In-Block Lateralization as a New Technique for Mobilization of the Inferior Alveolar Nerve: A Technique Case Series

Implant rehabilitation of the posterior mandible can be especially challenging given the frequent absence of bone volume associated with the anatomic position of the inferior alveolar nerve (IAN) within the mandibular body.^1–3  The IAN and the inferior alveolar artery, vein, and lymph vessels constitute the inferior alveolar neurovascular bundle, responsible for the sensory and vascular function of the mandible.^1,4  Because of its importance, the neurovascular bundle should be preserved during implant placement in the mandibular region, thus limiting surgical procedures.^2,5  Although the edentulous atrophic posterior mandible can be managed by a number of approaches,^3,6,7  most therapies require a minimum of 5 mm available bone over the IAN for successful rehabilitation.⁶  However, the remaining bone above the IAN is often of poor quality, compromising the long-term success of short implants.^3,8,9  Therefore, in cases of severe bone atrophy, with bone height below 4 mm, techniques for mobilization of the IAN have become an interesting option.^3,7 

There are 2 main techniques for IAN mobilization described in the literature, with some modifications to promote improvements and to reduce complications^10–13: (1) IAN lateralization through a lateral bone window posterior to the mental foramen aiming at the lateral exposure and mobilization of the IAN for the placement of implants in the remaining alveolar socket⁷ ; and (2) IAN transposition through a traditional lateral osteotomy that includes the mental foramen, in which the incisive branch is dissected and sectioned along the mental branch to allow wider lateral and distal branch retraction.^12,14,15  In both techniques, a buccal cortical window is recommended, ranging from 5 to 10 mm in height and from 25 to 30 mm in length,¹⁰  and the IAN is exposed and retracted laterally, held in this position during implant placement, and then repositioned to rest against the implants.⁷  The bone window can be filled with particulate autogenous bone graft, heterogenous bone graft, or membranes or even have the original block repositioned.^3,7,16  An advantage of IAN lateralization techniques is that they allows the placement of long implants, require a single appointment, provide a biomechanically favorable crown-to-implant ratio,¹⁷  reduce treatment time,¹⁸  and decrease the risk related to bone grafting or other additional procedures.

Studies have shown that the mandibular canal is often located closer to the lingual surface of the mandible,^1,2,19,20  in the posterior part of the body of the mandibular molar region, becoming progressively more superficial in relation to the buccal surface of the mandible as it approaches the mental foramen. This anatomic feature usually hinders surgical access to the IAN by traditional mobilization techniques in molar regions, requiring an extended osteotomy to safely access and retract the IAN.¹⁰  Another limitation of traditional techniques is the need for neurovascular bundle traction for implant placement, which may result in nerve lengthening and partial lesions of the axons and their myelin sheaths, causing loss of sensitivity.^8,15  However, if nerve manipulation is minimal and lengthening does not exceed 5%–7% of the total nerve length, normal sensory function should return within 4–6 weeks.²¹  In addition, an extended buccal cortical osteotomy weakens the mandibular body with potential risk of complications, such as mandibular fracture.^{3,14,15,19,22–24}  There is usually significant loss of structural integrity when a portion of the buccal cortex is removed during IAN lateralization, and this is potentiated by the already decreased volume of the mandibular body because of alveolar resorption.²⁵ 

In the current technique, we propose a modification of the traditional IAN lateralization technique, named in-block lateralization, to facilitate access to the IAN in the posterior mandibular region aiming to minimize nerve damage during manipulation, to reduce the extent of surgical access and osteotomy (and consequently mandibular bone fragility), and to restore the original mandibular anatomy with autogenous bone from the original bed, thereby accelerating bone healing and reducing treatment time for prosthetic rehabilitation.

Four female patients presented to a private dental clinic seeking corrective treatment for dentofacial deformity from 2013 to 2019. Patients complained of low self-esteem and asked for rehabilitation with implant-supported fixed dentures, rather than removable partial dentures, aiming to improve their quality of life. On clinical examination, all patients had an edentulous atrophic posterior mandible and were evaluated for implant rehabilitation in the posterior mandibular region. A cone-beam computed tomography (CBCT) scan showed severe bone atrophy in the posterior mandible, with bone height <4 mm in all cases. All patients had the anterior teeth in good periodontal health and with a mineralized structure. Of note, on clinical examination, all patients reported that they would like to avoid tooth extraction and more invasive treatments. After diagnosis and initial treatment planning, in-block lateralization of the IAN was indicated. Treatment consisted of 2 bilateral implants used for rehabilitation with a fixed dental prosthesis in both sides, where the most anterior implants were placed by the traditional technique and the posterior implants were placed by using the technique of in-block lateralization of the IAN. Research ethics committee approval was waived because of the case series nature of the study. Written informed permission was obtained from all patients to take and publish their photographs and images for scientific purposes. The operative steps are described below. Preoperative clinical presentation and intraoperative images are shown in Figures 1 through 4.

The procedure is performed under local anesthesia obtained by truncal infiltration of the IAN and the lingual nerve, associated with terminal infiltration of the buccinator nerve. An incision is made on the edentulous alveolar ridge to cut the remaining keratinized mucosa in half over the alveolar crest, extending it in a way to allow visualization of the mandibular body and the mental foramen. The mucosal incision usually reaches the mandibular canine region, where a vertical buccal incision is made. The mucoperiosteal flap is gently reflected, maintaining the integrity of the incised edges, to expose the surgical field. The exact location of implant placement is marked with a premarked fissure bur and a customized surgical guide.²⁶  The drilling sequence for implant installation is then performed, limiting the preparation depth to 1 mm above the IAN, based on prior CBCT information (Figure 1). The optimal implant direction is then determined in relation to the opposing teeth. Once the drilling is completed, a linear occlusal cortical osteotomy is performed with a piezoelectric device (Mectron Piezosurgery Device, Mectron) from the prepared implant bed into a medial and distal direction, extending 3–4 mm in length bilaterally, with a depth compatible with the existing thickness of the cortical bone. The occlusal osteotomy separates the implant bed into 2 relatively equal parts. Subsequently, anterior and posterior vertical osteotomies (8–10 mm in length) are performed on the buccal surface of the mandible with the piezoelectric device to a depth compatible with the thickness of the cortical bone. A horizontal osteotomy is then performed with the piezoelectric device at the apical level to unite the vertical osteotomies and define a square bone window (Figure 2b). The cortical bone block is removed with the aid of chisels and kept hydrated in saline solution.

After osteotomy, the mandibular canal is exposed, and the IAN is gently manipulated to remove cancellous bone fragments that may hinder a full mobilization of the IAN (Figure 2c). The nerve is retracted laterally and carefully protected in a way to allow implant bed preparation through its medial surface up to the desired bone depth. The implants are then inserted to the ideal locking depth, in accordance with the predetermined position and direction. The selected implants should be long enough to allow anchorage in the basal cortical bone of the mandibular body and provide adequate locking and stability. The IAN is repositioned to gently rest against the implants. Studies in rabbits have shown new bone formation at the nerve–implant interface,²⁷  obviating the need for additional procedures. The inner surface of the previously removed bone block can be shaped with specific piezoelectric tips for perfect adaptation to the defect at the end of surgery, thus establishing close bone-to-implant contact (Figure 3a). This provides an optimal inlay autogenous bone graft block, after which the technique is named.

The bone block is adapted and restored to its original position to close the bone defect (Figure 3C), covering the entire exposed implant surface without compressing the IAN.^7,16  The block can be fixed with 1.5-mm titanium miniscrews (Figure 3c) and/or miniplates (Figure 4c) inserted in the vertical osteotomies to stabilize the bone fragment. Occasional voids are filled with fine particulate heterogenous bone graft and amelogenin gel or platelet-rich fibrin. The use of barriers that protect the segments, select repair cells, and prevent bone resorption is recommended. Soft tissue is sutured using interrupted stitches with 4-0 or 5-0 nylon or 4-0 or 5-0 polyglactin.

Patients are given routine postoperative care instructions and advised to avoid chewing on the operated site. Soft and liquid diet is recommended during the implant osseointegration period. Patients with a strong masticatory pattern (bruxism, men, brachycephaly) should be treated preventively with botulinum toxin 10 days before surgery. Anti-inflammatory drugs, analgesics, and antibiotics are administered according to specific protocols. Neurostimulatory drugs, such as Etna, and B-complex vitamins can be administered shortly after surgery, depending on the patient's sensory perception and the IAN trauma extension.

Sutures are removed only after confirmation of edge repair, usually 12 days after surgery. Implants are loaded 90 to 120 days after insertion, placing implant-supported screw-retained partial prostheses. Soft-tissue procedures, such as volume augmentation and/or free grafting, can be performed concomitantly or after the lateralization procedure.

Follow-up ranged from 3 years to 5 years and 3 months. All patients were successfully rehabilitated with good functioning and reported satisfaction with the results. A 90-day follow-up CBCT scan is shown in Figure 5. Figures 6 through 9 show CBCT panoramic reconstructions of all 4 patients at the time of their last follow-up. CBCT scans comparing preoperative vs 3-year follow-up results are shown in Figures 10 through 12.

Alternatives for rehabilitation of the atrophic posterior mandible include the use of short implants, bone grafts, and surgery with IAN mobilization.^3,6,7  Studies comparing these alternatives have been conducted to evaluate the advantages of each approach.^3,6,17,29,30  Jayme et al¹⁷  compared the risk of marginal bone loss between short implants and conventional implants placed under IAN lateralization and concluded that, although IAN lateralization results were better in the models evaluated, variations in height had a similar impact on both treatments, thus accepting the hypothesis that the crown-to-implant ratio was not reliable for risk assessment. However, osseointegrated implants placed in combination with IAN lateralization are associated with a lower risk of bone loss than short implants placed in similar circumstances.^30,31 

Rehabilitation with short and extra-short implants is an effective treatment option,³²  but it requires a minimum alveolar bone height of 5 mm for implant placement. A bone height below 4 mm requires additional procedures for implant placement, such as vertical bone augmentation techniques (distraction osteogenesis, onlay bone grafting, bone ring, Khoury technique, and tenting technique).^3,30  The efficiency rates of these techniques vary according to the complexity of the procedures and require more than 1 appointment for implant placement and broader approaches, resulting in longer waiting times, higher costs, and greater risks related to grafting procedures.²¹ 

All patients undergoing IAN lateralization with simultaneous implant placement appear to develop some degree of neurosensory dysfunction, including anesthesia, paresthesia, dysesthesia, and hyperesthesia.^{3,8,9,31,33,34}  To minimize trauma to the IAN, alternative osteotomy designs using burs or saws have been suggested.¹¹  However, their use poses several risks, including bone overheating and damage to adjacent tissues.³ 

Although some studies have not proven histologic advantages of piezosurgery over other osteotomy systems,²⁸  the use of a piezoelectric device has provided more clinical precision to osteotomies and increased safety in the management of nerve trauma.^3,23,35  Piezosurgery has been introduced in the osteotomy process because it allows precise cutting of the hard tissue while preserving adjacent soft tissues, resulting in faster bone healing.^3,12,15,32  Comparative studies have shown that, despite the more invasive character of the piezoelectric device, the risk of injury with this device is lower than that with conventional diamond burs, with lesions being limited to the epineurium, whereas deeper structures remain unaffected.³⁵ 

Lateralization of the IAN is performed in mandibles with severe atrophy, especially in the posterior region; consequently, osteotomy is associated with a risk of mandibular fracture.^7,14,22  Also, the placement of multiple implants increases the risk of fracture, because the posterior mandible is under constant functional stress. Furthermore, nerve dysfunction may lead to proprioceptive changes that, in turn, may alter masticatory patterns, thus increasing the risk of fracture. Therefore, preserving the original bone structure at the end of the procedure favors bone healing, which is of particular clinical relevance for the recovery of bone strength.⁷ 

Implant survival rates in cases of IAN lateralization are high, ranging from 93.8% to 100%.^3,34  Both IAN mobilization techniques (lateralization and transposition) require manipulation and stretching of the nerve, but IAN lateralization is associated with lower rates of paresthesia, and follow-up studies have reported faster and more efficient recovery of neurosensory function with IAN lateralization rather than transposition.^33,36 

The possibility to restore the original mandibular anatomy with an inlay autogenous bone graft block, as presented here, increases the expectations of repair success, minimizes risks, and accelerates rehabilitation time.³⁷  Of utmost importance, before surgery, the diagnosis should be made by careful clinical examination combined with CBCT.^5,7,12  The amount and quality of bone in the region to be rehabilitated can be determined on CBCT scans, as well as the position and dimension of the IAN and its relationship with cortical surfaces. The IAN is usually in close proximity to the lingual surface of the mandible in the molar and retromolar regions.³⁸  The amount of bone between the mandibular canal and the inferior margin of the mandible ranges from 10.52 to 14.06 mm.³⁸  This condition usually allows the placement of longer implants, with anchorage in the basal cortical bone in cases in which IAN lateralization is performed. In general, areas with bone height <4 mm between the alveolar crest and mandibular canal are candidates for treatment with in-block lateralization, even if present between 2 teeth, provided the area is sufficiently large to perform the necessary osteotomies. Planning should preferably include a surgical guide to identify the ideal position of implants during insertion. Customized surgical guides can be manufactured based on virtual digital planning to determine the optimal implant tilt angle.²⁶ 

In conclusion, given the technological advances in osteotomy techniques, associated with biology-based techniques and the use of biologically effective biomaterials, rehabilitation with the technique of in-block lateralization of the IAN can be considered a new predictable, efficient treatment option with low morbidity and expected long-lasting results. Among the main clinical applications and advantages, we highlight the use in cases of posterior mandibular bone height below 4–5 mm, which preclude the safe indication of extra-short implants, and the possibility of maintaining the anterior teeth, thus avoiding dental mutilation as in treatment with complete fixed prostheses. Nevertheless, prospective studies are needed to evaluate the long-term outcomes of the surgical procedure.

Abbreviations

CBCT:

cone-beam computed tomography

IAN:

inferior alveolar nerve

The authors declare no conflicts of interest.