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The axis is the second vertebra of the cervical spine and indeed of the vertebral column as a whole. Although most of the cervical vertebrae have a similar structure, the axis is unique and differs from the rest of the cervical vertebrae. The axis articulates with the atlas, which is the first cervical vertebra, and with the third cervical vertebra. Like the atlas, the axis is specialized for bearing the weight of the head, in addition to allowing the movement of the head in all directions.
On the superior surface of the axis is a bony projection or process known as the odontoid process or dens. This process is situated anterior to the spinal cord and it functions a pivot for the rotation of the head. As the atlas sits directly above the axis, a ligament known as the transverse ligament of the atlas connects and holds together the anterior arch of the atlas and the odontoid process of the axis. The presence of this ligament between the dens and the atlas prevents the anterior displacement the atlas, while at the same time preventing the posterior displacement of the axis.
Odontoid fractures, which are fractures to the odontoid process of the axis, account for up to 15% of injuries of the cervical spine. Because the odontoid process can fracture in three different levels, there are three types of odontoid fractures that can be identified.
A type I odontoid fracture refers to a fracture that occurs only at the tip of the odontoid process. A type II odontoid fracture describes a fracture that involves the tip of the odontoid process as well as the base or the neck. Finally, a type III odontoid fracture describes a fracture that also affects the tip and neck or the odontoid process in such a way that the fracture extends into to the body of the atlas.
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