Cervical Pedicle Inlet Screws: A Novel Trajectory for Navigated Sub-axial Cervical Screw Placement With Improved Biomechanical Characteristics Compared to Lateral Mass Screws

Study Design: Cadaveric Biomechanical. Objective: To investigate the biomechanical characteristics of a new screw trajectory, which we have deemed the “pedicle inlet” trajectory. Methods: Four surgeons familiar with standard and image-guided cervical pedicle screw fixation techniques placed 3.5 mm diameter cervical fixation screws randomized at each sub-axial level to one of three fixation techniques: cervical pedicle, pedicle inlet, and lateral mass screw. A total of 180 screws were placed from C3 to C7 in 18 cadaveric spine samples. Maximal insertional torque was measured during the final seating of the screws. After confirmation of accuracy of screws with post-procedural CT-imaging, individual screws in the cadaveric samples were biomechanically tested for pull-out strength. Results: All screws were placed without breach into neurovascular spaces. The final insertional torque was equivalent (ANOVA, P > .05, 3.6 ± 1.7 Nm) across cervical pedicle screws, pedicle inlet, and lateral mass screws. Maximal pullout strength was observed for cervical pedicle screws (814.6 ± 387.3 N). Pedicle inlet screws had a 51% higher pullout strength as compared to lateral mass screws (593.2 ± 289.9 N v 392.4 ± 284.0 N, P < .01). All differences were statistically distinct from each other (ANOVA, P < .01). Conclusions: Use of image-guidance allows for safe placement along the pedicle inlet trajectory, with no recorded screw malposition, and a 51% improvement in pull out strength as compared to lateral mass screws. The pedicle inlet trajectory offers an alternative to traditional lateral mass screws with better fixation quality and may have particular application in percutaneous or minimally invasive posterior fusions.