This study was conducted in compliance with the principles of the Declaration of Helsinki, and was approved by the Committee for Ethics at Shinshu University School of Medicine. Patients who underwent dental rehabilitation by insertion of dental implants between 2000 and 2012 in our hospital were reviewed. Of these, seven patients underwent transposition of the IAN for dental implant placement and thus included in this retrospective assessment. These included six women and one man, with a median age of 64 years old (range 38–75 years old). The same surgical procedure, IAN transposition, was performed in each patient. All operations were done by same operator (H.K.). A crestal and anterior releasing incision was performed for visualization of the entire mental foramen and the lateral aspect of the mandible. The IAN was then extracted by gentle carving of the cortex and cancellous bone around the mental foramen and lateral wall of the inferior alveolar canal using a bone-cutting burr. The IAN was exposed and gently and minimally deflected laterally from the mental foramen to a distance 3–5 mm posterior to the most distal implant using a blunt curette. The incisor branch was cut in all cases. The implants could then be placed under direct visualization. After implant placement, the IAN was repositioned in the osseous canal against the implants. The mucoperiosteal flap was repositioned, and complete closure was achieved (Fig. 1). Implant exposure was performed 3 months after the surgical procedure, and prosthetic rehabilitation began thereafter.

The patients were recalled for evaluation of the function of the IAN in the range between 12 and 105 months (median follow-up time 49 months). We performed a relatively objective assessment of mental nerve paralysis by applying the modified SW perception test reported by Semmes and Weinstein [11, 12]. The presence or absence of sensation was tested with three nylon monofilaments of the same length but different diameters (0.165, 0.215, and 0.315 mm) (Fig. 2). Three representative points (labial commissure, lower lip, and mental region) were assessed (Fig. 3). According to Wernor et al. [13], whether the patients, with closed-eyes and in a horizontal position, could be aware of the stimulation was assessed in a quiet environment. The filament was pressed to the assessment points, and maintained for a few second. The use of SW perception tester started from a filament of 0.165 mm in diameter of the most weak force, and performed three times at one site. The threshold of the tester raised with filaments of 0.215 and 0.315 mm in diameter sequentially. Each point was tested three times separately by the same examiner and at the same pressure, and the presence of sensation was judged if the patients correctly expressed positive sensation more than twice. The presence of sensation was scored as 1 point, and total function of the IAN was graded by adding the scores from 0 to 9 points (3 points × 3 different diameters of nylon monofilaments). In addition, the quality of nerve paralysis was assessed according to the criteria reported by Highet [14] (Table 1).