The effect of locking fixation screws on the stability of anterior cervical plating

Study Design, Current anterior cervical plate systems were tested with lock ed and unlocked fixation screws and with unicortical and bicortical fixatio n screws to determine fixation rigidity and pull-off strengths. Objectives. To evaluate the effects of screw-plate locking and screw length on fixation strength and stability of anterior cervical plates. Summary of Background Data. New prate systems provide for rigid locking of the screw-plate interface, theoretically increasing construct rigidity, all owing unicortical fixation, and preventing screw back-out. There are few da ta on the effects of locking screws on the stability of anterior cervical p lating. Methods. Eighty fresh lamb vertebrae (C3-T1) were used. Test systems includ ed: Cervical Spine Locking Plate (CSLP; Synthes, Paoli, PA, Orion plate (So famor-Danek, Memphis, TN), and Acroplate (AcroMed, Cleveland, OH). The CSLP and Orion plates were tested with fixation screws, locked and unlocked, an d the AcroMed plate with unicortical and bicortical screw purchase. Biomech anical testing of the screw-prate constructs was performed to determine the initial bone-plate rigidity and purl-off strength. A 2.5-Nm cyclic bending moment was then applied to additional constructs for 10(5) cycles, and the se constructs retested. Results. Locked CSLP and Orion constructs were more rigid than all unlocked unicortical systems initially and after cyclic loading (P < 0.05). After c ycling, the rigidity of all unlocked unicortical constructs decreased signi ficantly (P < 0.05). There was no significant difference in pull-off streng ths between the CSLP, the Orion, and the unicortical AcroMed plate. However , all had significantly less pull-off strength than the AcroMed plate with bicortical screws. A negative correlation was observed between initial pull -off strength and sagittal vertebral body diameter. Conclusions. Locking screws significantly increased the rigidity of the tes ted screw-plate systems initially and after cyclic loading. Because pull-of f strength was affected by the vertebral body diameter, use of longer unico rtical screws may be clinically beneficial in the patient with larger cervi cal vertebrae.