Os odontoideum Orthotopic os odontoideum 2017-04-08T13:10:36Z 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 dystopic os odontoideum, the ossicle is located near the basion or is fused with the clivus.