## 4 Discussion  
In this experimental testing study, we found that, compared to a conventional pedicle screw, dual-threaded pedicle screws exhibited higher pullout strength in normal-quality bone and significantly  
Figure 6: Comparison of the pullout test results; the error bar shows standard deviation, and the blocks indicate mean values. (**a**) Comparison between Type I and III screws in Type A, B, C and D bone specimens (\(p=0.471\), \(p=0.106\), \(p=0.046\) and \(p=0.031\), respectively), and (**b**) comparison between Type I and Type IV screws in Type C and E bone specimens (\(p=0.006\) and \(p=0.105\), respectively). * Significant difference. lower pullout strength in osteoporporotic bone, after controlling for several factors such as screw length, diameter, and insertion technique. However, using an anterior cortex purchase insertion technique, the newly designed double dual-threaded screw exhibited better pullout biomechanics in osteoporporotic bone with bi-cortical bone. The pullout strength of pedicle screws, as one important mechanism responsible for implant failure, has been widely used to assess the strength of the bone-screw interface in different conditions [19; 20] and is a popular biomechanical testing parameter. Several factors, including the size and design of the screw, pedicle structures, bone quality, size of pilot holes, and insertion technique, may affect the pullout strength of a pedicle screw [13]. Therefore, these variable factors must be controlled for under similar situations during experimental testing to ensure a reliable comparison of differently designed screws. Polyurethane foam blocks with consistent properties are used to simulate a particular human bone density, and this commonly accepted testing material can limit the bias caused by the variation of human bone quality, pedicle structures, and screw-cortical interface [5]. Furthermore, the foam blocks, which are simple and easy to handle, could prevent deformation and breakage during biomechanical tests [18]. Therefore, to investigate the biomechanical performance of differently designed pedicle screws, we used laminated polyurethane foam blocks to compare the pullout strengths in the current experimental study. On the basis of the results of our tests, we found that, when compared to the conventional single-threaded screw (Type I), the dual-threaded screw (Type III) exhibited better pullout strength in normal cancellous bones, either with or without cortical (Type B bone specimen just contains cortical bone, which is different from Type E bone specimen consist of bi-cortical bone), although there were no significant differences. The pullout strength of the screw depends on several variables, most of which are related to the biomechanical properties of the bone-screw interface along the length of the screw. With uniformed dimensions in length, diameter, and thread profile of all screws in this study, the quality of bone and the thread pattern were the main factors affecting the pullout strength of the screws [21; 22]. As the normal cancellous bone contains bone of high quality, the thread pattern determining the properties of the bone-screw interface was critical for the holding strength of the pedicle screw. Because the double-threaded part of the screw affects the properties of the bone-screw interface more extensively, the screws with larger double-threaded regions may display a higher purchase, resisting the pedicle screw pullout in normal cancellous bone. This possibility is supported by the result of our test showing that the full double-threaded screw (Type II) displayed the highest pullout strength in normal cancellous bone with and without cortical bone (Type B and A bone specimens) (Table 2). However, the biomechanical testing in the osteoporporotic bone model yielded different results. Compared to the conventional single-threaded screw (Type I), the dual-threaded screw (Type III) showed significantly lower pullout strength in osteoporporotic bone, either with or without cortical bone (Type D and C bone specimens).