Os odontoideum is a rare anomaly of the Dens. Os odontoideum is an independent ossification center located proximal to the body of the axis in place of the dens[^1] [^2].
Etiology
There are two theories regarding the etiology of Os odontoideum[^3]:
Congenital: It is a congenital anomaly.
Acquired: It is an old fracture of the dens.
A true Os odontoideum is very rare and should be distinguished from the more common Ossiculum terminale, which represents a lack of bony fusion between the terminal end of the axis and the remainder of the dens of the axis.
Variants
Two anatomical variants of Os odontoideum are distinguished[^1] [^3]:
Orthotopic: Os odontoideum moves with the anterior arch of C1.
Dystopic: Os odontoideum is functionally fused with the Basion and may subluxate anteriorly to the C1 arch.