## 4 Discussion
\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** & **CDII-2** & **CDII-3** & **CDIII-1** & **CDIII-2** & **CDIII-3** \\ \hline Density & 43.125 & 53.799 & 27.165 & 20.881 & 33.66 & 16.985 & \(-7.784\) & 5.103 & \(-15.842\) \\ Core diameter & 69.31 & 71.541 & 62.792 & 69.31 & 71.541 & 62.792 & 69.31 & 71.541 & 62.792 \\ Conical angle & 30.569 & 30.569 & 30.569 & 27.363 & 27.363 & 27.363 & 24.677 & 24.677 & 24.677 \\ \hline \hline \multicolumn{10}{c}{**Grade 20 (0.32 g/cc)**} \\ \hline
**Screw design** & **CDI-1** & **CDI-2** & **CDI-3** & **CDII-1** & **CDII-2** & **CDII-3** & **CDIII-1** & **CDIII-2** & **CDIII-3** \\ \hline Density & 43.125 & 53.799 & 27.165 & 20.881 & 33.66 & 16.985 & \(-7.784\) & 5.103 & \(-15.842\) \\ Core diameter & 20.011 & 20.837 & 20.701 & 20.011 & 20.837 & 20.701 & 20.011 & 20.837 & 20.702 \\ Conical angle & 28.833 & 28.833 & 28.833 & 29.553 & 29.553 & 29.553 & 29.558 & 29.58 & 9.58 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Local sensitivity of density, core diameter, and conical angle on the artificial bone damage. In Figure 4, the slope of the linear section varied with density. This meant that the restoring force of the artificial bone against the stress increased in the elastic region during the pullout test when the density of artificial bone decreased. As shown in microstructure in Figure 1, the phenomenon was potentially caused by high stress absorption capacity because the internal cell size and distribution of grade 10 exceeded that of grade 20. However, the yield strength of grade 10 was lower than grade 20 due to a thinner cell wall and lower volume ratio of the cell wall. A gradual section after the linear section in the pullout curve was termed as the debonding section. Additionally, the load in the debonding section of grade 10 was lower than that in grade 20. Thus, the bonding section was related to the stiffness, and the debonding section was related to the yield strength and fracture strength of the artificial bone. Stiffness varied with density and screw design parameters (core diameter, conical angle). The stiffness increased with increases in the density since the stress absorption capacity of the cell and stress supporting capacity of the cell wall increased. Furthermore, a critical point angle existed with the highest stiffness among the parameters of the conical angle [12; 27]. Based on the aforementioned results, the main variable that affected the stiffness of the pull stress corresponded to the density of the artificial bone, and the parameter for the screw design corresponded to the conical angle. The increasing and decreasing tendency of the pullout strain energy was different according to the conical angle and core diameter as well as the density. The pullout strain energy corresponded to the stress absorbed in the artificial bone due to the stress applied to the screw [24; 25]. During the pullout test, the pullout load direction for the screw and reaction force direction in the artificial bone were opposite. However, when the conical angle increased, the reaction force in the direction opposite to the pullout load direction was dispersed along the angle of the screw. When the conical angle increased, the strain energy decreased due to the influence of the dispersed reaction force. Thus, more stress was applied to the artificial bone that contacted the screw thread. When stresses that exceeded the yield stress of the artificial bone were applied on it, deformation occurred in the contact area between the artificial bone and screw. When the maximum stress was exceeded, the artificial bone was broken at the contact lower part and upper part of the screw, and the screw was loosened and debonded [20]. Finally, when the conical angle increased, deformation and fracture of the artificial bone contacted screw rapidly occurred. When the load corresponded to the maximum in the pullout curve, it was termed as the pullout strength of the screw [28].