Odontoid Fracture Nonunion with Atlantoaxial Instability, Myelopathy, High-Riding Vertebral Artery, and Narrow Pars Treated with C1-C2-C3 Posterior-Only Fixation: First Case Report.

Nonunion is a known complication of odontoid fractures. However, coexisting instability and neurological involvement render odontoid fixation alone insufficient for effective management. This scenario necessitates atlantoaxial fusion, which can be achieved through anterior, posterior, or combined approaches. We report a case of a 51-year-old female with posttraumatic nonunion of type II odontoid fracture, atlantoaxial instability, high-riding vertebral artery, and narrow pars, presenting with spastic quadriparesis. She underwent C1-C3 posterior cervical fixation and fusion. The patient demonstrated significant neurological improvement at 1-year follow-up. This case highlights the importance of ruling out cervical myelopathy in such patients. There are no prior reports in the literature describing odontoid nonunion leading to atlantoaxial instability and cervical myelopathy in the presence of a high-riding vertebral artery. The case also underscores the ongoing dilemma regarding the ideal surgical approach in such complex anatomical scenarios at the C1-C2 junction, known for its numerous variations.