Biomechanical evaluation of In-Out-In pedicle screws for atlantoaxial posterior fixation in the presence of high-riding vertebral artery and narrow C2 pedicles: a finite element analysis.

Abstract

In patients with high-riding vertebral artery (HRVA) and narrow pedicles, placement of C2 pedicle screws carries a significant risk of vertebral artery injury. The in-out-in pedicle screws (IOIPS) technique offers an alternative posterior fixation strategy. However, the biomechanical performance remains unclear. A finite element model of the upper cervical spine (C0-C2) was developed to simulate HRVA and narrow pedicles. All posterior fixation constructs utilized C1 pedicle screws, while four different techniques were applied to C2: IOIPS, pedicle screws, pars screws, and translaminar screws. A vertical load of 50 N and a torque of 1.5 Nm were applied at C0 to assess and compare the range of motion (ROM) and stress distribution. All four fixation techniques effectively reduced ROMs at the atlantoaxial, with IOIPS demonstrating the lowest ROMs and translaminar screws showing the highest. Pedicle and pars screws exhibited higher von Mises stresses within the implants, while IOIPS achieved a more uniform stress distribution. Translaminar screws had the lowest implant stresses but provided the least stability. For patients with HRVA and narrow C2 pedicles, IOIPS offers superior stability along with relatively low and evenly distributed implant stress, suggesting it is a biomechanically viable fixation technique. However, its application should be reserved for carefully selected cases rather than adopted as a routine surgical strategy.