## 3 Results  
All of the screws were loaded to failure. Their structures were completely preserved after pullout. Defined as the experimentally measured peak load, the pullout strength of all types of screws is shown in Table 2.  
Comparison among Type I, Type II, and Type III Screws in Type A, B, C, and D Bone Specimens (in Normal and Osteoporotic Bone)  
The typical patterns of the load-displacement curves that illustrate the holding characteristics of different bone specimens combined with different screw types are presented in Figure 5a. All \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Bone Specimens**} & \multicolumn{4}{c}{**Screw Types**} \\ \cline{2-5}  & **Type I (KN)** & **Type II (KN)** & **Type III (KN)** & **Type IV (KN)** \\ \hline Type A & 2.23 \(\pm\) 0.07 & 2.38 \(\pm\) 0.18 & 2.28 \(\pm\) 0.13 & \\ Type B & 2.65 \(\pm\) 0.16 & 2.87 \(\pm\) 0.13 & 2.80 \(\pm\) 0.08 & \\ Type C & 1.56 \(\pm\) 0.23 & 1.26 \(\pm\) 0.06 & 1.32 \(\pm\) 0.13 & 0.98 \(\pm\) 0.15 \\ Type D & 1.99 \(\pm\) 0.11 & 1.80 \(\pm\) 0.14 & 1.82 \(\pm\) 0.09 & \\ Type E & 2.54 \(\pm\) 0.15 & 2.68 \(\pm\) 0.14 & 2.67 \(\pm\) 0.07 & \\ \hline \hline \end{tabular} The four types of screw were: Type I: single-threaded screw; Type II: double-threaded screw; Type III: dual-threaded screw; Type IV: double dual-threaded screw. The five types of bone specimens were: Type A: cancellous bone; Type B: cancellous bone with cortical bone in the upper margin; Type C: osteoporoporotic cancellous bone; Type D: osteoporoporotic cancellous bone with cortical bone in the upper and lower margins. KN: kilo Newton; values are mean \(\pm\) standard deviation. Disclosure: the screws were supplied by TJC life biotechnology company (Korea).  
\end{table} 
Table 2: Pullout strength of all types of screws in various bone specimens. Figure 4: Experimental configuration used for testing the axial pullout strength of pedicle screws. load-displacement curves exhibited similar trends, whereby the load increased sharply as the screws were extracted and then decreased rapidly once the screws were pulled out from the specimens. The displacement at the point of peak load was always less than 1.0 mm. In the Type A bone specimen, which represented normal cancellous bone, the pullout strength required for the Type I screw (conventional pedicle screw) was 2.23 \(\pm\) 0.07 kilo Newtons, while that for the Type III screw (dual-threaded screw), it was 2.28 \(\pm\) 0.13 kilo Newtons; the difference was not significant (\(p\) = 0.471). In the Type B bone specimen, which represented normal cancellous bone with cortical bone, the pullout strengths were 2.65 \(\pm\) 0.16 kilo Newtons and 2.80 \(\pm\) 0.08 kilo Newtons for the Type I and Type III screws, respectively, and there was no significant difference (\(p\) = 0.106). However, in the Type C bone specimen, which represented osteopororotic cancellous bone, the pullout strength required for the Type III screw was 1.32 \(\pm\) 0.13 kilo Newtons, which was significantly lower than that for the Type I screw (1.56 \(\pm\) 0.23 kilo Newtons) (\(p\) = 0.046). Furthermore, in the Type D bone specimen, which represented osteopororotic cancellous bone with cortical bone, the Type III screw exhibited a significantly lower pullout strength (1.82 \(\pm\) 0.09 kilo Newtons) compared to the Type I screw (1.99 \(\pm\) 0.11 kilo Newtons) (\(p\) = 0.031) (Figure 6a). Figure 5: Load–displacement curves from the axial pullout test. (**a**) Type I, II, and III screws in Type A, B, C and D bone specimens, and (**b**) Type I, II, and IV screws in Type C and E bone specimens. Comparison among Type I, Type II, and Type IV screws in Type C and E Bone Specim (in Osteoporotic Bone with and without Bi-cortical Bone)  
Typical load-displacement curves are shown in Figure 5b. The patterns of the load-displacement curves for all screws exhibited similar trends, as described in the above section.