Instability of the os odontoideum is demonstrated within the range of motion. On the extension view the anterior arch of C1 is positioned directly above the body of C2 and the os odontoideum is positioned just behind the posterior vertebral line. The clivus remains aligned with the os odontoideum indicating that the occiput, C1 and os odontoideum move together. On the flexed view the os odontoideum is positioned superior to the anterior one half of the C2 vertebral body and the anterior arch of C1 is positioned anterior to the anterior vertebral body.
No other abnormality is seen.