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Anatomy of Atlas


- See
      - Normal Variants
      - Development of Atlas:
      - Atlantooccipital disassociation
      - Jefferson frx

- Discussion:
    - C1 has no central body
    - C1 vertebra is ring of bone w/ large lateral masses that provide only two wt bearing articulations between skull & vertebral column;
    - superior articulation of C1 is concave anteroposteriorly to provide flexion and extension of the convex occipital condyle;
    - flexion arc is approximately 20 to 30 deg;
    - inferior articulations are concave mediolaterally to provide rotation on the convex C2 superior articular facet;
    - anterior tubercle of C1 vertebra is quite thin & is held adjacent to odontoid by transverse ligament posterior to odontoid;
             - this limits amount of rotation & anteroposterior excursion of C1 on C2;
    - posterior element of C1 is thin, bony ring, which completes neural arch;
    - ring of C1 is quite thin just posterior to facet joint;
    - this is because of depression in superior aspect of ring, which allows vertebral artery to pass between ring of C1 & occiput after it
             emerges from foramen transversarium of C1 w/o danger of compression;
    - approx 50% of cervical rotation occurs between atlas & axis, around laterally central but anteriorly eccentric odontoid process;
    - lateral wall of Atlas rotates to a considerable extent across canal of axis, physiologically decreasing opening between these two segments;
    - canal of atlas is about 3 cm in its AP diameter, & is occupied equally by spinal cord, the odontoid process, & free space, each of which measures approximately one centimeter;
             - anterior displacement of atlas that exceeds one centimeter may jeopardize the adjacent segment of the spinal cord;

- Surgical approach:
    - posterior element of C1 is thin & is hidden deeply between prominences of occiput and the large C2 spinous process;
    - careful dissection from below exposes posterior membrane between C1 and the C2

Effect of C1-C2 rotation on canal size.

Computed axial tomography in C1-C2 trauma.

Fractures of the atlas