Dynamic cervical plates: biomechanical evaluation of load sharing and stiffness.

STUDY DESIGN: An in vitro biomechanical study using a simulated cervical corpectomy model to compare the load-sharing properties and stiffnesses of two static and two dynamic cervical plates. OBJECTIVES: To evaluate the load-sharing properties of the instrumentation with a full-length graft and with 10% graft subsidence and to measure the stiffness of the instrumentation systems about the axes of flexion-extension, lateral bending, and axial torsion under these same conditions. SUMMARY OF BACKGROUND DATA: No published reports comparing conventional and dynamic cervical plates exist. METHODS: Six specimens of each of the four plate types were mounted on ultra-high molecular weight polyethylene-simulated vertebral bodies. A custom four-axis spine simulator applied pure flexion-extension, lateral bending, and axial torsion moments under a constant 50 N axial compressive load. Load sharing was calculated through a range of applied axial loads up to 120 N. The stiffness of each construct was calculated in response to +/-2.5 Nm moments about each axis of rotation with a full-length graft, a 10% shortened graft, and no graft. ANOVA and Fisher's post hoc test were used to determine statistical significance (alpha < or = 0.05). RESULTS: The two locked cervical plates (CSLP and Orion) and the ABC dynamic plate were similar in flexion-extension, lateral bending, and torsional stiffness. The DOC dynamic plate was consistently less stiff. The Orion plate load shared significantly less than the other three plates with a full graft. Both the ABC and the DOC plates were able to load share with a shortened graft, whereas the conventional plates were not. CONCLUSIONS: All plates tested effectively load share with a full-length graft, whereas the two dynamic cervical plates tested load share more effectively than the locked plates with simulated graft subsidence. The effect of dynamization on stiffness is dependent on plate design.