Introduction: In cases where there is a high riding or medialized vertebral artery, use of a longer screw could potentially place the vertebral artery at increased risk.
Methods: In vitro testing was performed using 15 cadaveric specimens (C0-C3). C1-C2 fixation using the standard Goel-Harms construct was used. All C1 screws were bicortical, 3.5 mm in diameter, and a length sufficient to have bicortical fixation in the lateral mass of C1. Specimens were randomized into three groups of 5 specimens each (Figure 1): (1) Bilateral long C2 pedicle screws (26-32 mm), (2) Bilateral short C2 pars screws (14mm in length), (3) Unilateral long and a contralateral short C2 pars screw. The propensity for screw loosening was simulated via repeated physiological loading on Spine Simulator (MTS Systems Inc, Eden Prarie, MN). Multidirectional flexibility before- and after-cyclic loading along with micro-motion of C2 screws was recorded. Screw pull-out testing using a custom-designed-fixture was performed to characterize the residual strength of each screw type following cyclic loading.
Results: Screw loosening was observed in three screws; two C2 pars-screws and one C1 lateral-mass screw. The spines became 28% more flexible compared to their intact state, following cyclic loading. Long C2 pedicle-screw exhibited greater stiffness compared to short C2 pars- scew post-cyclic loading, particularly in axial rotation. Bilateral long C2 pedicle-screws demonstrated greater reduction in mean pre-cyclic motion compared to bilateral short C2 pars-screw. The mean reduction in pre-cyclic motion achieved by combination of a long and a contralateral short C2 pars-screw was comparable to bilateral long C2 pedicle-screw. Bicortical long C2 pedicle-screws had significantly higher (94%) pullout strength compared to shorter unicortical C2 pars-screw
Conclusions: Long C2 pedicle-screw limited motion better compared to short C2 pars screw. Long C2 pedicle-screws provided significantly increased fixation strength compared to short C2 pars-screw during pull-out.
Patient Care: Such a study would provide valuable information to clinicians to weigh the risks and benefits of using a pedicle or pars screw in a C1-C2 construct.
Learning Objectives: By the conclusion of this study, participants should be able to compare the biomechanical characteristics of three different types of C2 fixation in a Goel-Harms C1-C2 posterior screw-rod fusion construct: (1) Bilateral long C2 pedicle screws, (2) Bilateral short C2 pars screws, (3) Unilateral long C2 pedicle screw with a contralateral short C2 pars screw.