1. Os odontoideum.
2. Ossiculum terminale. 
3. Nonunion of dens fracture.
4. Nontraumatic unfused odontoid synchondrosis

• Os odontoideum:

1. Sequela of odontoid synchondral fracture prior to union at age 5-7. 
2. The disruption occurs at the cartilaginous plate between dens and the body of axis.
3. Ossicle posterior to the transverse atlantal ligament (TAL). 
4. Atlantoaxial instability is more common with os odontoideum.

• Ossiculum terminale:

1. Congenital nonunion of the odontoid body from a terminal ossicle located above the TAL. 
2. No atlantoaxial instability in most of cases.

• Nonunion of dens fracture:

1. Occurring after age 7.
2. Gap of fracture is narrow.

• Nontraumatic unfused odontoid synchondrosis:

1. Smooth, well-corticated ovoid ossicle representing os odontoideum is separated from the body of axis by a large gap which begins above the superior facets of C2.