Delayed development of os odontoideum after traumatic cervical injury: support for a vascular etiology.

A previously healthy 2-year-old girl sustained a C1-2 ligamentous injury after a motor vehicle accident and underwent successful halo immobilization, with postimmobilization images showing good cervical alignment. At the time, plain radiography, CT scanning, and MR imaging showed a normal odontoid. Four years later, however, the patient was found to have an os odontoideum, evident on plain radiography and CT imaging. At the 10-year follow-up, the os odontoideum had not been surgically repaired, and the child had mild hypermobility. This is the first documented case in the modern imaging era of delayed os odontoideum formation after definitive CT scanning showed no fracture. As such, this suggests that os odontoideum may result from traumatic vascular interruption in the developing spine, with resulting osseous remodeling leading to an os odontoideum. This case argues against the congenital etiology of os odontoideum, as well as the strict posttraumatic theory whereby a trauma-induced odontoid fracture leads to osseous remodeling and subsequent development of an os odontoideum.