# Alternative Pedicle Screw Design via Biomechanical Evaluation
## 3 Mechanical Strength of the New Design
### Experimental Testing of Bending Strength
#### 3.1.2 Experimental Results and Discussion  
The pressing load for the compression distance is shown in Figure 4. Table 2 shows the loads at the point of rupture for the two conventional screws and the alternative screws, respectively. Each thread  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Diameter (mm)** & **Length (mm)** & **Pitch (mm)** & **Lead (mm)** \\ \hline Single thread & 4.5 & 35 & 2.5 & 2.5 \\ Double thread & 4.5 & 35 & 3.0 & 6.0 \\ Proposed thread & 4.5 & 35 & 2.5/1.25 & 5.0/5.0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Detailed dimensions of the tested screws.  
Figure 3: Experimental setup for bending strength testing: (**a**) The fused structure and (**b**) the material testing machine with the structure.  
was tested six times. As shown in Figure 4, when the load exceeded 160 N and the squeezed distance was more than 5 mm, the slope of the double thread was changed abruptly. From the results of Table 2, it is clear that a failure occurred near the load 160 N. Although this tendency was not clear with the single or proposed threads, it can be seen that there were also fractures at loads of 250-260 N and a compression distance of 10 mm.  
The double thread was vulnerable to bending loads despite the reduction in operation time. This is because it had a longer pitch (\(p\)) and twice the lead compared to the single screw, leading to a reduced number of turns during an operation. The double thread's basic diameter (\(d_{basic}\)), length, and shape were similar to those of the single thread, but the root of the thread got deeper, which made the effective cross-section of the screw smaller than that of the single screw because its pitch was longer.  
This was because the longer pitch makes the tensile stress area narrow, as follows [17]:  
\[A_{t}=k_{1}\frac{\pi}{4}(d_{basic}-k_{2}p)^{2}, \tag{1}\]  
where \(k_{1}\) and \(k_{2}\) are specific constants from the screw shape and dimensions. The small area led to weak strength against bending and loading.  
The alternative design showed the highest strength during bending loading in the experimental tests. The bending resistance of the alternative design was more than 55% higher than that of the conventional double-threaded screw. Thus, even though using it is convenient during surgery, it is difficult to apply it when it has such low strength. The alternative design was 12% stronger than the conventional single thread, although the difference was within the range of one standard deviation. However, the bending resistance of the alternative screw was similar to that of the conventional single thread but required fewer turns during surgery. Therefore, we can conclude that the alternative thread is a good solution for a pedicle screw from a bending resistance perspective.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Test Number** & **Single Thread (N)** & **Double Thread (N)** & **Alternative Thread (N)** \\ \hline
01 & 253.31 & 123.37 & 259.96 \\
02 & 232.67 & 167.33 & 244.38 \\
03 & 278.01 & 181.16 & 275.39 \\
04 & 253.31 & 178.19 & 237.94 \\
05 & 264.94 & 174.05 & 266.52 \\
06 & 252.95 & 175.84 & 270.13 \\ \hline Average & 255.87 & 166.66 & 259.05 \\ \hline Standard deviation & 15.05 & 21.71 & 14.88 \\ \hline \end{tabular}
\end{table}
Table 2: Bending test results: strength vs. bending.  
Figure 4: Typical bending test results: Pressing load vs. compression distance.