Odontoid process fracture with concomitant atlantoaxial instability

Approximately 30% of all type II dens fractures according to Anderson and D'Alonzo are accompanied by translational atlantoaxial instability. Whilst these comparatively rare injuries are observed as a result of high speed trauma in younger people, they also occur after low energy impact trauma in elderly patients, e.g. after falls. The sagittal reconstruction using a computed tomography (CT) scan of the dens is decisive as it accurately displays the frequent dorsal dislocation combined with atlantoaxial dislocation or subluxation. In all 30 cases of such injuries analyzed, translational atlantoaxial instability with posterior dislocation of C1-2 was present, in part with additional rotational instability. Avulsion fractures or rupture of the transverse ligament were not found in any of these cases. Anterior screw fixation of the odontoid process alone is insufficient when combined with segmental C1-2 instability. If anterior screw fixation is possible in elderly patients after repositioning of the dens fracture, the C1-2 joint and temporary retention, anterior transfixation of the C1-2 joint using the approach by Barbour and anterior screw fixation of the dens axis are recommended due to the empirically lower access morbidity. A posterior approach is recommended in young patients as reduction of the C1-2 joint and the dens fracture is achieved more easily and posterior fusion is possible.