Odontoid process agenesis

Last revised by Joshua Yap on 4 Apr 2023

Odontoid process agenesis is rare, occurring due to a lack of failure of ossification of the odontoid process. It is seen in patients with spondyloepiphyseal and spondylometaphyseal dysplasia 1,2.

Radiographic features

Plain radiograph

On plain film, odontoid process agenesis appears as a slight depression between the superior articular facets on open mouth view, or hypogenesis of the basal segment of the odontoid with a floating apical process. Both of these conditions are associated with atlantoaxial instability.


On dynamic CT, both flexion and extension views should be performed to identify posterior subluxation.

Treatment and prognosis

Atlantoaxial instability will lead to vascular compression and stroke because of ischemia 2,4. Treatment options include surgical fusion of C1 and C2.

See also

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