## 4 Discussion  
Fracture of pedicle screws can lead to considerable complications in the spine, such as loss of curvature and symptomatic pseudarthosis, which often requires reoperation. Screw fracture mostly occurs following high-energy impact injuries or metal fatigue from repetitive stress. [PERSON] et al. [16] demonstrated a reduction in the fatigue life and strength of pedicle screws when a portion of the screw threads was left exposed outside of the bone. This is echoed in the results of this study, where the EXP-FT-02 construct clearly had the lowest fatigue strength of all groups. However, Table 3 also shows that by omitting threads from the exposed portion of the screw (1/3 proximally unthreaded), the fatigue strength increased in comparison to a fully threaded screw. It is worth noting that the fatigue life of EXP-PU-02 was higher than EXP-FT-01, signifying that the fatigue strength of the proximally unthreaded (PU) screw when not fully inserted is higher than the fully threaded (FT) screw when fully inserted into the test block. In addition, whereas the construct with fully threaded screws failed through screw fracture, the construct with PU screws failed by fracture of the rods. This shows that the unthreaded portion (shank) of the PU screw plays an important role in the fatigue life and supports the hypothesis of this study that the fatigue strength would be superior to a fully threaded screw. A possible contributing factor to the greater fatigue strength is the diameter of the screw. The smooth shank on the PU screw had a diameter of 4.0 mm, whereas the inner/minor diameter of the fully threaded screw was 3.0 mm. The second axial moment of area of the PU screw was greater than the fully threaded screw at the point where the screws entered the test block. This might contribute to the better fatigue bending strength. Figure 4: The distribution of the maximum von Mises stress on (**a**) pedicle screws of four models and (**b**) rods of the FEM-FT-01 model under 170 N axial loading. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Axial Force (N)** & **170** & **220** \\ \hline
**Rod of FEM-FT-01 (MPa)** & 341.66 & 440.12 \\ \hline
**Rod of FEM-FT-02 (MPa)** & 369.67 & 475.36 \\ \hline
**Rod of FEM-PU-01 (MPa)** & 361.36 & 465.49 \\ \hline
**Rod of FEM-PU-02 (MPa)** & 362.24 & 468.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Maximum von Mises stress on rods. According to [PERSON] et al. [20], the most stressed site on a pedicle screw is the junction between the shank and threads, and the threads at the screw-bone interface tend to be less stressed than threads outside the interface. This is consistent with the findings of this study. Whether considering the FT or PU screw, the major stress occurred on the proximal part of screw, and the maximum von Mises stress occurred at the interface between the screw and the block. In all of the FT screw groups, the maximum von Mises stress on the screw exceeded that on the rod. Previous studies [20; 21; 22] have demonstrated an increase in stress at the screw head and a loss in fatigue strength with increasing unsupported screw length, which is consistent with the findings of this study. The FE model demonstrated that in comparison to the FT screw, the PU screw design produced a lower maximum von Mises stress on the screw and provided superior fatigue strength when partially inserted. This was supported by the fact that it was the rod rather than the screw that fractured during the dynamical compression test. Despite the clearly superior results obtained from the proximally unthreaded screw in this study, there are some limitations to the methods used. (i) The vertebrectomy model was developed in compliance with ASTM F1717, which is the correct approach to use for this form of study [22; 23; 24]. However, the simplifications incorporated into any such model cannot truly represent the multidirectional loading conditions in a normal human spine.