MedGen

The odontoid process, or dens, is a bony projection from the axis (C2) upward into the ring of the atlas (C1) at the top of the spine. During embryogenesis, the body of the odontoid derives from the centrum of the atlas and separates from the atlas, fusing with the superior portion of the axis. If the odontoid is hypoplastic or absent, the attachments for the apical and alar ligaments are missing, allowing for excessive rotation of the atlas, craniocervical instability, and possibly cord compression (summary by Stevens et al., 2009). [from OMIM]

Os odontoideum is classified into two anatomic types (orthotopic and dystopic). Os odontoideum is defined as an ossicle that consists of smooth and separate caudal portions of the odontoid process. With orthotopic os odontoideum, the ossicle moves with the anterior arch of the atlas, while the dystopic type consists of an ossicle near the basion, or one that is fused with the clivus [from HPO]