The axis (C2) is the second cervical vertebra, distinguished by the dens (odontoid process) projecting superiorly from its body. The C2 body is wider than a typical cervical body and provides attachment for the anterior longitudinal ligament, the cruciate ligament complex, and the apical and alar ligaments via the dens. The axis is the pivot for approximately 50% of cervical rotation.
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| Ossification | Four ossification centres, two for the body (fuse by age 3), one for the dens tip (apical ossification centre, fuses at age 12), and the dens primary centre which fuses to the body at the subdental synchondrosis by age 6. The subdental synchondrosis must not be confused with a fracture. |
Axis fractures are classified by the Anderson-D'Alonzo system for dens fractures (Type I: tip, Type II: waist, highest non-union risk, Type III: base into body) and Levine-Edwards for hangman's fractures (traumatic spondylolisthesis of the axis). Type II dens fractures in elderly patients have up to 60% non-union with conservative management, driving surgical fusion decisions. The C2 nerve root exits above the C2-3 facet and is at risk during C1-2 instrumentation.
A fracture extending into the cancellous body of C2, which generally heals well with external immobilisation.
Bilateral fracture through the pars interarticularis of C2 with forward slip of the C2 body on C3.
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