Odontoid Process — What is it and What Happens If You Injure It

Cervical spine, including the atlanto axial joint at the top of the neck.
Cervical spine, including the atlanto axial joint at the top of the neck. MediaForMedical/UIG/Universal Images Group/Getty Images

The Odontoid Process, aka, the Dens

The odontoid process, also known as the dens, is an upward projectile of bone that arises from the front part of the center of the axis vertebra. (The axis is the 2nd highest spinal bone.)

The atlas is the first bone of your neck; it sits on top of the axis. (The skull sits on top of the atlas.) The dens projects into a central space in the atlas, and this is how the bones of the atlantoaxial (atlanto relates to atlas, and axial relates to the axis bone) joint fit together.

Unlike most other spinal vertebrae, the atlas does not have a vertebral body. Instead, iis shaped like a ring that (as mentioned above,) is hollow in the center, and through which the odontoid process passes. This arrangement allows for a lot of freedom of motion between the combination of head, first vertebra (atlas), and 2nd vertebra (axis). In fact, the atlantoaxial joint is the most mobile (and the most complex) joint in the spine.

The odontoid process provides a pivot point — called an axis of motion — around which the skull and the first cervical vertebra (the atlas) rotate, twist and/or turn (these are really all the same thing.) So when it comes to turning your head so you can see behind you, or parallel park, you can, to a great extent, thank your dens for that. Muscles and the other joints in the neck play a role as well, but this related action between skull, first and second vertebra provide the underlying mechanics for the rotation to happen.

Injuries to the Odontoid Process: Warning — These are Serious Injuries

A number of injuries can affect the dens, including trauma that pushes the top of the skull down, which may in result in the dens being pushed into the brain stem. This will likely cause death.

Stress or injury to ligaments that hold the movement of the atlantoaxial joint in check can destabilize the dens, allowing it to disrupt the cervical spine.

This may cause paralysis.

Dislocation between the atlas and the axis. This is a very serious, rare injury that at the least will damage your spinal cord. Dislocations are a hyperflexion injury.

Other types of ligament injuries include stress or stretch that may cause excessive motion in the atlantoaxial joint.

And the dens can be fractured, either at the point at the top, at its base on the axis or on the body of the axis. Fractures are thought to be a shearing injury; sometimes they accompany a dislocation and other times not. When a dislocation is accompanied by a fracture, the odds of your spinal cord remaining intact are better than in the case of a dislocation between the atlas and axis only.

According to a 2005 study published in the journal Injury, odontoid fractures account for approximately 20% of all cervical fractures. Other types of neck fractures include, but are not limited to:clay shoveler's fracture and simple wedge fracture.

Authors of 2013 study published in the journal, Clinics, estimate the incidence of this injury to be a little lower, between 5% and 15%. 

As mentioned above, dens fractures are categorized by their height, which is a factor which may predict the prognosis as well as determine the treatment for the injury.

  • Type I: A fracture of the upper part of the process (the tip.) 
  • Type II: A Fracture at the base of the dens.
  • Type III: A fracture occurring in body of the axis underlying the dens.

Treatment may take the form of surgery or wearing a brace. It is, of course, best to consult with your spine specialist when deciding what to do about a dens injury, as this is a complicated injury in a very delicate area.

Living with a Dens Injury

Injury to the dens can  very significantly alter your life. Yu may have to adjust to life in a wheelchair overnight, for example. If you'd like to learn more about how people cope with this injury in their day to day existence, (and have a good cry while you're at it) read the book The Body Broken by Lynne Greenberg.

 

Source:

IIshii T, Mukai Y, Hosono N, Sakaura H, Fujii R, Nakajima Y, Tamura S, Sugamoto K, Yoshikawa H. Kinematics of the subaxial cervical spine in rotation in vivo three-dimensional analysis. Spine (Phila Pa 1976). 2004 Dec 15;29(24):2826-31.

Marcon, et. al., Fractures of the cervical spine. Clinics. November 2013. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812556/

Ochoa, et. al. Surgical management of odontoid fractures. Injury. July 2005. https://www.ncbi.nlm.nih.gov/pubmed/15993118

Continue Reading