Axis (C2)
Axis
location_on Second cervical vertebra, directly below the atlas
The axis is the second cervical vertebra (C2) and is distinguished by the dens (odontoid process), a tooth-like projection that extends superiorly from its body into the ring of the atlas. The dens acts as a pivot around which the atlas and skull rotate, allowing the head-turning (no) motion. The axis bears the most unusual morphology of any vertebra.
Key Anatomical Features
- Dens (odontoid process) projects superiorly and acts as the pivot for head rotation
- Bifid spinous process is the largest of the cervical vertebrae spinous processes
- Superior articular facets are convex and face superolaterally for the atlas
- Transverse foramina transmit the vertebral arteries with an outward bend
- Body is the thickest of the cervical vertebrae, incorporating what was the atlas body (the dens)
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Obliquus capitis inferior | Spinous process of axis | Rotates the atlas and head to the same side |
| Rectus capitis posterior major | Spinous process of axis | Extends the head and rotates to the same side |
| Semispinalis cervicis | Spinous process of axis | Extends and rotates the cervical spine |
| Longissimus cervicis | Transverse process of axis | Extends and laterally flexes the cervical spine |
| Levator scapulae | Transverse process of axis | Elevates the scapula and laterally flexes the neck |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Median atlantoaxial joint | Synovial pivot | Atlas (C1) via dens |
| Lateral atlantoaxial joints | Synovial plane | Atlas (C1) |
| C2-C3 intervertebral joint | Cartilaginous symphysis | C3 vertebra |
Common Pathologies
Odontoid (dens) fracture
The most common cervical fracture in patients over 70. Classified as Type I (tip), Type II (base of dens, most common and most problematic for healing), and Type III (extends into body of axis).
Hangman's fracture
Bilateral fracture through the pars interarticularis of the axis, historically caused by judicial hanging but now most commonly from motor vehicle accidents. Often paradoxically stable due to ring expansion.
Os odontoideum
A separated ossicle in place of the normal dens, either congenital or from a childhood dens fracture that failed to fuse. Can cause atlantoaxial instability.
Clinical Relevance
Type II odontoid fractures have a high rate of nonunion (up to 40%) due to the poor blood supply at the base of the dens. Treatment remains controversial between surgical fixation and halo immobilization. The transverse ligament of the atlas holds the dens against the anterior arch of the atlas; its rupture allows the dens to compress the spinal cord. The rule of thirds (Steel's rule) for the spinal canal at C1 states one-third is dens, one-third is cord, and one-third is space.
Development and Ossification
The axis ossifies from five centers: one for the body, one for each neural arch, one for the dens tip, and one for the dens base. The dens develops from the centrum of the atlas (proatlas) that fuses to the axis body. The dens tip ossicle appears around age 3 and fuses by age 12. The dens-body synchondrosis fuses by age 6.
Did You Know?
- The dens of the axis is developmentally the body of the atlas that migrated and fused to the axis
- About 50% of head rotation occurs at the atlantoaxial joint around the dens
- The hangman's fracture is named for the mechanism of injury in judicial hanging, though the fracture location differs from the upper cervical cord injury that was the actual cause of death
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