Medical Student Washington University School of Medicine Corpus Christi, Texas, United States
Introduction: In general, spinal fusion treats instability but does not spare motion, while cervical disc arthroplasty (CDA) spares motion but does not treat spinal instability. Instability is characterized by translational movement in flexion extension radiographs. An implant with higher shear stiffness would correspond to less translational movement in both flexion-extension and horizontal shear, theoretically limiting spinal instability. The purpose of this study was to characterize the mechanics of a novel, titanium spring-based, non-articulating cervical disc arthroplasty (CDA) designed with high shear stiffness.
Methods: A finite element analysis was run for loads of compression, bending, axial torsion, and shear using Fusion 360. The material choice was Ti-6Al-4V, the inferior endplate was rigidly fixed, and the superior endplate had the external force/moment distributed evenly over its superior surface. The theoretical range of motion and elastic stiffness of the design in compression, bending, and shear was quantified and compared to natural discs.
Results: The novel CDA has a compression range of motion and stiffness of 0.2 mm and 513 N/mm; a bending range of motion and stiffness of 4.14 degrees and 0.115 Nm/degree; an axial torsion range of motion and stiffness of 0.39 degrees and 5.0 Nm/degree; and a shear range of motion and stiffness of 0.077 mm and 5993 N/mm. Compared to a natural disc, the novel CDA is 4.3% stiffer in compression, 45.2% less stiff in bending, 11.9 stiffer in axial torsion, and 96.6 times stiffer in shear.
Conclusion : Compared to a natural disc, the novel CDA had compression and bending stiffness within an order of magnitude of natural values; meaning it reproduces natural biomechanics in these degrees of freedom. It is also almost two orders of magnitude stiffer in shear than natural discs. This will allow the disc to limit translational movement that is so often a cause of instability in the cervical spine, especially in flexion-extension radiographs. This novel CDA provides a middle ground between current disc replacement and fusion by sparing movement in the bending and compression degrees of freedom while providing stability in the translation directions.
