- Derived from the 4th occipital sclerotome but does not undergo fusion with the dens
- The dens is normal in size and shape
- If it enlarges, coupled with dens hypoplasia -- it is called os odontoideum
- Usually single, smooth
- Sometimes can show bony fragmentation mimicking a comminuted fracture (but one should be aware that an extensively comminuted fracture at the tip of the dens is extremely rare or nonexistent)
Swischuk L. Imaging of the cervical spine in children, 2004.