Anderson and D'Alonzo classification of odontoid process fracture

Last revised by Andrea Molinari on 19 Feb 2024

The Anderson and D'Alonzo classification is the most commonly used classification of fractures of the odontoid process of C2 1.

Another classification system is the Roy-Camille classification, which aids more in the management of odontoid fractures.


  • type I

    • rare

    • fracture of the upper part of the odontoid peg (generally oblique)

    • above the level of the transverse band of the cruciform ligament

    • usually considered stable

  • type II

    • most common

    • transverse course fracture at the base of the odontoid

    • below the level of the transverse band of the cruciform ligament

    • unstable

    • high risk of non-union

  • type IIa

    • ​type II fractures with comminution at the odontoid base

    • Hadley 2 described this type of fracture which has a significantly increased risk of nonunion when treated non-operatively compared with classical type II fractures

    • represents 5-10% of type II fractures

  • type III

    • through the odontoid and into the lateral masses of C2

    • relatively stable if not excessively displaced

    • best prognosis for healing because of the larger surface area of the fracture

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