In this research, we introduced the application of continuous intraoperative neuromonitoring (CIONM) to the surveillance of spinal nerve roots, marking the first instance of its use for this purpose. We successfully developed a consistent and replicable model for the continuous EMG monitoring of spinal nerve roots. Within the sham group, four nerve roots were monitored using CIONM alone, and it was determined that the application of a 1mA, 1Hz stimulus did not result in any observable nerve injury. The control group was established to confirm that the physical contact of the electrode with the nerve root, without any thermal or electrical stimulation, did not lead to any discernible harm or functional impairment of the nerve root.
Employing this method, we were able to observe the temporal onset of nerve root injury at varying temperatures. Our findings indicate that exposure to 50°C can lead to nerve injury within a 5-minute window, and as the temperature increases, the time frame for injury onset decreases. Following a recovery period of approximately 30 minutes, the tEMG signals, which had exhibited a decline in amplitude of over 50%, did not show signs of recovery. This persistent reduction in EMG signal amplitude probably indicates that the thermal injury to the nerves was severe enough to cause lasting damage.
The assessment of nerve injury was further validated through histological examination of tissue sections, providing a pathological confirmation of the observed electrophysiological changes. The observation that there were minimal nerve injuries on Day 1, in contrast to the nearly universal occurrence of nerve injuries in the groups exposed to 50°C and above on Day 7, suggests that the pathological changes induced by sustained heat injury may have a chronic nature. We propose that the pathological alterations resulting from nerve heat damage are likely to be of a chronic nature. However, the effects of this damage can be rapidly detected and manifested through CIONM. This real-time monitoring provides a critical tool for immediate assessment and subsequent clinical decision-making to prevent further nerve damage.
Currently, the application of CIONM is predominantly found in the context of thyroid surgery, where it is used to oversee the recurrent laryngeal nerve[8–11]. The operational mechanism involves the stimulation of the vagus nerve, which leads to the depolarization of nerve fibers. This process generates nerve impulses that travel downwards, prompting muscle movement that, in turn, produces EMG signals. These signals are then captured, amplified, and processed by receiving electrodes to create EMG waveforms. The alert threshold of a 50% decrease in amplitude is considered reliable, as evidenced by both clinical and experimental studies[9, 12]. The CIONM's primary advantage over traditional electrophysiological monitoring techniques is its capacity for real-time surveillance, offering a more sensitive and immediate reflection of EMG changes. This has been substantiated through meta-analytical studies that have confirmed the technique's efficacy and safety[13].
Given that the spinal nerve, much like the vagus nerve, is a peripheral nerve with motor fibers, the same principle of nerve stimulation and EMG signal generation can be applied to spinal nerve root monitoring. When a nerve root is stimulated with a current that exceeds the threshold level, it elicits a contraction in the innervated muscles, thereby generating EMG signals. This theoretical foundation underpins the rationale for utilizing CIONM in the monitoring of spinal nerve roots. In the present study, stimulation electrodes were strategically positioned at the nerve root, while recording electrodes were placed in the muscles corresponding to each nerve root to capture the EMG signals effectively.
Limited data are available regarding the specific temperature thresholds that correlate with thermal injury to spinal nerves. Konno et al.[14] have documented that a temperature of 40°C does not inflict injury on spinal nerves, in contrast to 70°C, which was found to completely impair nerve function within a 5-minute timeframe in a porcine model. Similarly, Tamai et al.[15] have identified a threshold of approximately 48.9°C for nerve injury in a rabbit model. Ohyama et al.[16] observed that following bipolar cauterization near the nerve root, where temperatures averaged 60.98°C, histological damage was evident in 47.8% of the nerves examined. Our study has delineated the critical temperature range to be approximately between 45°C and 50°C, beyond which spinal nerve injury becomes apparent. This finding aligns with the existing body of research. Furthermore, our investigation has provided preliminary insights into the tolerance times of nerve roots when exposed to varying temperatures. We observed that at 50°C, continuous exposure for 4–5 minutes was necessary to cause nerve injury, whereas at 55°C, this period was reduced to nearly 1 minute, and at 60°C and 65°C, injury occurred in approximately 20 seconds. These findings are corroborated by Lin et al.[17], who reported damage to the recurrent laryngeal nerve after a 20-second exposure to 60°C, a result that is congruent with our study's outcomes.
The integration of bone-powered surgical systems in spinal surgery presents a significant risk of spinal nerve thermal injury. Empirical evidence from prior research confirms that temperatures can exceed 50°C during osseous resection, a phenomenon observed irrespective of the tool employed—be it a high-speed drill or an ultrasonic bone cutter[7, 18]. Surgeons must remain vigilant about temperature management when utilizing these instruments to avert potential safety hazards. Surgeons often manually reduce the speed of cutting to minimize heat generation, particularly when elevated temperatures are suspected.
The advent of automated spinal robots is anticipated to partially supplant human operators in performing bone-cutting procedures in the future[1, 2, 19]. Unlike manual operations, which are dependent on the operator's experience, these robotic systems have the potential to exert more intelligent control over heat generation. Our study's findings are expected to inform and guide future research into the development of temperature warning systems for automated spinal robots. This advancement will be instrumental in assisting surgeons in meticulously planning and executing thermally sensitive procedures in the vicinity of the nerve roots, thereby enhancing patient safety and surgical precision.
The study presents several limitations. Firstly, due to its nature as an animal experiment with a small sample size, the extrapolation of these results to human applications may be limited. This is attributable to the potential existence of subtle neural differences between porcine models and humans, which could affect the comparability of the findings. Secondly, the study did not incorporate an assessment of postoperative motor function in the lower limbs. The evaluation was confined to electrophysiological and histological assessments. Given that multiple nerve roots were examined within each pig, any functional impairment of the lower limbs post-surgery would not specifically indicate which nerve roots were compromised. Nonetheless, this approach did contribute to a reduction in the number of animals used, thereby enhancing the ethical considerations and efficiency of the experimental design. Lastly, the method of direct thermal probe contact with the spinal nerve in this study may not perfectly replicate the clinical scenario where heat is transferred from the bone surface to the spinal nerve. This discrepancy could influence the accuracy of the study's findings in terms of real-world applicability. We believe that CIONM will represent a potent and effective method for the real-time surveillance of spinal nerve function.