CERVICAL PEDICLE SCREWS VERSUS LATERAL MASS SCREWS - ANATOMIC FEASIBILITY AND BIOMECHANICAL COMPARISON

Study Design. Biomechanical comparison of the pullout strengths of lat eral mass and pedicle screws in the human cervical spine. Measurements of pedicle dimensions and orientation were compiled. Objectives. To d etermine if transpedicular screws provide greater pull-out resistance than lateral mass screws and to investigate the anatomic feasibility o f pedicle screw insertion. Summary of Background Data. Cervical pedicl e screws have been reported in limited clinical and biomechanical stud ies, and some quantitative cervical pedicle anatomy has been reported. No direct biomechanical comparisons have been made between lateral ma ss and pedicle screws. Methods. Fifty-six fresh disarticulated human v ertebrae (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 w as 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 violatio ns were recorded, Screws were subjected to a uniaxial load to failure. Mean pedicle height, width, and angle with respect to the vertebral m idline were tabulated for each level. Results. The mean load-to-failur e was 677 N for the cervical pedicle screws and 355 N for the lateral mass screws. No significant correlations for either screw type were fo und between pull-out strength and bone density, screw length, or verte bral level. Pedicle and lateral mass dimensions were highly variable a nd not predictive of pull-out strength. Seven (13%) minor pedicle wall violations were observed. Conclusions. Cervical pedicle screws demons trated a significantly higher resistance to pull-out forces than did l ateral mass screws. The variability in pedicle morphometry and orienta tion requires careful preoperative assessment to determine the suitabi lity of pedicle screw insertion.