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

Abstract

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.

Figure 1

The three radiologic measurements used to define the presence of a high riding vertebral artery (HRVA) on cervical computed tomographic (CT) images. (A) A parasagittal CT image showing heights of the superior pars (black arrow) and isthmus (white arrow). (B) Axial CT image showing a narrow left C2 pedicle (black arrow). HRVA was defined as the superior pars height of ≤2 mm, and/or isthmus height of ≤5 mm, and/or pedicle width of ≤4 mm.

Figure 2

Trajectories and ideal placements of the three alternative C2 screw types on postoperative cervical CT images. (A) Translaminar screw. (B) and (C) Superior pars screw. (D) and (E) Inferior pars screw.

Figure 3

Types of cortical breaches as defined by the modification of the all India Institute of Medical Sciences outcomes classification for thoracic pedicle screws. The images of Type III breach are illustrative cases from different hospital. In the present study, Type III cortical breach did not occur during surgery. (A) and (B) Type IIa. (C) and (D) Type IIb. (E) and (F) Type IIc. (G) and (H) Type III.

Figure 4

The five C2 morphometric variables on sagittal and axial CT images. (A) Length (white) and height (yellow) of superior pars and length (red) of inferior pars. (B) Length (light blue) and thickness (green) of C2 lamina.

Figure 5

Values of the five C2 morphometric variables. (A) Mean superior pars height and laminar thicknesses (mean ± SD). (B) Mean lengths of the superior pars, inferior pars, and lamina (means ± SDs).