Os odontoideum Dystopic os odontoideum UMLS:C3552843 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