November 9, 2009

Os Terminale - Os Odontoideum Complex (1)

Diagram showing a range of dens anomalies from normal, os terminale and os odontoideum (hypoplastic dens). Adapted from Reference #1.

"I have always considered them [os terminale and os odontoideum] to be the same, believing that the os terminale becomes the os odontoideum when it enlarges in association with hypoplasia of the dens." - Leonard Swischuk, MD

Development of os terminale/os odontoideum complex
  • Os terminale is derived from the 4th occipital sclerotome
  • Os terminale develops and then fuses with the dens in most cases (becoming the tip of the dens)
  • If the os terminale does not fuse with the dens, it can overgrow and become the os odontoideum while the dens becomes hypoplasia. At the same time C1-2 stabilizing ligaments will be underdeveloped and predispose this section to hypermobility and instability
Imaging Appearance
  • Both os terminale and os odontoideum typically is a single, smooth ossicle
  • Sometimes, they can show bony fragmentation, bizarre and irregular in shape
  • If found posterior to its normal location, one can presume that there is some degree of instability
  • Anterior arch of C1 can overgrow (hyperplastic); this does not suggest that there is ununited fracture of the dens
1. Swischuk L. Imaging of the cervical spine in children, 2004.

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