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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.
Level of Evidence: Diagnostic Level IV. See Instructions to Authors for a complete description of levels of evidence.
Placement of lateral mass screws into the atlas (the first cervical vertebra) has become a popular means of treating atlantoaxial instability1. However, it can sometimes be a difficult procedure, as the region contains venous plexuses as well as the greater occipital nerve. To avoid these difficulties, some surgeons have recommended that, in the presence of a broad posterior arch of the atlas, the insertion of the screw be started in the dorsal aspect of the posterior arch instead of at the base of the lateral mass. We recently encountered an anatomical specimen containing an anomaly of the posterior arch of the atlas containing the vertebral artery. In this specimen, the sulcus or groove for the vertebral artery on the dorsal aspect of the atlas was completely covered by an anomalous ossification, termed the ponticulus postions (Latin for little posterior bridge)2. The resulting foramen contains the vertebral artery and is referred to as the arcuate foramen2. Many terms have been used in the literature to describe this anomaly, including Kimmerle anomaly3, foramen sagittale, and a retrocondylar vertebral artery. The purpose of this study was to determine the frequency of the anomaly and to identify its anatomical features in relation to surgical approaches to the posterior aspect of the atlas.
Materials and Methods
Radiographic Analysis