Accuracy evaluation of placements of three different alternative C2 screws using the freehand technique in patients with high riding vertebral artery

An observational study. To evaluate the safeties of placing three different alternative C2 screws using the freehand technique under high riding vertebral artery (HRVA) and to analyze the C2 morphometry in patients with HRVA. A retrospective analysis of radiologic data was performed on patients that underwent C2 instrumentation from September 2004 to December 2017. Two hundred fifty-one patients were included, and 90 of these patients (35.9%) had a unilateral or bilateral HRVA. We placed three alternative C2 screws including superior pars, inferior pars, and translaminar screws. Computed tomography was used to assess cortical breeches of screw placement and obtain morphometric measurements of C2 pars and lamina, that is, superior pars height/length, inferior pars length, and laminar thickness/length. We used the modification of the all India Institute of Medical Sciences outcome to define cortical breach. In total, 117 alternative C2 screws were inserted in 90 patients; 7 superior pars screws (6%), 69 inferior pars screws (59.0%), and 41 translaminar (35%) screws. Although cortical breaches occurred during 31 screw placements (26.5%), these were unacceptable in only two cases (1.7%). No symptomatic neurovascular complication was observed after screw placement in any case. Mean height of C2 superior pars was 3.8 +/- 1.8 mm and mean thickness of C2 lamina was 5.2 +/- 1.1 mm. Mean lengths of superior pars, inferior pars, and lamina were 17.8 +/- 3.0 mm, 13.6 +/- 2.2 mm, and 26.7 +/- 3.3 mm, respectively. Superior pars height and lamina thickness < 3.5 mm that was a minimal diameter of cervical screw were 49.6% and 6.8%, alternative C2 screw was not available in these cases. Placements of alternative C2 screws using the freehand technique were achieved accurately and safely in patients with HRVA. However, preoperative morphometric evaluation is essential to determine the best option for C2 instrumentation and C2 screw length to avoid neurovascular complications.