Rare atlantoaxial dislocation secondary to os odontoideum: A comparative study of orthotopic and dystopic variants with two representative cases.

Atlantoaxial dislocation often results in upper cervical spinal cord compression, which may lead to significant neurological impairment. Among these, os odontoideum (OO) is a rare anomaly where the odontoid is replaced by a separate ossicle. Case one describes a 36-year-old male with dystopic OO presented a 4-month history of progressive incomplete quadriparesis. Radiological evaluation confirmed the presence of atlantoaxial dislocation associated with basilar invagination and OO, demonstrating an ossified structure with functional fusion between the free odontoid segment and the basion. Case two involves a 56-year-old female with orthotopic OO manifesting as cervical pain and arm weakness. Imaging studies revealed OO with anterior atlantoaxial dislocation. Dynamic radiographs demonstrated synchronous movement of the free odontoid ossicle with the anterior arch of C1 during flexion-extension. This study compares the clinical manifestations, radiographic features, treatment approaches, and outcomes between orthotopic and dystopic OO variants in patients, providing clinically relevant insights for management decisions.