Dynamic cervical plates - Biomechanical evaluation of load sharing and stiffness

Study Design. An in vitro biomechanical study using a simulated cervical co rpectomy model to compare the load-sharing properties and stiffnesses of tw o 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 s tiffness 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 ultr a-high molecular weight polyethylene-simulated vertebral bodies. A custom f our-axis spine simulator applied pure flexion-extension, lateral bending, a nd 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 mo ments 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 determi ne statistical significance (alpha less than or equal to 0.05). Results. The two locked cervical plates (CSLP and Orion) and the ABC dynami c plate were similar in flexion-extension, lateral bending, and torsional s tiffness. The DOC dynamic plate was consistently less stiff. The Orion plat e load shared significantly less than the other three plates with a full gr aft. Both the ABC and The DOC plates were able to load share with a shorten ed graft, whereas the conventional plates were not. Conclusions. All plates tested effectively load share with a full-length gr aft, whereas the two dynamic cervical plates tested load share more effecti vely than the locked plates with simulated graft subsidence. The effect of dynamization on stiffness is dependent on plate design.