The Ponticulus Posticus as Risk Factor for Screw Insertion into the First Cervical Lateral Mass

Awareness of the osseous anomaly of ponticulus posticus (PP) is crucial in avoiding vertebral artery (VA) injuries during C1 instrumentation. The aim of this study was to investigate PP and its relationship with the VA with three-dimensional computed tomography angiography.

PP and the VA were investigated as intraoperative landmarks. The intersection of the VA to the posterior arch of C1 and the medial line and the posterior arch and VA curve around lateral masses were measured as intraoperative references.

PP was identified in 14.3% of samples. The anomaly was more common in women and on the right side. Of cases, 48.2% had PP bilaterally. In PP cases, the VA had variable courses through C2 before it passed under its bony bridges on the posterior arch of C1. Mean distances were found at the intersection of the VA to the posterior arch of C1 and the medial line to be larger and the posterior arch of C1 and the VA curve around lateral masses to be narrower than normal cases. Dual computed tomography scan data from C1 with PP were used to create three-dimensional patient-specific life-sized cervical spine models. Models revealed how the bone bridge affected the VA. The feasibility (>4 mm) of a safe lateral mass screw fixation was not influenced by PP anomalies.

The presence of the PP loop can limit space available for placement of the screw through the bony elements of C1. Standard screw techniques are contraindicated owing to the unacceptable high risk of VA injury.