This retrospective clinical study examined 79 patients between March 2003 and November 2016 who had undergone microsurgical repair of their lingual nerve in the removal of their mandibular third molars at the Department of Oral and Maxillofacial Surgery, Wakayama Medical University. This study followed the Declaration of Helsinki on medical protocol and ethics, and the regional ethical review board of Wakayama Medical University approved the study.
All cases of lingual nerve injury were caused in other facilities. The criteria for performing repairs of the lingual nerve via microneurosurgery included (1) a witnessed transection, (2) two-point discrimination (2-PD) > 20 mm in the affected area over 3 months after the injury, (3) no sensation observed during a temperature test in the affected area at over 3 months after the injury, (4) no sensation observed during a taste test in the affected area at over 3 months after the injury, (5) no sensation observed during a pin-prick test in the affected area at over 3 months after the injury, and (6) finding a difference for the Semmes-Weinstein monofilament test (SWM test) between the affected and non-affected side at over 3 months after the injury. Microneurosurgery of the lingual nerve repair was indicated if (1) or all of (2–6) were present . About the surgical procedure , the lingual nerve was exposed through an intraoral mucosal incision and lingual flap reflection. Optical magnifying glasses (250 mm) and an operating microscope (Superlux 301, Zeiss, Jena, Germany) were available during surgery. In all cases, the lingual nerves were completely disrupted and heavily trapped by dense scar tissue. Most cases showed neuromas at the torn nerve ends. The neuromas and peripheral scars surrounding the torn nerves were completely removed; after this procedure, the two nerve ends could touch without tension. As much scar tissue as possible was removed from the torn nerve, and the transected lingual nerve stumps were identified, mobilized, and trimmed to the point where the fascicles could be identified in the microsurgical field. In all cases, direct end-to-end epineural nerve sutures without tension were performed at eight or more sites around the stump, using 8-0 or 9-0 nylon. Nerve grafts were not required in any case.
The following data were collected and analyzed: sex, age, nerve injury side. The orthopantomographs of 26 cases were available and divided according to Winter’s classification, and the Pell and Gregory classification (Fig. 1) [13, 14].
The inclination of the longitudinal axis of the third molar was divided into distoangular, horizontal, mesioangular, vertical, inverted, buccoangular, and linguoangular based on Winter’s classification. Obtainment of the angle between the occlusal plane or line parallel to it and the longitudinal axis of the impacted third molar, in turn, allowed objective classification of the third molars according to the Winter classification. The subclasses used were as follows: (1) third molars with negative angles (< 0°) were considered to be inverted, (2) third molars with an angle between 0° and 30° were considered to be horizontal, (3) third molars with an angle between 31° and 60° were considered to be mesioangular, (4) third molars with an angle between 61° and 90° were considered to be vertical, and (5) third molars with an angle > 90° were considered to be distoangular .
The depth of the impacted third molar with the occlusal plane and the available space with respect to the ascending mandibular ramus were divided into positions A, B, and C, and into classes I, II, and III according to the Pell and Gregory classification.
We compared the ratios of the respective data in our lingual nerve injury group (LNIG) to the ratios of the respective data in the past literature (Fig. 1), and which was used as the control group (CG) [9, 11, 15,16,17,18]. This data came from studies that examined the various complications encountered during the removal of the mandibular third molar.
Statistical analyses of the ratio differences between each parameter of two groups, namely lingual nerve injury group and control group, were performed using Software JMP® Pro version 12.2.0 (SAS Institute Inc., Cary, NC). Chi-squared test and t test were used for these analyses, with P < 0.05 designated as being significant.