Os odontoideum (OO) is a rare anomaly of the second cervical vertebrae characterised by a separation of a portion of the odontoid process (also called the dens) from the body of the axis. The little bone is marked by a small oval corticated ossicle of which the size can range. The OO can be situated in the position of the odontoid process (orthotropic) or in the vicinity of the base of the occipital bone, more precisely in the area of the foramen magnum (dystopic) with no bony link to the body of the axis.
The classification of the odontoid fractures is known as the Anderson/Alonzo classification: