D. Greg Anderson, MD, Philadelphia, PA, USA; Lawrence R. Wierzbowski, PhD, Daniel M. Schwartz, PhD, Bala Cynwyd, PA, USA; Alan S. Hilibrand, MD, Alexander R. Vaccaro, MD, Todd J. Albert, MD, Philadelphia, PA, USAPurpose: Stimulus-evoked electromyograph (EMG) testing has become a common procedure used to verify the accuracy of lumbar pedicle screw placement. Current protocols for stimulus-evoked EMG testing assume that the electrical resistance of a pedicle screw being tested is negligible. Theoretically, a high-resistance pedicle screw could lead to a false-negative testing result, because the level of current flow through the circuit might be insufficient to stimulate an adjacent nerve root in the setting of a pedicle breech. In this study, we sought to determine the electrical resistance of eight commonly used pedicle screws by measuring the resistance across various portions of the screw to determine if a high-resistance circuit was likely to occur during clinical testing using stimulus-evoked EMG.Methods: Eight types of pedicle screws—Acromed Isola (7 × 40 mm), DePuy Moss-Miami (7 × 40 mm), Stryker Xia (7.5 × 40 mm), Sofmor Danek Multi-Axial (7.5 × 40 mm), Synthes Side-Opening USS (7 × 40 mm), Synthes Variable-Axis USS (7 × 40 mm) Spinal Concepts Flexible Angle BacFix (7 × 40 mm) and Spinal Concepts Multi-Axial BacFix (7 × 40 mm)—were subjected to resistance testing across various portions of the screw. Five screws of each type (40 screws total) were tested using a standard Fluke volt-ohm meter. Resistance testing was performed by securing each screw with a clamp device and then placing direct current (DC) electrodes on the screw in various locations. Direct current resistance testing of screws was chosen, because DC current is used during intraoperative stimulus-evoked EMG testing. Each portion of the screw was tested three times to account for variability in electrode placement. All screws underwent resistance testing across the screw shank. Fixed screws (Isola, Synthes Side-Opening and Flexible Angle BacFix) were tested across the length of the screw by placing electrodes on the proximal and distal ends of each screw. Screws with mobile crowns (Moss-Miami, Xia, Danek Multi-Axial, Synthes Variable-Axis and Multi-Axial BacFix) were tested between the mobile crown and screw shank and between the hexagonal port (small screwdriver socket) and screw shank. Testing between the mobile crown and screw shank was performed with the mobile crown placed in three random positions to determine if the resistance is affected by the position of the screw crown.Results: The screw shank resistance of all tested screws fell into the 0- to 36.4-Ohm range. The resistance between the proximal and distal ends of fixed screws ranged from 0.1 to 31.8 Ohms. The resistance between the hexagonal port and screw shank of mobile crown screws ranged from 0 to 25 Ohms. In contrast, the resistance between the mobile crown and screw shank was widely variable ranging from 0.1 Ohms to an open circuit. Overall, screws with mobile crowns demonstrated an open circuit in 24% of the measurements taken and a high resistance circuit (greater than 1,000 Ohms) in 7% of the measurements. In general, the resistance of a given screw was noted to vary widely, depending on the position of the mobile crown.Discussion: The accuracy of stimulus-evoked EMG testing is dependent on electrical stimulation of surrounding nerve roots in the setting of a pedicle breech. Nerve root stimulation is detected by observing compound action potentials in the distal muscle groups on EMG monitoring. To stimulate nerve roots, a voltage and current threshold must be reached. High resistance in the electrical circuit formed during testing will impede electrical current and could prevent the threshold for nerve root stimulation from being reached in the setting of a pedicle breech leading to false-negative testing results. Several factors are known to affect accuracy of stimulus-evoked EMG testing, including depth of anesthesia, blood in the surgical field and chronic compression of nerve roots. However, it has not been previously shown that some pedicle screw implants have the potential to create a high resistance circuit and could lead to false-negative testing with stimulus-evoked EMG. With a screw implanted in the pedicle, the accessible portions of the screw for clinical testing include the crown and hexagonal port. This study has shown that the crown should not be chosen as the testing site with mobile crown screws, because testing of the crown can lead to a high resistance circuit in some positions. Resistance between the hexagonal port and the screw shank was low in all of the screws tested and should provide an accurate location for probe placement during stimulus-evoked EMG testing.