What is an os odontoideum?
An os odontoideum describes separation of the dens from the body of C2. It appears as a well-circumscribed ossicle with a smooth, thin cortex that is separated from the adjacent dens by a radiolucent gap. It was originally thought that this entity represented failure of fusion of the secondary ossification center with the remaining dens. Such a congenital anomaly is now known as persistent ossiculum terminale, however, and appears as a small ossicle at the tip of the dens. Most authorities now agree that os odontoideum is more likely an acquired post-traumatic abnormality. Regardless, this entity can be differentiated from an acute odontoid fracture by the presence of a sclerotic cortical margin and occasional hypertrophy of the anterior arch of axis. Os odontoideum can cause atlantoaxial instability. Some studies have shown increased frequency of this entity in patients with Down syndrome.