Analgesic effects of ultrasound-guided inferior alveolar nerve block for extraction of impacted mandibular third molar: a retrospective cohort study

Introduction

Background

Impacted mandibular third molar (IMTM) extraction is the most common oral and maxillofacial surgery, with a reported worldwide prevalence of 25% (1). IMTM is associated with severe postoperative pain despite various analgesic strategies (2-8).

Rationale and knowledge gap

Traditionally, the intraoral inferior alveolar nerve block (IANB) has been used in dentistry for pain management (6-10); however, this procedure has several limitations. IANB is performed by blindly advancing the needle tip intraorally just above the mandibular sulcus or lingula (9). Its success rate is 75–85%, and the risk of blood aspiration is reported in 2.9–22% of cases (9,10). For these limitations, performing the traditional IANB without patient feedback under general anesthesia is considered risky in our institution. This block targets the pterygomandibular space (PMS), a cleft surrounded laterally by the mandible and the masseter, and medially by the pterygoid muscles, through which the mandibular nerve branches pass. The ultrasound-guided inferior alveolar nerve block (UGIANB) is a new approach for accessing this area from the body surface. The advantages of UGIANB over IANB are the real-time visualization of the needle tip, local anesthetic, and important structures such as arteries using ultrasound equipment. The efficacy and safety of ultrasound-guided nerve blocks in the extremities and trunk have been widely reported. More recently, high success rates and efficacy of UGIANB for mandibular surgery under general anesthesia have also been noted (11,12). However, there is limited evidence regarding its use in IMTM extraction.

Objective

Our institution has been performing UGIANB since May 2022. We hypothesized that its application would lead to better postoperative pain management compared to general anesthesia alone. Therefore, this study aimed to evaluate the postoperative analgesic effect of UGIANB for IMTM extraction. We present this article in accordance with the STROBE reporting checklist (available at https://joma.amegroups.com/article/view/10.21037/joma-24-19/rc) (13).

Methods

Patient selection

This single-center retrospective cohort study analyzed data from patients who underwent bilateral or unilateral IMTM extraction under general anesthesia at the Hinode Makomanai Dental Hospital in Sapporo, Japan, between June 2020 and September 2022. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). This study was approved by the Institutional Review Board of Hinode Dental Makomanai Hospital (approval code: 22-5, approval date: November 2, 2022) and registered with the Japan Registry of Clinical Trials (clinical trial plan number: jRCT101122028, date: December 16, 2022) and individual consent for this retrospective analysis was waived.

The data analysis and statistical plan were established before extracting the electronic medical records data. Patients (aged ≥16 years) with IMTM extracted under general anesthesia were screened for eligibility. Exclusion criteria included patients with an American Society of Anesthesiologists physical status classification of ≥ III, intellectual disability, or additional dental procedures (e.g., composite fillings, prosthetic inlays).

Primary intervention

The primary intervention was performing the UGIANB with general anesthesia (UGIANB group), and in the control, not performing the UGIANB with general anesthesia (Control group). From June 2020 to April 2021, before the introduction of the UGIANB technique, all cases were in the Control group. From May 2021 to September 2022, after the introduction, both groups were included. The flowchart of patient allocation is depicted in Figure 1. The primary outcome was the time to the first postoperative rescue analgesic administration. The secondary outcomes were the numeric rating scale (NRS) score for pain intensity at 24 hours, postoperatively, and complications of the nerve block, such as local anesthetic systemic toxicity (LAST), hematoma, and local infection.

Figure 1 Study flow diagram. ASA-PS, American Society of Anesthesiologists physical status; UGIANB; ultrasound-guided inferior alveolar nerve block.

General anesthesia management

Anesthesiologists determined general anesthesia management, including airway management, maintenance anesthetics, opioids, and non-opioid analgesics. Airway management was selected based on the patient’s aspiration risk, procedural difficulty, and operative time, using either tracheal intubation (Spiral Tube; Fuji System Japan, Tokyo, Japan) or supraglottic airway insertion (AuraOnce^TM; Ambu, Copenhagen, Denmark). General anesthesia was maintained with sevoflurane, desflurane, propofol, or remimazolam. Intraoperative analgesia included fentanyl and remifentanil, with acetaminophen or flurbiprofen as non-opioid options as needed. For local infiltration anesthesia, 2% lidocaine with 1:80,000 adrenaline was used in all patients to prevent bleeding, and 1.8 mL was administered preoperatively per tooth extraction.

Analgesic procedure

The intervention variable was the analgesic procedure (UGIANB vs. Control group). In the UGIANB group, all patients received UGIANB on each side of the surgical area, postoperatively, under general anesthesia by one dental anesthesiologist (D.O.). A 0.375% levobupivacaine was prepared by mixing 75 mg/10 mL of levobupivacaine (Popscaine^® 0.75% in, Maruishi Pharmaceutical Co., Osaka, Japan) with 10 mL of normal saline. A high-frequency linear probe (Venue Go; GE Healthcare, Chicago, IL, USA) was positioned transversely under the zygomatic arch to visualize the PMS. A mouth opener was used to open the mouth fully (12), and a 22-G, 5-cm echogenic needle (SonolectNeedle USG-type corner cube reflector; Hakko Co., Nagano, Japan) was inserted from the cranial side of the probe using an out-of-plane technique. We administered a small amount of local anesthetic when the needle tip was seen to pass beyond the masseter muscle. If the entire lateral pterygoid muscle posterior to the PMS was seen to have been pushed down on ultrasound images, we considered the dose to have been administered intra-PMS. We administered 5 mL of 0.375% levobupivacaine to the facial region corresponding to the site(s) of surgery (Figure 2).

Figure 2 Ultrasound images and procedure of UGIANB. (A) Ultrasound image showing the anatomical landmarks for UGIANB. (B) Anatomical model demonstrating the insertion point for UGIANB. (C) Clinical application of UGIANB with ultrasound guidance. The yellow arrow indicates the injection site of the solution. The orange areas indicate, from top to bottom, the masseter muscle, temporalis muscle, and lateral pterygoid muscle. The yellow area represents the pterygomandibular space. The red line indicates the maxillary artery. UGIANB, ultrasound-guided inferior alveolar nerve block.

Surgery procedure

All surgeries were performed by two experienced oral surgeons, each with over 20 years of experience (one of them being a co-author S.O.). All tooth extractions were performed in accordance with the conventional method. The mucoperiosteal flap extended from the mesial corner of the second molar distally to the retromolar region. Bone was removed using both a round and a fissure bur in a high-speed handpiece. The tooth was sectioned by the fissure bur and dislocated and removed using an elevator. The wound was irrigated with physiologic saline solution. A 4-0 nylon suture was used to close the wound without tension.

Postoperative care

The physician selected postoperative oral analgesics as either 1,000 mg acetaminophen or 60 mg loxoprofen sodium hydrate, depending on the expected intensity of postoperative pain and the patient’s condition. Both drugs were administered every 8 hours postoperatively. All patients received a 100 mg cefditoren pivoxil tablet as an antibiotic every 8 hours. If additional rescue analgesia was required, the type and dosage of analgesia were selected by the physician and recorded in the medical records. Pain intensity was assessed preoperatively and at discharge, 24 hours post-surgery using the NRS score.

Statistical analysis

Regarding the primary outcome, postoperative rescue analgesics, 45% of patients in the conventional general anesthesia group at our facility did not require analgesics within 24 hours. After introducing UGIANB, 5 out of the first 6 patients (83%) did not require rescue analgesics. Based on this data, we calculated the required sample size as N=27, assuming an alpha error of 0.05 and a power of 80%. To account for a 10% dropout rate, we determined a final sample size of 30 patients per group. The calculation was performed using EZR, which is based on R (R Foundation for Statistical Computing, Bienna, Austria). Covariates included demographics [age, sex, body mass index, American Society of Anesthesiologists physical status, impaction scale for mandibular third molar (14)], clinical data (preoperative NRS pain score, operative time, anesthesia time, number of teeth extracted, airway management, total consumption of fentanyl and remifentanil, type of non-opioid analgesics), and postoperative data (time to first rescue analgesia, type of regular oral analgesics, postoperative NRS pain score at 24 hours, complications). All data were obtained from electronic medical and anesthesia records.

For continuous data, normality was assessed using the Shapiro-Wilk test. Data with a normal distribution are presented as mean and standard deviation, whereas those without a normal distribution are presented as median and interquartile range. Categorical variables are presented as absolute frequencies and percentages.

Bivariate analysis was performed to evaluate the differences in baseline characteristics between patients in the UGIANB and Control groups. Welch’s t-test or the Mann-Whitney U-test was used to compare quantitative variables, depending on the distribution. Categorical variables were analyzed using the Chi-square test or Fisher’s exact test, depending on the statistical assumptions. After comparing the baseline characteristics of the two clinical groups, comparisons were made for the first postoperative rescue analgesic time (minutes), postoperative pain assessment according to NRS at 24 hours, and complications of nerve block (LAST, hematoma, and local infection). Kaplan-Meier survival analysis was performed to evaluate the first postoperative rescue analgesic time. The log-rank test was then used to evaluate the statistical significance of the differences in the survival curves. Statistical significance was set with a P value <0.05. All analyses were performed using JMP Pro (JMP^® Pro version 17, SAS Institute Japan Co., Tokyo, Japan).

Results

Among the 227 patients enrolled in this study, patients were excluded based on the exclusion criteria (n=63) and missing data (n=105), to proceed with 59 patients finally [UGIANB group (n=29) and Control group (n=30)] in the analysis (Figure 1). The total of 108 IMTM was extracted under general anesthesia, and we found that the impacted scale in the UGIANB group and the Control group was as follows: A (18.9% vs. 14.6%), B (66.0% vs. 70.8%), and C (15.1% vs. 14.6%) (P=0.89). The baseline characteristics of the study population are presented in Table 1. We found a difference in the percentage of females between the UGIANB and Control groups (72.4% vs. 46.7%, P=0.04).

Perioperative data of the study population are presented in Table 2. Several variables were unbalanced between the groups. The mean (standard deviation) operative time was 31.8 (14.1) minutes for UGIANB and 40.0 (15.7) minutes for Control (P=0.04). The median (interquartile range) fentanyl consumption in the groups was UGIANB 0 (0, 0) and Control 0 (0, 100) mcg, respectively (P<0.001), while the median (interquartile range) remifentanil consumption was UGIANB 0.5 (0.3, 0.6) and Control 0.0 (0.0, 0.4) mg, respectively (P<0.001). The absolute frequency and percentages of acetaminophen administered as intraoperative adjuvant analgesia were 23 (79.3%) for the UGIANB group and 1 (3.3%) for the Control group, while those of flurbiprofen were 6 (20.7%) for the UGIANB group and 29 (96.7%) for the Control group (P<0.001). The percentages of acetaminophen administered as postoperative adjuvant analgesia correspondingly were 14 (48.3%) for the UGIANB group and 8 (26.7%) for the Control group, while those of loxoprofen sodium hydrate were 15 (51.7%) for the UGIANB group and 22 (73.3%) for the Control group (P=0.09).

The median [interquartile range] time to the first postoperative rescue analgesia was longer with UGIANB than with Control {461 [363–511] vs. 266 [157–495] min; P=0.049} (Table 3, Figure 3). The median [interquartile range] NRS pain score 24 hours after surgery was significantly lower in the UGIANB group than in the Control group {3 [1.5, 5] vs. 4.5 [3, 6]; P=0.02}. No complications were associated with the UGIANB and Control procedures in either group.

Figure 3 Kaplan-Meier analysis for postoperative time to first rescue analgesia of the UGIANB and Control groups. UGIANB, ultrasound guided inferior alveolar nerve block.

Discussion

Key findings

In this retrospective study, we compared the postoperative analgesic effects of UGIANB with the local infiltration anesthesia alone for patients undergoing IMTM extraction under general anesthesia. In the primary analysis, UGIANB was associated with a significantly prolonged time to the first rescue analgesia, and a lower NRS score at 24 hours postoperatively compared to the Control group, without any UGIANB-related complications.

Explanations of findings

This study indicates that administering UGIANB after IMTM extraction surgery prolonged the median time to use rescue analgesics by nearly 200 minutes compared to conventional analgesic methods, allowing patients to go over 7 hours without needing rescue analgesics. Additionally, it reduced the median NRS pain score on the first postoperative day by 1.5 points. IMTM extraction is one of the most common surgeries in the oral and maxillofacial region and is known to involve very severe postoperative pain.

Postoperative pain for the extraction of IMTM in Japanese patients, the highest pain intensity on postoperative day one while under local infiltration anesthesia was 84.8 (15.8) [visual analog scale (0–100), mean (standard deviation)] (5), while under local infiltration anesthesia, the intraoral IANB ranged from 50 to 64 [visual analog scale (0–100), mean] (7). The National Comprehensive Cancer Network guidelines reported that an NRS range of 0–3 indicates mild pain, 4–7 indicates moderate pain, and 8–10 indicates severe pain (15). According to these findings, Japanese patients who underwent this procedure experienced moderate-to-severe postoperative pain. However, our study showed that UGIANB effectively reduced the pain intensity from a moderate-to-severe level to a mild level.

During the study period, no patients underwent maxillary nerve block. There are several reasons for this. First, it is well-known among dentists that postoperative pain is more severe following the extraction of IMTM than maxillary third molar. The mandible has less spongy bone and thicker cortical bone than the maxilla, leading to more significant inflammation post-extraction (16). Secondly, performing nerve blocks for both the maxilla and mandible together, along with local infiltration anesthesia, increases the risk of LAST due to the higher total dosage.

This study revealed that nerve block for the mandible alone could effectively provide postoperative analgesia, and no patients exhibited signs of LAST.

Comparison with similar research

Many studies have challenged postoperative analgesic management for this surgical procedure, such as the fixed doses of ibuprofen and acetaminophen (2), the local application of bupivacaine (3), and the low-level laser and light-emitting diode light irradiation (4). However, a gold standard for pain management during this surgery is yet to be established.

Intraoral IANB is the conventional analgesic procedure in dentistry (6-10). Recently, Amorim et al. (6) reported effective postoperative analgesia for 48 hours using intraoral IANB with 0.75% ropivacaine compared to 2.0% lidocaine with 1:100,000 adrenaline. The anterior IANB technique involves injecting anesthesia into the PMS at the level of the mandibular sulcus and is safer than the conventional technique (8). However, this technique has the drawback of being performed blindly, resulting in a success rate of only 75% (8). In our study using Thiel-embalmed cadavers, 8 UGIANB procedures were performed, and dye was correctly administered within the PMS in 7 cases (87.5%), significantly exceeding conventional intraoral IANB’s success rate without complication (17). This study also demonstrated that UGIANB has a very high potential to affect the inferior alveolar, buccal, and lingual nerves within the PMS. Therefore, UGIANB may provide the necessary analgesia for extracting mandibular IMTM.

Iwanaga et al. (10) reported that the distance between the posterior edge of the lingual nerve and the anterior edge of the inferior alveolar nerve at the level of the mandibular foramen ranged from 1.62 to 8.36 mm [ 5.33 (1.88) mm, mean (standard deviation)] in fresh-frozen specimens. These findings highlight the risk of lingual nerve injury during intraoral IANB, which may be inappropriate under general anesthesia. In this study, female patients received more UGIANB than male patients. Nermo et al. (18) reported that the prevalence of dental anxiety is higher in female patients, and surgeons and anesthesiologists may decide to perform UGIANB to address preoperative anxiety.

Strengths and limitations

Early discharge from the hospital is required after IMTM extraction surgery. Therefore, an analgesic technique that ensures effective pain relief and safety without complications is essential. In this study, 53 UGIANB procedures were performed on 29 patients, with no complications such as accidental arterial puncture. As previously reported, the use of a mouth opener during UGIANB improves the visibility of the ultrasound equipment, thereby enhancing safety (12).

This study demonstrates the analgesic effect of adding UGIANB to conventional analgesic methods. However, several biases in intraoperative and postoperative characteristic variables between the groups were found. Surgeon and anesthesiologist allocation bias likely occurred in both groups, possibly leading to medication selection bias. The patients in the UGIANB group had shorter operative times than those in the Control group (31.8 vs. 40.0 minutes, P=0.04). However, the percentage of bilateral teeth extracted was similar between the UGIANB and the Control groups (82.8% vs. 83.3%, P>0.99), and anesthesia time were comparable (55.1 vs. 61.4 minutes, P=0.16), indicating that the UGIANB procedure took approximately 10 minutes. Patients in the UGIANB group were administered less intraoperative fentanyl and more intraoperative remifentanil than those in the Control group. Anesthesiologists may have preferred fentanyl in the Control group due to its longer duration of action, enhancing the effect of their anesthetics. Patients in the UGIANB group were also more likely to receive intra- and postoperative acetaminophen than those in the Control group. Although nonsteroidal anti-inflammatory drugs are more effective than acetaminophen for treating postoperative dental pain (19), they are not without side effects.

Implications and actions needed

This report has many cases with missing NRS scores, resulting in a small final sample size. Although the NRS score is not the primary endpoint, postoperative NRS scores are essential data demonstrating the efficacy of UGIANB. Further prospective, evidence-based studies are needed.

Conclusions

In this single-center cohort study, we observed that UGIANB provided effective postoperative analgesia in patients undergoing IMTM extraction under general anesthesia. We found that UGIANB resulted in a longer time to first rescue analgesia and a lower NRS score at 24 hours postoperatively compared to the conventional method, with no complications associated with the block.

Acknowledgments

We would like to thank Editage (www.editage.jp) for English language editing.

Funding: None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://joma.amegroups.com/article/view/10.21037/joma-24-19/rc

Data Sharing Statement: Available at https://joma.amegroups.com/article/view/10.21037/joma-24-19/dss

Peer Review File: Available at https://joma.amegroups.com/article/view/10.21037/joma-24-19/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://joma.amegroups.com/article/view/10.21037/joma-24-19/coif). The authors have no conflicts of interest to declare.

Ethical statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). This study was approved by the Institutional Review Board of Hinode Dental Makomanai Hospital (approval code: 22-5, approval date: November 2, 2022) and registered with the Japan Registry of Clinical Trials (clinical trial plan number: jRCT101122028, date: December 16, 2022) and individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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