Which condition describes separation of the odontoid process leading to atlantoaxial instability?

• A. Os odontoideum
• B. Odontoid agenesis
• C. Basilar invagination
• D. Retroflexed odontoid

Answer: (A)

The key idea here is that instability at the C1–C2 junction arises when the dens (odontoid process) is no longer firmly attached to the body of the axis. The odontoid acts as the pivot that allows the atlas (C1) to rotate freely around the axis (C2). When the odontoid becomes detached from C2, you lose that stable peg, and the atlas can translate or rotate excessively on the axis, producing atlantoaxial instability. This detached odontoid fragment with a smooth, well-corticated edge is characteristic of os odontoideum, which can be congenital or post-traumatic in origin.

Odontoid agenesis would mean there is no dens at all, not a separated piece, so it doesn’t describe a separation causing instability. Basilar invagination involves upward displacement of the upper cervical spine into the skull base rather than a detached dens. Retroflexed odontoid refers to the angulation of a normally attached dens, which can cause compression but does not involve separation from the axis.