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A Computed Tomographic Study to Document Anatomical Variations of First Cervical Vertebra: Prevalence, Classification and Possible Clinical Implications

‐ First cervical vertebra has different anatomical features as compared to other cervical vertebrae. It holds globe of skull and lacks body and spine. It is composed of 2 lateral masses linked by anterior and posterior arches. Posterior atlanto‐occipital membrane is attached to posterior arch. Lateral edge of this membrane sometimes ossifies thus converting groove into canal. Consequently, neurovascular groove gets converted into a bony ring “ponticulus posticus”. It is considered as potential cause of neck pain and headache. Knowledge of this variation is important during procedures like instrumentation of atlas including C1 lateral mass screw placement which requires mobilization of C2 nerve root and screw insertion inferior to bony arch of atlas directly into lateral mass. In patients with ponticulus posticus, if not identified pre‐operatively may give impression of wide bony arch for screw placement, which may result in vertebral artery injury due to a breach of this variation.