Modified classification system of high-riding vertebral artery for the C2 screw placement strategy: a large-scale, cross-sectional study.

High-riding vertebral artery (HRVA) increases vertebral artery injury risk during C2 instrumentation. We conducted a cross-sectional study of cervical computed tomography angiography (CTA) in southern Chinese adults (18-60 years; four centers, 2019-2025) to validate a modified HRVA classification. Measurements included C2 isthmus height (C2IsH) and C2 internal height (C2InH) on sagittal reconstructions 3 mm lateral to the spinal canal border, and C2 pedicle width (C2PW) and C2 lamina width (C2LW) in axial images. The modified classification includes two previously unrecognized variants: Type 4 (isthmio-absent; C2IsH = 0 mm) and Type 5 (internal-absent; C2InH = 0 mm). Among 1,804 participants, yielding 3,608 potential screw insertion sites (PSIS), HRVA prevalence was 29.60%. Mean C2IsH and C2InH were 7.00 and 6.02 mm, significantly lower than those for Caucasians (7.73 and 7.06 mm; P < 0.01), and HRVA prevalence also differed (P < 0.05). Types 4 and 5 comprised 0.14% and 0.08% of PSIS. Narrow pedicles and laminae were more frequent in HRVA than non-HRVA (both P < 0.01). Intraobserver and interobserver agreements were satisfactory. The modified classification demonstrates high reliability and clinical utility, supporting individualized preoperative assessments and tailored C2 screw placement strategies to reduce vertebral artery injury risk.Research registration: This study has been registered with the Chinese Clinical Trial Registry (ChiCTR2500096630, https://www.chictr.org.cn/ ).