## 4 Discussion  
The initial stability of pedicle screws has been shown to be positively associated with long-term fixation, stabilizing the bone until fusion occurs [15]. The stability of the initial fixation is typically evaluated using a pullout test, and is heavily influenced by factors such as bone quality, screw design, surgical technique, and insertion torque [8; 9; 10]. In practice, surgeons have found that double-lead pedicle screws can be inserted quicker and with greater stability than their single-lead counterparts. However, there is little experimental data available to support this. The present study used torque analysis to determine the effects of different thread designs on pullout capability. During the initial revolution (1 st revolution), the insertion torque of the double-lead screws was nearly three times that of the single-lead screws with the same number of flutes and same cutting length, but the average final maximum torque of the double-lead screw was only slightly higher than that of the single-lead screw. Similarly, [PERSON] et al. reported a greater insertion torque with double-lead dental implants [16]. For surgeons placing pedicle screws in clinical settings, the torque required to insert the screws is believed to be a good predictor of initial stability and pullout strength [17]. An in vitro biomechanical test by [PERSON] et al. [13] showed that double-lead pedicle screws could reduce insertion time, but did not improve the pullout strength. This result is similar to the findings of this study, where the double-lead designs had a lower pullout strength than their single-lead counterparts (Figure 5). There are two main reasons for this. First, the single- and double-lead screws had helical angles, with the high helical angle of the double-lead screws producing a lower helical cross-sectional area, leading to a reduction in axial force resistance. [PERSON] et al. [18] indicated that the thread helix angle affects the stability of vertically and horizontally loaded implants, and as the helix angle is increased, the implant's resistance to loading is reduced. Second, an analysis of the contact interfaces was performed using the method described by [PERSON] et al. [16] and showed that the double-lead screws had a higher level of defects. However, quantifying the defects is difficult, especially for small caves on the contact interfaces (Figure 6). Figure 6: Observations of the artificial bone implant for Screw D (single-lead) and Screw H (double-lead). In this study, the number of cutting flutes was not found to significantly affect the screw torque in any of the single-lead screw groups (A/B and C/D) or in the short flute double-lead screws (E/F). In addition, the maximum insertion torque of the 9.5 mm double-lead screw with three cutting flutes was less than the same screw with two cutting flutes. In terms of insertion torque, this result indicates that there would be negligible benefit from increasing the number of cutting flutes in single-lead screws when using a pre-drilled hole. However, increasing the number of cutting flutes in the double-lead design provided a significant reduction in insertion torque. Group H, which had the largest cutting surface area of all groups with three cutting flutes, recorded a maximum insertion torque that was 7.81% lower than Group G with two cutting flutes. The greater cutting area offset the additional resistance from the double-lead design as the screw advanced through the test block. Theoretically, cutting flutes can facilitate insertion, and [PERSON] et al. [10] indicated that screws with fewer than three cutting flutes had difficulty cutting threads as the screw rotated into the test material. However, our study did not find any significant difference in initial insertion torque due to different numbers of cutting flutes, possibly due to a pilot hole being pre-drilled in the material before screw insertion.