Biomechanical evaluation of stand-alone interbody fusion cages in the cervical spine

An in vitro biomechanical investigation of the immediate stability in cervical reconstruction.

The purpose of this study was to compare the segmental stability afforded by the interbody fusion cage, the anterior locking plate, and the "gold standard" autograft.

Recently, interbody fusion cage devices have been developed and used for cervical reconstruction, but to the authors' knowledge no studies have investigated the biomechanical properties of the stand-alone interbody cage device in the cervical spine.

Using six human cervical specimens, nondestructive biomechanical testing were performed, including axial rotation (+/-1.5 Nm, 50 N preload), flexion/extension (+/-1.5 Nm) and lateral bending (+/-1.5 Nm) loading modes. After C4-C5 discectomy, each specimen was reconstructed in the following order: RABEA cage (cage), tricortical bone graft (autograft), cervical spine locking plate system (plate). Unconstrained three-dimensional segmental range of motion at C4-C5 and above and below were evaluated.

In flexion/extension, the plate demonstrated significantly lower range of motion than did the cage and the autograft (P < 0.005), and the cage showed a significantly higher range of motion than did the intact spine (P < 0.05). Under axial rotation, the plate indicated a significantly lower range of motion than did all other groups (P < 0.05). No significant differences were indicated in lateral bending. Adjacent to C4-C5, an increased range of motion was observed.

The increased motion adjacent to C4-C5 may provide an argument for acceleration of disc degeneration. From the biomechanical point of view, this study suggests that the cervical interbody fusion cage should be supplemented with additional external or internal supports to prevent excessive motion in flexion-extension.