Os odontoideum (OO) is an anatomical anomaly in which odontoid process separates from the body of C

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and appears as an ossicle with smooth circumferential cortical margins. OO may be incidental finding in asymptomatic patients. OO may also lead to instability of the atlanto-axial joint and can present with a wide range of symptoms and signs. The axis vertebra has a unique developmental organization. The final axis shape is formed by the sequential fusion of the apical cap of the dens

the body of dens and the axis body. The etiology of OO remains controversial in the literature. Some authors considered it was from congenital fusion failure between the dens and the body of axis. Some authors supported the acquired hypothesis that OO resulted from an unrecognized fracture and subsequently avascular necrosis and osseous remodeling. The etiology for OO may be multifactorial which may include congenital and traumatic causes.