Biomechanical Analysis of the Cervical Spine Following Disc Degeneration, Disc Fusion, and Disc Replacement: A Finite Element Study

Background: Discectomy and fusion is considered the "gold standard'' treatment for clinical manifestations of degenerative disc disease in the cervical spine. However, clinical and biomechanical studies suggest that fusion may lead to adjacent-segment disease. Cervical disc arthroplasty preserves the motion at the operated level and may potentially decrease the occurrence of adjacent segment degeneration. The purpose of this study was to investigate the effect of disc generation, fusion, and disc replacement on the motion, disc stresses, and facet forces on the cervical spine by using the finite element method. Methods: A validated, intact, 3-dimensional finite element model of the cervical spine (C2-T1) was modified to simulate single-level (C5-C6) and 2-level (C5-C7) degeneration. The single-level degenerative model was modified to simulate both single-level fusion and arthroplasty (total disc replacement [TDR]) using the Bryan and Prestige LP discs. The 2-level degenerative model was modified to simulate a 2-level fusion, 2-level arthroplasty, and single-level disc replacement adjacent to single-level fusion (hybrid). The intact models were loaded by applying a moment of +/- 2 Nm in flexion-extension, lateral bending, and axial rotation. The motion in each direction was noted and the other modified models were loaded by increasing the moment until the primary C2-T1 motion matched that of the intact (healthy) C2-T1 motion. Results: Both Bryan and Prestige discs preserved motion at the implanted level and maintained normal motions at the adjacent nonoperative levels. A fusion resulted in a decrease in motion at the fused level and an increase in motion at the unfused levels. In the hybrid construct, the TDR (both) preserved motion adjacent to the fusion, thus reducing the demand on the other levels. The disc stresses followed the same trends as motion. Facet forces increased considerably at the index level following a TDR. Conclusion: The Bryan and Prestige LP TDRs both preserved motion at the implanted level and maintained normal motion and disc stresses at the adjacent levels. The motion patterns of the spine with a TDR more closely resembled that of the intact spine than those of the degenerative or fused models.