American Board of Surgery Qualifying Exam (ABS QE) Practice Test

Which type of C-2 odontoid fracture is unstable?

1. Type 1

2. Type 2

3. Type 3

4. Type 4

The correct answer is: Type 2

The Type 2 odontoid fracture is recognized as unstable because it typically occurs at the base of the odontoid process (the peg of the C-2 vertebra) and entails a separation of the bony fragment from the rest of the vertebra. This type of fracture often results in instability due to its anatomical location and the nature of the fracture itself. In particular, Type 2 fractures are at high risk for nonunion, primarily owing to the limited blood supply to that area, which can impede healing and contribute to persistent instability. In contrast, Type 1 fractures occur at the tip of the odontoid and generally do not involve the axis body, thus maintaining stability. Type 3 fractures extend into the body of the axis but usually retain stable alignment. Type 4 would imply further classifications beyond typical discussions of odontoid fractures and is not typically addressed in standard teaching for the different types. Understanding the stability associated with these fractures is critical for surgical decision-making and management, as unstable fractures may require surgical intervention to prevent neurologic injury and to promote healing.