Enhancement of Intraoperative Transcranial Motor-evoked Potential Derivation with the Use of a Fibular Head Reference Electrode Placement Method in Spine Surgery

Intraoperative transcranial motor-evoked potential recordings commonly utilize the belly tendon montage method; however, its reliability in large lower limb muscles such as the quadriceps femoris and hamstring muscles can be limited. We hypothesized that placing the reference electrode on the fibular head (fibular head reference electrode method) would improve waveform clarity and stability compared to belly tendon montage. This retrospective study analyzed 101 patients who underwent transcranial motor-evoked potential monitoring during spinal surgery at Nagoya University Hospital from October 2021 to August 2023. We compared compound muscle action potential amplitude, baseline waveform derivation success rate, noise amplitude, signal-to-noise ratio, and mean consecutive ratio between belly tendon montage and fibular head reference electrode method. Fibular head reference electrode method showed significantly higher median compound muscle action potential amplitudes in quadricep femoris (202.5 μV vs. 52.5 μV) and hamstring muscles (131.0 μV vs. 33.3 μV) with p < 0.001. Baseline waveform derivation success rates also improved with fibular head reference electrode method (quadricep femoris: 76.5% vs. 50.5%; hamstring muscles: 73.5% vs. 39.8%; p < 0.001). When noise amplitude increased, the signal-to-noise ratio improved significantly (p < 0.001), and mean consecutive ratio indicated better waveform stability in hamstring muscles. For predicting postoperative paralysis, fibular head reference electrode method yielded a sensitivity of 100% and a negative predictive value of 100% in both quadricep femoris and hamstring muscles, with specificities of 93.5% and 95.6%, respectively. These findings suggest that fibular head reference electrode method improves the reliability, clarity, and predictive value of transcranial motor-evoked potential monitoring in spinal surgery and may represent a superior alternative to conventional belly tendon montage.