Ultrasonic Versus High-Speed Drilling for C2 Pedicle Screw Placement with High-Riding Vertebral Artery: A Comparative Study of Vascular Safety and Thermal Injury.

BACKGROUND AND OBJECTIVE: High-riding vertebral artery (HRVA) anatomy markedly narrows the corridor for C2 pedicle screw placement and increases the risk of vertebral artery injury. Ultrasonic bone drilling (UBD) may improve vascular safety compared with high-speed drilling (HSD), but its thermal effects and influence on early fixation remain unclear. This study compared UBD and HSD for C2 pedicle screw tract preparation in HRVA anatomy. METHODS: This integrated clinical-experimental study included a retrospective clinical cohort of 33 patients with 43 HRVA pedicles, a perfused 3D-printed C2 benchtop model with porcine arteries, and a paired rabbit model. Clinical outcomes included conversion to alternative fixation due to failure, substantial venous plexus bleeding, and vascular injury. In the benchtop model, standardized 1-, 2-, and 3-mm pedicle breaches were created to assess arterial injury risk. In rabbits, intraoperative bone temperature, acute osteonecrosis depth, bone healing by micro-CT, and screw pull-out strength at 0, 4, and 12 weeks were evaluated. RESULTS: In the clinical cohort, UBD was associated with lower rates of conversion to alternative fixation (3.8% vs. 35.3%, p = 0.022) and substantial venous bleeding (7.7% vs. 35.3%, p = 0.034); one minor vertebral artery injury occurred in the HSD group, although the low event rate limits statistical power (p = 0.395). In benchtop testing, HSD caused more frequent full-thickness arterial perforation at 2 mm and 3 mm breaches. In rabbits, UBD generated higher peak bone temperatures and deeper acute osteonecrosis, with a trend toward delayed bone healing. Pull-out strength was significantly lower in the UBD group at 4 weeks; at 12 weeks, no statistically significant difference was detected between groups. CONCLUSION: UBD was associated with improved intraoperative feasibility in this retrospective HRVA cohort and showed fewer vessel perforations in a benchtop model, but prospective validation is required. Its greater thermal injury and lower pull-out strength at 4 weeks highlight the need for optimized drilling parameters and effective irrigation.