## 4 Discussion
The effect of screw geometry on pullout strength increased as bone density increased [17]. Previous studies reported an effect of the outer/inner diameter shape or thread type on the pullout strength, leading to the conclusion that a conical core compressed the surrounding bone during screw insertion, thereby increasing the insertional torque and pullout strength [31; 32]; this was further confirmed with finite element analyses [33; 34]. [PERSON] et al. used the concept of the flank overlap area, the contact surface between the implant and engaged bone, to illustrate that decreasing the inner diameter and maintaining the outer diameter of the screw would increase the compression of adjacent bone and, thus, increase the pullout strength [35]. The novelty of our study is that we quantified the EBV for various screw geometries and further compared the EBV with the pullout strength. A strong positive correlation (R\({}^{2}\) = 0.8595) was found between the EBV and the pullout strength, and Cy/Co-V showed the highest EBV and pullout strength values among all the screw geometries. Our results demonstrated that the pullout strength could be evaluated by EBV for bone with a homogenous density. Our biomechanical data show that smaller pilot holes lead to higher insertional torques and pullout strengths. Although these results are similar to those of previous studies [15; 20; 22], our study is novel in the control of the screw size, bone density and pilot-hole tapping method. [PERSON] et al. inserted 10 different commercialized pedicle screws into 20-pcf test blocks using tapped (3.17 mm) or untapped pilot holes and found that the pullout strength was lower for tapped than for untapped holes [15]. However, consistent results are difficult to obtain due to highly variable screw geometries and sizes. [PERSON] et al. used self-tapping cortical bone screws inserted into synthetic osteopororotic bone with pilot holes of different sizes, ranging from 70 to 80% of the outer diameter, and found that a pilot-hole size of 71.5% of the outer diameter was a critical threshold for pullout strength [20]. These results are not comparable to those of our study because of the osteopororotic density and unclear thread types. [PERSON] et al. evaluated the insertional torque and pullout force of two sizes of pedicle screws with different pilot-hole sizes and found that the fixation stability was influenced even when using pilot holes that were smaller than the screw sizes [22]. A rough conclusion may be drawn by using pilot-holes that are only one size smaller than the outer screw diameter. The largest pilot hole used in the present study was 3.7 mm, which was smaller than the inner diameter of the tested screws and allowed the use of the EBV to evaluate the pullout strength. For all three pilot-hole sizes, the pullout strength was higher for the V-threaded screws than for the S-threaded screws, and Cy/Co-V showed the highest pullout strength among the other screw geometries, which corresponds to the graph showing the correlation between the pullout force and the EBV. There was no significant difference in the insertional torque between the V-threaded and S-threaded groups for either density or for any of the three pilot-hole sizes, which means that the manual sensation experienced during clinical screw insertion and the value of insertional torque cannot represent the true screw fixation stability. For example, the lowest torque exhibited by Cy/Cy-V did not correspond to the lowest pullout strength among all six configurations. Spinal fusion surgeries are performed for a wide spectrum of indications, including trauma, infection, tumor, deformity and congenital anomalies. The goal of spinal fusion is to realign normal anatomy and restore biomechanical stability. Inadequate fixation and subsequent motion may cause pseudarthrosis-induced axial back pain, subsequent implant failure or interbody cage retrepulsion [36; 37]. The superior biomechanical performance of Cy/Co-V screws could be clinically recommended to ensure secure fixation.