Congenital Absence of Posterior Elements of C2 Vertebra with Atlanto-Axial Dislocation and Basilar Invagination: A Case Report and Review of Literature.

Developmental anomalies of the axis are commonly encountered, especially anomalies involving the odontoid process. Anomalies of the posterior elements are uncommon. We describe a unique case of agenesis of posterior elements of C2 with basilar invagination and atlanto-axial dislocation. An obese 8-year-old boy presented with symptoms of cervical myelopathy. Radiological workup revealed a craniovertebral junction anomaly with occipitalised atlas, absent posterior elements of axis, and hypertrophied C3 spinous process. Atlanto-axial instability and basilar invagination was present. Magnetic resonance angiography revealed hypoplastic left vertebral artery. Traction with cervical tongs failed to improve the alignment and symptoms. Anterior trans-oral release, followed by posterior decompression and custom-made instrumentation, was done. The patient recovered completely and was asymptomatic at the end of two years. X-ray and computed tomography scan demonstrated reduction of basilar invagination and maintenance of alignment. This is the first case to be reported of agenesis of posterior elements of axis associated with basilar invagination. One should look for this condition in patients with hypertrophied spinous process of C3. Utilization of hypoplastic pedicle of axis serves as an additional fixation point to increase the stability of the construct.