Comparison of the Optimal Design of Spinal Hybrid Elastic Rod for Dynamic Stabilization: A Finite Element Analysis

Abstract Introduction: Dynamic stabilization can fix deformities, fractures, and disc degeneration of the spine. Nitinol and poly(carbonate urethane) (PCU) are applicable elastic biomedical materials. The spinal hybrid elastic (SHE) rod is a semirigid pedicle screw-based rod intended for use with universal pedicle screws. This study aimed to investigate the biomechanical effects of different ratios of SHE rod using finite element analysis (FEA). Methods: A 3-dimensional nonlinear FEA of an intact lumbar spine model (INT) was constructed. The system was composed of pedicle screws, an inner nitinol stick (NS), and an outer PCU shell (PS). Four groups of models were constructed with the same outer diameter (5.5 mm) as the SHE rod. These groups had different NS diameter/PS thickness ratios: Nt45 (4.5/1.0), Nt35 (3.5/2.0), Nt25 (2.5/3.0), and Nt15 (1.5/4.0). After implanting at L3-L4, they were compared with INT. The resultant intervertebral range of motion (ROM), disc stress, facet joint contact force, screw stress, NS stress, and PCU stress were analyzed. Results: The overall trend in results indicated that ROM, disc stress, and facet force decreased moderately in the implanted L3-L4 levels, and increased slightly in the adjacent L2-L3 levels. NS stress and the NS diameter trended towards inverse proportionality. Changing the ratio did not markedly influence screw or PS stress. Conclusions: The SHE rod system provided sufficient spinal support and increased gentle adjacent-segment stress. The optimal NS diameter/PS thickness ratio of the SHE rod system is 4.5/1.0 mm or 3.5/2.0 mm due to the lower stress concentrations for the durability of the implant.