- Email: [email protected]
Course of the Mandibular Incisive Canal and Its Impact on Harvesting Symphysis Bone Grafts
DENTAL IMPLANTS
Course of the Mandibular Incisive Canal and Its Impact on Harvesting Symphysis Bone Grafts David D. Vu, PharmD, DDS, MD,* Hans C. Brockhoff II, DDS, MD,y David M. Yates, DMD, MD,z Richard Finn, DDS,x and Ceib Phillips, MPH, PhDk Purpose:
To characterize the anatomic course of the mandibular incisive canal to define parameters for harvesting autogenous bone from the symphysis of the mandible.
Materials and Methods:
A series of osteotomies were completed between the mental foramina in the anterior mandibles of 19 cadavers. Methylene blue dye was used to help identify the incisive canal. From the canal, distances to key adjacent landmarks were measured with a Boley gauge to 0.1 mm. Measurements included distances from the mandibular incisive canal to the buccal cortex, the lingual cortex, the inferior border of the mandible, the apices of the teeth, and the buccal cementoenamel junction (CEJ) of the teeth.
Results:
The canal decreased in diameter from lateral to medial. It tended to be closer to the buccal cortical bone than to the lingual cortex (P < .001) and was, at times, directly abutting the buccal cortex (average distance to buccal cortex, 3.5 mm). The canal maintained a relatively constant distance from the apices of the teeth (approximately 7 to 8 mm), coursing inferiorly under the longer canines bilaterally. The canal became increasingly difficult to identify toward the midline, likely dispersing into microscopic tributaries.
Conclusions: The authors suggest several modifications to the standard surgical approach to the symphysis area during the harvest of bone grafts. When the goal is to avoid the mandibular incisive canal, osteotomies should not exceed a depth of 4 mm, should be at least 5 mm anterior to the mental foramen, and 9 mm below the root apices (or 23 mm below the lowest facial CEJ) and should maintain the contour of the mandible’s inferior border. Alternatively, some degree of canal compromise can be accepted and larger grafts can be obtained by increasing the depth of the harvest in the horizontal dimension or decreasing the distance from the osteotomy to the root apices (or the CEJ) in the vertical dimension. Ó 2015 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 73:258.e1-258.e12, 2015
There are a diversity of intra- and extraoral options when harvesting autogenous bone for reconstruction and preprosthetic treatment for dental rehabilitation.
Sites include the calvarium, iliac crest, tibia, ribs, zygoma, maxilla, and mandible.1 Autogenous grafting has long been considered the gold standard, providing
*Resident, Division of Oral and Maxillofacial Surgery, Department
Orthodontics, School of Dentistry, University of North Carolina,
of Surgery, University of Texas Southwestern Medical Center/
Chapel Hill, NC.
Parkland, Dallas, TX.
Address correspondence and reprint requests to Dr Vu: Division
yFellow, Oral and Maxillofacial Head and Neck Oncology Surgery, University of Michigan, Ann Arbor, MI.
of Oral and Maxillofacial Surgery, University of Texas, Southwestern Medical Center, 5201 Harry Hines Boulevard, CS 3.104, Dallas, TX
zFellow, Cleft Lip/Palate and Craniofacial Surgery, Louisiana State
75235; e-mail: [email protected]
University, Shreveport, LA.
Received August 24 2014
xProfessor, Division of Oral and Maxillofacial Surgery, Department of Surgery, and Department of Cell Biology, University
Ó 2015 American Association of Oral and Maxillofacial Surgeons
of Texas Southwestern Medical Center, Dallas; Chief, Oral and
0278-2391/14/01597-3
Maxillofacial Surgery, Veteran’s Affairs North Texas Health Care
http://dx.doi.org/10.1016/j.joms.2014.10.005
Accepted October 4 2014
System, Dallas, TX. kAssistant Dean, Advanced Education/Graduate Programs; Director, Curriculum in Oral Biology; Professor, Department of
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osteoconductive, osteoinductive, and osteogenic properties. Intraoral sites provide the same ectomesenchymal tissue present at recipient sites and thus are favored over extraoral mesenchymal sources, which are slower to revascularize and thus exhibit greater resorption.2,3 Yates et al4 recently quantified how much bone can be obtained from several intraoral sites: the mandibular symphysis, mandibular ramus and body, mandibular coronoid process, and the zygomaticomaxillary buttress. The largest quantity of bone was obtained from the ramus and body (2.02 mL), followed by the symphysis (1.15 mL). The coronoid process and zygoma provided much less bone (0.17 and 0.11 mL respectively). Thus, the mandibular symphysis is a good option for autogenous bone grafting when a moderate quantity of bone is required (ie, alveolar deficits at $1 dental implant site). It might not be sufficient where larger grafts are needed, such as large defects in the cleft palate.5 It is less technically challenging to harvest from the symphysis, offering greater ease of access compared with the other 3 intraoral harvest sites. It avoids a visible external or cutaneous scar, is believed to offer less morbidity, decreases operating time, and decreases hospitalization time.6 However, harvesting bone from this region does carry some risks, including hemorrhage, injury to existing dentition (resulting in pulp necrosis or devitalization), loss of unerupted permanent teeth, and injury to nerves, resulting in sensory disturbances, such as hypoesthesia and paresthesia. Neurologic deficits have been reported in up to 80% of patients postoperatively (average, 39.6% across 6 studies). These symptoms persisted in up to 20% of patients (average, 8.4% across the same 6 studies).2,7-9 The key neurovascular bundles to consider include the incisive canal and mental nerves and the ‘‘anterior loop,’’ which is an extension or bridge to the mental nerve from the mandibular and incisive canals.10 Although Yates et al4 described the volume of bone that could be harvested from the anterior mandible, Pommer et al2 described safety margins based on the course of the mandibular incisive canal on computed tomography (CT). Previous margins have followed a rule of ‘‘5’s,’’ with osteotomies placed 5 mm from adjacent anatomic structures: 5 mm anterior to the mental foramen, 5 mm below the root apices, and 5 mm above the inferior border of the mandible.11 After evaluating 50 dentate mandibles with CT and calculating risk of injury to the incisive canal, Pommer et al2 recommended safety margins as follows: 4 mm maximum depth of the bone graft, 5 mm anterior to the mental foramen, 8 mm below the root apex, and leaving the inferior border of the mandible intact (especially at the midline, to maintain facial contours). Of note, there is much greater leeway to the root apex dimension if the graft depth is no deeper than the recommended
4 mm2 (ie, the osteotomy theoretically should be anterior to the canal and therefore one can harvest a larger amount of bone in the superior dimension toward the roots). Understanding the anatomy of the neurovascular bundle of the anterior mandible (and its course in the incisive canal) is essential for harvesting grafts and for other common surgical procedures, such as genioplasties or the placement of dental implants. The anatomic description of the incisive canal has been inconsistent, with some investigators describing it as a discrete canal continuing anteriorly from the mental foramen, others describing it as a plexus of nerves that supply the teeth in the anterior mandible, and still others neglecting to mention it at all.8,12-15 This may, in part, be because conventional radiography (orthopantomography) is poor at detecting the incisive canal and harbors an inherent distortion in images. Other imaging modalities, such as magnetic resonance imaging (MRI) and cone-beam CT (CBCT), have more recently helped to confirm the existence of the canal and describe its anatomy.12,16-18 Current research has suggested that the most common configuration is likely somewhere between the 2 descriptions, with a true incisive canal continuing anteromedially from the mandibular canal, which eventually resembles more of a plexus at or near the midline.8,12 The aim of this study was to describe the anatomy and course of the mandibular incisive canal and thus estimate safe margins for harvesting of bone grafts.
Materials and Methods The subjects in this study were not identifiable. Institutional approval was obtained to collect data. Institutional review board approval was not required according to the standard protocols of the authors’ institution. Table 1 lists the materials used in this study. Mandibles were harvested from 19 cadavers previously fixed with formalin. The remaining attached soft tissues were removed. The dentate status of the mandibles was recorded. For each specimen, the length of the mandibular incisive canal was measured. This was achieved by placing a string along the inferior border of the mandible between the bilateral mental foramina. The string was marked at each endpoint using a hemostat and then measured with a standard ruler consisting of 1-mm intervals (Fig 1). Methylene blue solution was used to stain the internal components of the incisive canal and aid in identification of the neurovascular bundle. The dye was drawn into a 20-mL syringe and injected with light digital pressure into the mental foramina bilaterally with a 23-gauge needle (Fig 2). Vertical osteotomies were
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Table 1. MATERIALS
Methylene blue, CAS# 61-73-4, C16H18N3SCl Boley gauge Ruler Hemostat Cotton string Osteomed (Addison, TX) saggital saw Osteomed reciprocating saw Osteomed handpiece Handpiece burs: pineapple, 701, #8 round Becton Dickinson (Franklin Lakes, NJ) 20-mL syringe Becton Dickinson 23-gauge needle Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
subsequently performed between the interdental spaces using an Osteomed (Addison, TX) handpiece with a reciprocating saw (blade width, 0.4 mm) to completely section the anterior mandible. The position of the first osteotomy was between the first and second premolars, closely coinciding with the anterior extent of the mental foramen. Subsequent osteotomies occurred by tooth until the contralateral mental foramen was reached. Before each section, additional methylene blue was injected to aid in identification of the neurovascular bundle and canal (Figs 3-7). After each section, the (stained) neurovascular bundle was identified visually and by probing with the syringe needle to confirm position and size. Extravasation of methylene blue was minimal and did not obscure identification or measurements. The canal diameter was measured using a Boley gauge to the closest tenth of a millimeter. Distances
FIGURE 2. Injection of methylene blue dye into the incisive canal. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
to adjacent landmarks were measured next. The distance from the neurovascular bundle to the outer surface of the buccal cortical bone was recorded, followed by the distance to the outer surface of the lingual cortical bone, and then the distance to the inferior border of the mandible. Two measurements were obtained superiorly: the distance to the root apex and the distance to the cementoenamel junction (CEJ) at the buccal surface of the associated tooth (Fig 8). The handpiece was used with pineapple, 701, and #8 round burs when necessary to aid in exposure of the tooth root and root apex. Of note, each set of measurements occurred distal of the associated tooth for
FIGURE 1. Estimation of the length of the incisive canal.
FIGURE 3. Site of first osteotomy (saw blade width, 0.4 mm). Note its relation to the mental foramen.
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FIGURE 4. First osteotomy complete. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
consistency, except at the midline. This is the only set of measurements that occurred mesial to any tooth. In addition, because there are right and left central incisors, there were 2 possible values for this position. Because the bundle and canal were often difficult to identify at the central incisor, the side that best displayed the bundle and canal was used, if any. Linear mixed models with a compound covariance structure were used to assess the effect of direction (buccal vs lingual for the horizontal dimension; inferior border vs CEJ for the vertical dimension) and location (premolar, lateral incisor, canine) on the distance from the canal and to assess the effect of side and location on diameter measurements. Interaction terms were not considered. The least square means for the factors of direction and location were calculated. All analyses were performed using SAS 9.2 (SAS Institute, Cary, NC).
FIGURE 5. Cut surfaces exposed. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
FIGURE 6. Second osteotomy complete. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
Results Characteristics of the 19 mandibles are presented in Table 2. All specimens had teeth from at least the first premolar to the first premolar, with most having teeth through the first and even second molars present. Two specimens had sufficient teeth only to provide data for a hemimandible. Measurements were not completed at edentulous sites for these specimens, and the alveolus was judged to be adequately robust without severe or noteworthy atrophy for measurements to be included. Cause of death included malignancy, cardiovascular disease, respiratory failure, and renal failure. A 32-year-old died of a drug overdose. The neurovascular bundle and incisive canal were easier to identify laterally. These structures became increasingly difficult to identify, resulting in fewer values toward the midline (approximately half the specimens at the midline and at the central incisor yielded an identifiable canal from which data could be recorded). A lingual foramen was identified in 2 specimens (Figs 9, 10). The length of the canal from mental foramen to mental foramen (estimated by the length of the inferior border of the mandible) ranged from 46.5 to 62.3 mm (mean, 53.2 mm; standard deviation, 4.4 mm). Table 3 lists the distances from the neurovascular bundle to selected anatomic landmarks. The average distance of the neurovascular bundle to the outer surface of the buccal cortex was statistically significantly different from the distance to the outer surface of the lingual cortex (horizontal axis; P < .001) after adjusting for tooth position, indicating that the canal was closer to the buccal cortical bone (3.5 mm) than to the lingual cortical bone (5.1 mm). The average
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FIGURE 7. A, B, All osteotomies complete. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
distance also differed significantly (P < .004) among tooth positions after adjusting for direction. Because there were inadequate samples at the central incisor,
there were no comparisons to this position or the midline. Similarly, in the vertical axis, the average distance of the neurovascular bundle to the inferior border of the mandible was significantly different from the distance to the CEJ (P < .001). The average distance to the inferior border was approximately 10.7 mm, whereas the average distance to the CEJ was approximately 21.7 mm. The average distance also differed significantly (P = .04) among the tooth positions. The diameter of the canal was statistically significantly different among tooth positions (premolar diameter, 1.8 mm; canine, 1.4 mm; lateral incisor, 1.2 mm; P < .0001).
Discussion The present results and the graphic representations aid in describing the course of the mandibular incisive canal. As expected, the canal diameter decreases from lateral to medial. The neurovascular bundle becomes smaller toward the midline, as it gives off branches to individual teeth (Fig 11). The canal is initially relatively close to the labial surface and buccal cortical bone when near the mental foramen at the first premolar. Travelling anteromedially, it gains shelter within the cortical bone and additional cancellous bone, occasionally returning close to the buccal surface along its course, particularly at the midline. There also is relatively abundant cancellous bone posterior to the canal toward the lingual cortex (Figs 12, 13).
Table 2. CADAVER CHARACTERISTICS (N = 19)
FIGURE 8. Distances measured. a, Distance to buccal outer cortex; b, distance to lingual outer cortex; c, distance to inferior border of mandible; d, distance to root apex; e, distance to buccal cementoenamel junction; x, canal. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
Age (yr) Mean Range Gender Male Female Race Caucasian Remaining teeth, n Mean Range
73.4 32-91 9 10 19 10.8 4-14
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FIGURE 9. Lingual foramen (arrow). Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
FIGURE 10. Lingual foramen (arrows). Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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Table 3. SUMMARY OF DISTANCES
Position L first premolar L canine L lateral L central Midline R central R lateral R canine R first premolar
Canal Diameter
Distance to Outer Buccal Cortex
Distance to Outer Lingual Cortex
Distance to Mandibular Inferior Border
Distance to Root Apex
Distance to CEJ
1.78 (0.44) 1.33 (0.36) 1.21 (0.32) 1.03 (0.18) 0.68 (0.18) 0.97 (0.14) 1.17 (0.2) 1.45 (0.3) 1.74 (0.33)
2.34 (1.04) 3.55 (1.56) 3.87 (1.22) 3.77 (1.65) 3.41 (1.42) 3.73 (1.52) 4.04 (1.45) 4.18 (1.78) 2.96 (2.1)
4.96 (1.87) 4.81 (2.02) 5.51 (1.66) 6.43 (1.71) 6.86 (3.12) 6.41 (1.86) 5.61 (2.04) 4.77 (1.61) 4.92 (2.27)
11.36 (1.99) 10.59 (1.74) 9.85 (2.25) 12.71 (3.06) 13.99 (3.48) 12.51 (3.42) 10.69 (2.46) 10.18 (2.18) 11.23 (1.9)
7.53 (3.36) 7.18 (3.34) 11.29 (4.14) 8.62 (4.54) 7.71 (4.23) 9.91 (4.05) 12.18 (6.99) 7.22 (3.96) 7.12 (3.26)
20.01 (3.24) 20.38 (5.82) 23.51 (3.66) 20.32 (4.65) 19.57 (3.98) 20.93 (4.75) 23.66 (4.4) 22.37 (3.78) 20.31 (3.63)
Note: Values are millimeters and are presented as mean (standard deviation). Abbreviations: CEJ, cementoenamel junction; L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
The canal remains a good distance from the inferior border of the mandible (10 to 14 mm; Fig 14) and maintains a relatively constant distance below the root apices traveling medially (Fig 15). Because of this, it appears to descend slightly from the mental foramen, traveling beneath the canine, and as it approaches the midline, ascends toward the more medially positioned incisors, maintaining roughly 7 to 8 mm of distance (distances are longer at the incisors, because their roots are shorter than the canine roots and the canal does not quite ‘‘catch up’’ to them). It should be reiterated that, at or near the midline, the canal was difficult to identify in several specimens. This was likely because it was too small, but it also could have been forming a plexus-like network, with small tributaries, rather than having a single discrete canal. This also was a limitation of the use of methylene blue, which helped guide measurements with ease when the canal and associated neurovascular
bundles were grossly larger proximally. Distally, however, there was a decrease in uptake, despite repeated injections, as canal size decreased. The canal might have crossed the midline in some specimens, communicating with the contralateral canal, whereas in others, it ended in the small tributaries described earlier or a single canal that veered up toward the ipsilateral central incisor (without crossing the midline; Figs 16, 17). Krasny et al12 found using MRI that most (70%) incisive canals crossed the midline and anastomosed and approximately one fourth of those were ‘‘plexus-like.’’ This is in contrast to the present findings, where the incisive canal could not be identified at the midline in approximately half the specimens. The lingual foramen also should be mentioned (typically situated at or above the mental spines), which might contain neural or vascular contents (or both). Its clinical importance might include hemorrhage from
FIGURE 11. Canal diameter. Depicted are mean plus 1 standard deviation (top of box), mean (line bisecting box), mean minus 1 standard deviation (bottom of box), and range of values (whiskers). L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 12. Distance to buccal cortex. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
dissection as a possible complication.8,16 This foramen also might be one possible source of redundant neurovascular supply that helps a patient recover after surgical disruption of the incisive canal. The authors did identify the lingual foramen twice, because their osteotomies coincided with this structure in 2 specimens (Figs 9, 10). Table 4 presents a comparison of data from the present study with 4 other studies. Two groups used CBCT for their measurements17,18: Pommer et al2 used conventional CT and Krasny et al12 used MRI. Distance to the root apex was measured in only 2 of the studies. Pommer et al recorded the shortest distance (5.2 to 6.6 mm), and the authors recorded the longest distance (7.1 to 9.9 mm). Only in the present study was the CEJ used as a landmark in which the distance from the CEJ to the canal was recorded. The distance to the buccal cortex was shortest in the present study and the study by Al-Ani et al.17 This is reflected in Figure 18, which illustrates how the canal can be essentially abutting the cortical bone, and in Figure 7, which shows how even at the midline the canal course can
come close to the inner surface of the buccal cortical bone. Distance to the lingual cortex was similar across studies (Pommer et al did not record this distance). Distance to the inferior border also was similar across studies. Given the data, the authors expect that when harvesting a graft from the mandibular symphysis, it is quite likely that the incisive canal is compromised and might even be resected with bone, particularly in cases in which a thick graft is taken. An analysis of the present osteotomies in all possible dimensions will aid in the assessment and determination of reasonable safety margins if, in fact, total avoidance of the incisive canal is the desired goal. A depth of 4 mm from the buccal surface as recommended by Pommer et al2 will aid in obtaining a moderate volume of bone, but the present data show that the distance to the canal from the buccal surface is shorter than 4 mm, on average, at all tooth positions except for 2 (right lateral and right canine), which are only slightly longer than 4 mm (Table 3). Three of the other studies suggested slightly more
FIGURE 13. Distance to lingual cortex. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 14. Distance to mandibular inferior border. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
buccal bone, but not much (approximately 1 mm additional at most). Yates et al4 also showed that up to 7.8 mm of bone thickness is obtainable from symphysis grafts, a depth that would likely encroach on and violate a noteworthy number of incisive canals if the osteotomy overlaps it (horizontally) and is not inferior to it (vertically). Therefore, depth of harvest should be considered with distance from the root apex, which is discussed next. The authors still advocate 4 mm of depth to procure sufficient bone. Increasing the safety margin to the tooth root apex according to the recommendations of Pommer et al2 from 5 to 8 mm is roughly in agreement with the present data, because the distance from the canal to roots was 7 to 8 mm for most teeth (it was 11 to 12 mm for the lateral incisor, but this tooth root is shorter than the adjacent canine; thus, measuring clearances and planning osteotomies from the canine would be prudent). However, because their measured distance from the canal to the root apex ranged from 5.2 to 6.6 mm, they are giving at least an additional 1.4 mm of clearance with this proposed safety margin
(accounting for canal diameter). Therefore, an osteotomy would have to be 9 mm below the root apex to afford a similar level of clearance with the present results. Of note, the authors found a high degree of variation in distances to the root apex (range, 0 to 28 mm); thus, recommendations in this parameter are soft at best. An additional landmark for the vertical dimension, if unsure of the position of the root apex, is the CEJ. Based on the present results, a margin of approximately 23 mm from the most inferior CEJ would afford a similar level of safety as would 9 mm below the root apex (Fig 19). The CEJ also is a useful clinical landmark that can be seen under direct vision. Next, harvest position is considered with regard to the mental foramen. An osteotomy 5 mm anterior to the mental foramen would be just anterior to the first premolar. The first premolar is where the incisive canal tends to be closest to the buccal cortical bone given that the neurovascular bundle courses to the surface to branch off as the mental nerve (often as the anterior loop) and then continues forward and
FIGURE 15. Distance to root apex. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 16. Schematic for an incisive canal that crosses the midline with branches to individual teeth. Smaller tributaries not illustrated. The course of the canal mimics the root profile, descending around the longer canine root.
FIGURE 17. Schematic for an incisive canal that does not cross the midline. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
medially as the incisive nerve. Uchida et al10 found the average anterior loop length to be approximately 2 mm; thus, 5 mm generally provides an adequate margin of safety in avoiding the anterior loop (if present) or the incisive nerve. The longest loop length they noted was 9 mm; thus, if a particularly robust anterior loop is seen on imaging (CT), a larger margin may be necessary. For the inferior osteotomy, the authors agree that the inferior border of the mandible should be maintained for facial contour. Alternatively, it is acceptable to follow the traditional safety margin of 5 mm, although this would decrease the potential quantity of the harvest.2
One should keep in mind the potential for lip or chin ptosis with large grafts, particularly those requiring extensive subperiosteal dissection of the mentalis muscle. This can result in loss of normal facial contour and esthetics, although the literature suggests that this is not a common clinical concern.19,20 Interestingly, even with complete disruption of the incisive canal, Hutchinson and MacGregor21 found restoration of vitality of almost all teeth after 6 to 12 months. They evaluated 181 teeth of 24 patients who underwent subapical osteotomies and transposition. Procedures of the mandible involved the anterior teeth. Testing for vitality consisted of thermal and electrical pulp testing (thermal testing consisted of cotton
Table 4. COMPARISON OF STUDIES
Parameter Patients, N Method Race Distance to root apex (mm) Distance to CEJ (mm) Distance to buccal cortex (mm) Distance to lingual cortex (mm) Distance to mandibular inferior border (mm)
Present Study
Pommer et al2
Krasny et al12
Al-Ani et al17
Makris et al18
36* cadaveric Caucasian 7.1-9.9z 19.6-23.7 2.3-4.2 4.8-6.9 9.9-14
50 CT Caucasian 5.2-6.6 NM 3.4-4.4 NM 10.3-14
64 MRI NS 6.1-9.5 NM 4-4.8 5.5-6.8 8.7-13.4
60 CBCT Asiany NM NM 2.9-3.4 4.6-5.2 9.1-10.8
100 CBCT NS NM NM 3.6-4.8 5.3-5.7 9.4-11.2
Abbreviations: CBCT, cone-beam computed tomography; CEJ, cementoenamel junction; CT, computed tomography; MRI, magnetic resonance imaging; NM, not measured; NS, not stated. * Thirty-six hemimandibles (19 cadavers). y Malay, Indian, and Chinese. z Excludes value at lateral incisor, which is 12.2 mm. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 18. Note the proximity of the canal to the buccal surface. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
pledgets soaked in ethyl chloride for cold and warmed gutta percha for heat). At 6 months, 20 of 20 mandibular teeth in 10 patients had a 100% response to electrical pulp testing and cold stimulation, whereas 90% responded to heat stimulation. They found similar, although slightly less robust, results in the maxilla. The few patients who remained insensate were indifferent to their symptoms. Noia et al22 evaluated soft tissue neurosensory morbidity in a prospective trial (chin, mental nerve cutaneous distribution) in 30 patients who underwent symphyseal bone graft harvesting. The safety margins for the graft were 5 mm from the apices of the teeth, 5 mm medial to the mental foramen, and 5 mm superior to the inferior border of the mandible. The chin was divided into 6 zones, each of which was tested for 2-point discrimination, brush directional stroke, pinprick, thermal cold discrimination, and thermal hot discrimination. Approximately 50% of patients
had deficits in the first month, with subsequent resolution of symptoms in all patients after 1 year. These findings suggest that accessory neurovascular pathways can provide a redundancy, or ‘‘back-up’’ system, when complete disruption occurs (as opposed to neurapraxia or other reversible mechanisms). Possible supplementary neurovascular sources include the sublingual and mylohyoid branches, lingual foramina, etc. Maintenance of the periodontal ligament and its neural systems also can help decrease perception of deficits. These studies suggest that patients who undergo osteotomies tend to do well, whether the procedure is for harvesting of bone grafts, genioplasty, or even to aid in rapid orthodontic movements.23 In the latter, this technique has evolved into the use of corticotomies in which alveolar bone is decorticated up to 0.5 mm, with selective medullary penetration to enhance bleeding. This is termed the regional acceleratory phenomenon in which the injured sites recruit osteoclasts and osteoblasts to increase remodeling and thus increase orthodontic movements by 3 to 4 times that of conventional means.24 The dimensions of corticotomies, by definition, being relatively superficial, would generally spare key structures such as the mandibular incisive canal, further securing their safety from injury. Thus, safety margins can serve as guides, rather than as rules, to keep surgeons cognizant of the incisive canal. The proximity of the canal to the buccal surface (Fig 18), in particular, stands out, so depth of harvest might be the most important dimension to consider when harvesting grafts. Veneer grafts of 4 mm or wider, for instance, will typically harvest the incisive canal in the graft. If one is fairly certain that one is inferior to the incisive canal, then one can be more generous with depth (>4 mm) without direct canal compromise. Conversely, if graft depth is less than or equal to the recommended 4 mm, the superior osteotomy might be considerably closer to the root apices and the CEJ than the recommended values of 9 mm below the
FIGURE 19. Distance to the cementoenamel junction. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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Table 5. RECOMMENDED SAFETY MARGINS
Parameter Depth of graft harvest Distance anterior to mental foramen Distance below root apex Distance below CEJ Distance above inferior border of mandible
Recommended Margin (mm) 4 5 9 23 5 (or maintain inferior border contour)
Abbreviation: CEJ, cementoenamel junction. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
root apices or 23 mm below the CEJ. One must keep in mind that the anatomy of the canal can vary quite widely as shown in the present ranges. Anatomic studies of the human body typically show that large ranges for measurements tend to be the rule and not the exception, emphasizing the uniqueness of individual anatomy. Thus, clinician judgment and appropriate imaging should ultimately guide therapy. Table 5 presents suggested safety margins for the clinician who wishes to make a conscious effort to avoid the incisive canal. Others might accept that the canal is usually unavoidable and plan to obtain larger grafts. Acknowledgments The handpiece and equipment used in the cadaver dissection were supplied by Osteomed.
References 1. Misch CM: Comparison of intraoral donor sites for onlay grafting prior to implant placement. Int J Oral Maxillofac Implants 12: 767, 1997 2. Pommer B, Tepper G, Gahleitner A, et al: New safety margins for chin bone harvesting based on the course of the mandibular incisive canal in CT. Clin Oral Implants Res 19:1312, 2008 3. Koole R: Ectomesenchymal mandibular symphysis bone graft: An improvement in alveolar cleft grafting? Cleft Palate Craniofac J 31:217, 1994 4. Yates DM, Brockhoff HC, Finn R, et al: Comparison of intraoral harvest sites for corticocancellous bone grafts. J Oral Maxillofac Surg 71:497, 2013 5. Bahr W, Coulon JP: Limits of the mandibular symphysis as a donor site for bone grafts in early secondary cleft palate osteoplasty. Int J Oral Maxillofac Surg 25:389, 1996
6. Sindet-Pedersen S, Enemark H: Mandibular bone grafts for reconstruction of alveolar clefts. J Oral Maxillofac Surg 46:533, 1988 7. Rawashdeh M, Telfah H: Secondary alveolar bone grafting: The dilemma of donor site selection and morbidity. Br J Oral Maxillofac Surg 46:665, 2008 8. Mraiwa N, Jacobs R, Steenberghe DV, et al: Clinical assessment and surgical implications of anatomic challenges in the anterior mandible. Clin Implant Dent Relat Res 5:219, 2003 9. Hoppenreijs TJ, Nijdam ES, Freihofer HP: The chin as a donor site in early secondary osteoplasty: A retrospective clinical and radiological evaluation. J Craniomaxillofac Surg 20:119, 1992 10. Uchida Y, Noguchi N, Goto M, et al: Measurement of anterior loop length for the mandibular canal and diameter of the mandibular incisive canal to avoid nerve damage when installing endosseous implants in the interforaminal region: A second attempt introducing cone beam computed tomography. J Oral Maxillofac Surg 67:744, 2009 11. Hunt DR, Jovanovic SA: Autogenous bone harvesting: A chin graft technique for particulate and monocortical bone blocks. Int J Periodontics Restorative Dent 19:165, 1999 12. Krasny A, Krasny N, Prescher A: Study of inferior dental canal and its contents using high-resolution magnetic resonance imaging. Surg Radiol Anat 34:687, 2012 13. Mraiwa N, Jacobs R, Moerman P, et al: Presence and course of the incisive canal in the human mandibular interforaminal region: Two-dimensional imaging versus anatomical observations. Surg Radiol Anat 25:416, 2003 14. Polland K, Munro S, Reford G, et al: The mandibular canal of the edentulous jaw. Clin Anat 14:445, 2001 15. Krasny A, Krasny N, Prescher A: Anatomic variations of the neural canal structures of the mandible observed by 3-Tesla magnetic resonance imaging. J Comput Assist Tomogr 36:150, 2012 16. Reinhilde J, Lambrichts I, Liang X, et al: Neurovascularization of the anterior jaw bones revisited using high-resolution magnetic resonance imaging. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 103:683, 2007 17. Al-Ani O, Nambiar P, Ha K, et al: Safe zone for bone harvesting from the interforaminal region of the mandible. Clin Oral Implants Res 24:115, 2013 18. Makris N, Stamatakis H, Syriopoulos K, et al: Evaluation of the visibility and the course of the mandibular incisive canal and the lingual foramen using cone-beam computed tomography. Clin Oral Implants Res 21:766, 2010 19. Desai AJ, Thomas R, Kumar T, et al: Current concepts and guidelines in chin graft harvesting: A literature review. Int J Oral Health Sci 3:16, 2013 20. Toscano N, Shumaker N, Holtzclaw D: The art of block grafting: A review of the surgical protocol for reconstruction of alveolar ridge deficiency. J Implant Adv Clin Dent 2:45, 2010 21. Hutchinson D, MacGregor AJ: Tooth survival following various methods of sub-apical osteotomy. Int J Oral Surg 1:81, 1972 22. Noia CF, Ortega-Lopes R, Olate S, et al: Prospective clinical assessment of morbidity after chin bone harvest. J Craniofac Surg 22:2195, 2011 23. Bolding SL, Roblee RR, Sandor G, et al: Optimizing orthodontic therapy with dentoalveolar distraction osteogenesis, in Reyes M, (ed.): Distraction Osteogenesis of the Facial Skeleton. Hamilton, Ontario, Canada, BC Decker, 2007, pp 187–197 24. Hassan AH, Al-Fraidi AA, Al-Saeed SH: Corticotomy-assisted orthodontic treatment: Review. Open Dent J 4:159, 2010
Course of the Mandibular Incisive Canal and Its Impact on Harvesting Symphysis Bone Grafts David D. Vu, PharmD, DDS, MD,* Hans C. Brockhoff II, DDS, MD,y David M. Yates, DMD, MD,z Richard Finn, DDS,x and Ceib Phillips, MPH, PhDk Purpose:
To characterize the anatomic course of the mandibular incisive canal to define parameters for harvesting autogenous bone from the symphysis of the mandible.
Materials and Methods:
A series of osteotomies were completed between the mental foramina in the anterior mandibles of 19 cadavers. Methylene blue dye was used to help identify the incisive canal. From the canal, distances to key adjacent landmarks were measured with a Boley gauge to 0.1 mm. Measurements included distances from the mandibular incisive canal to the buccal cortex, the lingual cortex, the inferior border of the mandible, the apices of the teeth, and the buccal cementoenamel junction (CEJ) of the teeth.
Results:
The canal decreased in diameter from lateral to medial. It tended to be closer to the buccal cortical bone than to the lingual cortex (P < .001) and was, at times, directly abutting the buccal cortex (average distance to buccal cortex, 3.5 mm). The canal maintained a relatively constant distance from the apices of the teeth (approximately 7 to 8 mm), coursing inferiorly under the longer canines bilaterally. The canal became increasingly difficult to identify toward the midline, likely dispersing into microscopic tributaries.
Conclusions: The authors suggest several modifications to the standard surgical approach to the symphysis area during the harvest of bone grafts. When the goal is to avoid the mandibular incisive canal, osteotomies should not exceed a depth of 4 mm, should be at least 5 mm anterior to the mental foramen, and 9 mm below the root apices (or 23 mm below the lowest facial CEJ) and should maintain the contour of the mandible’s inferior border. Alternatively, some degree of canal compromise can be accepted and larger grafts can be obtained by increasing the depth of the harvest in the horizontal dimension or decreasing the distance from the osteotomy to the root apices (or the CEJ) in the vertical dimension. Ó 2015 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 73:258.e1-258.e12, 2015
There are a diversity of intra- and extraoral options when harvesting autogenous bone for reconstruction and preprosthetic treatment for dental rehabilitation.
Sites include the calvarium, iliac crest, tibia, ribs, zygoma, maxilla, and mandible.1 Autogenous grafting has long been considered the gold standard, providing
*Resident, Division of Oral and Maxillofacial Surgery, Department
Orthodontics, School of Dentistry, University of North Carolina,
of Surgery, University of Texas Southwestern Medical Center/
Chapel Hill, NC.
Parkland, Dallas, TX.
Address correspondence and reprint requests to Dr Vu: Division
yFellow, Oral and Maxillofacial Head and Neck Oncology Surgery, University of Michigan, Ann Arbor, MI.
of Oral and Maxillofacial Surgery, University of Texas, Southwestern Medical Center, 5201 Harry Hines Boulevard, CS 3.104, Dallas, TX
zFellow, Cleft Lip/Palate and Craniofacial Surgery, Louisiana State
75235; e-mail: [email protected]
University, Shreveport, LA.
Received August 24 2014
xProfessor, Division of Oral and Maxillofacial Surgery, Department of Surgery, and Department of Cell Biology, University
Ó 2015 American Association of Oral and Maxillofacial Surgeons
of Texas Southwestern Medical Center, Dallas; Chief, Oral and
0278-2391/14/01597-3
Maxillofacial Surgery, Veteran’s Affairs North Texas Health Care
http://dx.doi.org/10.1016/j.joms.2014.10.005
Accepted October 4 2014
System, Dallas, TX. kAssistant Dean, Advanced Education/Graduate Programs; Director, Curriculum in Oral Biology; Professor, Department of
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osteoconductive, osteoinductive, and osteogenic properties. Intraoral sites provide the same ectomesenchymal tissue present at recipient sites and thus are favored over extraoral mesenchymal sources, which are slower to revascularize and thus exhibit greater resorption.2,3 Yates et al4 recently quantified how much bone can be obtained from several intraoral sites: the mandibular symphysis, mandibular ramus and body, mandibular coronoid process, and the zygomaticomaxillary buttress. The largest quantity of bone was obtained from the ramus and body (2.02 mL), followed by the symphysis (1.15 mL). The coronoid process and zygoma provided much less bone (0.17 and 0.11 mL respectively). Thus, the mandibular symphysis is a good option for autogenous bone grafting when a moderate quantity of bone is required (ie, alveolar deficits at $1 dental implant site). It might not be sufficient where larger grafts are needed, such as large defects in the cleft palate.5 It is less technically challenging to harvest from the symphysis, offering greater ease of access compared with the other 3 intraoral harvest sites. It avoids a visible external or cutaneous scar, is believed to offer less morbidity, decreases operating time, and decreases hospitalization time.6 However, harvesting bone from this region does carry some risks, including hemorrhage, injury to existing dentition (resulting in pulp necrosis or devitalization), loss of unerupted permanent teeth, and injury to nerves, resulting in sensory disturbances, such as hypoesthesia and paresthesia. Neurologic deficits have been reported in up to 80% of patients postoperatively (average, 39.6% across 6 studies). These symptoms persisted in up to 20% of patients (average, 8.4% across the same 6 studies).2,7-9 The key neurovascular bundles to consider include the incisive canal and mental nerves and the ‘‘anterior loop,’’ which is an extension or bridge to the mental nerve from the mandibular and incisive canals.10 Although Yates et al4 described the volume of bone that could be harvested from the anterior mandible, Pommer et al2 described safety margins based on the course of the mandibular incisive canal on computed tomography (CT). Previous margins have followed a rule of ‘‘5’s,’’ with osteotomies placed 5 mm from adjacent anatomic structures: 5 mm anterior to the mental foramen, 5 mm below the root apices, and 5 mm above the inferior border of the mandible.11 After evaluating 50 dentate mandibles with CT and calculating risk of injury to the incisive canal, Pommer et al2 recommended safety margins as follows: 4 mm maximum depth of the bone graft, 5 mm anterior to the mental foramen, 8 mm below the root apex, and leaving the inferior border of the mandible intact (especially at the midline, to maintain facial contours). Of note, there is much greater leeway to the root apex dimension if the graft depth is no deeper than the recommended
4 mm2 (ie, the osteotomy theoretically should be anterior to the canal and therefore one can harvest a larger amount of bone in the superior dimension toward the roots). Understanding the anatomy of the neurovascular bundle of the anterior mandible (and its course in the incisive canal) is essential for harvesting grafts and for other common surgical procedures, such as genioplasties or the placement of dental implants. The anatomic description of the incisive canal has been inconsistent, with some investigators describing it as a discrete canal continuing anteriorly from the mental foramen, others describing it as a plexus of nerves that supply the teeth in the anterior mandible, and still others neglecting to mention it at all.8,12-15 This may, in part, be because conventional radiography (orthopantomography) is poor at detecting the incisive canal and harbors an inherent distortion in images. Other imaging modalities, such as magnetic resonance imaging (MRI) and cone-beam CT (CBCT), have more recently helped to confirm the existence of the canal and describe its anatomy.12,16-18 Current research has suggested that the most common configuration is likely somewhere between the 2 descriptions, with a true incisive canal continuing anteromedially from the mandibular canal, which eventually resembles more of a plexus at or near the midline.8,12 The aim of this study was to describe the anatomy and course of the mandibular incisive canal and thus estimate safe margins for harvesting of bone grafts.
Materials and Methods The subjects in this study were not identifiable. Institutional approval was obtained to collect data. Institutional review board approval was not required according to the standard protocols of the authors’ institution. Table 1 lists the materials used in this study. Mandibles were harvested from 19 cadavers previously fixed with formalin. The remaining attached soft tissues were removed. The dentate status of the mandibles was recorded. For each specimen, the length of the mandibular incisive canal was measured. This was achieved by placing a string along the inferior border of the mandible between the bilateral mental foramina. The string was marked at each endpoint using a hemostat and then measured with a standard ruler consisting of 1-mm intervals (Fig 1). Methylene blue solution was used to stain the internal components of the incisive canal and aid in identification of the neurovascular bundle. The dye was drawn into a 20-mL syringe and injected with light digital pressure into the mental foramina bilaterally with a 23-gauge needle (Fig 2). Vertical osteotomies were
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Table 1. MATERIALS
Methylene blue, CAS# 61-73-4, C16H18N3SCl Boley gauge Ruler Hemostat Cotton string Osteomed (Addison, TX) saggital saw Osteomed reciprocating saw Osteomed handpiece Handpiece burs: pineapple, 701, #8 round Becton Dickinson (Franklin Lakes, NJ) 20-mL syringe Becton Dickinson 23-gauge needle Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
subsequently performed between the interdental spaces using an Osteomed (Addison, TX) handpiece with a reciprocating saw (blade width, 0.4 mm) to completely section the anterior mandible. The position of the first osteotomy was between the first and second premolars, closely coinciding with the anterior extent of the mental foramen. Subsequent osteotomies occurred by tooth until the contralateral mental foramen was reached. Before each section, additional methylene blue was injected to aid in identification of the neurovascular bundle and canal (Figs 3-7). After each section, the (stained) neurovascular bundle was identified visually and by probing with the syringe needle to confirm position and size. Extravasation of methylene blue was minimal and did not obscure identification or measurements. The canal diameter was measured using a Boley gauge to the closest tenth of a millimeter. Distances
FIGURE 2. Injection of methylene blue dye into the incisive canal. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
to adjacent landmarks were measured next. The distance from the neurovascular bundle to the outer surface of the buccal cortical bone was recorded, followed by the distance to the outer surface of the lingual cortical bone, and then the distance to the inferior border of the mandible. Two measurements were obtained superiorly: the distance to the root apex and the distance to the cementoenamel junction (CEJ) at the buccal surface of the associated tooth (Fig 8). The handpiece was used with pineapple, 701, and #8 round burs when necessary to aid in exposure of the tooth root and root apex. Of note, each set of measurements occurred distal of the associated tooth for
FIGURE 1. Estimation of the length of the incisive canal.
FIGURE 3. Site of first osteotomy (saw blade width, 0.4 mm). Note its relation to the mental foramen.
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FIGURE 4. First osteotomy complete. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
consistency, except at the midline. This is the only set of measurements that occurred mesial to any tooth. In addition, because there are right and left central incisors, there were 2 possible values for this position. Because the bundle and canal were often difficult to identify at the central incisor, the side that best displayed the bundle and canal was used, if any. Linear mixed models with a compound covariance structure were used to assess the effect of direction (buccal vs lingual for the horizontal dimension; inferior border vs CEJ for the vertical dimension) and location (premolar, lateral incisor, canine) on the distance from the canal and to assess the effect of side and location on diameter measurements. Interaction terms were not considered. The least square means for the factors of direction and location were calculated. All analyses were performed using SAS 9.2 (SAS Institute, Cary, NC).
FIGURE 5. Cut surfaces exposed. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
FIGURE 6. Second osteotomy complete. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
Results Characteristics of the 19 mandibles are presented in Table 2. All specimens had teeth from at least the first premolar to the first premolar, with most having teeth through the first and even second molars present. Two specimens had sufficient teeth only to provide data for a hemimandible. Measurements were not completed at edentulous sites for these specimens, and the alveolus was judged to be adequately robust without severe or noteworthy atrophy for measurements to be included. Cause of death included malignancy, cardiovascular disease, respiratory failure, and renal failure. A 32-year-old died of a drug overdose. The neurovascular bundle and incisive canal were easier to identify laterally. These structures became increasingly difficult to identify, resulting in fewer values toward the midline (approximately half the specimens at the midline and at the central incisor yielded an identifiable canal from which data could be recorded). A lingual foramen was identified in 2 specimens (Figs 9, 10). The length of the canal from mental foramen to mental foramen (estimated by the length of the inferior border of the mandible) ranged from 46.5 to 62.3 mm (mean, 53.2 mm; standard deviation, 4.4 mm). Table 3 lists the distances from the neurovascular bundle to selected anatomic landmarks. The average distance of the neurovascular bundle to the outer surface of the buccal cortex was statistically significantly different from the distance to the outer surface of the lingual cortex (horizontal axis; P < .001) after adjusting for tooth position, indicating that the canal was closer to the buccal cortical bone (3.5 mm) than to the lingual cortical bone (5.1 mm). The average
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FIGURE 7. A, B, All osteotomies complete. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
distance also differed significantly (P < .004) among tooth positions after adjusting for direction. Because there were inadequate samples at the central incisor,
there were no comparisons to this position or the midline. Similarly, in the vertical axis, the average distance of the neurovascular bundle to the inferior border of the mandible was significantly different from the distance to the CEJ (P < .001). The average distance to the inferior border was approximately 10.7 mm, whereas the average distance to the CEJ was approximately 21.7 mm. The average distance also differed significantly (P = .04) among the tooth positions. The diameter of the canal was statistically significantly different among tooth positions (premolar diameter, 1.8 mm; canine, 1.4 mm; lateral incisor, 1.2 mm; P < .0001).
Discussion The present results and the graphic representations aid in describing the course of the mandibular incisive canal. As expected, the canal diameter decreases from lateral to medial. The neurovascular bundle becomes smaller toward the midline, as it gives off branches to individual teeth (Fig 11). The canal is initially relatively close to the labial surface and buccal cortical bone when near the mental foramen at the first premolar. Travelling anteromedially, it gains shelter within the cortical bone and additional cancellous bone, occasionally returning close to the buccal surface along its course, particularly at the midline. There also is relatively abundant cancellous bone posterior to the canal toward the lingual cortex (Figs 12, 13).
Table 2. CADAVER CHARACTERISTICS (N = 19)
FIGURE 8. Distances measured. a, Distance to buccal outer cortex; b, distance to lingual outer cortex; c, distance to inferior border of mandible; d, distance to root apex; e, distance to buccal cementoenamel junction; x, canal. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
Age (yr) Mean Range Gender Male Female Race Caucasian Remaining teeth, n Mean Range
73.4 32-91 9 10 19 10.8 4-14
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FIGURE 9. Lingual foramen (arrow). Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
FIGURE 10. Lingual foramen (arrows). Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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Table 3. SUMMARY OF DISTANCES
Position L first premolar L canine L lateral L central Midline R central R lateral R canine R first premolar
Canal Diameter
Distance to Outer Buccal Cortex
Distance to Outer Lingual Cortex
Distance to Mandibular Inferior Border
Distance to Root Apex
Distance to CEJ
1.78 (0.44) 1.33 (0.36) 1.21 (0.32) 1.03 (0.18) 0.68 (0.18) 0.97 (0.14) 1.17 (0.2) 1.45 (0.3) 1.74 (0.33)
2.34 (1.04) 3.55 (1.56) 3.87 (1.22) 3.77 (1.65) 3.41 (1.42) 3.73 (1.52) 4.04 (1.45) 4.18 (1.78) 2.96 (2.1)
4.96 (1.87) 4.81 (2.02) 5.51 (1.66) 6.43 (1.71) 6.86 (3.12) 6.41 (1.86) 5.61 (2.04) 4.77 (1.61) 4.92 (2.27)
11.36 (1.99) 10.59 (1.74) 9.85 (2.25) 12.71 (3.06) 13.99 (3.48) 12.51 (3.42) 10.69 (2.46) 10.18 (2.18) 11.23 (1.9)
7.53 (3.36) 7.18 (3.34) 11.29 (4.14) 8.62 (4.54) 7.71 (4.23) 9.91 (4.05) 12.18 (6.99) 7.22 (3.96) 7.12 (3.26)
20.01 (3.24) 20.38 (5.82) 23.51 (3.66) 20.32 (4.65) 19.57 (3.98) 20.93 (4.75) 23.66 (4.4) 22.37 (3.78) 20.31 (3.63)
Note: Values are millimeters and are presented as mean (standard deviation). Abbreviations: CEJ, cementoenamel junction; L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
The canal remains a good distance from the inferior border of the mandible (10 to 14 mm; Fig 14) and maintains a relatively constant distance below the root apices traveling medially (Fig 15). Because of this, it appears to descend slightly from the mental foramen, traveling beneath the canine, and as it approaches the midline, ascends toward the more medially positioned incisors, maintaining roughly 7 to 8 mm of distance (distances are longer at the incisors, because their roots are shorter than the canine roots and the canal does not quite ‘‘catch up’’ to them). It should be reiterated that, at or near the midline, the canal was difficult to identify in several specimens. This was likely because it was too small, but it also could have been forming a plexus-like network, with small tributaries, rather than having a single discrete canal. This also was a limitation of the use of methylene blue, which helped guide measurements with ease when the canal and associated neurovascular
bundles were grossly larger proximally. Distally, however, there was a decrease in uptake, despite repeated injections, as canal size decreased. The canal might have crossed the midline in some specimens, communicating with the contralateral canal, whereas in others, it ended in the small tributaries described earlier or a single canal that veered up toward the ipsilateral central incisor (without crossing the midline; Figs 16, 17). Krasny et al12 found using MRI that most (70%) incisive canals crossed the midline and anastomosed and approximately one fourth of those were ‘‘plexus-like.’’ This is in contrast to the present findings, where the incisive canal could not be identified at the midline in approximately half the specimens. The lingual foramen also should be mentioned (typically situated at or above the mental spines), which might contain neural or vascular contents (or both). Its clinical importance might include hemorrhage from
FIGURE 11. Canal diameter. Depicted are mean plus 1 standard deviation (top of box), mean (line bisecting box), mean minus 1 standard deviation (bottom of box), and range of values (whiskers). L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 12. Distance to buccal cortex. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
dissection as a possible complication.8,16 This foramen also might be one possible source of redundant neurovascular supply that helps a patient recover after surgical disruption of the incisive canal. The authors did identify the lingual foramen twice, because their osteotomies coincided with this structure in 2 specimens (Figs 9, 10). Table 4 presents a comparison of data from the present study with 4 other studies. Two groups used CBCT for their measurements17,18: Pommer et al2 used conventional CT and Krasny et al12 used MRI. Distance to the root apex was measured in only 2 of the studies. Pommer et al recorded the shortest distance (5.2 to 6.6 mm), and the authors recorded the longest distance (7.1 to 9.9 mm). Only in the present study was the CEJ used as a landmark in which the distance from the CEJ to the canal was recorded. The distance to the buccal cortex was shortest in the present study and the study by Al-Ani et al.17 This is reflected in Figure 18, which illustrates how the canal can be essentially abutting the cortical bone, and in Figure 7, which shows how even at the midline the canal course can
come close to the inner surface of the buccal cortical bone. Distance to the lingual cortex was similar across studies (Pommer et al did not record this distance). Distance to the inferior border also was similar across studies. Given the data, the authors expect that when harvesting a graft from the mandibular symphysis, it is quite likely that the incisive canal is compromised and might even be resected with bone, particularly in cases in which a thick graft is taken. An analysis of the present osteotomies in all possible dimensions will aid in the assessment and determination of reasonable safety margins if, in fact, total avoidance of the incisive canal is the desired goal. A depth of 4 mm from the buccal surface as recommended by Pommer et al2 will aid in obtaining a moderate volume of bone, but the present data show that the distance to the canal from the buccal surface is shorter than 4 mm, on average, at all tooth positions except for 2 (right lateral and right canine), which are only slightly longer than 4 mm (Table 3). Three of the other studies suggested slightly more
FIGURE 13. Distance to lingual cortex. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 14. Distance to mandibular inferior border. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
buccal bone, but not much (approximately 1 mm additional at most). Yates et al4 also showed that up to 7.8 mm of bone thickness is obtainable from symphysis grafts, a depth that would likely encroach on and violate a noteworthy number of incisive canals if the osteotomy overlaps it (horizontally) and is not inferior to it (vertically). Therefore, depth of harvest should be considered with distance from the root apex, which is discussed next. The authors still advocate 4 mm of depth to procure sufficient bone. Increasing the safety margin to the tooth root apex according to the recommendations of Pommer et al2 from 5 to 8 mm is roughly in agreement with the present data, because the distance from the canal to roots was 7 to 8 mm for most teeth (it was 11 to 12 mm for the lateral incisor, but this tooth root is shorter than the adjacent canine; thus, measuring clearances and planning osteotomies from the canine would be prudent). However, because their measured distance from the canal to the root apex ranged from 5.2 to 6.6 mm, they are giving at least an additional 1.4 mm of clearance with this proposed safety margin
(accounting for canal diameter). Therefore, an osteotomy would have to be 9 mm below the root apex to afford a similar level of clearance with the present results. Of note, the authors found a high degree of variation in distances to the root apex (range, 0 to 28 mm); thus, recommendations in this parameter are soft at best. An additional landmark for the vertical dimension, if unsure of the position of the root apex, is the CEJ. Based on the present results, a margin of approximately 23 mm from the most inferior CEJ would afford a similar level of safety as would 9 mm below the root apex (Fig 19). The CEJ also is a useful clinical landmark that can be seen under direct vision. Next, harvest position is considered with regard to the mental foramen. An osteotomy 5 mm anterior to the mental foramen would be just anterior to the first premolar. The first premolar is where the incisive canal tends to be closest to the buccal cortical bone given that the neurovascular bundle courses to the surface to branch off as the mental nerve (often as the anterior loop) and then continues forward and
FIGURE 15. Distance to root apex. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 16. Schematic for an incisive canal that crosses the midline with branches to individual teeth. Smaller tributaries not illustrated. The course of the canal mimics the root profile, descending around the longer canine root.
FIGURE 17. Schematic for an incisive canal that does not cross the midline. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
medially as the incisive nerve. Uchida et al10 found the average anterior loop length to be approximately 2 mm; thus, 5 mm generally provides an adequate margin of safety in avoiding the anterior loop (if present) or the incisive nerve. The longest loop length they noted was 9 mm; thus, if a particularly robust anterior loop is seen on imaging (CT), a larger margin may be necessary. For the inferior osteotomy, the authors agree that the inferior border of the mandible should be maintained for facial contour. Alternatively, it is acceptable to follow the traditional safety margin of 5 mm, although this would decrease the potential quantity of the harvest.2
One should keep in mind the potential for lip or chin ptosis with large grafts, particularly those requiring extensive subperiosteal dissection of the mentalis muscle. This can result in loss of normal facial contour and esthetics, although the literature suggests that this is not a common clinical concern.19,20 Interestingly, even with complete disruption of the incisive canal, Hutchinson and MacGregor21 found restoration of vitality of almost all teeth after 6 to 12 months. They evaluated 181 teeth of 24 patients who underwent subapical osteotomies and transposition. Procedures of the mandible involved the anterior teeth. Testing for vitality consisted of thermal and electrical pulp testing (thermal testing consisted of cotton
Table 4. COMPARISON OF STUDIES
Parameter Patients, N Method Race Distance to root apex (mm) Distance to CEJ (mm) Distance to buccal cortex (mm) Distance to lingual cortex (mm) Distance to mandibular inferior border (mm)
Present Study
Pommer et al2
Krasny et al12
Al-Ani et al17
Makris et al18
36* cadaveric Caucasian 7.1-9.9z 19.6-23.7 2.3-4.2 4.8-6.9 9.9-14
50 CT Caucasian 5.2-6.6 NM 3.4-4.4 NM 10.3-14
64 MRI NS 6.1-9.5 NM 4-4.8 5.5-6.8 8.7-13.4
60 CBCT Asiany NM NM 2.9-3.4 4.6-5.2 9.1-10.8
100 CBCT NS NM NM 3.6-4.8 5.3-5.7 9.4-11.2
Abbreviations: CBCT, cone-beam computed tomography; CEJ, cementoenamel junction; CT, computed tomography; MRI, magnetic resonance imaging; NM, not measured; NS, not stated. * Thirty-six hemimandibles (19 cadavers). y Malay, Indian, and Chinese. z Excludes value at lateral incisor, which is 12.2 mm. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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FIGURE 18. Note the proximity of the canal to the buccal surface. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
pledgets soaked in ethyl chloride for cold and warmed gutta percha for heat). At 6 months, 20 of 20 mandibular teeth in 10 patients had a 100% response to electrical pulp testing and cold stimulation, whereas 90% responded to heat stimulation. They found similar, although slightly less robust, results in the maxilla. The few patients who remained insensate were indifferent to their symptoms. Noia et al22 evaluated soft tissue neurosensory morbidity in a prospective trial (chin, mental nerve cutaneous distribution) in 30 patients who underwent symphyseal bone graft harvesting. The safety margins for the graft were 5 mm from the apices of the teeth, 5 mm medial to the mental foramen, and 5 mm superior to the inferior border of the mandible. The chin was divided into 6 zones, each of which was tested for 2-point discrimination, brush directional stroke, pinprick, thermal cold discrimination, and thermal hot discrimination. Approximately 50% of patients
had deficits in the first month, with subsequent resolution of symptoms in all patients after 1 year. These findings suggest that accessory neurovascular pathways can provide a redundancy, or ‘‘back-up’’ system, when complete disruption occurs (as opposed to neurapraxia or other reversible mechanisms). Possible supplementary neurovascular sources include the sublingual and mylohyoid branches, lingual foramina, etc. Maintenance of the periodontal ligament and its neural systems also can help decrease perception of deficits. These studies suggest that patients who undergo osteotomies tend to do well, whether the procedure is for harvesting of bone grafts, genioplasty, or even to aid in rapid orthodontic movements.23 In the latter, this technique has evolved into the use of corticotomies in which alveolar bone is decorticated up to 0.5 mm, with selective medullary penetration to enhance bleeding. This is termed the regional acceleratory phenomenon in which the injured sites recruit osteoclasts and osteoblasts to increase remodeling and thus increase orthodontic movements by 3 to 4 times that of conventional means.24 The dimensions of corticotomies, by definition, being relatively superficial, would generally spare key structures such as the mandibular incisive canal, further securing their safety from injury. Thus, safety margins can serve as guides, rather than as rules, to keep surgeons cognizant of the incisive canal. The proximity of the canal to the buccal surface (Fig 18), in particular, stands out, so depth of harvest might be the most important dimension to consider when harvesting grafts. Veneer grafts of 4 mm or wider, for instance, will typically harvest the incisive canal in the graft. If one is fairly certain that one is inferior to the incisive canal, then one can be more generous with depth (>4 mm) without direct canal compromise. Conversely, if graft depth is less than or equal to the recommended 4 mm, the superior osteotomy might be considerably closer to the root apices and the CEJ than the recommended values of 9 mm below the
FIGURE 19. Distance to the cementoenamel junction. L, left; R, right. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
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Table 5. RECOMMENDED SAFETY MARGINS
Parameter Depth of graft harvest Distance anterior to mental foramen Distance below root apex Distance below CEJ Distance above inferior border of mandible
Recommended Margin (mm) 4 5 9 23 5 (or maintain inferior border contour)
Abbreviation: CEJ, cementoenamel junction. Vu et al. Mandibular Incisive Canal and Bone Harvesting. J Oral Maxillofac Surg 2015.
root apices or 23 mm below the CEJ. One must keep in mind that the anatomy of the canal can vary quite widely as shown in the present ranges. Anatomic studies of the human body typically show that large ranges for measurements tend to be the rule and not the exception, emphasizing the uniqueness of individual anatomy. Thus, clinician judgment and appropriate imaging should ultimately guide therapy. Table 5 presents suggested safety margins for the clinician who wishes to make a conscious effort to avoid the incisive canal. Others might accept that the canal is usually unavoidable and plan to obtain larger grafts. Acknowledgments The handpiece and equipment used in the cadaver dissection were supplied by Osteomed.
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