The ponticulus posticus: implications for screw insertion into the first cervical lateral mass.

BACKGROUND The arcuate foramen is an important osseous anomaly of the first cervical vertebra (the atlas) that must be taken into consideration during placement of lateral mass screws into the atlas. METHODS The prevalence of this anomaly in our patient population was determined through a retrospective review of 464 lateral radiographs of the neck. The anatomy of the arcuate foramen was identified in a study of cadavers. RESULTS Seventy-two arcuate foramina, complete or incomplete, were identified on the 464 lateral radiographs of the neck; thus, the prevalence was 15.5% in our patient population. CONCLUSIONS Although the arcuate foramen is a common anomaly, it is often not recognized. Proper identification of this anomaly on preoperative lateral radiographs should alert the surgeon to avoid using the ponticulus posticus as a starting point for a lateral mass screw in order to not injure the vertebral artery.