## 3 Results
Figure 5: Schematic diagram of pullout behavior. Figure 3: (**a**) pedicle screw 3D model, (**b**) products, (**c**) parameters of pedicle screw and (**d**) schematic diagram of pullout test. Figure 6a shows the stiffness measured in the linear elastic region of the pullout curve based on each screw design. The stiffness corresponded to the stress wherein the material retained its shape and implied the resist force on the pull stress applied to the screw in the pullout test [25]. The stiffness increased 2.5 times when the density doubled. Additionally, there was no significant change in stiffness with increases in the core-diameter although stiffness increased with increases in the conical angle and decreased again at a conical angle corresponding to 8.58\({}^{\circ}\). Conversely, the area under the pullout curve from the start of pullout test to the maximum pullout load is termed as pullout strain energy [25]. The pullout strain energy based on the design of the screw was calculated and shown graphically in Figure 6b. The pullout strain energy of grade 20 exceeded that of grade 10. Furthermore, at grade 20, and the change in the strain energy sharply decreased when the core diameter increased to 5.5a. Furthermore, the degree of strain energy changed with the changes in conical angle in CDI and CDII group was significantly reduced. Conversely, at grade 10, the strain energy did not change significantly based on the screw design parameters. Overall, the pullout strain energy tended to decrease when the conical angle decreased irrespective of the density. The pullout strength based on each pedicle screw design is shown graphically in Figure 7. At the density of grade 10, the pullout force power was higher in the order of CDII-2, CDI-2, and CDI-3. In grade 20, the pullout force was higher in the order of CDII-2, CDI-1, and CDI-2. Figure 6: Comparison of (**a**) pullout stiffness and (**b**) pullout strain energy with variables of density and pedicle screw design. Figure 7: Comparison of pullout force with respect to the variables of density and pedicle screw design. The pullout force of the screw designed in the study was compared with those in previous studies as shown in Figure 8, and the results indicate that it exhibited a significant improvement when compared to that in previous studies [12; 24]. The pullout force of grade 10 improved by approximately more than 200-500 N. Specifically, with respect to grade 20 that corresponds to normal bone density, the pullout force improved by approximately 2-2.5 times that in other study results. In order to more precisely analyze the pullout behavior with screw designs, a cross-section of the artificial bone after the test was analyzed by using a micro-CT. Figures 9 and 10 show the cross-sectional images of an artificial bone specimen (grade 10 and grade 20, respectively) in which the screw was removed after first debonding with each screw design. In the grade 10 section, damage along the shape of the screw and fracture of the screw tip were commonly observed. However, when the core diameter increased (from CDI to CDIII), the fracture part of the upper region of screw increased and exhibited a distinct difference. In the CDI and CDII  
Figure 8: Comparison of pullout force with previous studies with respect to the variables of density and pedicle screw design. Figure 9: Cross sectional images of the artificial bone (grade 10) with the screw removed after the pullout test. groups, when the conical angle increased, the length of the upper damaged area increased, and the density also increased due to damage to the artificial bone. Conversely, in the CDIII group, the degree of damage wherein length and density increased in the upper damaged area was similar irrespective of the conical angle. Grade 20 showed a tendency similar to that of grade 10. The CDI and CDII groups exhibited an increase in the upper damage area, and density increases in the artificial bone when the conical angle increased.