Lightning Podiums: Spinal Gumbo - 803A

Presented by: P. Halverson


P. Halverson(1), D. Hawkes(1)

(1) Nexus TDR, Salt Lake City, UT, United States


Introduction: The cervical spine relies on storage of elastic energy to provide motion and stability. During total disc replacement, at least a portion of the disc and supporting ligaments are excised. As a result, the overall stiffness of the spinal segment significantly decreases and the segment becomes increasingly unstable. Unless the excised stabilizing tissues are replaced by a device capable of elastic energy storage, the surrounding soft tissue must compensate. Elevated strain-energy density in the remaining soft tissues has been linked to Heterotopic ossification; thus, elastic energy storage may be a key component in preventing HO. The goal of the current study is to develop an algorithm to facilitate design of a device capable of elastic energy storage. Further, the algorithm should account for the variability that occurs as a result of genetic diversity, spinal level, degree of degeneration, surgical technique, etc.

Material and Methods: A 2D closed-form model of the FlexBAC-C Total Disc Replacement (Nexus TDR) was created to determine moment-rotation, stress, and strain-energy storage in Flexion-Extension, Lateral Bending, and Axial Rotation. Parametrized models are then linked to a Generalized Reduced Gradient optimization routine. The optimization routine is executed to find design specific parameters that fit the selected non-linear stiffness curve based on genetic diversity, spinal level, degree of degeneration, and surgical technique. Finally, the resulting design is analyzed using 3-dimensional Finite Element Analysis to verify that the device operates within acceptable stress limits.

Results: The optimization platform was validated by comparing the empirical moment-rotation response of the physical prototype with the closed form and FEA solutions. Agreement between the benchtop and closed form solutions was 5% for Flexion-Extension and Lateral Bending, and 8% for axial rotation. The Agreement between FEA and Benchtop testing was 3% for Flexion-Extension and Lateral Bending, and 6% for Axial rotation. The duration of the analysis and optimization was 46 minutes on an i7-6700 Quad Core CPU.

Conclusion: With this study, we were able to develop and validate an alogorithm and implant design for a total disc replacement with tailorable stiffness to provide storage of elastic energy mimicking that of an intact spinal disc. The algorithm and design provide for a device that may be able account for natural and iatrogenic changes in stiffness of a spinal segment. Further testing is required to evaluate the clinical significance.