## 1 Introduction  
During spinal surgery requiring the instrumentation of the vertebral column, there is a non-negligible risk of damaging the nervous structures [1]. In order to avoid such undesired outcomes, intraoperative neuromonitoring (IONM) techniques may be used to guarantee safety of procedures. Several methods are available, including intraoperative fluoroscopy, trans-cranial stimulation, somatosensory evoked potentials and motor evoked potentials [2].  
In the specific case of intrapedicular instrumentation of the lumbar or thoracic vertebas, the electrical stimulation of either the pedicle track or the pedicle screw is a possible IONM approach. This method is based on the principle that a medial pedicle breach will provide a low-resistance path through which a given electrical current may flow and stimulate a compound muscle potential of the muscles innervated by the neighbouring nerve roots [3]. The electrical current stimulation threshold that would indicate a correct insertion was initially proposed to be 7.0 or 10.0 mA, depending if the pedicle track or the pedicle screw was stimulated [3]. However, other authors have reported wider ranges [4; 5; 6; 7]. While some systematic reviews have identified the current thresholds associated to higher sensitivities of this IONM method [8; 9; 10], there is no complete explanation yet with respect to the high levels of heterogeneity and lack of agreement between studies on the best current stimulation threshold.  
This is a multifactorial issue caused by the multiple situations that may alter the normal nerve stimulation, which not only includes patient intrinsic aspects such as compressed nerve roots [11] but also extrinsic ones, such as the properties of the pedicle screw [12]. Although pedicle screws have been extensively characterized and analysed in terms of their biomechanical [13] and mechanical properties [14; 15], the study of their electrical properties is still scarce. The available studies have shown that electrical properties of screws can differ due to factors such as the manufacturer [16], shaft's hollowness [12] or the application of special coatings such as hydroxyapatite [17]. However, these studies used different coupling techniques to establish a contact between the pedicle screw and the measuring devices' leads, either by using unipolar probes [16], alligator clips [12; 18; 19] or conductive epoxy glues [16]. Since contact resistance is a well-known issue that can increase measured electrical resistance [20] and other contributing factors for increased electrical resistance can also be present in each of the above mentioned methods, the identification or development of an adequate coupling method is necessary. Such coupling is the first requisite to enable accurate standardized measurements and the development of appropriate sensing apparatus for laboratory-based studies of the pedicle screw's electrical properties.  
Therefore, the objective of this study is to (i) present the development of a new coupling method between the pedicle screw and the measuring device's leads by means of thermo-retractable sleeves; (ii) to analyse its measurement accuracy by comparing it with other common measurement methods and the tabled electrical properties of the titanium alloy used to manufacture the pedicle screw; and (iii) to determine if it presents the necessary discriminant ability to measure the titanium alloy electrical resistivity from two different pedicle screws.