## 4 Discussion  
The electrical properties of pedicle screws have been reported in the scientific literature as measured with different coupling methods. However, not all methods are the same when ensuring an adequate coupling between the pedicle screw and measuring leads. The actual resistance of the screw is a recurrent concern and suspected of being one the causes of false-negatives in intra-operative electrical stimulation [19]; thus, the use of adequate coupling methods while characterizing the pedicle screws is of utmost importance. This study explored a selection of coupling methods to measure the electrical properties of pedicle screws and tested it on two screw models of the same material and anodization process. The electrical resistivity of the Ti-6 Al-4V alloy is known to be 1.71 u\(\Omega\)-m [23], meaning that the expected resistance considering the screws' diameter and measurement region length is 1.075 m\(\Omega\) for the Spinelock and 0.871 m\(\Omega\) for the Spinecall. Since electrical resistance and resistivity are intrinsic to the material and, as such, should remain constant across different coupling methods, a method that provides results closer to these tabled values should be the most adequate for pedicle screw electrical characterization. The application of multimeter probes to the pedicle screw is probably the easiest method for measuring its electrical properties, and the comparatively small probe diameter allows accurate placement over a given section of the screw. While this method provided  
Figure 4: Electrical resistivity measured by each coupling method for each analyzed pedicle screw. Coupling methods with no statistical difference (\(p>0.05\)) within (*) and between (#) screws are denoted, as well as the existence of outliers (+). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Mann–Whitney U** & **Z** & **\(p\) Value** & **\(r\)** \\ \hline PROBE & 491.000 & 5.237 & \(<\)0.001 & 0.524 \\ CLIPS & 559.500 & 4.761 & \(<\)0.001 & 0.476 \\ WRAP & 544.000 & 4.868 & \(<\)0.001 & 0.487 \\ CAPSULE & 910.500 & 2.343 & 0.019 & 0.234 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the resistivity of each pedicle screw according to the measuring method. the second lowest resistance measurements on the Spinelock screw, there was a need to apply a considerable amount of force to ensure contact between the probes and the pedicle screw. If an insufficient amount of force was applied, the voltmeter values would not stabilize. This is probably the result of the reduced contact area between the probe and the screw due to both structures presenting a round surface. Alternatively, alligator clips are commonly reported as the chosen coupling method in this type of study [12, 18, 19]. The jagged shape of the clip's teeth and the bracing force provided by the spring allow an apparent good grip to the screw' threads. However, the gripping force exerted will vary according to the screw's diameter, and the teeth may not conform to the thread's shape, resulting in reduced contact surfaces and hence higher electrical resistance. The intrinsic instability of this coupling was also noted by [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [18], who reported that rotating the screw under the clips or increasing its pressure resulted in electrical resistance changes. These assertions are corroborated by our results, with the overall increase in resistance and the higher inter-quartile range due to the reapplication of the clips or rotation of the screws, being observed in both screws specimens. These attributes make the CLIPS a poorly reliable coupling method for resistance measurements. If the contact area and the conformation to the shape of the screw are critical aspects of a good coupling, wrapping a thigh conductive ring around its shaft should provide better results.