Troubleshooting a Rare Anatomic Variation with Intraoperative Navigation in a Patient with Bilateral C2 Pars Fractures.

C2 pars fractures occur most commonly after traumatic hyperextension injuries. Although a significant number of cases may heal with conservative measures, some require surgical intervention. Anatomical variations of the V3 segment of the vertebral artery are not uncommon and may present an obstacle to safe instrumentation. Intraoperative CT-guided navigation is a useful tool in these cases, but the limitations of accuracy in the upper cervical spine especially in the context of unstable injuries must be understood to avoid complication. In this case we present a rare anatomic variation of the vertebral artery size and position in conjunction with bilateral C2 pars fractures treated successfully by surgical fixation. This article highlights the important technical details of the posterior instrumentation of unstable atlas pars fractures with the aid of intraoperative navigation.