DIAGNOSIS: Standards There is insufficient evidence to support diagnostic standards. Guidelines There is insufficient evidence to support diagnostic guidelines. Options Plain x-rays of the cervical spine (anteroposterior, open-mouth odontoid, and lateral) and plain dynamic lateral x-rays performed in flexion and extension are recommended. Tomography (computed or plain) and/or magnetic resonance imaging of the craniocervical junction may be considered. MANAGEMENT: Standards There is insufficient evidence to support treatment standards. Guidelines There is insufficient evidence to support treatment guidelines. Options Patients with os odontoideum, either with or without C1-C2 instability, who have neither symptoms nor neurological signs may be managed with clinical and radiographic surveillance.Patients with os odontoideum, particularly with neurological symptoms and/or signs, and C1-C2 instability may be managed with posterior C1-C2 internal fixation and fusion.Postoperative halo immobilization as an adjunct to posterior internal fixation and fusion is recommended unless successful C1-C2 transarticular screw fixation and fusion can be accomplished.Occipitocervical fusion with or without C1 laminectomy may be considered in patients with os odontoideum who have irreducible cervicomedullary compression and/or evidence of associated occipitoatlantal instability.Transoral decompression may be considered in patients with os odontoideum who have irreducible ventral cervicomedullary compression.