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

Reference:

1. Swischuk L. Imaging of the cervical spine in children, 2004.