Table 1. Summary of current and previous odontoid fracture classification systems with their drawbacks.

System	Based on	Classification	Drawback
Schatzker’s (1971) et al. [6]	Fracture position and location in relation to the attachment of the accessory ligaments	Low fracture either at the level or below the level of the attachment of the accessory ligaments	1. Diversity of terminologies surrounding the location of the fracture       2. Some lack of internal fracture morphology and type of fracture         3. Discrepancies regarding fracture stability, comminution and displacement which ultimately affect management recommendation         4. Often no distinct anatomic borders defined between fracture types
		High fracture on one or both sides, above the attachment of the accessory ligaments
Anderson and D’Alonzo’s (1974) et al. [8]	Fracture position and location	Type I oblique fracture at the tip of the odontoid process
		Type II at the base of the dens and the junction with the body
		Type III at the junction of the dens with the anterior portion of the axial body extending into the body and lateral masses of C2
Althoff’s (1979) et al. [7]	Fracture position and location	Type A above the neck
		Type B through the neck including superior C2 body
		Type C includes the medial part of one C2 superior articulating process
		Type D includes fractures through the body of C2, including the medial part of both superior articulating processes
Hadley’s (1988) et al. [10]	Subclassification of Anderson and D’Alonzo’s	Type I and type III the same as Anderson and D’Alonzo’s
		Type IIA additional bone fragment at the dens fracture site
Grauer’s (2005) et al. [9]	Subclassification of Anderson and D’Alonzo’s	Type I and type III same as Anderson and D’Alonzo’s
		Type IIA not displaced from their site of origin
		Type IIB displaced and/or extend transversely from anterior superior to posterior inferior
		Type IIC comminuted and/or extend from anterior inferior to posterior superior
Roy-Camille’s (1979) et al. [11]	The direction of the fracture line and the amount of displacement	Type I obliquely slanted from posterior to anterior displacing the odontoid process anteriorly	1. Fails to describe fracture location     2. Did not address the relationship of fractures involving the body of C2
		Type II oblique angle slants anterior to posterior displacing the odontoid process posteriorly
		Type II horizontal fracture with no angulation displacing the odontoid process anteriorly or posteriorly
Current system	Embryological development and anatomy of the odontoid process	Type I involving the upper ¼ of the dens (zone I)	1. Only fractures involving and related to the anatomical odontoid process
		IA non displaced
		IB displaced and/or angulated odontoid process anteriorly or posteriorly
		Type II involving the lower ¾ of the dens (zone II)
		IIA non displaced
		IIB displaced and/or angulated odontoid process anteriorly or posteriorly