Application of the Finite Element Technique in the Design and Evaluation of the Artificial Facets for the Lumbar Spine

An experimentally validated three-dimensional (3D) finite element (FE) model of the ligamentous L3–S1 segment was used to study the effects of artificial facet designs on the segment biomechanics (motion, facet loads, and stresses). The intact model was modified to simulate several artificial facet designs across the L4–L5 segment including capping with and without screws and pedicle screw based designs with sliding articulating surfaces. For the pedicle screw based design, the effect of increasing the connecting shaft thickness and increasing width surrounding the pedicle screw, butted against the vertebral pedicle for further support, was studied. All of the FE models were evaluated in response to 6 Nm moment in extension, flexion, bending, and rotation. The predicted increases in motion, compared to the intact case, were smaller. The predicted facet loads decreased up to 25.7% in extension and 25.1% in bending at the implanted level as compared to intact spine segment. For all of the loading modes, the stresses in both implant designs were less than the yield stress of titanium. Therefore, the implants are unlikely to fail. Additional cadaver and other experimental protocols are essential for the evaluations of the most appropriate designs identified through the FE investigations.