For clinical assessment of a patient who is a candidate for dental implants and suffers from a reduced mandibular alveolar ridge height, first study casts should be prepared and then the occlusal relationship should be evaluated and recorded. The following points should also be considered:

Babbush mentioned several indications for nerve repositioning including the following: placement of removable prosthetics, stabilizing the ­remaining anterior teeth, stabilizing the TMJ, and establishing muscular balance following reconstruction of the dentoalveolar system. He also discussed some related limitations, primarily that this procedure is technically difficult and requires adequate expertise. The surgeon should have advanced surgical experience with this technique, as well as sufficient anatomic knowledge and necessary skills to fully manage perioperative and postoperative complications if they occur. According to Babbush, the most significant risk of surgery is nerve injury due to surgical manipulation of the IAN as a result of the surgical procedure itself. Although rare, mandibular fracture should also be considered as a risk, especially in cases with severe mandibular atrophy [25] (Fig. 7.4).

Rosenquist et al. in their studies on the nerve repositioning procedure mentioned the following indications and contraindications for this operation:

Before choosing nerve repositioning, the required criteria must be assessed. According to the literature, 100 % of patients who undergo nerve repositioning develop various degrees of sensory nerve dysfunction of the IAN sensory distribution [1, 27–29]. Therefore, the patient and family members should be well informed relevant to the phases of treatment, duration of surgery, and postoperative general complications, and most importantly they should be provided with knowledge about the postoperative lower lip and chin (and possibly lower incisor) paresthesia which will definitely occur and may last for up to 6 months, and in some cases, if it lasts longer or is very severe, it may require microneurosurgical exploration and repair [1, 27, 29–31].

Despite the issues mentioned above, the patient may not fully comprehend the sensation of paresthesia, and in such cases, it is recommended to perform an IAN anesthetic block for the patient using bupivacaine for long-lasting anesthesia induction so that the patient can experience anesthesia and paresthesia for up to 8–12 h. The advantages of this treatment ­modality should also be explained to the patient including shorter treatment duration, no need for autogenous bone grafting and no donor site morbidity, minimum use of other bone replacement materials, and avoiding the need for additional surgeries [1, 2, 7, 27, 29].

The incision is made on alveolar crest starting from the anterior border of the ramus forward. At the mesial surface of the mandibular canine, a releasing incision is made anteriorly and toward the vestibular sulcus in order to avoid injuring mental nerve branches. If the anterior-releasing incision is not made at the mesial of the canine, there is a chance of injuring the mental nerve branches present in the vestibular soft tissues [12, 28, 29, 32]. When retracting the mucoperiosteal flap, the mental foramen is completely exposed and the dissection is extended toward the inferior border of the mandible. Considering the radiographic and CBCT evaluations along with the fact that the IAC is usually located 2 mm below the mental foramen, it is necessary to expose the lateral surface of the body of the mandible and release the periosteum around the mental nerve [6, 12, 17] (Figs. 7.5 and 7.6).

The MN lies in the mandibular buccal soft tissues and is at risk of injury during the incision(s). 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. In some patients there is an actual dehiscence of the IAC, and the IAN and the MN come to lie on the alveolar ridge crest. Placement of an incision must, therefore, take these anatomic changes into consideration. During the retraction of the mucoperiosteal flap, it is possible to exert continuous undue pressure on the underlying MN with resultant neuropraxic (Sunderland grade I) injury. Gentle soft tissue retraction with frequent brief relaxations of retraction pressure is suggested [33].

Nerve injury is most likely to happen when performing the ostectomy to expose the IAC. According to the existing research, the technique of performing the ostectomy is effective in reducing postsurgical complications. There are two general techniques to accomplish this portion of the surgery.

In the first technique, which is usually performed for treatments other than dental implants, a piece of bone is removed as a block and then the IAC is exposed. This method can be indicated for simultaneous implant surgery where there is adequate bone height over the canal. In such cases, even after resecting a bone block, a sufficient amount of bone still remains at the lateral side of implant [7]. In the second technique, the ostectomy is performed with the “groove ostectomy” method, since more bone will be saved and primary implant stability will also be improved. These techniques can be modified to accomplish either nerve lateralization or transpositioning.

The portion of the surgery when the nerve is released and withdrawn from the IAC is one of the stages that may lead to nerve damage. The neurovascular bundle inside the canal is released using special curettes, and it is moved laterally using a blunt nerve hook. Bone removal in close vicinity to the neurovascular bundle should be performed patiently and thoroughly. This is usually performed using special curettes parallel to the surface of nerve bundles in an anteroposterior direction. Tiny bone spicules around the nerve should be removed. The area should be thoroughly irrigated so that the nerve bundle can be clearly seen. Spongy bone surrounding the nerve is removed using a special curved curette. The nerve is released and slowly retracted from the canal using a nerve hook. The hook should be rounded at the end and polished (Fig. 7.8). Care is taken to avoid any tension or sudden movement of the nerve, which may cause injury.

Another important stage that may cause damage to the IAN is when the nerve is outside of the canal. Some studies have suggested various techniques for protecting the nerve outside of the canal and retracting it while drilling for implant placement. Such techniques include the use of a looped suture [6], small green cloth [34], piece of a suture cover [13], half of the rubber piston of a dental cartridge [35], umbilical tape [25], vessel loops [10, 16, 32], and elastic bands (author’s preference).

The important strategy for preventing nerve damage in this stage relies on increasing the contact surface in order to decrease pressure on the nerve. The more the contact surface decreases, the more the nerve damage will occur (pressure  =  force/area). So, avoid exerting too much traction upon the nerve when lateralizing the nerve and during nerve repositioning; attempt to transform the contact point to a contact area (Fig. 7.9). Then, a 10-mm-wide gauze cord or elastic band is passed below the IAN retracting it away from the surgical site decreasing ischemic trauma to the nerve. It also retracts the nerve away from the surgical site during the operation reducing the risk of iatrogenic nerve damage [2, 13, 30, 32] (Figs. 7.10 and 7.11). The advantage of using a wide elastic band is that it counteracts the tension of the nerve to some extent and also decreases the final pressure on the nerve due to its elasticity. Also, the retracted neurovascular bundle must be constantly moistened by normal saline.

Finally, the last stage of possible nerve damage refers to the process of nerve repositioning, when the nerve is replaced back into the IAC, and the contact between the nerve and the implant occurs. Prior to this phase, the surgeon should decide whether or not to place material(s) between the implant and the nerve. There is a lot of controversy in this regard and some studies have been performed on animal models in this respect. In a study by Yoshimoto et al. with rabbits, no ­difference was observed microscopically after placing and not placing a membrane between the implant and the nerve bundle [36]. On the other hand, in the Kahnberg et al. dog study, healing was not complete after 14 weeks, but none of the implants were exposed. Histologic examination showed that in cases where a membrane had not been placed, a small contact was present between the nerve bundle and the implant. When a membrane is used, the distance between the nerve bundle and the implant will be four to eight times greater. There is no contact between the nerve and the implant when using a membrane, but the bone is not seen around the implant either [37]. An autogenous bone graft, either from the bone removed to unroof the IAC or elsewhere, or banked bone, is always placed between the repositioned nerve and the associated implants to prevent direct contact of the IAN and thermal transmission or metallic sensitivity with the implant(s). Also, artificial material, such as ­calcium hydroxyapatite, should never be placed in direct contact with the nerve. A severe inflammatory reaction in the nerve, similar to a chemical burn with dense scarring, accompanied by considerable pain, is often the unfortunate result. Surgical treatment of such injuries is problematic [33]. The author’s preference is to place a collagen membrane or autogenous bone graft in between the implant and IAN [10, 33]. A potential advantage of bone grafting over membrane placement is that if proper healing occurs in the area, the contact area of implant and bone will increase for osseointegration (Fig. 7.12).

Therefore, attention should be paid during and after surgery to minimize the factors responsible for ischemia and mechanical neurotrauma. Such factors in the order of most common to the least common during nerve repositioning surgery include: