|Second cervical vertebra, or epistropheus, from above. (Dens labeled at center top.)|
|Second cervical vertebra, epistropheus, or axis, from the side. (Odontoid process labeled at center top.)|
|Gray's||subject #21 99|
The dens (also odontoid process or odontoid peg) is a protuberance (process or projection) of the axis (second cervical vertebra). It exhibits a slight constriction or neck, where it joins the main body of the vertebra. The condition, where the dens is separated from the body of the axis, is called os odontoideum, and may cause nerve and circulation compression syndrome.
On its anterior surface is an oval or nearly circular facet for articulation with that on the anterior arch of the atlas.
On the back of the neck, and frequently extending on to its lateral surfaces, is a shallow groove for the transverse atlantal ligament which retains the process in position.
The apex is pointed, and gives attachment to the apical odontoid ligament; below the apex the process is somewhat enlarged, and presents on either side a rough impression for the attachment of the alar ligament; these ligaments connect the process to the occipital bone.
The internal structure of the odontoid process is more compact than that of the body.
The odontoid peg is the ascension of the atlas fused to the ascension of the axis. The peg has an articular facet at its front and forms part of a joint with the anterior arch of the atlas. It is a non-weight bearing joint. The alar ligaments, together with the apical ligaments, are attached from the sloping upper edge of the odontoid peg to the margins of the foramen magnum. The inner ligaments limit rotation of the head and are very strong. The weak apical ligament lies in front of the upper longitudinal bone of the cruciform ligament, and joins the apex of the deltoid peg to the anterior margin of the foramen magnum. It is the fibrous remnant of the notochord.
Contact sports are contraindicated for individuals with an anomalous dens, as any violent impact may result in a catastrophic injury. This is because a malformed odontoid process may lead to instability between the atlas and axis (the C1 and C2 cervical vertebrae).
Fractures of the dens, not to be confused with Hangman's fractures, are classified into three categories according to the Anderson / D'Alonso system:
- Type I Fracture - Extends through the tip of the dens. This type is usually stable.
- Type II Fracture - Extends through the base of the dens. It is the most commonly encountered fracture for this region of the axis. This type is unstable and has a high rate of non-union.
- Type III Fracture - Extends through the vertebral body of the axis. This type can be stable or unstable and may require surgery.
- Odontoid process at the US National Library of Medicine Medical Subject Headings (MeSH)* Diagram at umich.edu (Odontoid process is #3)
- Atlas of anatomy at UMich back_bone11 - "C2, 2nd Cervical Vertebra or Axis - Superior View (Dens is #1)"
- Atlas of anatomy at UMich back_bone12 - "C2, 2nd Cervical Vertebra or Axis - Posterior View (Dens is #1)"
- SUNY Labs 31:st-0204 - "Pharynx - Bones"
- Walid MS et al.: A Variation Of Type III Odontoid Fracture Presenting As Isolated Jaw Pain. The Internet Journal of Orthopedic Surgery. 2007. Volume 7 Number 1.