# Alternative Pedicle Screw Design via Biomechanical Evaluation
## 4 Discussion and Conclusions  
In a previous study, the ASTM F1717 standard was used to compare the bending strength of polyaxial pedicle screws embedded in a polyurethane block that mimicked vertebrae. Their test results were similar to the results of the present study, as shown in Figure 4[27]. Therefore, the presented method is considered appropriate. There have been examples of measuring the pull-out strength of several types of pedicle screws, varying from 420 to 1676 N [4; 20]. It is difficult to directly compare these results with this study because the formers were not obtained using an FEA model. However, there has been a report that standard pedicle screws can withstand more than 796 N [20]. When considering safety, it is meaningful to analyze the pull-out characteristics under loading of 600 N in the proposed FEA model.  
The alternative design of a pedicle screw for spinal fusion was introduced and a design platform was constructed to improve structural robustness and surgical convenience, which was confirmed by a combined experimental and analytical approach. The alternative pedicle screw consisting of a head, shank, sub-thread, and main thread was based on modifying conventional pedicle screws to enhance the convenience of their use during surgery and their structural robustness. The smooth contours, short shank, and fine pitch increased their mechanical strength, while the quadruple thread adopted as a sub-thread could reduce the surgery time.  
A series of experiments was carried out on each screw to investigate the bending strength. The alternative design showed the highest resistance against bending load, which was more than 55% higher than the pedicle screw with a conventional dual thread. For the pull-out resistance, an FEA model was first established and then the stress distribution of each screw was investigated. The stress value of the alternative pedicle screw was certainly acceptable because it was below the strength of each material. However, the design could be modified by a gradual change of the cross-sectional area and longer quadruple thread to lower the generated stress under loading.  
The alternative design can effectively resist bending and pull-out loadings. Moreover, the generated stress can be lowered via a small modification of the design to change the cross-sectional area gradually and employ a longer quadruple thread. Therefore, it can be concluded that the alternative dual screw design could be a more appropriate design for spinal fusion.  
The alternative pedicle screw was designed and evaluated using the proposed comprehensive design platform, which makes it possible to simultaneously improve the bending and pull-out strength. Moreover, the stress distribution on the vertebra and screw, and both together, can be closely analyzed. This plays an important role in predicting and improving problems that can occur at an early stage after surgery, as well as the fatigue issue that can occur after a somewhat longer time after the surgery. Moreover, the platform can be used to optimally design other medical devices in the future.  
Conceptualization, M.H.H. and M.G.L.; methodology, M.H.H.; validation, J.K. and M.H.H.; formal analysis, M.H.H.; writing--original draft preparation, J.K.; writing--review and editing, M.G.L.; supervision, M.G.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Research Foundation of Korea (NRF) grant, funded by the Korean Government (MSIT) (NRF-2018R1A2B2002683).  
The authors declare there are no conflicts of interest.