The axis is the second cervical vertebra, unique for its upward-projecting dens (odontoid process) that acts as a pivot around which the atlas, and therefore the head. Rotates to produce the 'no' head-shaking movement.
| Articulations |
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| Landmarks |
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| Ossification | The axis has four primary centres: one for the dens, two for the body, and one for the arch. The dens has a separate tip (os terminale) ossification centre. The dentocentral synchondrosis fuses by age 6. |
The odontoid process (dens) is the most common site of cervical spine fracture in the elderly, from low-energy falls that produce a sudden head flexion force. Anderson and D'Alonzo classify these as type I (rare, through the tip), type II (most common, through the base, highest non-union rate), and type III (into the body). Hangman fractures are traumatic spondylolisthesis of C2 through the pars interarticularis from hyperextension-distraction injuries.
Fractures of the dens are graded I to III by level; type II fractures at the base have the highest non-union rate and are common in older adults after simple falls.
Traumatic spondylolisthesis of the axis is a bilateral fracture through the pars interarticularis of C2 caused by hyperextension, as in a head striking a windscreen.
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