Ossification
Atlas
Three 1° ossification centers
- two posterior arch with anterior arch
- anterior arch appears at 1 year
- anterior and posterior arches fuse to body at 7 years
Axis
Five 1° ossification centers - two neural arches / lateral masses / two halves dens / body
Five 2° centers - Os Terminale / inferior ring apophysis / 2 transverse processes / spinous process
- body of odontoid fuses by birth
- dens separated from body by dentocentral synchondrosis / fuses by 6 years
- tip of odontoid ossifies by age 3 and fuses by age 12
Failure of fusion
Os Odontoideum - failure of fusion of base of odontoid / appears like Type II fracture / atlanto-axial instability
Odontoid hypoplasia - congenitally small odontoid / Morquio Syndrome & SED / atlanto-axial instability
Os terminale - unfused terminal apophysis
Atlanto-axial instability
www.boneschool.com/cervical-spine/atlanto-axial-instability