C1 Vertebra or Atlas

Description

Centered on the upper cervical spine, the atlas (C1) appears as a ring formed by anterior and posterior arches joined to paired lateral masses, with a large vertebral foramen occupying the center. Superiorly, the concave superior articular facets sit on the lateral masses to receive the occipital condyles, while the inferior articular facets face inferomedially toward the superior facets of the axis (C2). The transverse processes project laterally and contain the transverse foramina, and the posterior arch bears a groove for the vertebral artery as it courses medially toward the foramen magnum. C2 is included immediately inferior to C1, with the dens rising superiorly behind the anterior arch of the atlas. Atlas morphology matters because it is the bony interface for both nodding at the atlanto-occipital joint and rotation at the atlanto-axial joint, and small changes in alignment can have outsized neurologic consequences. Fracture patterns such as a Jefferson burst fracture split the anterior and posterior arches and can be assessed by lateral mass displacement relative to C2, a key teaching point when correlating with open-mouth odontoid radiographs and CT. Ligament relationships are implicit in this view: the dens is restrained against the anterior arch by the transverse ligament of the atlas, and failure here underlies atlanto-axial instability in rheumatoid arthritis and in Down syndrome screening. Educators will find this image useful for gross anatomy labs, osteology practicals, and radiology teaching files that pair 3D morphology with axial and coronal CT of the craniovertebral junction. It also suits neurosurgical and orthopedic spine publications discussing C1 lateral mass screw placement corridors and vertebral artery risk along the posterior arch groove. Anatomical accuracy verified by SciePro's Medical Advisory Board.