Os odontoideum is considered as an anatomic variant of the odontoid process ofC2 and must be put in differential diagnosis with the type 2 odontoid fracture and from the persistent ossiculum terminale.
Coronal and sagittal reformat CT of the upper cervical spine shows Os odontoideum and backward dislodgment of the C2 vertebral body [Figure 1a-c].
Keywords: Craniocervical junction anomalies, os odontoideum, myelopathy
Os odontoideum is a rare craniocervical junction anomaly with a smooth cortical edge, located behind the anterior arc of the atlas and separated from the main body of the odontoid process (1).
The exam revealed a smooth and well-corticated bone ossicle measuring 14 mm and located superiorly to the odontoid process corresponding to an os odontoideum (Figures 1(a)-1(c)).
The main diagnosis was a noninflammatory ROP developed on atlantoaxial instability, secondary to an os odontoideum. The main differential diagnosis was pseudoarthrosis of an old fracture of the dens of axis.
 Failure of fusion of XY complex with Z at the dento synchondrosis or maintenance of transitory intervertebral disc at this point, produces an os odontoideum
thought to be induced by excessive movement at the time of ossification of dens.
Os odontoideum is the most common anomaly of the odontoid process.
The radiographic report, read by a chiropractic radiologist, gave the following information "Dystopic os odontoideum with dynamic stenosis at C1-C2 and lateral hypermobility with secondary degenerative joint disease of the left lateral atlanto-axial joint.
; duzgun dairesel yuzeyli, aksis cismi ile kemiksel devamliligi olmayan ossikulumun normal odontoid prosesin yerini aldigi servikal vertebra anomalisidir.
The original instability resulted from rheumatoid arthritis in nine, os odontoideum
in five, Type II nonunions in eight, unstable bursts in seven, transverse ligament rupture in six, Type III ondontoid fractures in five.
KEY WORDS: Os odontoideum
, atlantoaxial instability, manipulation.