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Normal Variants: of Occiput – C1 – C2

- Discussion:
    - various stages of Fusion of the the basiocciput may be seen;
    - ranging from partial fusion, either anterior or posterior, to complete occipitalatlantal fusion;
    - atlantal-axial fusion anomalies are seen w/ equal frequency;
    - aplasia / Hypoplasia may involve any portion of the ring of C1 and C2;
    - may simulate fracture on radiographs;
    - smooth sclerotic margin at ring anomaly suggest congenital deficiency;
    - fractures tend to have lucent irregular margins;
- Normal Variants of Dens:  
    - rare but may be misinterpreted as a fracture;
    - dens may be completely absent, hypoplastic, or incompletely fused to body of C2 (lesion called Os Odontoideum)
         - Os Odontoideum is smaller than normal dens and is fixed to anterior ring of C1; (2 move as a unit);
- Normal Variant of the Lower Cervical Spine:
    - fusion of the vertebral bodies or fusion of the posterior elements
    - Spina Bifida occulta (incomplete fusion of the posterior ring)
    - has smooth sclerotic margins that help differentiate from frx;
    - hypoplastic lesions in lateral masses are best appreciated w/ AP
         - look for thinning of hypoplastic portion & relative hypertrophy and sclerosis of the contralateral lateral mass

Effect of C1-C2 rotation on canal size.