The efficacy of the electrical nerve stimulation method for localizing the inferior alveolar nerve (IAN) within the mandibular bone was evaluated. Six New Zealand rabbits were used (both sides of the mandible). The IAN was stimulated through the mandibular bone and compound action potentials (CAPs) were recorded proximally from the main trunk of the nerve. Stimulation current pulse widths were set at 0.05, 0.1, 0.3, 0.5, and 1 ms. The minimum current magnitude that generated a CAP with a criterion level (300 mV peak-to-peak amplitude) was measured in the range of 0.05–5 mA. Correlations between the distance of the IAN from the active electrode site and the minimum current magnitudes were studied for each pulse width. The correlation coefficients were 0.678, 0.807, 0.893, 0.851, and 0.890 for the pulse widths of 0.05, 0.1, 0.3, 0.5, and 1 ms, respectively. The minimum current producing the criterion CAP response in the IAN was significantly ( P < 0.0001 for all pulse widths) and highly correlated with the distance between the stimulation site and the nerve. The results suggest that electrical nerve stimulation is a promising method that can be used for the localization of the IAN, especially during mandibular implant surgery.
Inferior alveolar nerve (IAN) injury associated with mandibular surgery is a complication contributing to altered sensation of the lower lip, chin, and buccal gingiva. The outcomes of such an iatrogenic nerve injury are usually devastating for the patient and this remains a complex clinical problem with major medico-legal implications. The risk of iatrogenic IAN injury depends on the procedure performed, the technique used, and the surgeon’s experience. Careful preoperative examination of the location and the course of the IAN is essential for mandibular surgeries associated with the risk of nerve injury, e.g. impacted third molar extraction, dental implant surgery, orthognathic surgery, and excision of pathological lesions.
Current techniques used to detect the location and the course of the IAN include direct dental radiographs, panoramic radiographs, computerized radiographs, surgical navigation, and cone beam computerized radiographs. All of these techniques require additional radiation exposure (if used solely for nerve localization in addition to standard preoperative diagnostic techniques) and are associated with difficulties in the intraoperative localization of the nerve bundle. Although direct radiographs can be used intraoperatively, this is a time-consuming method and may be associated with radiographic errors due to incorrect positioning of the sensor. Surgical navigation provides intraoperative detection of the location of the IAN. The disadvantages of surgical navigation include the requirement for sophisticated preoperative planning and setup, and the possibility of navigation errors. Intraoperative neurophysiological monitoring (IONM) may be an additional and objective method for localization of the IAN during mandibular surgeries.
IONM has been used widely during surgical procedures since the 1970s. IONM is based on the evoked potentials that are obtained by an electrical stimulation. The electrical nerve stimulation method is considered the gold standard for locating peripheral nerves. Electrical stimulation of the peripheral nerve results in muscular twitching or paresthesia of the innervated region, depending on the characteristics of the nerve. By using electrical nerve stimulation, it is possible to confirm the proximity of the peripheral nerve. Despite the long history of clinical use of the electrical nerve stimulation technique, it does not appear to have been studied in the mandible for locating the IAN.
The aim of the present study was to evaluate the efficacy of the electrical nerve stimulation method for the localization of the IAN within the mandibular bone. This was achieved by stimulating the nerve passing through the mandibular canal and recording from the main trunk of the IAN.
Materials and methods
Subjects and anaesthesia
The study was reviewed and approved by the Institutional Ethics Committee for the Local Use of Animals in Experiments of Boğaziçi University. Six skeletally mature male New Zealand white rabbits, weighing between 2.1 and 3.7 kg (mean 2.69 ± 0.46 kg), were used. Preoperatively, all rabbits were monitored daily for 2 weeks with regard to their general health and food intake. All rabbits survived this phase without any significant systemic or local pathology or impairment in their general health. Each rabbit underwent the same surgical procedure and the electrical nerve stimulation method.
The rabbits were anaesthetized with 35 mg/kg ketamine (Alfamyne; Egevet, Izmir, Turkey) and 5 mg/kg xylazine (Rompun, Bayer, Istanbul, Turkey), initially via intramuscular route. After induction, the anaesthesia level was maintained with 2% isoflurane (Forane; Abbott, Istanbul, Turkey) in oxygen. The surgical site was shaved and prepared with 10% povidone–iodine solution. After securing the animal in a supine position, the submandibular region was prepared and draped under aseptic conditions. The inferior border of the mandible was approached via submandibular incision. Vital signs of the rabbits were monitored continuously during the operation.
Surgery and identification of related structures
A skin incision of approximately 2 cm in length was made parallel to the inferior–lateral surface of the mandible. After incising the skin and the platysma muscle, the overlying fascia was dissected. The facial artery and vein were ligated. The periosteum was reflected and the inferior border of the mandible was exposed. The medial pterygoid muscle was dissected bluntly and the lingual and the inferior alveolar nerves were approached at the posterior aspect. The periosteal dissection was continued anteriorly and the mental nerve bundle identified. Finally, the IAN, the mental nerve, and the inferior border of the mandible were exposed.
Nerve stimulation and recording setup
A commercially available nerve stimulator device (Tracer III Portex; Smiths Medical, Kent, UK) was used to electrically stimulate the IAN. The device delivers electrical current between 0.05 and 5 mA at 0.05–1 ms pulse widths and at 1 or 2 Hz pulse frequency. The current output, pulse width, and pulse frequency are selectable and can be controlled with a foot pedal. Initially, the output of the device was attached to the mental nerve bundle via stainless steel needle electrodes (30 AWG) to check that the entire stimulation and recording system was working properly. On the posterior aspect, stainless steel hook electrodes (diameter 0.254 mm) were attached to the IAN (1 cm distance between recording electrodes) and the evoked potential signals were amplified by a custom-made differential AC amplifier (gain 100/1000, low-pass filter cut-off frequency 5 kHz). The ground electrode was placed under the skin away from the surgical site. The voltage across the stimulation electrodes and the evoked-potential amplifier output were monitored with an oscilloscope (TDS 220; Tektronix, Wilsonville, OR, USA). For some trials, the digitized data of triggered oscilloscope channels were saved directly to a USB memory stick. Since this took a considerable amount of time (sometimes several minutes), the digitized data were not saved in every trial to avoid delays during the experiment.
After initial testing, the IAN stimulation was performed through the mandibular bone between the mental foramen and foramen linguae. For this purpose, an annealed stainless steel wire (diameter 0.254 mm) was insulated with capillary glass (outer diameter 1.0 mm) using cyanoacrylate glue and acted as the cathode. The stimulating metal end was cut flush with the glass, and depth markings were made. The anode was a stainless steel needle (30 AWG) placed anteriorly at the mental nerve. Figure 1 a shows a diagram of the IAN stimulation and recording setup.