CHAPTER 14 INFERIOR ALVEOLAR NERVE LATERALIZATION AND MENTAL NEUROVASCULAR DISTALIZATION
Since the development of endosteal implant reconstruction in the early 1960s, the severely atrophic posterior mandible has presented challenges for the implant reconstruction team. Most obviously, the presence of the inferior alveolar canal and its contents has required that the implant practitioner take precautions to avoid damaging the canal’s vital structures (Figures 14-1 and 14-2). One early solution was to surgically enter the posterior mandible, explore and isolate the inferior alveolar nerve and, in some instances, the mental neurovascular bundle, and relocate the nerve on a permanent or temporary basis as endosteal implants were simultaneously placed.1
Figure 14-1. A, Diagrammatic view of a longitudinal cut of the mandible demonstrates the relationship of the inferior alveolar canal and mental foramen to the apices of the teeth. The decrease in vertical height of bone superior to the canal after the loss of teeth posteriorly is evident. B, Coronal section of the head with the inferior dental nerve located away from both the buccal and lingual cortical bone because there is no atrophy of this specimen.C, Transverse section of the head with the inferior dental nerve in close proximity to both the buccal and lingual cortical bone in a very severe horizontal atrophic ridge (as compared to part B, which is not atrophic). D, A mandible with less than 2 mm of vertical bone superior to the inferior alveolar canal is demonstrated. E, A mandible demonstrates a dehiscence of the bone over the inferior alveolar canal at the region of the bicuspids and continuous with the mental foramen.F, Edentulous mandibles demonstrate the relative position of the mental foramen after various degrees of atrophy have occurred. G, Serial cross-sections of a mandible visualize the path of the inferior alveolar canal and mental foramen, including the section to the right of the foramen, which demonstrates the canal for the anterior loop of the nerve. H, A cross-section of a mandible demonstrates the well-circumscribed cortical bone of the inferior alveolar canal. I, Cross-sections of both the right and left mandible demonstrate bilateral symmetry of architecture, canal position, and bone density. J and K, Cross-sections of two mandibles demonstrate extreme alveolar ridge atrophy, causing the mental foramen to be located on the crest of the residual ridge. L, High-powered view shows a mental foramen with a circular shape and dense bone. M, High-powered view shows a mental foramen with an oval shape and several foramina inside for the path of the incisive branches of the nerve.
(B and C, From Grant JCB: An atlas of anatomy, ed 4, Baltimore, 1956, Williams & Wilkins.)
Figure 14-2. A, Panoramic radiographic view demonstrates a mandible with wires inserted into the inferior alveolar canal. Adequate vertical residual bone could accommodate implant reconstruction (right). Minimal vertical residual bone is depicted on the opposite side. To place implants in the posterior quadrants, nerve repositioning is necessary (left). B, Panoramic radiograph with ball-bearing marker is used as a preliminary screening procedure to evaluate distortion error when calculating vertical bone relative to the inferior alveolar nerve. C, Computed tomographic scan with three-dimensional reformatted image reviewed with SIM/Plant computer software accurately determines residual bone dimensions within 0.1 mm. D, The panoramic radiographic view discloses a two-dimensional evaluation of the patient in part C and shows built-in distortions.
Because of its technical difficulty, this reconstruction procedure was not widely used at first.2,3 In the past 10 years, however, it has begun to receive significant attention from various practitioners around the world.4–11 It still requires that the implant practitioner has complete familiarity with the specific anatomy (see Chapter 7) and the surgical handling of the neurovascular structures, even though technologic advancements have begun to facilitate nerve repositioning. With this consideration in mind, this chapter reviews the indications for and limitations of two related procedures: (1) inferior alveolar nerve lateralization, and (2) distalization of the mental neurovascular bundle. It also provides a detailed description of how the procedures are performed.12–14
Figure 14-3. A, Patient was evaluated and treatment was planned using only a periapical radiographic film taken several years before implant treatment and with no ability to evaluate magnification. An implant was placed into the canal, resulting in permanent anesthesia. B, This panoramic film demonstrates the excessive length of the distal implant.
The preoperative and postoperative care and management of patients who have undergone inferior alveolar nerve lateralization or distalization of the mental neurovascular bundle is complex. Patient acceptance rate is low. Three or four meetings are commonly required before the initiation and acceptance of either treatment, including consultation with other members of the family, as well as members of the surgical and prosthetic teams. As part of this treatment presentation phase, an injection of bupivacaine (Marcaine 0.5%, Cook-Waite, Abbott Laboratories, North Chicago, IL) to achieve a mandibular block may be useful as a screening measure. Such an injection will provide the patient with 8 to 16 hours of anesthesia of the neurovascular complex as a preview of one of the types of nerve deficit that may ensue. The patient’s refusal or acceptance of this screening measure can be documented in the patient’s chart as a risk management tool.
A higher degree of neurological deficiency is associated with distalization of the mental neurovascular bundle than with lateralization of the inferior alveolar nerve.11,13 An obvious reason for this difference in outcome is the fact that the surgical procedure for the former requires the use of high-speed or regular rotary instruments to circumvent completely the bone of the mental foramen from the buccal aspect of the mandible. Also, the incisive nerve that branches off from the mental nerve must be severed to mobilize the mental nerve and move it distally.15 Both factors require more manipulation and increase the risk of injury, therefore increasing postsurgical edema and potential complications.
Solar et al. and Ulm et al.16–30 documented the position and classification of the intraosseous path of the mental nerve. They reported the course of the mental nerve within the mandible as observed in 37 dried human specimens. Two types of pathways were documented. In 22 cases a siphon configuration was noted at the end of the mental canal and exit point. The investigators observed an arch that travels laterally and cranially. This area is wide, compared with the 4 to 5 mm before the exit of the foramen itself. This type of pathway was classified as Type I. A distance of up to 5 mm was measured between the mental foramen and the most anterior point (anterior loop) of the canal. No correlation was made between this distance and the degree of atrophy of the jaw.
In the other 15 of the 37 specimens, the mental canal was observed as ascending directly from the mandibular canal to the mental foramen without curving forward (i.e., no anterior loop). This type of pathway or course was classified as Type II. The average angle of inclination between the plane through which the mental canal (anterior loop) courses and the horizontal plane was measured at 50 degrees (Figure 14-4).
Figure 14-4. A, Solar’s Type I pathway of the mental nerve travels laterally and cranially; an anterior loop is usually present. B, Solar’s Type II pathway of the mental nerve ascends directly from the mandibular canal to the mental foramen without curving forward; therefore, no anterior loop is present.
(A and B [left] redrawn from Babbush CA: Transpositioning and repositioning the inferior alveolar and mental nerves in conjunction with endosteal implant reconstruction, Periodontology 2000 17:183, 1998. Munksgaard International Publishers Ltd, Copenhagen, Denmark.)
Based on these data, it is recommended that a distance of at least 6 mm anterior to the mental foramen be maintained when performing surgery in this region. This distance refers to the center point of the most distal implant or the implant in closest proximity to the mental foramen when performing surgery in the mandibular premolar-canine region. These findings provided a clearer presentation of the anatomy of these areas.
Regular radiographic studies, whether panoramic lateral cephalometric, occlusal, or periapical views, will not define a medial-lateral position of the inferior alveolar canal or mental foramen. They will only define the inferior-superior position as it relates to the residual crest of the ridge and the inferior border of the mandible.28,30–35
In some cases the nerve may be located significantly toward the medial or lateral cortical plates. This situation precludes the need to lateralize the neurovascular bundle during implant placement. In other words, the implant can be primarily placed medial or lateral to the canal without surgically repositioning these vital structures (Figure 14-5).36 A preoperative computed tomography (CT) scan with three-dimensional reformatted images is recommended to define clearly the medial and lateral positions of the canal and foramen in all cases involving lateralization or distalization of these nerves.37
Figure 14-5. Cross-section of the mandible clearly demonstrates the inferior alveolar canal located in the lingual cortical bone. An implant could be placed buccal and superior to this area without causing neurological sequelae.
It is now possible to analyze these images by using SIM/Plant interactive software (Materialise, Leuven, Belgium). Once the images are loaded into the computer, the nerve pathway and position can be defined easily and the neurovascular pathway can be highlighted in various colors (Figure 14-6, A and B). When the position and degree of the loop of the mental nerve and the position and pathway of the inferior alveolar nerve have been ascertained, the case can be planned with a high degree of accuracy. In addition, the computer can define the density, dimension, size, shape, and position of the bone and the number, size, and shape of planned implants (see Chapter 8 and Figure 14-6, C and D).12