Cervical spine fusion: Biomechanics of a three-level cadaver model comparing anterior plate versus stand-alone cage

Study Design—Biomechanical cadaveric study. Objective—Long anterior cervical plate and cage (APC) constructs have a risk of pseudarthrosis with minor bone resorp-tion. Stand-alone cages (SACs) allow settling. The biomechanics of SAC have been investigated, but not multilevel, compression screw SAC. The purpose of this study is to evaluate the biomechanical safety of three-level SAC versus APC. Methods—Discectomies at three levels of five human cadaver spines (T1-C3) were fixed with SAC. A 0.18 mm thick shim was interposed between the cage and the superior endplate, and a pressure transducer map was placed between the cage and the inferior endplate. Tests were performed in flexion-extension and then repeated after removing the shims to simulate minor bone resorption. Subsequently, APC was applied and experiments were repeated. The pressure between each cage and endplate and motion of the implants were measured. Results—The range of motion (ROM) of SAC and APC constructs were comparable. The contact area and pressure between cage and endplate did not significantly change during motion with SAC. Shim removal did not significantly affect ROM, contact area, or average pressure measures. For APC, both contact area and pressure decreased from extension to flexion. Shim removal caused a significant loss of contact area and pressure. Conclusions—SAC provided comparable rigidity to the conventional APC construct while maintaining compression at the endplate-cage interface throughout flexion-extension and after minor bone resorption.