## 4 Discussion  
The distribution of insertional torque and pullout strength differed by density, with the Cy/Co-V screw geometry exhibiting the highest value for both. The pullout strength was significantly higher for the V-threaded screws than for the S-threaded screws in the 20- and 30-pcf groups, but this phenomenon was not observed for insertional torque. The relationship between insertional torque and pullout strength is still under debate. The pullout strength, which refers to the strength required to extract a screw through its longitudinal axis, has been described as a critical parameter for judging the fixation stability of pedicle screws. However, there is no method for testing the pullout strength during surgery without affecting fixation. Another easy but subjective parameter is insertion torque, which is a subjective value determined by the surgeon during screw insertion. Controversies between pullout strength and insertional toque do exist. In the study, both the pullout strength and the insertional toque could be mutually compared using this standardized polyurethane foam. [PERSON] et al. performed biomechanical testing using screws with threads of varying pitch in osteoporotic test blocks and found a higher maximal insertional torque with increasing pitch but no change in pullout strength [16]. The results obtained using the osteoporotic test blocks in their study may not be comparable to those obtained using the normal and higher-than-normal bone density blocks in our study. [PERSON] et al. tested screws of three different pitches in blocks of three different densities to clarify the correlation between bone density and screw pitch [17]. In their study, torque was related to pullout strength in all configurations, and medium-sized screws showed the highest biomechanical strength. However, the cannulated, partially threaded screws used  
Figure 9: (**A**) The EBV and (**B**) the relationship between the pullout strength and EBV for various screw geometries. A strong positive correlation (R\({}^{2}\) = 0.8595) was noted after comparing the pullout strength of all screws with the EBV. in their study were not comparable to the non-cannulated, fully threaded screws used in our study. [PERSON] et al. found that both the maximum insertional torque and pullout strength were significantly and positively correlated when tested using blocks with a bone density that was higher than normal [19]. However, the geometric design of the orthodontic mini-screws used in their study was completely different from that of the pedicle screws used in our study. [PERSON] et al. evaluated the pullout strength of nine pedicle screw geometries in standardized polyurethane foams of grades 5, 15 and 20 and concluded that the value was higher in the V-threaded group and highest for the Cy/Co-V type [21]. The conflicting pullout strength results obtained using osteopororotic test blocks [16; 21] and the screw fatigue induced by the decreased inner diameter of the neck design in their study [30] limited further clinical applications. The varying results for insertional torque may also have been caused by differences in measurement methods. The insertional torque reflected the manual sensation experienced when inserting screws, and the last portion of the threading process was supposed to be much more important than the whole insertion process. The insertional torque was measured during the last portion of the threading process in our study; however, in other studies, maximal values were recorded by digital meters throughout the screw insertion process [16; 19], which is less clinically practical. In our study, both the pullout strength and the insertional toute were mutually compared, and the highest value from both methods was found for the Cy/Co-V screw geometry. A significant difference was observed in the pullout strength between the different bone densities. The pullout strength was significantly higher for Cy/Co-V than for Co/Co-V and Cy/Cy-V in the 30-pcf group but it was only higher than that of Cy/Cy-V in the 20-pcf group.