Repositioning the Inferior Alveolar Nerve for Placement of Endosseous Implants: Technical NoteDennis G. Smiler, DDS, MScD
Placement of implants in the posterior mandible is limited by the height of bone between the alveolar crest and the inferior alveolar canal. This paper discusses a surgical technique to reposition and protect the neurovascular bundle so that endosseous implants may be placed. A rectangular window is cut in the cortical bone posterior to the mental foramen. The mental foramen is not violated and the mental nerve is not relieved peripherally into the soft tissue. Cancellous bone is removed from the window and the canal is uncovered. A vessel loop placed around the bundle repositions and protects the nerve laterally. After implant placement, the bundle is replaced within the cortical window and the mucoperiosteal flap is sutured. Avoiding manipulation of the terminal branches of the inferior alveolar nerve reduces the risk of permanent nerve damage. (INT J ORAL MAXILLOFAC IMPLANTS 1993;8:145-150.)
Key words: implants, inferior alveolar nerve, surgery
After extraction of teeth there is progressive loss of ridge height. Bone resorption continues in a medial direction and progresses from moderate to severe atrophy of the mandible.1 Resorption diminishes bone height between the alveolar bone crest and the inferior alveolar canal.2 However, the region of the mandible beneath the canal is not significantly affected by bone resorption.3 The placement of endosseous implants in the posterior mandible frequently involves operative decisions related to the neurovascular bundle.
Prior attempts to laterally reposition the inferior alveolar neurovascular bundle involved the removal of bone at the mental foramen and dissection of the terminal branches of the mental nerve. Removal of the Osseous mental foramen has a risk of nerve damage and subsequent sequelae. Dissection of mental nerve branches into the mucosa to relieve tension on the main body of the bundle also carries the risk of nerve damage. A nerve-repositioning surgical protocol that does not remove bone at the mental foramen or involve dissection of its terminal branches may increase the success rate of the operation without inducing temporary or permanent nerve changes.
A surgical procedure to laterally reposition the mandibular nerve in preparation for placement of implants removes the inferior alveolar neurovascular bundle from its canal. The nerve is displaced laterally and is protected in this buccal/lateral position away from cutting drills. Adequate bone quality and quantity necessary for
stabilization and healing of posterior mandibular implants is now available above and below the mandibular canal. Implants are placed according to good surgical protocol. Repositioning the neurovascular bundle prior to drilling implant receptor sites and placing implants reduces the risk of nerve transection or damage.
Surgical ProcedureAnesthesia is obtained with a mandibular nerve block and local infiltration injections. Panoramic radiographs and/or computed tomography scans can demonstrate the course of the canal and its relation to crestal bone. A crestal incision is made extending from the retromolar pad anteriorly to the canine region (Fig 1). The incision length must provide adequate tissue relaxation for reflection of the mucoperiosteal flap to the mandibular inferior border. The mental foramen and its neurovascular bundle are identified. Gentle dissection inferior to the canal and under the bundle will relieve tissue tension anteriorly.
A vertical osteotomy, approximately 5 to 7 mm in length, is made 3 to 4 mm posterior to the mental foramen. Using a no. 4 round bur or a no. 701 fissure bur, this bone cut is made only through the outer cortex. The posterior vertical osteotomy is made over the canal in the second molar region. Horizontal osteotomies connect the vertical bone cuts. Care is taken to assure that the bone cuts are made through the cortical bone and just into the softer and bleeding cancellous bone (Fig 2a).
After confirming that all bone cuts are through the outer cortex, a Molt elevator is inserted in the cortical osteotomy window (Fig 2b). Lever action against the mandible removes the entire outer rectangular cortical window, which is discarded.
The underlying cancellous bone is exposed through this cortical bone window (Fig 3a). The outer cancellous bone covering the canal is gently removed with the small end of the Molt curet or a spoon curet (Fig 3b). As cancellous bone is gently removed, the lateral aspect of the canal is exposed. A small curet is then positioned within the confines of the canal with the sharp edges turned outward. The cancellous bone covering the lateral aspect of the canal is removed in small increments. No pressure is placed on the neurovascular bundle. With upward movement of the curet, small increments of bone are removed over the bundle. The bundle is exposed for the entire length of the rectangular cortical window. This broad exposure permits release of the neurovascular bundle without tension.
The bundle is released for the entire length of the osseous cortical window and gently removed from its canal with curets or nerve hooks. A small curved hemostat is placed under the nerve at its midpoint within the window (Fig 4). The tip of this hemostat is opened slightly after it is passed under the bundle. One end of the vessel loop is placed within the opened beaks. The hemostat is closed and the vessel loop is gently pulled under and around the bundle. The vessel loop now passes around and under the bundle and displaces it laterally. The vessel loop is moved anteriorly or posteriorly to displace the bundle laterally away from proposed implant surgical sites
Implant receptor sites are prepared according to good surgical principles. Implant guide pins are used to assess the final implant position and verify the width and length of the implants. After irrigation of the receptor sites, the implants are placed (Fig 5b). The vessel loop is removed. The osseous window is irrigated and covered with CollaCote (Colla-Tec, Plainsboro, NJ), a resorbable collagen wound dressing (Fig 6). The previously removed outer cortical plate is not replaced. No particulate materials are placed over the exposed bundle or within the cortical window. The mucoperiosteal flap is repositioned and sutured. Postoperative panoramic radiographs are taken.
Case ReportsPatient 1. A 64-year-old man was seen for consultation and implant treatment planning. The mandibular arch was edentulous bilaterally. Panoramic radiographic examination revealed insufficient bone height between the inferior alveolar canal and ridge crest for implant placement. Surgery consisted of bilateral repositioning of the neurovascular bundles. The right neurovascular bundle was very thin and threadlike in diameter. The left neurovascular bundle was thicker. Two Sustain (Orthomatrix, Minneapolis, MN) implants were placed in the right and left sides (Fig 7a). Immediately after surgery the patient experienced slight numbness of the lower right lip. The lower left lip had no sensory deficit. After 3 months of healing, the implants were uncovered and fixed abutments were placed (Fig 7b). The patient was referred to the restorative clinician for fixed prosthesis reconstruction. Six months postoperatively, the patient had no neurosensory deficit involving the left inferior alveolar nerve and very mild sensory loss of the right lip.
Patient 2. A 69-year-old woman had a partially edentulous mandible and completely edentulous maxilla. The maxillary arch was treated with bilateral sinus lift graft procedures and placement of implants. Two Integral (Calcitek, Carlsbad, CA) implants were placed in the mandibular left posterior quadrant and reconstructed with two crown restorations joined with a nonrigid connector. The inferior alveolar canal was exposed on the right side and the neurovascular bundle was laterally repositioned. Two Integral implants were placed and the bundle was replaced in its bony compartment (Fig 8a). Figure 8b is a panoramic radiograph illustrating the right posterior implant through the canal to the inferior border of the mandible. There was no sensory loss of the right inferior alveolar nerve.
Patient 3. Fixed partial prosthesis reconstruction supported by implants was planned for this 43-year-old woman. Preoperative radiographs demonstrated insufficient bone height between the ridge crest and inferior alveolar canal for implant placement. Following bilateral surgical repositioning of the neurovascular bundles, receptor sites for three cylinder implants in the mandibular right side and one cylinder implant in the left side were prepared. After implant seating, the neurovascular bundle was replaced within the cancellous bone window (Fig 9a). The
cortical osseous window was covered with CollaCote (Fig 9b) and the mucoperiosteal flap was sutured. Four months after implant surgery, the patient was reconstructed with fixed partial prostheses (Figs 10a and 10b). There was no sensory loss of the right or left inferior alveolar nerves.
ComplicationsApproximately 10 nerve repositioning procedures have been performed according to the above surgical protocol. As yet there have been no long-term adverse sequelae from this surgical procedure. Two patients had unilateral mild neuropraxia that resolved within 3 weeks. All other patients had no temporary or permanent loss of sensation over the distribution of the inferior alveolar nerve bundle.
Bleeding and hemorrhage have not been a problem. The amount of blood loss has averaged less than 25 mL and definitely less than that lost from removal of difficult impacted mandibular third molars. Hemorrhage is possible from transection or tearing of the neurovascular bundle. Although this complication has not occurred, the recommended procedure would be to clamp and tie the vessel. Firm pressure might also alleviate an acute hemorrhage from tearing of the neurovascular bundle.
DiscussionOne surgical procedure suggested to laterally reposition the inferior alveolar neurovascular bundle involves removal of bone at the posterior aspect of the mental foramen. Another surgical procedure removes the entire mental foramen via a series of connected holes circumferentially drilled around the foramen. These and other procedures involve dissection and manipulation of the more terminal branches of the mental nerve. Some procedures advocate dissection of the terminal branches into the mucosa to relieve tension on the main body of the nerve bundle. Dissection and manipulation of the smaller more terminal branches of any nerve carries an increased risk of nerve damage greater than manipulation of the thicker or main body of the nerve.
As described in this article, lateral repositioning of the inferior alveolar neurovascular bundle through a posterior cortical window provides a relatively safe surgical method for protecting the nerve during implant site preparation and placement. This operation is preferred over other surgical procedures to lateralize the bundle, because it involves manipulation of only the thicker bundle that lies within the inferior alveolar canal. Manipulation of the thicker component and not the smaller, more terminal branches of the nerve may reduce the incidence of postoperative neuropraxia and permanent paresthesia or anesthesia.
Fig. 1 Crestal incision from the retromolar pad area to the canine region provides relaxation for reflecting the mucoperiosteal flap to the mandibular inferior border.
Fig. 2a Two vertical osteotomies are made: one distal to the mental foreman and the other in the second molar region. Horizontal osteotomies connect vertical bone cuts.
Fig. 2b Molt curet is inserted and the cortical window is removed.
Fig. 4 Closed small hemostat is placed under the bundle at the midpoint of the cortical window. The beaks are opened slightly to close one end of a vessel loop.
Fig. 5a Hemostat is pulled under the bundle and the vessel loop displaces the bundle laterally. The vessel loop is moved to displace and protect the bundle when the implant receptor site is drilled.
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