Atlas (C1 vertebra)

What it is

The atlas is the first cervical vertebra (C1), the topmost vertebra that supports the skull. It allows the nodding motion of the head and is involved in the alignment and stability of the craniocervical junction. Part of it (the posterior arch) is often removed during Chiari decompression surgery.

Understanding Atlas (C1 vertebra)

The atlas is a ring-shaped vertebra that sits directly beneath the skull and articulates with it, allowing the head to nod. Because the upper spinal cord and the descending cerebellar tonsils pass through this region, surgeons performing a posterior fossa decompression often remove the posterior (back) arch of the atlas (a C1 laminectomy) to provide additional room and relieve crowding at the craniocervical junction. The atlas is also relevant to craniocervical conditions: its alignment with the skull and the axis (C2) matters for stability, and abnormalities (such as atlanto-occipital assimilation, where it is fused to the skull) can be associated with Chiari. Understanding the atlas clarifies a vertebra involved in both Chiari surgery and craniocervical alignment.

Why it matters

Understanding the atlas helps families see which vertebra is partly removed during Chiari surgery to create room, and how it is involved in the alignment and stability of the craniocervical junction.