Cervical pedicle screws versus lateral mass screws - Anatomic feasibility and biomechanical comparison

Study Design. Biomechanical comparison of the pullout strengths of lateral mass and pedicle screws in the human cervical spine. Measurements of pedicle dimensions and orientation were compiled. Objectives. To determine if transpedicular screws provide greater pull-out resistance than lateral mass screws and to investigate the anatomic feasibility of pedicle screw insertion. Summary of Background Data. Cervical pedicle screws have been reported in limited clinical and biomechanical studies, and some quantitative cervical pedicle anatomy has been reported. No direct biomechanical comparisons have been made between lateral mass and pedicle screws. Methods. Fifty-six fresh disarticulated human vertebrae (C2-C7) were evaluated with computed tomography to determine morphometry and vertebral body bone density. Lateral mass and pedicle screws were randomized to left versus right. A 3.5-mm cortical screw was used for both techniques, unless a pedicle was narrower than 5.0 mm; then a 2.7-mm cortical screw was used instead. Pedicle wall violations were recorded, Screws were subjected to a uniaxial load to failure. Mean pedicle height, width, and angle with respect to the vertebral midline were tabulated for each level. Results. The mean load-to-failure was 677 N for the cervical pedicle screws and 355 N for the lateral mass screws. No significant correlations for either screw type were found between pull-out strength and bone density, screw length, or vertebral level. Pedicle and lateral mass dimensions were highly variable and not predictive of pull-out strength. Seven (13%) minor pedicle wall violations were observed. Conclusions. Cervical pedicle screws demonstrated a significantly higher resistance to pull-out forces than did lateral mass screws. The variability in pedicle morphometry and orientation requires careful preoperative assessment to determine the suitability of pedicle screw insertion.