Extraction of impacted mandibular third molars in close proximity to the inferior alveolar canal with coronectomy-miniscrew traction to avoid nerve injury

Extraction of impacted mandibular third molars (IMTMs) is the most common surgery performed in the Department of Oral and Maxillofacial Surgery. Inferior alveolar nerve (IAN) injury is a rare but severe complication, and the risk is significantly higher in cases of IMTM near the inferior alveolar canal (IAC). The existing surgical method to extract such IMTMs is either not safe enough or is time-consuming. A better surgical design is needed. From August 2019 to June 2022, 23 patients underwent IMTM extraction by Dr. Zhao at Nanjing Stomatological Hospital, Affiliated Hospital of Medical School, Nanjing University, and were found to have IMTMs in close proximity to the IAC. Due to high IAN injury risk, these patients underwent coronectomy-miniscrew traction to extract their IMTMs. The time between coronectomy-miniscrew insertion and complete removal of the IMTM was 32.65 ± 2.110 days, which was significantly shorter than that of traditional orthodontic traction. Two-point discrimination testing revealed no IAN injury, and no injury was reported by patients during follow-up. Other complications, such as severe swelling, severe bleeding, dry socket, and limited mouth opening, were not observed. Postoperative pain levels were not significantly higher in the coronectomy-miniscrew traction group than in the traditional IMTM extraction group. For IMTMs that are in close proximity to the IAC and must be extracted, coronectomy-miniscrew traction is a novel approach to minimize the risk of IAN injury in a less time-consuming way with a lower possibility of complications.


Introduction
Extraction of impacted mandibular third molars (IMTMs) is among the most common surgeries performed in dentistry [1]. Inferior alveolar nerve (IAN) injury, which has been reported to occur in approximately 0.35-8.4% of IMTM extractions, [2] is a rare but serious complication. Recent evidence has revealed that a close anatomic relationship between IMTM and the inferior alveolar canal (IAC) increases the risk of IAN injury by 8-22% [1,3,4]. Oral surgeons have attempted to minimize the possibility of IAN injury during surgical removal for such IMTM.
Coronectomy, orthodontic extrusion, and pericoronal ostectomy are the most commonly applied methods for IMTM extraction. Coronectomy, also known as deliberate vital root retention, has been proposed to remove the crown but leave the roots undisturbed, thereby reducing the risk of nerve injury. However, IAN injury can still occur due to periapical periodontitis of the remaining tooth [5]. Orthodontic extrusion can be applied to extrude the IMTM away from the nerve, but as a major drawback, the orthodontic traction device is complex, and the treatment is often time-consuming and expensive [6,7]. On the other hand, pericoronal ostectomy is rather unpredictable with limited indications [8]. Modern dentistry calls for safer, cheaper, and quicker methods to completely extract IMTM association with a high risk of IAN injury.
In the present study, we introduced a novel approach with coronectomy-miniscrew traction to extract IMTMs with radiographic overlap with the IAC. In our 23 patients, IMTMs were successfully extracted, and no IAN injuries were observed. As a combination of coronectomy, orthodontic extrusion and pericoronal ostectomy, our method is a safe way to completely extract IMTMs rapidly and inexpensively. We hope this technique can be an alternative option for the extraction of IMTMs in close proximity to the IAC.

Patients
From August 2019 to June 2022, among patients that came to Nanjing Stomatological Hospital to extract their IMTMs, and received by Dr. Sufeng Zhao, 23 patients were included in our study. Panoramic tomography showed that their IMTMs were in close proximity to the IAN, which was further confirmed through CBCT. Due to personal requests or medical reasons, these patients wanted their IMTMs to be safely and completely extracted and agreed to join our study. Detailed inclusion and exclusion criteria are listed in the supplementary data.
Among the 23 patients, 7 were male and 16 were female. The mean age was 27.26 ± 1.193 years, and the follow-up period was 3 to 34 (12.26 ± 2.093) months. The impaction types were as follows: 2 vertical impaction, 1 horizontal impaction, 19 mesial impaction, 1 distal impaction. Detailed information is listed in Table 1. All surgeries were performed by a single surgeon (Dr. Zhao). This study was approved by the Research Ethics Committee of Nanjing Stomatological Hospital, Affiliated Hospital of Medical School, Nanjing University (approval number: KY-2020NL-060).

Primary surgery
The first surgery was performed as follows ( Fig. 2, Table 2):

Coronectomy and pericoronal osteotomy
After anesthesia of the IAN block with lidocaine, an envelope mucoperiosteal flap was made to fully expose the surgical area ( Fig. 2A a-b). Part of the pericoronal alveolar bone was removed to fully expose the whole crown and to remove bone resistance along the extraction direction ( Fig. 2A c-d, Table 2). Then, coronectomy was carried out to ensure sufficient traction space and to reveal the pulp chamber according to the impacted type of IMTM (Fig. 1A, Table 2).
Traditional coronectomy focuses on the removal of crown resistance. In this method, we removed crown resistance and ensured that the pulp chamber was properly exposed, with a relatively flat surface with a proper angle for miniscrew implantation (Fig. 1). Moreover, to ensure quick root migration during traction, a modified pericoronal osteotomy was performed. Pericoronal bone resistance was released to fully expose the crown before coronectomy, and the bone connecting the remaining tooth was removed by a high-speed surgical drill to broaden the clearance between the tooth and bone ( Fig. 2A c).

Miniscrew implantation and wound closure
After pericoronal osteotomy and coronectomy, a miniscrew (Tomas-pin SD N 08, DENTAURUM GmbH&Co KG, Germany) was inserted into the exposed pulp chamber of the remaining tooth according to the impacted type and number of roots of the IMTM ( Table 2, Fig. 1B). It would be better to insert the miniscrew parallel to the major axis of the IMTM. Another miniscrew was inserted into the buccal cortex between the left second molar and the first molar or into the mandibular ramus. A miniscrew on the mandibular  2.365 ± 0.05918 ramus should be implanted on the buccal side to avoid being a burden to occlusion (Supplementary Fig. 3A-e). A 2-to 3-mm incision was made in the gingiva to expose the miniscrew upon wound closure. After the implantation of miniscrews, an elastic tension spring (GRIKIN Advanced Materials Co., Ltd, China), which generated a force of approximately 200 g forwards/ backwards (depending on the IMTM type) and upwards, was placed on the two miniscrews. No special treatment was performed on the exposed dental pulp. The wound was then irrigated with sterile saline and primarily closed. A panorama was taken immediately after surgery.

Secondary surgery and follow-up
Patients were required to return for 2 to 3 follow-ups at 1 week, 2 weeks, and 2-4 weeks (depending on the migration of the remaining root) after the first surgery. At the 1-week follow-up, the surgeon checked if the spring was properly stretched. At the 2-week follow-up, a panoramic radiograph was taken to evaluate the migration of the remaining root, and CBCT was taken immediately or later depending on the root migration shown on the panoramic radiograph. The actual follow-up time differed because of patient compliance.
After root migration was confirmed by CBCT (which indicated reduced IAN injury risk), a secondary surgery was performed to remove the remaining IMTM and the extraction device. After root migration, the remaining tooth was subluxated (Supplementary video) and thus was easy to remove. Patients were required to return for follow-up 7 days and 1 month after the secondary surgery or any time if any complications occurred.

Two-point discrimination testing
IAN injury was assessed by two-point discrimination (2PD) testing of the lower lip and the adjacent skin. Patients were required to point out whether the contact was felt, and a score of 1 point or 2 points was given. If the patient was uncertain about the number of contact points, it was recorded as 1 point. The 2PD score was recorded as the distance between the two points and was recorded as the longest distance when the patient felt there was one point. The lower lip was divided into 2 equal parts from the midline and marked as L1 (R1). From the midline to the corner of the mouth, the skin was divided into 2 equal parts and recorded as L2 and L3 on the left and R2 and R3 on the right. The same dimension of L3 (R3) extending outwards from the corners of the mouth was recorded as L4 and R4 ( Supplementary Fig. 1). The 2PD scores were recorded for each region on the same side of the IMTM before the first surgery and at the 7-day

Complications
To compare the complication rate and level of pain between the new method and traditional method, common complications (including limited mouth opening, bleeding,

Data analysis
The data were analyzed by SPSS v19 (SPSS, Chicago, IL, USA). Two-way ANOVA was used to compare NRS data. P < 0.05 was considered to indicate statistical significance for all tests.

Patient 1: mesially inclined IMTM
A 26-year-old female came to our department to extract her left IMTM. Radiography revealed a close relationship between tooth 38 and IAC (Fig. 2B upper and C a). Under proper local anesthesia, the crown and bone resistance were removed ( Fig. 2A c-d). Miniscrews were planted into the remaining tooth and in the mandible between teeth 36 and 37 and were connected using an elastic tension spring ( Fig. 2A). Thirty-three days later, CBCT showed that the 38 roots moved away from the IAC by 2.4 mm ( Fig. 2B and C, Table 1). Postoperative 3D reconstruction showed satisfactory treatment results (Fig. 2C). Upon secondary surgery, the remaining tooth was subluxated before extraction (Supplementary Fig. 2 and supplementary video). The miniscrew and retraction device and the remaining tooth were removed. No IAN injury was observed by 2PD testing or reported at the 34-month follow-up.

Patient 2: mesially inclined IMTM
A 21-year-old female came to our department to extract her left IMTM. CBCT revealed that tooth 38 was mesially impacted and was closely related to the IAC (Fig. 3A  a). The space between her teeth 36 and 37 was limited. To minimize the risk of 36/37 root injury upon miniscrew implantation, a navigation plate was designed preoperatively (Fig. 3B). During the primary surgery, the plate was used to guide the implantation of the miniscrew in the mandible (Fig. 3C). Another miniscrew was inserted in the remaining 38 (Fig. 3A b). Twenty-two days later, CBCT showed that 38 moved away from the IAC by 2.7 mm (Table 1), and the remaining 38 and the extraction device were then removed smoothly. No IAN injury was observed by 2PD testing or reported by the patient during the 19-month follow-up.

Patient 3: distally inclined IMTM
A 30-year-old male came to our department to extract his left IMTM. Clinical examination showed that tooth 38 was distally inclined. CBCT showed that the distal root was long, thin, and curved distally into the IAC (Supplementary  Fig. 3B-upper). During the first surgery, the distal crown and pericoronal bone resistance were removed (Supplementary Fig. 3A a-c). Considering the distal dilaceration of the root, two miniscrews were inserted into the distal root and the mandibular ramus so that the remaining tooth could be rotated along the direction of the root. The miniscrew on the ramus was implanted on the buccal side of the ramus to prevent interference with the occlusion (Supplementary Fig. 3A  d-f). After 57 days of traction, the remaining 38 was successfully moved by 2.2 mm from the IAC (Supplementary Fig. 3B-lower, Table 1) and then successfully removed. No IAN injury was reported by the patient during the 17-month follow-up.

Patient 4: horizontal IMTM
A 21-year-old female came to our department to extract her left IMTM. CBCT showed that tooth 38 was horizontally impacted, with radiographic overlap with the mandibular canal ( Supplementary Fig. 4A). Under proper local anesthesia and exposure, most of the crown and part of the distal alveolar were removed. Two miniscrews were inserted into the remaining root and on the mandible between teeth 36 and 37. Fourteen days later, the remaining 38 was successfully moved by 2.5 mm from the IAC (Supplementary Fig. 4B, Table 1). The remaining root and the extraction device were removed in the secondary surgery. No IAN injury was observed by 2PD testing or reported by the patient during the 19-month follow-up.

Patient 5: vertical IMTM
A 27-year-old female came to our department to extract her left IMTM. Her tooth 38 was vertically impacted, with radiographic overlap with the mandibular canal (Supplementary Fig. 4C-upper). Two miniscrews were inserted into the remaining tooth and between tooth 36 and 37 on the mandible after coronectomy and pericoronal osteotomy, respectively. After 48 days of traction, the remaining 38 was moved by 2.9 mm from the IAC (Supplementary Fig. 4C-lower, Table 1). The remaining root and the extraction device were then removed. No IAN injury was observed by 2PD testing or reported by the patient during the 34-month follow-up.

Roots of IMTM migrated enough for extraction in approximately 5 weeks
The average time interval between the first and second surgeries was less than 5 weeks (32.65 ± 2.110 days). The average root migration distance was 2.365 ± 0.05918 mm.

No IAN injury was observed, and the postoperative pain was not higher than that of the traditional surgical method
No lower lip numbness was reported during follow-up among the 23 patients. The 2PD score difference was less than 2 mm before and after treatment for all patients. Complications such as adjacent tooth injury, pulp disease, limited mouth opening, severe swelling, severe postoperative bleeding, and dry socket were not observed in the coronectomyminiscrew traction group or in the control group (Table 1,  Supplementary Table 1). Moreover, the postoperative pain level in the coronectomy-miniscrew traction group after the primary surgery was lower (p = 0.0056), and the pain level after the secondary surgery was not greater (p = 0.6878) than that in the control group (Fig. 4). However, 3 out of 23 patients in the coronectomy-miniscrew traction group complained of oral ulcers in the buccal mucosa around the extraction device during traction, which was not observed in the control group.

Discussion
Extraction of the inferior third molar is the reason for 90% of all IAN injury cases [9]. Among all IAN injury cases, 1-8% were temporary, and 0.5-3.6% were permanent [10]. The actual incidence may be higher than reported due to the lack of complaints from patients with mild symptoms and the lack of postoperative follow-up. Therefore, it is important to minimize the risk of nerve injury for the extraction Fig. 4 Pain level after different surgery method by NRS. Control: traditional IMTM extraction. Primary surgery: coronectomy and implantation of miniscrews. Secondary surgery: removal of the remaining tooth and traction device of IMTM, especially when the IMTM is closely related to the IAC. In recent years, methods including coronectomy [11], orthodontic extraction [7,12], pericoronal ostectomy [8], and their combination have been proposed for IMTM in close proximity to IAC.
First described in 1984 [13], coronectomy is the removal of a tooth crown while leaving the roots in situ. However, periapical inflammation could still happen to the root, and lead to IAN injury [14], infection, [15,16] reoperation, root migration, root exposure [10,17], and hypersensitivity. Furthermore, food impaction can also occur with this method. Coronectomy is also not suitable for an IMTM that must be completely removed due to personal or medical needs. Comparatively, our method is much safer, with a lower risk of complications.
Bonett et al. [12] applied orthodontic extraction to extract vertically or mesially inclined IMTMs. In a systematic review performed by Motamedi [7] in 2015, general orthodontic traction lasted for approximately 6 months. Among the 13 studies/case reports (123 impacted teeth) included in this review, only 2 case reports (fewer than 10 teeth) had traction times shorter than 2 months [7]. Many of the patients complained of discomfort during the progression of IMTM extrusion [7,18]. Generally, orthodontic traction is expensive and time-consuming and usually requires the participation of an orthodontist, leading to limited application. Comparatively, our method is much less expensive, shorter in time (32.65 ± 2.110 days), and can be completed independently by a single surgeon.
Pericoronal ostectomy deliberately removes pericoronal alveolar bone to adjust the eruption path of the IMTM. The indication of pericoronal ostectomy is also limited [8].
Our method is a combination of coronectomy, modified pericoronal ostectomy, and improved orthodontic extraction.
There are three goals of our coronectomy: (1) to remove crown resistance of IMTM extraction; (2) to create enough space for miniscrew traction; and (3) to create a suitable surface of miniscrew plantation. On the other hand, in traditional coronectomy, the remaining root should be buried in the gingiva to minimize future complications, which could be more aggressive than our method.
We also performed a modified pericoronal ostectomy by removing only a small amount of bone around the remaining crown with a fissure burr to broaden the clearance between the tooth and bone. Through our modified pericoronal ostectomy, regional acceleratory phenomenon (RAP) could occur to facilitate tooth movement. First introduced by Frost [19], RAP is a concept that describes when an intentional surgical injury happens to the bone, a cascade of physiologic events could lead to an increase in bone turnover with concomitant demineralization and new bone formation at the site of the bone injury [20,21]. This concept has been widely applied to explain orthodontic treatments facilitated by surgical treatments [22]. In our modified pericoronal ostectomy, we removed the pericoronal bone resistance and accelerated root migration during traction, and the total amount of bone removed was limited. Ostectomy should be more conservative on the buccal and lingual sides to the lingual nerve and the rather thin lingual alveolar bone. Mesial bone removal may be performed depending on the impaction type of the IMTM and the relationship between the adjacent tooth and the IMTM. Additionally, distal bone removal can be comparably more aggressive when needed, as the distal bone is often thick without an important anatomic structure.
Instead of traditional orthodontic extraction, we used an orthodontic miniscrew and orthodontic spring to exert extraction force. The extraction force was exerted by a spring, and the direction was designed based on the direction of coronectomy and plantation of the miniscrews. It is better to implant the miniscrew parallel to the long axis of the IMTM; therefore, the direction of extraction would be more efficient and more controllable to avoid undesired rotation. However, the miniscrew often cannot be implanted perfectly parallel to the long axis of the IMTM. In such cases, extraction can still be performed, but surgeons should pay more attention to check if the miniscrews loosen upon follow-up. The design of miniscrew extraction is simple and can be carried out independently by the surgeon.
After proper coronectomy, pericoronal bone removal, and miniscrew implantation, with proper extraction force exerted by the spring, the extraction time (32.65 ± 2.110 days) of our method can be significantly shorter than that of traditional orthodontic extraction, which can last for months [12].
There was no significant difference between the 2PD scores during and after treatment, and no lower lip numbness was reported during follow-up among the 23 patients, indicating that no IAN injury occurred after IMTM extraction. Common complications, including limited mouth opening, swelling, bleeding, and dry socket, were not observed among patients with this method.
It is also worth noting that the level of pain was not higher than that in the control group (Fig. 4). This might be the result of limited surgical wounds in both the primary and secondary surgeries. With coronectomy and only very limited bone removal in pericoronal ostectomy, the wound was mainly limited to the mucosa in the primary surgery. In the secondary surgery, the root was gradually removed from the IAC, making the wound after extraction smaller than that after the traditional method, thus reducing postoperative pain. However, after the first surgery, the miniscrew and traction device could cause oral ulcers in some patients (3 out of 23). Generally, patients found the pain and discomfort caused by this method bearable and were satisfied with the surgical outcome.

Conclusion
To conclude, compared with the reported methods, our method can minimize the IAN injury risk and has the advantages of a shorter course of treatment, lower treatment cost, and minimal patient discomfort in the operative area. This method requires high surgical skill and clinical experience; thus, it is only recommended for skilled surgeons. Although future studies with larger patient groups are needed, our study provides preliminary evidence that this method is highly competitive and has broad applicability prospects.
Author contribution Sufeng Zhao: contributed to conception, design, data acquisition, analysis and interpretation, performed all the surgeries, and critically revised the manuscript.
Yujia Wang: contributed to conception, data analysis and interpretation, drafted the manuscript.
Xiaoyue Yang, Xinyao Zhou, and Zezheng Wang: contributed to data acquisition, helped perform the surgeries.
Kun Zhang: contributed to design, helped with traction design, and critically revised the manuscript.
Xudong Yang: contributed to conception, design, data analysis, and interpretation and critically revised the manuscript.
All authors gave their final approval and agreed to be accountable for all aspects of the work.

Data availability
The datasets analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Ethics approval This study was approved by the institutional review board of the Nanjing Stomatological Hospital, Affiliated Hospital of Medical School, Nanjing University (KY-2020NL-060). Written informed consent was obtained from patients involved prior to treatment.

Competing interests
The authors declare no competing interests.