The "challenging" fractures of the odontoid process: a review of the classification schemes.

Fractures of the odontoid process seem to have an unclear and not well-understood behavior. This is well demonstrated in the literature as there is a disagreement in many fields, including the patterns of fractures, the percentage of pseudarthrosis or the appropriate treatment that is required. This situation makes the fracture itself a "challenging" one. Indeed, despite the existence of extensive literature, there are still many unresolved problems concerning these fractures. We recognize six main classification schemes of the odontoid process fractures which clearly demonstrate the presence of differences but, also, the disadvantages that these classifications are related to. The most important factors that render these fractures "enigmatic" are the anatomy of the odontoid, the diversity of fractures encountered, the radiological evaluation, the stability and the inability to incorporate the whole spectrum of fractures in one classification. The progress made the last decades in the field of the radiology and their follow-up permitted to better analyze these lesions and to extract useful conclusions.