In odontoid fractures, which type has the highest nonunion risk?

• A. Type I apex fracture
• B. Type II base of dens
• C. Type III through the odontoid into the body
• D. Type IV (unclassifiable)

Answer: (B)

Healing potential in odontoid fractures depends on where the break occurs, because blood supply and joint motion affect how well the bone can unite. The fracture through the base of the dens disrupts a zone with limited vascularity and sits right at the C1–C2 junction where there is a lot of motion, so it tends to heal poorly. That combination—poor blood supply at the fracture site plus high mechanical stress—drives the highest risk of nonunion.

In contrast, a fracture at the tip of the dens (apex) is usually stable and has better healing with immobilization, and a fracture that extends into the body of the axis (through the odontoid into the vertebral body) involves bone with better regenerative potential and tends to unite more reliably. The unclassifiable type is not part of the standard patterns and carries variable prognosis, but it isn’t the pattern most associated with nonunion.