Description of a New Odontoid Fracture Variant: Proposal for a Modification of the Anderson and D`Alonzo Odontoid Fracture Classification

Introduction: Odontoid fractures are common cervical spine injuries. The classification system described by Anderson and D`Alonzo is the most frequently used to describe odontoid fractures. Type 3 odontoid fractures are usually amenable to conservative management and type 2 fracture management is controversial. We have observed an odontoid fracture variant consisting of an odontoid fracture and a concurrent fracture that separates the pars-interarticularis from the rest of the C2 body. The purpose of this paper is to describe this odontoid fracture variant and designate it as a Type 4 odontoid fracture.

Methods: A more precise distinction between odontoid fractures with and without a fracture that separates the pars-interarticularis from the C2 body is proposed. Criteria that define this fracture variant include:

1) Coronal fracture line separating pars-interarticularis from rest of the C2 body

2) Any type of odontoid fracture

3) Unilateral vs. Bilateral pars involvement

A Cohen kappa test with a power analysis was performed to evaluate inter-observer reliability among 2 spine surgeons for the proposed classification system modification.

Results: 16 odontoid fracture variants were reviewed, of which 13 were acutely treated operatively. Of the three fractures treated conservatively, two cases failed and displaced and one failed halo management and was treated operatively. Kappa test for inter observer reliability was found to be 0.686, indicating "good" agreement.

Conclusion: Odontoid fractures in which one or both pars-interarticularis are separated from the vertebral body are clinically distinct fracture patterns, which differ from the other odontoid fracture types in the Anderson and D`Alonzo classification. In one study, where they were described as "complex C2 fractures", they compromised 19% of all C2 fractures. We feel that this odontoid fracture variant indicates a greater degree of instability and may benefit from surgical intervention. Identification of this fracture variant has been reproduced among different surgeons. In light of its frequency and its influence on clinical decision making we believe that it should be designated as a Type 4 odontoid fracture.