The IAN or LN can be injured by needle contact secondary to the injection of a local anesthetic into the pterygomandibular space [27, 28] or the MN when injecting in the area of the mental foramen. Although the exact pathophysiology of this injury remains unknown, there are three possible causes: (1) direct intraneural injection with mechanical injury to the nerve (i.e., severance of axons, partial or total, scar tissue or neuroma formation, Wallerian degeneration), (2) interruption of vessels of the mesoneurium with peri- and intraneural hemorrhage and secondary scar formation, and (3) chemical toxicity of the anesthetic solution, or from a contaminant (sterilizing solution in a storage container) that is able to enter into a leaky anesthetic cartridge [13]. Regardless of the cause, it is recommended that aspiration be performed prior to all local anesthetic injections. If there is a bloody aspirate, or the patient complains of a paresthesia (typically, an “electric shock-like” sensation), the needle is withdrawn a few millimeters and aspiration is repeated. If there is now no bloody aspirate, it can be assumed that the needle tip is no longer in contact with a blood vessel or nerve, and the injection is completed. A note of such an occurrence should be routinely entered in the patient’s chart. This technique may prevent direct injection into a vascular space, but does not necessarily prevent deposition of the anesthetic within the epineurium (the diameter of the IAN is four to five times greater than the associated inferior alveolar artery or vein). Nerve injury secondary to local anesthetic injection, although uncommon, has a reported incidence of 1:26,762 to 1:160,571. It can be difficult to differentiate from injury related to the placement of the dental implants, especially if sedation or general anesthesia was used and, therefore, the patient was unable to report a paresthesia at the time of the injection(s). Without obvious clinical or radiographic signs of injury to the nerve from the dental implant procedure itself, the possibility of needle injection injury cannot be eliminated. Unfortunately, a small number of patients who have suffered an injection-related injury can be misdiagnosed with injury related to the dental implant surgery and subsequently undergo either inappropriate removal of the implant or fruitless exploratory surgical procedures that reveal no visible nerve injury at the implant location.

Injury to the IAN as a consequence of bone preparation or implant placement can be due to errors in radiographic planning, drilling, or direct contact of the implant with the nerve. Drilling injuries to the IAN can be difficult to diagnose. Despite correct position of the implant vis-a-vis the IAC on the postoperative radiograph appearance of the implant, osseous preparation with the drill may have been performed beyond the planned implant depth causing injury to the nerve (Fig. 6.2). In addition to the possibility that one of the implant drills entered the IAC and injured the IAN, it is also possible that the drill caused vascular trauma to the inferior alveolar artery (IAA) or inferior alveolar vein (IAV) and resulted in intra-canal bleeding. This bleeding may be noted during the osteotomy preparation by visualization of oozing that is more significant than normal marrow oozing. Once the implant is placed, the bleeding is tamponaded with resultant pressure placed upon the IAN, resulting in paresthesia, and even dysesthesia. This error can be prevented by measurement from correctly calibrated radiographs of the distance from the alveolar ridge crest to the superior aspect of the IAC, the use of drilling equipment with predetermined depth stops, and careful technique to prevent drilling beyond the planned depth. Irrigation with adequate coolant to dispel heat generated by bone drilling may also prevent a thermal injury in the absence of direct contact with the nerve. Frequent intraoperative reverification of the drill dimensions (diameter and length) is also helpful.

In addition to injury caused by drilling, the extent of injury to the IAN due to the implant itself is related to the degree of encroachment of the implant into the IAC or its direct contact with the IAN (Fig. 6.3). Nerve injury due to implant placement may occur despite proper osseous preparation, when the implant is inserted beyond the vertical confines of the prepared bone, compressing or breaching the superior wall of the IAC and forcing bone into the canal (Fig. 6.4a). Alternately, extension of drilling into the IAC may facilitate overinsertion of the implant beyond its intended depth and into the IAC, making direct contact between the implant and the IAN (Fig. 6.4b, c). Finally, delayed osseous healing and remodeling from localized injury can cause excessive bone formation during the healing phases and compromise the IAC cross-sectional diameter resulting in nerve compression (Fig. 6.4d) [8].

The mental nerve (MN) lies in the mandibular buccal soft tissues and is at risk for iatrogenic injury during a vestibular incision. Recognition of the changing anatomy of the edentulous mandible is particularly helpful in minimizing the risk of injury to the MN. As the patient ages, the alveolar bone in an edentulous area resorbs, and the position of the mental foramen becomes closer to the crest of the alveolar ridge (Fig. 6.5a). In some patients there is actual dehiscence of the IAC, and the IAN and the MN come to lie on the alveolar ridge crest (Fig. 6.5b). Placement of an incision must, therefore, take these anatomic changes into consideration. During the retraction of a mucoperiosteal flap, it is possible to exert continuous, undue pressure on the underlying MN. Gentle soft tissue retraction with frequent brief relaxation of retraction pressure is suggested (Fig. 6.5c).

Less common causes of nerve injury are related to placement of bone grafts (autologous, allogeneic, xenogeneic) during simultaneous implant placement. In cases of complex implant reconstruction, the bone graft material may be placed into the donor site with excessive force, thus severely compressing or even crushing the IAN. It is also possible that particulate bone materials placed in the vicinity of the mental foramen may migrate or become dislodged to impinge upon the MN as it exits the foramen, and this may cause significant scarring around the nerve and resultant paresthesia, including dysesthesia.

A list of microneurosurgical procedures that can provide surgical management of IAN injuries from dental implants is provided in Table 6.2. Figure 6.7 shows various microneurosurgical operations (note: these should include only nerve repairs secondary to dental implant-associated injuries). Although it is beyond the scope of this chapter to discuss all the techniques listed in Table 6.2, in a review of 167 IAN injuries [6], the most commonly performed operation was autogenous (sural or great auricular) nerve grafts (n  =  71, 38.2 %) for reconstruction of a nerve gap, followed by internal neurolysis (n  =  60, 32.3 %) when the nerve was not discontinuous. The need for reconstruction of a nerve gap was much more frequent with the IAN than that of the LN [4]. This has to do with the greater ease with which the proximal and distal stumps of the LN, contained within soft tissue, are able to be mobilized and brought into approximation for suturing without tension, than is the case with the IAN contained within a bony canal. This certainly has implications for the dental implant patient with a nerve injury, the majority of which are related to the IAN, and not the LN.