Pedicle Screws Can be 4 Times Stronger Than Lateral Mass Screws for Insertion in the Midcervical Spine A Biomechanical Study on Strength of Fixation

Study Design: A biomechanical study. Objective: To compare, in cervical vertebrae (C3-C6), the pullout strengths of pedicle screws and lateral mass screws after both types of screw had been subjected to a period of cyclic loading in 2 planes. Summary of Background Data: In posterior subaxial cervical fixation systems, screws are usually inserted into the lateral mass. As an alternative to lateral mass fixation, pedicle screw fixation became popular in the 1990s and was first used for lower cervical spine trauma cases. However, it is controversial as to whether lateral mass screw fixation in the upper-middle cervical spine offers as much biomechanical security as compared with pedicle screw fixation. Methods: For each of the 32 vertebrae, 1 side was randomly chosen to receive a pedicle screw and the other side a lateral mass screw. The pedicle or lateral mass screws inserted into the first 16 vertebrae were cyclically loaded to simulate torsion and the remaining 16 vertebrae were cyclically loaded to simulate flexion/extension of the spine. At the end of the cyclic loading each screw was pulled out along its long axis. Results: For the torsion group, the mean pullout strength of the pedicle screws was nearly 4 times greater than the mean pullout strength of the lateral mass screws (cf 762 N with 191 N). In contrast, the mean pullout strength of the pedicle screws in the flexion/extension group was only twice the mean pullout strength of the lateral mass screws (cf 571 N with 289 N). Conclusions: Not forgetting the potential risks of inserting pedicle screws in cervical vertebrae, pedicle screws are a better biomechanical choice than lateral mass screws for cervical fixation at the levels C3 through to C6.