Accidental identification of accessory mental nerve and foramen during implant surgery.

The accessory mental nerve and the corresponding foramen are not a very common occurrence. In the current case report, we present the notice of an accessory mental nerve in the mandibular molar area during implant placement. The case was managed well without any complications.

INTRODUCTION

The concept of osseointegration has made implant therapy the most preferred mode for tooth replacement.[1] The increasing use of implants has along with it brought about series of reports on potential complications related to implant placement.[23] The availability of adequate bone volume and height is a prerequisite for implant placement; however, this may not always be possible. Implant placement into compromised sites needs adjunctive procedures like augmentation of bone either prior to or simultaneously during the implant placement.[4-7] Simultaneous implant placement and bone grafting procedure reduces the total duration of the treatment as compared to a staged approach of grafting and graft integration time of 4-6 monthsfollowed by implant placement and restorations 3-4 months later. Studies have shown that guided bone regeneration procedure at the time of implant placement can form and maintain bone formation around titanium implants.[47-12] Although all the clinical and radiographic evaluations are undertaken prior to implant placement so that the available bone height and anatomic structures like foramen and bony canals are visualized, some times the finer structures do get missed out but get identified during the surgical procedure.

The following report is of a case wherein we accidentally identified accessory mental foramen (AMF) and an accessory mental nerve (AMN) exiting it while carrying out a guided bone regeneration procedure using a titanium mesh in combination with bovine anorganic bone for covering a dehiscence defect around an implant in the mandibular molar area.

CASE REPORT

A 25-year-old male patient was referred to the Department of Implantology, SDM College of Dental Sciences and Hospital, Dharwad, India, for replacement of his missing tooth. On clinical examination it was noted that the patient had a missing mandibular right first molar (tooth number 46) [Figure 1]. The tooth was extracted due to endodontic failure 8 months back. The alveolar ridge appeared high well rounded. The overlying mucosa appeared healthy. The orthopantmogram revealed adequate amount of bone available for implant placement. The available height was 18 mm (1:3 magnifications). It was decided to place a 5.3 diameter, 13 mm long implant [Figure 2].

Figure 1

Pre-operative

Figure 2

Pre-operative radiograph

The patient was given a 0.2% chlorhexidine rinse for 2 minutes and local anesthesia was secured on the buccal and lingual sides. A mid-crestal incision extending from the distal aspect of the 2nd premolar to the mesial aspect of the 2nd molar was made and a full thickness mucoperiosteal flap was elevated. The alveolar crest was visualized and the osteotomy was started with a pilot drill for a depth of 13 mm. Sequential drilling was done to enlarge the osteotomy to facilitate the placement of 5.3 diameter, 13 mm long implant [Uniti® (Uniti®, Equinox, Holland)]. Implant torqued into the osteotomy site. We noticed that there was a dehiscence exposing 2–3 mm of the implant on the buccal aspect. It was decided to cover the dehiscence with graft and titanium mesh. A vertical incision was placed along the distoincisal line angle of the second premolar, to facilitate the grafting. While the flap was being reflected we noticed a fine fiber exiting a foramen located at a distance of approximately 3–4 mm apical to the alveolar crest. The fiber was carefully dissected from the adjacent tissues and was left undamaged. The foramen and the fiber were identified as accessory mental foramen and nerve [Figure 3]. The main trunk and the mental foramen were located in the area between the apices of the two premolars [Figure 2]. The exposed threads were covered with bovine anorganic bone [Bio-oss, Geistlich Pharma AG, Wolhusen, Switzerland] and stabilized with a titanium mesh screen [Synthes, CO, USA], of 1.0-mm sized pores which was contoured to the defect [Figure 4] During grafting, care was taken so that the nerve fiber was not traumatized. The flaps were approximated and closed with 4-0 vicryl [Ethicon, Johnson and Johnson, Aurangabad, India] sutures and primary closure was attained. The patient was advised not to take any analgesic till the anesthetic effect had worn off, and he was monitored continuously to check for any signs of nerve injury till 6 hours after the surgery. A radiograph was made to assess the placement of implant in relation to the inferior alveolar canal [Figure 5]. Once it was ascertained that no nerve damage had happened the patient was discharged from the clinic. Three months after the implant placement the patient was recalled and the 2nd stage procedure was carried out. After 15 days of healing period final impressions were made in vinyl polysiloxane impression material [Aquasil, Dentsply, De Trey, Konstanz, Germany]. The final restoration was cemented [Figures 6 and 7] and the patient was put on a follow-up care. The patient is being currently followed up since the past two years. No changes in the hard and soft tissue levels have been noticed.

Figure 3

Accessory mental nerve and foramen along with the buccal dehiscence on the implant

Figure 4

Dehiscence covered with titanium mesh and bio-oss

Figure 5

Immediate post implant placement radiograph

Figure 6

Final restoration

Figure 7

Radiograph of the final restoration

DISCUSSION

The knowledge of the local anatomy plays an important role in the treatment planning process. Clinical examination and findings from radiographs do not always ensure all the anatomic structures in a particular area are identified. Accidental identification of anatomic structures is not new, but the management of cases in these circumstances is a challenge.[6]

In the present case, AMN and AMF were accidentally detected in the mandibular 1st molar area during implant surgery and were managed successfully.

AMN is a branch of the inferior alveolar nerve, which exits from the main bundle of the nerve prior to the mental foramen, from an accessory foramen known as accessory mental foramen. AMN can be detected any location along the course of the mental nerve.[6] Its occurrence has been reported with an incidence of 3.52% of cases on the left and 4.22% on the right side of the mandible, in a study done on South Indian mandibles by Roopa et al.[13] Shankland[14] reported an incidence of 6.62% of accessory mental foramen in Asian Indians. Injury to any of the nerve during surgical procedures in this area results in paresthesia of the area supplied by it.[36]

In the present case, a 2–3 mm dehiscence was observed on the buccal aspect of the implant. The dehiscence was covered by GBR procedure, with a combination of bovine anorganic bone and titanium mesh.GBR technique was chosen because it has been shown that GBR is the best technique available to cover the dehiscence around implants. Dahlen et al, have shown that guided bone regeneration can be predictably carried out around titanium implants.[410] Studies by Zitzmann et al, showed that if the initial defect size is more than 2 mm then GBR is indicated in coverage of the defect which can be successfully treated by a combination of bovine anorganic bone along with collagen membrane and the results can be predictability maintained over long periods.[12] Although the autogenous bone grafts are gold standard for grafting procedures and are used in combination with bovine bone mineral for augmentation,[1516] in the current case as the defect was small (approx 2 mm) bovine particulate bone mineral was used. Bovine bone mineral when used as an onlay bone graft has shown bone formation between the particles of the graft.[916-18]

Apart from the graft material, the stability of the graft is very important for regeneration. The titanium mesh not only protects the graft but also maintains the shape around the defect which attributed to the stiffness of the mesh and can be easily contoured to achieve a three dimensional adaptation around the ridge.[11] The biocompatibility of the mesh also makes it suited for long term implantation and also less risk of infection even after premature exposure as compared to the non-resorbable membranes.[1619]

In current case, AMN was seen exiting the AMF at a distance of 3–4 mm from the alveolar crest leading to the thought whether the nerve was traumatized during the preparation of the osteotomy site or during implant placement. During the grafting procedure care was taken so that the AMN was not traumatized. The patient was not medicated with post operative analgesics and till the anesthetic effect waned off. In case any signs of nerve damage were observed it was decided to remove the implant immediately. As none of the signs of nerve injury or damage were detected the patient was discharged.

To summarize the case, it can be said that optimal knowledge of the local anatomy and radiographic identification of the anatomic structures is important, but there are certain conditions when these structures are missed out and are identified during surgical procedure. It is important to manage the aberrant anatomical structures carefully so that no permanent damage ensues, because any damage to these structures need not necessarily lead to implant failure but is very commonly associated with legal action against the practitioner.[3]

To conclude it can be stated that the case wherein an accessory mental nerve exiting an accessory mental foramen were identified in an area of the mandibular 1st molar on the right side was managed without causing any nerve injury. However, imaging techniques like CT scan that provides a better picture of the operating areas should be carried out for better planning and avoiding unnecessary complications during implant surgery.