(11) Lingual nerve impairment/injury after retrieval of the displaced mandibular third molar into the floor of the mouth

Abstract: Statement of the problem: Iatrogenic nerve injury due to the extraction of the mandibular third molar (MTM) has been well documented i.e., inferior alveolar nerve (IAN) and lingual nerve (LN) injuries. Iatrogenic LN injury caused not directly by the extraction but by displacement of the MTM or its fragment (dMTM) into adjacent anatomical spaces is a rare entity with a reported incidence of only 0.023% (2/8887).1 When an intraoral retrieval is selected, two different approaches can be used- a lingual mucoperiosteal flap (LMF) or the direct approach (DA). The former takes advantage of the scar in which the dMTM penetrated the periosteum when it was displaced, and the latter makes a new incision right above the dMTM when it can be palpated. However, it is still difficult for surgeons to predict the chance of LN impairment/injury by retrieval as there are no such reports on the true incidence of LN impairment/injury caused by the retrieval or what the safest intraoral approach is for the dMTM. Therefore, the goal of this study is to provide the incidence of iatrogenic LN impairment/injury caused by the retrieval of the dMTM in the floor of the mouth, not by the displacement, based on a review of the existing literature.
Materials and methods: To clarify the incidence of the lingual nerve impairment/injury caused by the retrieval of the dMTM into the floor of the mouth, the retrieval cases were collected by the searching the following words using PubMed, Google Scholar, and the CENTRAL Cochrane Library database:
(("wisdom tooth") OR ("third molar")) AND ((displacement)) AND (("sublingual space") OR ("submandibular space")). After the title, abstract and full-text assessment, a total of 38 cases from 25 studies were eligible for further evaluation and inclusion into the study.
Methods of data analysis: Differences in the age of the patients with presence and absence of LN impairment/injury were analyzed using the Mann-Whitney U test, and all other factors (sex, bilateral differences, anesthetic procedure, and operative procedure) were analyzed with the 2 × 2 chi square test. Statistical analyses were performed with EZR, which is a graphical user interface for the open-source statistical program “R” (The R Foundation for Statistical Computing, Vienna, Austria). More precisely, EZR is a modified version of R commander designed to add statistical functions frequently used in biostatistics. Statistical significance was set at p < 0.05.

Results: LN impairment/injury due to the retrieval was found in six cases (6/38: 15.8%). In three cases, the tooth was retrieved under general anesthesia, and three cases underwent a procedure with local anesthesia (p = 0.869).

Outcomes Data: The LN impairment/injury was temporary in all six cases and all of them recovered between 3 to 6 months after retrieval. In all six cases of LN impairment/injury, the tooth was retrieved using LMF (p = 0.570). There were no significant differences with age, sex or side (p = 0.513, p = 0.761, and p = 0.904).

Conclusions: Overall, temporary LN impairment/injury caused by retrieval of displaced mandibular third molar fragment was 15.8% (6/38) of the retrieved cases which did not have LN impairment/injury before retrieval. The permanent iatrogenic LN impairment/injury due to retrieval of the dMTM is considered rare as long as the appropriate surgical approach is chosen based on the preoperative diagnosis and anatomical knowledge.

Reference
1. Masui M et al. Displacement of lower third molar into floor of mouth: a single institutional experience. Oral Maxillofac Surg. 2021. doi: 10.1007/s10006-021-01012-3.