## 2 Materials and Methods  
### Animal Experiment  
The protocol of the experiment was permitted by the Institutional Animal Care and Use Committee of Pusan National University, Yangsan Hospital (2019-015-A1C0(0)). The experiments were conducted on five adult female pigs. The pigs were sedated with an intramuscular injection of ketamine hydrochloride (10 mg/kg) and xylazine (2 mg/kg). Anesthesia was continued with inhalation of 3% isoflurane. The neuromuscular blockage was not used at any moment during the procedure. After completion of the experiment, potassium chloride was intravenously administered for euthanizing the pigs. One of the authors attached pairs of monopolar needle electrodes of the EMG bilaterally to the muscle bellies of the quadriceps, tibialis anterior, medial hamstring, and medial gastrocnemius, each 3 cm apart. A ground plate was attached to the skin between the stimulating and recording sites. A conventional stimulator (Direct nerve probe 100 mm, Medtronic Xomed, Jacksonville, FL, USA) and the novel magnetic stimulator were used for the nerve stimulating cathode on pedicle screws (Figure 1). We developed the magnetic stimulator by taking the wire through which the conventional probe's current flows and replacing only the stimulator part with a magnet that could be attached to other surgical instruments (Figure 2). A nerve integrity monitor (NIM-Response 3.0 system: Medtronic Xomed, Jacksonville, FL, USA) was used to stimulate and record the triggered EMG. A constant current stimulation was delivered, and the intensity of the current was gradually increased (0.1-80.0 mA) until a reaction was obtained. The responses were recorded using a 10-20 kHz filter setting, an amplifier sensitivity of 50 \(\upmu\)V per division, and a 50 ms time base. A subperiosteal approach was adopted posteriorly until the lamina, facet joint, and transverse processes on both sides of the fourth and fifth lumbar vertebrae were exposed. Pedicle screws (5.5 \(\times\) 35 mm) were usually inserted (Group A, 20 pedicle screws) bilaterally  
Figure 1: Magnetic nerve stimulating probe attached with various surgical instruments. The black arrow points to the magnet of the magnetic probe. (**a**) Gearshift probe. (**b**) Ball-tipped probe. (**c**) Pedicle screwdriver. (**d**) Long forcep. Figure 2: Schematic diagram comparing conventional probe and novel magnetic probe. The conventional probe is integrated with the wire, whereas the magnetic probe has the stimulator part replaced with a magnet and could be attached to other surgical instruments. In contrast, the wire carrying the electrical signal is the same for both probes. into the pedicles of the fourth and fifth lumbar vertebrae by using an anatomical landmark (Figures 3 and 4). We performed the pedicle screw fixation in five steps. The first step is to find the point where the horizontal line that bisects the transverse process and the vertical line passing the lateral border of the superior articular facet meet and make an entry using a high-speed burr. The second step is to make a path from the posterior element through the pedicle to the anterior body using a gearshift probe through the created entry. Then, using a ball-tipped probe (Figure 1b), check whether the pedicle wall is intact. If it is intact, it works to widen the path created by using the tapper. Next, determine the length of the screw to be fixed while finally checking the integrity of the pedicle wall using a ball-tipped probe. Finally, we fix the pedicle screws. After fixing the screw, we experimented with measuring the trigger threshold while attaching the magnetic probe to the screwdriver and contacting the conventional probe to the driver tip to where the magnet was attached, as shown in Figure 1c. The screws were stimulated using our novel magnetic stimulating probe and the conventional probe, respectively, increasing the current until a t-EMG reaction was observed in the peripheral muscles. The minimum current (triggered threshold) that provoked a consistent and repetitive response was recorded.