The C2 Pedicle Width, Pars Length, and Laminar Thickness in Concurrent Ipsilateral Ponticulus Posticus and High-Riding Vertebral Artery: A Radiological Computed Tomography Scan-Based Study

STUDY DESIGN: Retrospective radiological study. PURPOSE: We aimed to determine the prevalence of ponticulus posticus (PP) and high-riding vertebral artery (HRVA) occurring simultaneously on the same side (PP+HRVA) and in cases of PP+HRVA, to assess C2 radio-anatomical measurements for C2 pars length, pedicle width, and laminar thickness. OVERVIEW OF LITERATURE: PP and HRVA predispose individuals to vertebral artery injuries during atlantoaxial fixation. In cases of PP+HRVA, the construct options thus become limited. METHODS: Consecutive computed tomography scans (n=210) were reviewed for PP and HRVA (defined as an internal height of <2 mm and an isthmus height of <5 mm). In scans with PP+HRVA, we measured the ipsilateral pedicle width, pars length, and laminar thickness and compared them with controls (those without PP or HRVA). RESULTS: PP was present in 14.76% and HRVA in 20% of scans. Of the 420 sides in 210 scans, PP+HRVA was present on 13 sides (seven right and six left). In scans with PP+HRVA, the C2 pars length was shorter compared with controls (13.69 mm in PP+HRVA vs. 20.65 mm in controls, p<0.001). The mean C2 pedicle width was 2.53 mm in scans with PP+HRVA vs. 5.83 mm in controls (p<0.001). The mean laminar thickness was 4.92 and 5.48 mm in scans with PP+HRVA and controls, respectively (p=0.209). CONCLUSIONS: The prevalence of PP+HRVA was approximately 3% in the present study. Our data suggest that, in such situations, C2 pedicle width and pars length create important safety limitations for a proposed screw, whereas the translaminar thickness appears safe for a proposed screw.