## 4 Discussion
This finding is consistent with those of a previous study [23], reasoning that osteoporporotic cancellous bone with fragile trabecular would be destroyed easily by the screw thread during insertion. Therefore, the screws with a larger region of double threads would damage more extensively the bone-screw interface, leading to a decreased resistance to the pullout force. This proposed mechanism may explain why the dual-threaded screw exhibited significantly lower pullout strength relative to the conventional single-threaded screw in osteoporporotic cancellous bone with and without cortical bone. A previous study is in concordance with the present results and reported that dual-threaded screws increase the insertion torque in poor-quality bone compared to single-threaded screws, without increasing the axial pullout strength [15]. Notably, cortical bone is less affected by osteoporosis relative to cancellous bone [24], and 60% of pullout strength was provided by cortical bone in the pedicle region [15]. Therefore, we designed the double dual-threaded screw with double threads in both the proximal and distal shafts to augment the holding strength in the pedicle and anterior cortex of the vertebral body region. To simulate the screw insertion in a real vertebral body using an anterior cortex purchase technique (Figure 3), we used the foam block with a smaller size (Type C and E bone specimens), in which the tip of the screw could pass through the bottom. Although cancellous bone may experience much larger destruction during insertion of a screw with double threads, the interaction of cortical bone with the double threads in proximal and distal areas could provide enhanced holding power and thus resist the pullout. That may explain why the double dual-threaded screw (Type IV) exhibited higher pullout strength in osteopororotic bone with bi-cortical bone (Type E bone specimen), compared to the conventional single-threaded screw (Type I). Although there was no significant difference because of the small sample size, the trend of statistical significance indicates that the biomechanical performance of the double dual-threaded screw would potentially be superior to that of the conventional pedicle screw. Although a growing number of spinal surgery interventions are using dual-threaded screws as fixation devices to improve the fixation capability for osteopororotic patients, the prioritization of these devices over conventional pedicle screws remains controversial. According to the results of our study, the dual-threaded screw exhibited significantly lower pullout strength in osteopororotic bone, which indicates that it would not be the optimal fixation device for osteopororotic patients. However, the increased biomechanical performance of the double dual-threaded screw in osteopororotic bone with bi-cortical bone margin indicates that this device could be an alternative option for patients with osteoporosis, using an anterior cortex purchase technique. Therefore, the present experimental study may provide spine surgeons with selection strategies for optimal fixation devices according to patients' bone quality and screw insertion technique. Nevertheless, it should be kept in mind that these recommendations are based on results obtained under axial loading conditions, and no conclusion could be made for the other loads applied to screws intraoperatively or postoperatively. Recently, many surgeons have focalized their attention on the selection of pedicle screw material, because corrosion of the external implant would be one of the causes of fixation failure leading to severe side effects in the human body [7; 9]. As a solution for implant corrosion, several materials and coating techniques, including PVD/CVD, micro-arc-oxidation (MAO), and sol-gel coating, were explored to improve corrosion resistance and subsequently reduce harmful ion release [10; 12; 25].