Dens (anatomy)
Encyclopedia
The dens 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
.
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 never stable.
Type III Fracture - Extends through the vertebral body of the axis. This type can be stable or unstable and may require surgery. http://www.wheelessonline.com/ortho/dens_fracture
Axis (anatomy)
In anatomy, the second cervical vertebra of the spine is named the axis or epistropheus.It forms the pivot upon which the first cervical vertebra , which carries the head, rotates....
(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
Anterior arch of the atlas
The anterior arch of the atlas forms about one-fifth of the ring of the atlas: its anterior surface is convex, and presents at its center the anterior tubercle for the attachment of the Longus colli muscles; posteriorly it is concave, and marked by a smooth, oval or circular facet , for...
.
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
Alar ligament
The alar ligaments connect the sides of the dens to tubercles on the medial side of the occipital condyle....
; these ligaments connect the process to the occipital bone
Occipital bone
The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...
.
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
Weight bearing
In orthopedics, weight-bearing is the amount of weight a patient puts on the leg on which surgery has been performed. In general, it is described as a percentage of the body weight, because each leg of a healthy person carries the full body weight when walking, in an alternating fashion.After...
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
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
.
Fracture types
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 never stable.
Type III Fracture - Extends through the vertebral body of the axis. This type can be stable or unstable and may require surgery. http://www.wheelessonline.com/ortho/dens_fracture
External links
* Diagram at umich.edu (Odontoid process is #3) - "C2, 2nd Cervical Vertebra or Axis - Superior View (Dens is #1)" - "C2, 2nd Cervical Vertebra or Axis - Posterior View (Dens is #1)" - "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. http://www.ispub.com/ostia/index.php?xmlPrinter=true&xmlFilePath=journals/ijos/vol7n1/odontoid.xml