The AO Spine Upper Cervical Injury Classification System (AO UCIC) - An Umbrella Review of Traumatic Axis Injuries Factors that May Affect Treatment Decision.

Study design: An umbrella systematic review. Objective: To identify historically recognized injury characteristics that may affect treatment decisions of traumatic C2 injuries and help improve the description of the "modifiers" presented in the AO Upper Cervical Injury Classification (UCIC). Methods: We performed an umbrella review of systematic reviews evaluating studies about the management of axis fractures that identify potential modifiers in the treatment of these injuries. These modifiers were grouped according to the new AO UCIC. Results: Eight systematic reviews were included. They were divided into three groups: (1) Axis body fractures - one study, (2) Hangman´s fractures - one study, and (3) Odontoid fractures, six studies. For axis body fractures, most injuries were treated non-operatively, except some Benzel type 3 fractures (AO Type A) with displacement or severe comminution (M1). Hangman´s fractures classified as Effendi I and Levine-Edwards I and II were treated non-operatively with success, with no modifiers identified for non-union or instability. For Levine-Edwards type IIA and III surgery was generally recommended, but these should be classified as AO type B and C respectively without a need for modifiers. For odontoid fractures, fractures in the dens base, with displacement, or in elderly patients were associated with non-union (M1) and patients' specific factors (surgical condition) seem to affect the decision of treatment (M3) for considering surgery. Conclusions: We identified from the literature some axis injury characteristics that seem to affect the treatment decision in historical series. Knowledge of these modifiers may further enhance the system's clinical utility.