Intraoperative neuromonitoring (IONM) is a key technology for minimizing nerve damage during spinal surgery, but electrode selection remains controversial: subcutaneous needle electrodes (NE) offer superior signals but are invasive, while surface-mount electrodes (SE) are noninvasive but offer unstable signals, and there is a lack of standardized selection criteria.
Peking University People's Hospital, in collaboration with multiple institutions, compared the IONM performance of NE and SE (with and without conductive paste) in 40 spinal surgery patients, measuring impedance, myoelectric root mean square (RMS), and motor evoked potential (MEP) amplitude. The results were published in the Journal of Orthopaedic Surgery and Research in 2025.
The study included patients undergoing various spinal surgeries, using total intravenous anesthesia to avoid interference with muscle relaxants. MEPs were induced through transcranial electrical stimulation, and bilateral abductor pollicis brevis myoelectric signals were simultaneously recorded. Nonparametric analyses were used to analyze the data. Results showed that the impedance of the NE (1.95 kΩ) was significantly lower than that of the SE (36.15 kΩ with conductive paste and 95.6 kΩ without conductive paste). The RMS value of the SE (1.57-1.63 mV) was higher than that of the NE (0.55 mV), indicating greater susceptibility to noise interference. Impedance and RMS were positively correlated. The MEP amplitude (159.48 ± 182.64 mV) and signal-to-noise ratio (SNR) of the NE were both higher than those of the SE. The MEP induction success rate was 100%. In 18 cases, no waveform was induced in the SE, and the first MEP was recorded earlier in the NE. The conductive paste did not increase the MEP amplitude in the SE.
Furthermore, there was no significant correlation between BMI and MEP parameters. The SE was susceptible to electromagnetic interference in the operating room, which affected waveform recognition.
Research has clearly established that NE is the preferred IONM technique for spinal surgery due to its low impedance, high signal quality, and high induction success rate. While SE is noninvasive, it has high impedance and is susceptible to interference, especially when monitoring deep muscles, which can result in signal leakage. Conductive paste cannot compensate for this deficiency. Clinically, electrode selection should be tailored to the specific situation: NE is preferred for complex surgeries, while SE can be used cautiously for superficial muscle monitoring and in conjunction with signal amplification technology.
Although this study has limitations such as a small sample size and single-center location, it provides guidance for electrode optimization. Future multicenter studies are needed to explore novel surface electrodes that balance minimal invasiveness with signal reliability to improve surgical safety.