Dystopic Os Odontoideum With Chronic Posterior Atlantoaxial Subluxation: A Case Report.

Os odontoideum is considered a rare cervical spine anomaly. Depending on its type, it may or may not result in atlantoaxial instability, which can lead to anterior subluxation (the most common type) or posterior subluxation, with the latter being extremely rare. Posterior atlantoaxial subluxation is particularly dangerous, as it can progressively lead to myelopathy through various mechanisms and may ultimately result in quadriplegia or even death.