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Development of Axis

- Anatomy:
    - Body:
            - one center (occassionally 2) appears by the fifth fetal month;
    - Neural Arches:
            - appear bilaterally by the 7 th fetal month
            - fuse posteriorly by the 2nd or 3 rd year;
    - Bifid tip of the Spinous Process
            - occassionally a secondary center is present in each tip
    - Neurocentral synchondrosis;
            - fuses at 3 to 6 years;
    - Inferior Epiphyseal Ring:
            - appears at puberty and fuses at approximately 25 years;
    - Summit ossification center for odontoid
            - appears at 3 to 6 years and fuses with the odontoid by 12 years;
            - basilar synchondrosis fuses by age 7 years, on average;
            - subdental synchondrosis scar may remain as a remnant of this fusion;
    - Odontoid (dens)
            - two separate centers appear by the fifth fetal month and fuse w/ each other by the 7 th fetal month;
    - Synchondrosis between odontoid and neural arch:
            - fuses at 3 to 6 years;
    - Synchondrosis between odontoid and body;
            - fuses at 3 to 6 years;

- Congenital Anomalies of the Odontoid
    - anomalies of the odontoid include complete aplasia, hypoplasia, and os odontoideum;
    - hypoplasia of odontoid may be seen in conjunction w/ dysmorphic conditions such as achondroplasia or spondyloepiphyseal dysplasia;
    - os odontoideum, the most common anomaly, occurs on both congenital and traumatic basis;
    - all three conditions can produce symptoms and serious neurologic sequelae as a result of atlantoaxial instability