Can an Endplate‐conformed Cervical Cage Provide a Better Biomechanical Environment than a Typical Non‐conformed Cage?

摘要

ObjectivesTo evaluate the biomechanical characteristics of endplate-conformed cervical cages by finite element method (FEM) analysis and cadaver study. MethodsTwelve specimens (Csub2/sub–Csub7/sub) and a finite element model (Csub3/sub–Csub7/sub) were subjected to biomechanical evaluations. In the cadaver study, specimens were randomly assigned to intact (I), endplate-conformed (C) and non-conformed (N) groups with Csub4–5/sub discs as the treated segments. The morphologies of the endplate-conformed cages were individualized according to CT images of group C and the cages fabricated with a 3-D printer. The non-conformed cages were wedge-shaped and similar to commercially available grafts. Axial pre-compression loads of 73.6 N and moment of 1.8 Nm were used to simulate flexion (FLE), extension (EXT), lateral bending (LB) and axial rotation (AR). Range of motion (ROM) at Csub4–5/sub of each specimen was recorded and film sensors fixed between the cages and Csub5/sub superior endplates were used to detect interface stress. A finite element model was built based on the CT data of a healthy male volunteer. The morphologies of the endplate-conformed and wedge-shaped, non-conformed cervical cages were both simulated by a reverse engineering technique and implanted at the segment of Csub4–5/sub in the finite element model for biomechanical evaluation. Force loading and grouping were similar to those applied in the cadaver study. ROM of Csub4–5/sub in group I were recorded to validate the finite element model. Additionally, maximum cage-endplate interface stresses, stress distribution contours on adjoining endplates, intra-disc stresses and facet loadings at adjacent segments were measured and compared between groups. ResultsIn the cadaver study, Group C showed a much lower interface stress in all directions of motion (all P 4–5/sub was smaller in FLE-EXT ( P = 0.001) but larger in AR ( P = 0.017). FEM analysis produced similar results: the model implanted with an endplate-conformed cage presented a lower interface stress with a more uniform stress distribution than that implanted with a non-conformed cage. Additionally, intra-disc stress and facet loading at the adjacent segments were obviously increased in both groups C and N, especially those at the supra-jacent segments. However, stress increase was milder in group C than in group N for all directions of motion. ConclusionsEndplate-conformed cages can decrease cage-endplate interface stress in all directions of motion and increase cervical stability in FLE–EXT. Additionally, adjacent segments are possibly protected because intra-disc stress and facet loading are smaller after endplate-conformed cage implantation. However, axial stability was reduced in group C, indicating that endplate-conformed cage should not be used alone and an anterior plate system is still important in anterior cervical discectomy and fusion.