Computer-Guided Inferior Alveolar Nerve Lateralization With Simultaneous Implant Placement: A Preliminary Report

One of the most important factors for successful placement of dental implants is the vertical dimension of the available bone at the desired implant location. At the posterior mandible, the minimum required residual alveolar bone height to allow placement of short implants with a 2-mm safe zone between the nerve and the implant apex is 7 mm, with at least 6 mm of alveolar ridge width.¹ 

Rehabilitation of vertically deficient posterior mandible is one of the challenging situations in dental implantology. Different options have been discussed in the literature for the treatment of vertical inadequacy at the posterior mandible, such as short implants, vertical cortical onlay, vertical tent pole, guided bone regeneration, and segmental sandwich osteotomy (inlay technique). Several drawbacks were declared with these procedures, such as donor site morbidity, bone graft resorption, the need for another surgery for implant placement, and peri-implant marginal bone resorption.² 

Although all of these techniques showed a varied range of success, still, most of them could not be applied in severe vertical resorption at the posterior mandible. Moreover, until now, no long-term survival rate of dental implants has been established for any of those techniques.^3,4 

The classic inferior alveolar nerve (IAN) repositioning surgery had been long proposed for treatment of inadequate vertical height in the posterior mandible. This procedure has the advantage of offering immediate implant placement with primary stability from the native basal bone along with a reduction in treatment time compared with the previously mentioned techniques. However, the major inconvenience of this technique is the altered sensory function of the nerve in conjunction with the weakening of the mandibular body, especially when multiple implants are placed simultaneously with nerve lateralization.^5–7 

Many modifications were introduced to this technique with the aim of lowering the neurosensory dysfunction of the nerve. Repositioning of the IAN was performed via lateralization or transposition surgery techniques, with lateralization yielding lower degrees of neurosensory disturbance. Despite its time consumption, some surgeons have used piezo surgery because of its good results in reducing the risk of injury to the nerve during its exposure.^8–11 

One of the serious complications reported with IAN repositioning procedures is mandibular body fracture during implant placement after nerve retraction. This can be attributed to the complete removal of the buccal cortical plate of the mandibular body and the severing of the crestal and inferior body cortex, leaving only the lingual cortex intact, which compromises the integrity of the mandibular body, especially if the implant is placed where the lingual cortex is quite thin.^12–14 

Hence, the aim of the present study was to create a surgical guiding stent to accurately guide the outline of the access window for IAN lateralization simultaneously with the position of the implant in order to minimize the risk of mandibular body fracture and postoperative neurosensory dysfunction.

Seven male patients ranging in age from 32 to 53 years were included in this study. All had a unilateral edentulous posterior mandible with residual alveolar bone height of less than 8 mm above the IAN (Figure 1).

None of the patients had any previous trials for grafting procedures or implant placement in the site planned for nerve lateralization. All patients were free from any systemic disease that could affect bone metabolism.

A comprehensive intraoral examination was performed to examine the covering mucoperiosteum condition, ridge thickness, and interarch space.

A preoperative panoramic radiograph was taken for each patient as a primary survey to obtain the data concerning the approximate height of the residual alveolar bone above the IAN and to detect any remaining roots or localized bony pathosis.

Multislice computerized tomography (CT) was performed for the selected patients following predetermined fixed parameters (axial cuts only, bony window, slice thickness of 0.5 mm, and slice interval of 0.5 mm).

Axial cuts of the CT scan in the form of Digital Imaging and Communications in Medicine files were imported into surgical planning computer software (Mimics, Materialise, Leuven, Belgium) where multiplanar reformatting was carried out to generate coronal and sagittal cuts.

A segmentation process was accomplished by using the 2-dimensional CT cuts to define the image thresholds based on Hounsfield's units, excluding soft tissue and only highlighting the hard tissue (bone and teeth). The maxilla/mandible region was further isolated by cropping and/or region-growing functions to remove artifacts and select only the area of interest. This was followed by the 3-dimensional (3D) calculation of the selected regions to export the 3D models in the format of stereo lithographic (STL) files (Figure 2).

The STL files were imported into computer-aided design software (3Matic, Materialise) to create the design of the guiding stent. The stent body was drawn on the surface of the posterior mandibular region and then extruded to a thickness of 5 mm. The superior and inferior cutting paths were drawn to be exactly above and below the traced IAN and then subtracted from the virtual stent body. Guiding channels 2.0 mm in diameter were designed to guide the position of implants during insertion.

The STL printing files of the designed stents were sent to a 3D printing lab for manufacturing using a fused deposition modeling machine from ABS plastic material (Figure 3).

Under local anesthesia, a mucoperiosteal flap was reflected to expose the mandibular body and the mental nerve. The prefabricated surgical guiding stent was fixed in place with monocortical screws.

A premarked fissure bur, according to the thickness of the stent and the cortical bone, was used, on a contra angle hand piece, to make the superior and the inferior cuts of the cortical osteotomy according to the planned depth. The part of the stent between the 2 horizontal slots was then removed to make the cortical vertical cuts (Figure 4). In some cases, the outline of the superior and inferior cuts was marked through the stent, and then a carbide disc with a predetermined diameter was used to make the cuts through the cortical bone only (Figure 5).

The stent was removed to facilitate the separation of the cortical window using a bibeveled chisel placed at the superior cut and gentle malleting to allow its separation completely without being fractured (Figure 6). Nerve releasing was performed until it could be retracted laterally with the minimum tension. The stent was then returned to place, and the nerve was retracted using blunt instruments during implant osteotomy creation guided by the fixed stent (Figure 7).

After implant installation and stent removal, a small piece of thin collagen membrane was placed between the implant surface and the nerve bundle. The cortical bony window thickness was minimized and repositioned back to its place, as a graft, and fixed with a titanium mesh (Figure 8). Finally, the flap was sutured back to its place with 3/0 Vicryl interrupted sutures.

Postoperative instructions and medication were prescribed for the patients. Patients were examined on a weekly basis for the first month, then at months 2, 4, and 6. Postoperative data consisted of clinical measurements, and complications were recorded and assessed.

Removal of the titanium mesh followed by the prosthetic permanent loading of the installed implants was performed 6 months postoperatively.

Similar to other studies,^8,15,16  clinical evaluation of the nerve sensory function was performed using subjective and objective tests, which included light touch test, heat test, pain test, and 2-points tactile discrimination test. The operated sites were compared with the normal sites and recorded on a scale of 0–2, ranging from no sensation at all to normal sensation, respectively.

All tests were performed by 1 examiner at each visit through the scheduled follow-up visits.

Seven men with less than 8 mm unilateral residual alveolar bone height above the IAN were included in this study. They underwent guided unilateral IAN lateralization with simultaneous placement of endosseous dental implants (Figure 9).

All cases proceeded uneventfully with complete healing of the surgical site and the resolution of all expected postoperative inflammatory signs and symptoms. There were no infections, wound dehiscences, fractures, or other major complications.

Upon postoperative clinical examination, only 1 patient showed immediate sensation recovery after the resolution of the local anesthetic effect while the other patients experienced postoperative deficient sensation.

All patients recorded total sensory recovery after only 3 weeks, and none of the patients showed any signs or symptoms of permanent neurosensory dysfunction during the 6-month follow-up period.

After 6 months, all implants showed clinical stability and osseointegration, and permanent prosthetic loading was performed after the removal of the titanium mesh.

Rehabilitation of severely atrophic posterior mandible poses a difficult challenge for dental implant placement. Repositioning of the IAN has been used widely as an alternative to short implants or bone grafts for osseointegrated implant placement in the posterior atrophic mandible.

This technique is superior in decreasing the need for bone grafting with the immediate insertion of dental implants in the same surgery, thus reducing the overall treatment time, cost, and donor site morbidity.¹⁷ 

Since the major risk of IAN lateralization is neurologic deficiencies of the inferior alveolar bundle and its terminal branches, it should be performed under a strict and meticulous protocol.

In 1992, Rosenquist¹⁸  reported 10 interventions for IAN transposition with simultaneous implant insertion. Six months after surgery, 20% of the cases showed persistent nerve dysfunction in the operated regions, albeit with a return to normal in all cases after 1 year.

On the other hand, Morrison et al,¹⁹  who performed 26 IAN lateralization procedures, recorded a complaint of only initial sensitivity in the area served by the mental nerve in all patients. These disturbances resolved progressively in less than 1 month. However, in only 4 cases (15%), sensitivity was improved in 6 months.

Accordingly, and consistently with other studies,⁶  this study used the IAN lateralization technique to limit any possible outcome of neurosensory deficiencies.

It was stated that the improper selection of the site of the outer window to uncover the nerve bundle compromises a large percentage of the nerve injury during the procedure.²⁰  In this study, a customized 3D-printed surgical guide to delineate the osteotomies of the outer window and also guide the placement of the dental implants was fabricated and used to minimize the size cortical window required to access to the nerve and reduce the risk of IAN injury during cutting through the cortical bone.

Temporary weakening of the mandible due to the removal of a cortical bony window for gaining access to the mandibular canal was another drawback previously mentioned in the literature. This is in addition to the implant placement, which may lead to mandibular fracture at the operative site.^12–14  Conversely, in this study, all cases proceeded uneventfully with no evidence of mandibular fracture. This may be due to the implied surgical technique that was guaranteed to save the maximum amount of cortical bone by minimizing the size of the decorticated window to expose the nerve and positioning the implants in areas with maximum cortical thickness.

In the current study, guiding stents were designed to have bony support over the crestal and buccal cortex of the mandibular body just posterior to the mental foramen; however, the authors recommend extending the boundaries of the stent to be supported by teeth and bone if possible to avoid the possibility of stent misfit over the bone. However, in cases of full edentulism, the stent design would be only bony supported. For all patients in this study, upon postoperative examination, none showed any signs of prolonged neurosensory disturbance, even with the use of bony-supported guiding stents, as they provided direct access to the IAN and allowed for its easier identification with subsequent meticulous and careful retraction.

Several methods of cutting the cortical bone have been introduced in the literature, including the use of carbide fissures, discs, and finally piezo devices, which greatly minimize the risk of injury to the nerve during cutting of the cortical window. The use of premarked fissures to cut exactly through the thickness of the guiding channels and cortical bone was not always easy; even with wide mucoperiosteal flap reflection, the accessibility of the contra-angle hand piece with the installed cutting fissure was limited. On the contrary, marking the outline of the window through the guiding stent followed by the use of carbide discs with a predetermined radius to cut directly through the exact thickness of the cortex was more practical. Especially with the need to accomplish the cuts through the cortex only and no need to extend the cutting depth to the cancellous bone, the cutting angle will not endanger the nerve, but it would still be safer to make the cuts with the piezo surgery device, even if it takes a longer time to cut through the dense cortical bone of the mandible.^5,8 

Barbu et al²⁰  were against repositioning the removed cortical bony window back in place and recommended the use of additional bone grafting around the nerve. They attributed this to the risk of nerve injury caused by replacement of the removed block. Contrarily, in this study, the outer cortical wall of the bony window was repositioned back in place after implant placement and fixed using a mesh with no reported complications.

Upon implant examination, all installed implants in all patients proved to be clinically stable and osseointegrated at the end of the follow-up period and upon permanent prosthetic loading, which was in accordance with the work of Hirsch and Branemark,²¹  who declared the success of implant osseointegration and stability following the use of IAN lateralization technique in their study.

The use of a surgical guide to delineate the outline of the buccal access window during the IAN lateralization procedure greatly enhanced the process of nerve identification while decreasing the intraoperative time and demonstrating decreased risks for (1) IAN injury, (2) postoperative nerve dysfunction, and (3) intraoperative mandibular body fracture. However, further studies with larger samples and modifications of the stent design are required to allow an easier and more predictable nerve lateralization technique.

Abbreviations

3D

3-dimensional

CT

computerized tomography

IAN

inferior alveolar nerve

STL

stereo lithographic

The authors stated that there is no conflict of interest.