Typical cervical vertebrae are characterized mainly by their small size and the presence of a foramen in each transverse process. Spinal processes are typically short and bifid.
The atlas (C1), axis (C2), and vertebra prominens (C7) are considered atypical cervical vertebrae because of their distinguishing features.
The first cervical vertebra is also known as the atlas. It articulates with the skull and with the adjacent vertebra - the axis. The major distinguishing feature of the atlas is that it does not have a vertebral body.
The atlas is ring shaped when viewed from above and has two lateral masses interconnected by an anterior arch and a posterior arch. Each lateral mass articulates with an occipital condyle above (atlanto-occipital joint) and with a superior articular process of the axis below (atlanto-axial joints). The posterior surface of the anterior arch has an impression for the dens, which is a projection from the vertebral body of the axis. The transverse processes of the atlas are large and project further laterally than those of other cervical vertebrae and they act as levers for muscle action.
The axis is the second cervical vertebra and it is characterized by a large tooth-like projection called the dens, which extends superiorly from the vertebral body. The anterior surface of the dens articulates with the anterior arch of the atlas, while the two superolateral surfaces have impressions that serve as attachment sites for alar ligaments.
Vertebra prominens (C7)
The vertebra prominens is the name of the seventh cervical vertebra. It similar to vertebrae C3-C6 but has some distinct features and thus is considered an atypical vertebrae. Its main distinguishing feature is its spinous process, which is longer than those of all cervical vertebrae, is not bifid and is easily palpable.