Simultaneous Congenital Fusion Defect of Anterior and Posterior Arch of Atlas (C1): Case Report.

The atlas (C1), the most important cervical vertebra, occupies a crucial anatomical and functional position. It is distinguished by its unique structure, composed of anterior and posterior arches and two lateral masses, each with a transverse foramen for the passage of the vertebral artery at its segmental transition. Congenital anomalies of the posterior arch of the atlas are rare (0.69–4%) and can range from clefts to hypoplasia or aplasia. The combination of simultaneous anterior and posterior arch defects is rarely described in the literature. This case exemplifies the need to differentiate congenital malformations of the atlas from acquired pathologies, as well as to evaluate for dynamic compromise of the craniocervical junction and rule out spinal cord and/or upper cervical nerve root involvement.