Biomechanical comparison of unicortical versus bicortical C1 lateral mass screw fixation

Study Design: Biontechanical study of pullout strength of unicortical versus bicortical C1 lateral mass screws using a cadaveric cervical spine model. Objective: To compare pullout strength of unicortical versus bicortical Cl lateral mass screws. Summary of Background Data: The internal carotid artery and hypoglossal nerve lie over the anterior aspect of the lateral mass of the atlas and are at risk from bicortical Cl lateral mass screws. Unicortical screws would reduce the risk of injury to these neurovascular structures; however, no data are available on the relative strength of unicortical versus bicortical C1 lateral mass screws. Methods: Fifteen cadaveric cervical spine specimens underwent axial pullout testing of Cl lateral mass screws. A unicortical Cl lateral mass screw was placed on I side with a contralateral bicortical screw. Results: The mean pullout strengths of the unicortical screws and bicortical screws were 588N (range, 212 to 1234N) and 807 N (range, 163 to 1460 N), respectively (P = 0.008). Conclusions: Bicortical C I lateral mass screws were significantly stronger than unicortical screws; however, the mean pullout strength of both the unicortical and bicortical Cl screws were greater than previously reported values for subaxial lateral mass screws. On the basis of these data, the clinical necessity for using bicortical screw fixation in all patients must be questioned. If similar strength can be achieved using unicortical C1 lateral mass screw to that currently accepted in the subaxial spine, bicortical screws might not be justified for the C I lateral mass. However, the ability to extrapolate C1-C2 data to subaxial spine data is uncertain because of the difference in normal physiologic loading at these levels.