## 2 Materials and Methods  
This study assessed eight different screw designs (Table 1, Figure 1). All screws were made of a titanium alloy (Ti6 Al4V ELI) and had a solid and conical core, full-length threaded shaft, 6.0 mm major diameter, 4.4 mm minor diameter, 35 mm shaft length, and a V-shaped thread with a 2.5 mm pitch for single-lead screws and 5.0 mm pitch for double-lead screws. The control screws (Groups A and E) were commercially available pedicle screws commonly used in spinal surgery (OCTOPODA, Bricon GmbH, Wurmlingen, Germany) and included two full-length flutes spaced at 2.9 mm. The six other screws (Groups B to D and F to H) were experimental prototypes designed explicitly for use in this study by the investigators and manufactured by Bricon GmbH.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Screw Group** & **Lead of Thread** & **Number of Cutting Flutes** & **Length of Flutes (mm)** \\ \hline A & & 2 & 2.9 \\ B & single-lead & 3 & 2.9 \\ \hline C & & 2 & 9.5 \\ D & & 3 & \\ \hline E & & 2 & 2.9 \\ F & double-lead & 3 & 9.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Design features of eight pedicle screws evaluated in this study.  
This study aimed to replicate implantation in a healthy human vertebra. To achieve this, synthetic sawbones (Sawbones, Pacific Research Laboratories, Vashon, WA, USA) were used which had a similar density (0.32 g/cm\({}^{3}\), 20 PCF, solid polyurethane foam) to the pedicles of vertebrae [14]. The sawbones were shaped into sample blocks of size 60 mm \(\times\) 40 mm \(\times\) 40 mm (width \(\times\) depth \(\times\) height). Each insertion site was pre-drilled using a 2.7 mm drill bit to a depth of 35 mm to prepare the trajectory of the pedicle probe according to the surgical procedure recommended by Bricon GmbH.  
A universal testing machine (eXpert 8602, ADMET, Norwood, MA, USA) equipped with a custom-made pulling jig was used to perform the insertion torque and pullout strength tests in accordance with ASTM F543 [3] (Figure 2). The machine had an axial force capability of 2.2 kN and torque capacity of 20 Nm. To insert the screws into the test block, each screw was rotated at a rate of 5 rev/min under an axial load of 11.18 N and inserted to a depth of 35 mm. The insertion torque was recorded during insertion from the initial to final revolution of the screw. Once fully inserted, the pullout test was performed by pulling the screw away from the test block at a loading rate of 1 mm/s. The fixation strength of each screw was defined by the maximum load recorded prior to failure by any means. Each test setup was performed five times with separate screws and separate sawbone samples.  
[PERSON]'s post hoc test with a level of significance of 0.05 was used to ascertain significant differences between individual means when the analysis of variance resulted in significant differences.  
Figure 1: Screws with varying leads of thread and cutting flute length. A differential screw uses a spindle with two screw threads of differing leads, a single-lead means equal to the thread pitch and double-lead means twice the thread pitch. Major diameter is the diameter at the thickest part of the thread. Minor diameter is the thickness at the base of the screw.