## 4 Discussion
Indeed, a significantly lower median resistance and inter-quartile range was observed at the Spinecall screw when using this method but not at the Spinelock screw, which did not provide significant differences with the CLIPS methods. This different outcome between screws may be related to specimen preparation, which could have resulted in a less tight conductive ring in the Spinelock screw and, therefore, a lower contact area. Similarly, more robust methods such as the use of conductive epoxy [16] are available, which transforms any contact imperfection between the conductive ring and the screw into a conductive medium while keeping the leads in place. However, such materials are often expensive, and removal is not practical. The CAPSULE method is very similar to WRAP, as the coupling is performed by a conductive ring. However, instead of using a glue to keep the ring in place, a thermo-retractable sleeve was used. This allows the application of an almost uniform pressure around all sides of the conductive ring, thus ensuring a more homogenous large contact surface. This coupling also better mimics real use conditions where the pedicle screw will be tightly surrounded by tissue during its electrical stimulation. With this coupling, a lower resistance and resistivity was achieved in both screw models, and although they are not as low as the reported for Ti-6 Al-4V alloys [23], it is the method with the closest results, thus being the most accurate coupling tested in this study. The differences with respect to the tabled electrical properties of Ti-6 Al-4V may be due to testing conditions, namely ambient temperature, surface features and measuring equipment characteristics. However, the study of [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [17], using more complex apparatus in a material testing laboratory, showed resistivity values identical to those of CAPSULE. Screw resistivity and the accuracy of its measurement are also important for understanding the effect of the anodization as it may influence the overall resistance of the screw [19]. In this study, only the CAPSULE method allowed the measurement of identical resistivity values between pedicle screw models. This is particularly significant, as this is a material dependent property, and both pedicle screw specimens were manufactured with the same titanium alloy and anodization process. This demonstrates that the CAPSULE, when compared to the other coupling methods of this study, is the only method accurate enough to ensure the measurement of pedicle screw electrical properties. It should be noted that the results of this study present some limitations. First, these methods were not tested with pedicle screws from different manufacturers or with different models, such as poliaxial screws, which have been reported to have higher electrical resistance due to the mobile crown [19]. While the specimens used in this study were submitted to the same anodization procedure, it was not verified if they had an identical layer of titanium oxide, which can also contribute to different resistivity. Additionally, it is worthy to mention that this study was a laboratory-based development of a coupling method to measure resistance of pedicle screws. However, once inserted into living tissue, other interactions may happen; therefore, the study of the electrical properties on animal models should be considered as a next step for further characterization of pedicle screws.