## 4 Discussion  
As a result of the measurement of the compressive strength according to the density shown in Figures 1 and 2, the yield compressive stress was higher as the density increased. This was because the thickness of the wall increased due to the thickness and distribution of the cell wall and cell size in the artificial bone, and yield compressive stress increased since the distribution was uniform [26]. Actually, due to increases in the compressive strength as the higher density of the artificial bone, energy absorption ratio in plateau region increased. As the amount of the energy absorption increased, the amount of the elastic recovery increased. Based on previous studies, the amount of elastic recovery decreased when the density of artificial bone increased under the same compression amount. This was because the internal cell volume increased and the amount of absorbed energy increased due to increases in the internal cell size when the density decreased. Therefore, the amount of recovery of the artificial bone increased when the compression load was removed. Increases in the density increased the difference in strain energy based on each change in the recovery amount because the supported stress increased due to increases in the cell wall ratio [26]. In the case of a pedicle screw inserted into the human body based on the characteristics of an artificial bone, it was expected to exhibit different pullout strengths based on the bone density and quality of the patients. The design variable of the pedicle screw was the most common factor that affected the pullout force. In the study, a pedicle screw was designed and fabricated with variables of core diameter and conical angle among various design variables such as thread shape, thread width, and pitch. In the case of a pedicle screw inserted into the human body based on the characteristics of an artificial bone, it was expected to exhibit different pullout strengths based on the bone density and quality of the patients. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & & & & & & & & \\ \hline
**Screw Design** & **CDI-1** & **CDI-2** & **CDI-3** & **CDII-1** & **CDI-2** & **CDII-3** & **CDIII-1** & **CDIII-2** & **CDIII-3** \\ \hline Density & 86.092 & 80.976 & 68.831 & 78.003 & 87.484 & 77.278 & 78.81 & 75.068 & 74.759 \\ Core diameter & \(-6.467\) & \(-7.586\) & \(-9.086\) & \(-6.457\) & \(-7.586\) & \(-9.086\) & \(-6.457\) & \(-7.756\) & 9.082 \\ Conical angle & \(-1.211\) & \(-1.211\) & \(-1.211\) & \(-3.970\) & \(-3.970\) & \(-3.970\) & \(-2.581\) & \(-2.582\) & \(-2.582\) \\ \hline \hline \multicolumn{10}{c}{**Grade 20 (0.32 g/cc)**} \\ \hline
**Screw design** & **CDI-1** & **CDI-2** & **CDI-3** & **CDII-1** & **CDI-2** & **CDII-3** & **CDIII-1** & **CDIII-2** & **CDIII-3** \\ \hline Density & 86.092 & 80.976 & 68.831 & 78.003 & 87.484 & 77.278 & 78.81 & 75.068 & 74.759 \\ Core diameter & \(-13.738\) & \(-20.048\) & \(-9.636\) & \(-13.738\) & \(-20.048\) & \(-9.636\) & \(-13.738\) & \(-20.048\) & \(-9.636\) \\ Conical angle & \(-17.218\) & \(-17.218\) & \(-17.218\) & \(-17.218\) & \(-13.53\) & \(-13.53\) & \(-6.633\) & \(-6.633\) & \(-6.633\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Local sensitivity of density, core diameter, and conical angle on the pullout force.