Ossification
Atlas
Three 1° Ossification centres
- two lateral masses with posterior arch in between
- anterior arch which appears at 1 year
- posterior arch fuses at 4 years
- anterior & posterior fuses to body at 7 years
Axis
Five 1° Ossification centres
- two neural arches / lateral masses
- two 1/2 dens
- body
Five 2° Centres
- Os Terminale (Appears 3, fuses 12)
- inferior ring apophysis
- 2 transverse processes
- 1 spinous process
Body of odontoid fuses by birth from two centres
- failure = Dens Bicornis
Dens separated from body by Dentocentral Synchondrosis
- present at birth
- fuses by 6 years
Tip of Odontoid not ossified at birth
- ossifies by age 3
- fuses by age 12
- Ossicum Terminale Persistens if doesn't fuse
Deformities
1. Os Odontoideum
- failure of fusion of base of odontoid
- normally fuses by age 6
- appears like Type II fracture
2. Odontoid Hypoplasia
- congenitally small odontoid
- asssociated with Morquio Syndrome & SED
- can get AAI 2° Hypoplasia
3. Odontoid Aplasia
- absent odontoid
4. Os Terminale Persistens
- unfused terminal apophysis
- normally fuses by age 12
Os Ondontoideum
Theories
Congenital
Developmental
- ? os odontoideum is a traumatic event
Clinical Features
Usually asymptomatic
May present in adulthood with
- pain (Usually dull ache)
- neurologic symptoms
- Lhermitte sign / lightning pain
- long tract signs
- risk of sudden death
X-ray
Open-mouth view
Instability
- > 3.5 mm translation on FE views
- ADI > 5 mm
- SAC also important if < 14 mm
Surgical Indications
Non syndromic
1. > 10 mm FE motion
2. Painful neck with instability
3. Neurology
Prophylactic neck fusion is controversial
- if asymptomatic avoid contact sport
SED / Morquio
ADI > 5mm = Fusion
Technique
Gallie Fusion
- involves using notched bone graft
- sitting it between the posterior arch of C1 and the spinous process and medial lamina arch of C2
- secured by sublaminar wires
- this is not inherently rotationally stable
- usually requires halo immobilisation