Considering Pedicle Screw Resistance in Electromyography of the Spine.

Background: Evoked electromyographic (EMG) monitoring of pedicle screws has been shown to be an effective adjuvant to image guidance or direct visualization of pedicle screw placement. Electrical stimulation is delivered to the head of the screw at various intensities until a muscle potential is evoked. This practice is based on the fact that an intact pedicle effectively shields nerve roots from electrical stimulus. Several factors have been debated that may affect the interpretation of results; however, to the best of our knowledge, the electrical resistance of modern manufactured pedicle screws and stimulation devices has not been studied.

Objective: To determine if pedicle screw resistances affect triggered EMG.

Methods: Samples of the most commonly implanted pedicle screws in Canada were obtained, with diameters ranging from 4.5 to 7 mm. The resistance between the screw head and thread and core at the midpoint and tip of the screw was recorded using a Multimeter in accordance with IEEE standards. For screws with variable threads, the midpoint was considered the point at which the thread pitch changed.

Results: All screws had low impedances when tested at the point of the screw, but much higher when the cup is tested. The resistance of different manufactures' screws was significantly different, ranging from 0.514 to 2156 Ohms.

Conclusion: Despite differences in resistance, most screws had resistances in ranges that allow for triggered EMG pedicle integrity testing. Resistance from pedicle screws is an important consideration in EMG monitoring of the spine.