## 4 Discussion  
The SHE rod dynamic stabilization system is intended to balance the stability and motion of the spine after surgery. In this study, the overall trend indicated that ROM, disc stress, and facet force decreased moderately in the implanted L3-L4 levels and increased mildly in the adjacent L2-L3 levels. Nevertheless, the stress shielding effect on adjacent  
Figure 4: The von Mises stress on the inner NS markedly increased on the upper and lower portion. The maximum occurred on the N25 in torsion. levels is mild. The present results demonstrate that the SHE rod can provide good spinal stability and avoid high loading at the adjacent levels. PEEK (polyether ether ketone) is a low elastic modulus, similar to that of bone (3.6 GPa), and less rigid than titanium (115 GPa) [21]. The PEEK rod is nonrigid and has recently been used to stabilize the spine dynamically. PEEK rods closely approximate physiologic load sharing [22]. However, the hard surface, durability, and fatigue fracture of PEEK rods should be of concern. They were associated with initiating scratching at the rod-screw interface and needed larger-diameter, locked-in, specific pedicle screws [23]. Compared to the PEEK rod, the outer PCU shell is scratch resistant. The SHE rod system has a 5.5 mm diameter for universal pedicle screws. However, it is still necessary to compare the two materials in further biomechanical research. Nitinol is an elastic shape memory alloy that minimizes stress shielding and has even been shown to trend toward superior fatigue resistance compared with titanium [24]. However, nitinol is not a popular material in orthopedic applications because of its poor corrosion resistance and anti-wear properties [25]. To this end, we introduced a PCU polymer as an insulator. The outer PCU shell enveloped the inner Nitinol stick, providing perfect insulation against the titanium screw heads and nuts. In the present FEA, the NS stress increased markedly in the upper and lower portions. This phenomenon is directly related to the connection between the screw head and nut. Nevertheless, the NS stress was always lower than the yield strength of nitinol (816 MPa) [26]. NS in the SHE system has a low potential implant failure rate. Even in the worst-case scenario of the thinnest PS of the SHE rod system (Nt45) after rod fracture, the biomechanical effects still afford nearly sufficient spine support and gentle adjacent segment stress [27]. The spacer in the Dynesys system is made of PCU, which is the same material as PS [19]. The stabilization of the pretension provided by the spacer and cord in Dynesys differs from that in the SHE rod system. In addition, PCU is less rigid than nitinol. The outer PS stress decreased under the same load owing to the stress shielding by the inner NS. As such, we observed no obvious trend in PS stress as the NS diameter was changed. In most of the present models, the PS stress decreased. The maximum PS stress was still much lower than the mean tensile strength of the PCU (30 MPa) [13]. Though the materials in Dynesys were different to those in the SHE rod system, they both have the same intended use [28]. The SHE rod system provides no inferior spinal support compared with the Dynesys system. No other single clinical material can exhibit biphasic rigidity and flexibility. There-fore, we propose the hybrid use of semi-rigid and flexible materials. The semi-rigid nitinol is used for the inner stick, and the flexible PCU is used for the outer shell. A commercially available SHE rod was devised based on the results of the precise ratio of the hybrid used in this study. Considering the industrial technologies, the PCU shell is too thin in Nt45 to manufacture. The optimal NS diameter/PS thickness ratio of the SHE rod system should be 3.5/2.0 mm due to the lower stress concentrations for the durability of the implant. This study had some limitations. All spinal models were healthy with no pathological properties or defects.