension (nodding — “yes” movement) | Transverse axis | Flexion: Longus capitis, rectus capitis anterior; Extension: Rectus capitis posterior major and minor, obliquus capitis superior, semispinalis capitis, splenius capitis, upper part of trapezius |
| Lateral flexion (slight) | Anteroposterior axis | Rectus capitis lateralis; assisted by semispinalis capitis, splenius capitis, sternocleidomastoid, and trapezius |
Rotation is NOT possible at the atlanto-occipital joint — rotation occurs at the atlantoaxial joints.
The line of gravity of the weight of the head passes in front of the atlanto-occipital joints. Therefore, in the erect position, the head tends to fall forwards, and its position is maintained by the constant traction of the extensor muscles — particularly the semispinalis capitis.
Blood and Nerve Supply
| Supply | Source |
|---|---|
| Arterial | Vertebral artery |
| Nerve | 1st cervical nerve (suboccipital nerve) |
Applied Anatomy
Fracture-Dislocation at the Atlanto-Occipital Region
The membrana tectoria, alar ligaments, and apical ligament of the dens together stabilise the atlanto-occipital joint from above. Severe trauma may cause dislocation — invariably fatal as it compresses the medulla oblongata.
Hanging
Death in judicial hanging results from dislocation of the dens following rupture of the transverse ligament of the atlas — the displaced dens crushes the spinal cord and medulla. Alternatively, hanging may cause fracture through the axis (Hangman’s fracture) or separation of the axis from C3.
Suboccipital Triangle and Vertebral Artery
The posterior atlanto-occipital membrane arches over the vertebral artery and 1st cervical nerve as they pass in the groove on the posterior arch of the atlas. Ossification of this membrane can compress the vertebral artery, causing ischaemia of the posterior fossa.
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