The Mandibular Symphysis in 3D

LITERATURE REVIEW

Revised Maxillofacial Anatomy: The Mandibular Symphysis in 3D

Robert J. Miller, MA, DDS1 / Warren C. Edwards, DDS2 / Carlos Boudet, DDS3 / Jonathan H. Cohen, DDS4

1Chairman of the Department of Oral Implantology at the Atlantic Coast Dental Research Clinic in Palm Beach, Florida, and director of The Center for Advanced Aesthetic and Implant Dentistry in Delray Beach, Florida. 2Private practice, Melbourne, Florida, and senior resident Atlantic Coast Dental Research Clinic, Palm Beach, Florida. 3Private practice, West Palm Beach, Florida, and senior resident Atlantic Coast Dental Research Clinic, Palm Beach, Florida. 4Private practice, Coral Springs, Florida

Abstract: Placement of dental implants in the anterior mandible is considered by many clinicians to be a relatively low risk procedure. However, hemorrhagic episodes following implant placement in the mandibular symphysis are regularly reported and can have serious consequences. The use of high resolution focused cone beam scanners has given us the ability to visualize the intricate neurovascular network of the intraforaminal region without distortion and in greater detail. Knowledge of the arterial supply, and navigated implant placement in the mandibular symphysis, can help to avoid these potentially life-threatening emergencies. TITANIUM 2009 1(2): 0-0

Key words: incisive canal, lingual artery, mylohyoid artery, submental artery, superior genial foramen.

Correspondence to: Dr. Robert J. Miller, 16244 S. Military Trl., Ste. 260 Delray Beach, FL 33484 561-499-5665 email: [email protected]

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INTRODUCTION can now be visualized without data loss. Preoperative assessment of bone For the first time, anatomical structures density and volume is a critical compo- that have been anecdotally reported nent of dental implant surgery. For most in the literature may now be routinely of the history of this discipline, periapical examined. or panaoramic X-rays have been used to evaluate implant sites. Limitations The Incisive Canal of these radiographic modalities are For decades, the intraforaminal distortion, magnification, and a missing region of the mandibular arch has been third dimension of bone volume. The called “the zone of safety”. This term introduction of computed axial tomog- has been used because of the relative raphy (CT) revolutionized our ability to absence of sensory deficit to the lip and Figure 1: Cross-sectionals, anterior to the mental foramen, demonstrating the course virtually dissect maxillofacial structures chin after placement of dental implants of the incisive canal. and to determine osseous architecture as compared to the region posterior without distortion.1 However, CT imaging has 3 major drawbacks. First, is the relatively high radiation dose during the scanning procedure.2 Second, is the high degree of background scatter around metallic restorations and implants.3 T h i r d , is the significant burnout of medullary bone that is directly proportional to the radiation dose.4 These parameters often to the mental foramen. It is now clear, Figure 2: Panoramic X-ray view of incisive canal, obscure fine osseous structures and however, that there are additional con- demonstrating contralateral anastamosis. eliminate soft tissue profile. Our lack of siderations based on the presence of appreciation for the complex anatomy critical structures in this area. of the mandibular symphysis occasionally The literature has reported that leads to unintended consequences or there is a loop of the mental foramen even life-threatening emergencies. There that extends 5-7 mm anteriorly13 and it are many case reports in the literature may often appear on panoramic images. describing hemorrhagic episodes fol- In some cadaver studies, however, it has lowing surgical implant placement that been reported that this loop exists in resulted in near-fatal airway obstruc- only a few specimens examined and tion.6-11 Some authors have even sug- that its importance is marginalized.14 Figure 3: Bucco-lingual position of incisive canal approaching the midline. gested that a CT scan should be routinely Digital volumetric tomography of our performed prior to the placement of patients indicates that not only is there implants in the intraforaminal region. an anterior extension of the mental The more recent introduction of foramen in 100% of these scans, but low radiation focused cone beam scan- that it is not a loop. It is a continuation ners has enabled us to view osseous of the inferior alveolar nerve known as architecture in a highly detailed format, the incisive branch (Fig. 1). without burnout, and with greater con- When we examine the body of the 12 2 trast. In addition, the ability to create mandible, we find that / 3 of the inferior tomographic slices down to .08 mm alveolar nerve (IAN) exits at the mental Figure 4: Harvesting of symphysis block gives us a true volumetric representation foramen. This is the neurovascular sup- grafts exposing the incisive canal. of the arch. Fine osseous architecture ply to the lip and chin on that side. One

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third of the IAN continues through the patients still had altered sensation with incisive canal and anastamoses with the symphysis donor sites while there were Figure 5: Lingual contralateral side (Fig. 2). no symptoms with the ramus group.19 artery anastamosing This incisive branch is the neurovas- with the incisive canal at mid-symphysis. cular supply to all of the anterior teeth Lingual Artery and the chin closer to the midline.15 T h e In addition to the incisive neuro- incisive nerve begins on the facial side vascular bundle, there are additional of the body of the mandible and tends vascular structures that anastamose with to move towards the lingual of the this branch. The genial tubercle at mid- mandible at mid-symphysis when seen symphysis is the attachment point for in cross section (Fig. 3). the genioglossus muscle.20 It also houses Clinicians performing autogenous the lingual foramen through which block graft procedures often prefer the lingual artery courses.21 This artery is symphyseal bone for the shape and approximately 1-2 mm in diameter and can volume of the graft needed.16 Following be seen to anastamose with the incisive removal of the bone block, some sur- canal in cross sectional views22 (Fig. 5). geons may harvest the medullary bone Drilling procedures for placement and hemopoetic tissue available (Fig. of dental implants can potentially resect 4). In doing so, they may inadvertently these blood vessels. If this occurs, the resect the incisive nerve and blood sup- artery may prolapse back into the floor ply. After healing, patients often complain of the mouth.23 The sublingual space will of an altered sensation in the affected fill with arterial blood, raising the tongue Figure 6: Consequence of severe bleeding anterior teeth. This iatrogenic result can until the airway is compromised.24 subsequent to implant placement in the mandibular symphysis. be avoided by resisting the temptation Immediate emergency care is indicated to aggressively manipulate the medul- with the possibility of tracheostomy lary bone after harvesting the graft. until blood flow is controlled (Fig. 6). The There are also reports in the litera- placement of implants at mid-symphysis Figure 7: Submental ture of dysesthesia following placement should be carried out with knowledge artery at mid-symphysis, anastamosing with of dental implants within the region 10 of the position of the lingual and sub- the incisive canal. mm anterior to the mental foramen.17 mental arteries as well as ridge trajec- This may be caused by injury to the tory. If there is insufficient bone above incisive nerve in the cuspid position. The these vascular components, placement patient will not exhibit parasthesia of the of implants at mid-symphysis should be lip or chin because the injury is anterior avoided. to the sensory division to those areas. In one study, computed tomograms However, Wallerian degeneration of of 70 patients were examined for visible the nerve may result in pain or a burn- vascular canals in the mandible as well ing sensation following transection or as for their localization, incidence, diam- compression.18 This type of injury can be eter, and content. All patients examined avoided by pre-surgical evaluation of the showed at least 1 lingual perforating position of this nerve on cross section. bone canal in the mandible. In a cadaver One study, that compared post study, the number of intraforaminal operative neuropraxia following block lingual vascular canals ranged from 1 to harvesting of grafts from the ramus ver- 4. At least 1 lingual vascular canal was 25 Figure 8: Antero-posterior course of mylo- sus the symphysis, demonstrated that found in 80% of the mandibles studied. hyoid artery. after 18 months, more than 50% of the Since such vascular canals are encoun-

4 | TITANIUM LITERATURE REVIEW tered regularly, the authors recommend artery tends to course from the lingual a routine CT or CBCT (focused cone cortex at the bicuspid region and finally beam computed axial tomography) anatamosing with the incisive canal at Figure 9: Anastamosis examination prior to implant surgery the cuspid position (Figs. 8-10). of mylohyoid artery to help avoid severe bleeding complica- with the incisive canal. tions during the placement of implants Superior Genial Foramen in this region. All radiographic imaging techniques demonstrate the mental foramen. Submental Artery Occasionally, periapical or panoramic More variable than the lingual artery, films may indicate an anterior exten- branches of the submental artery are sion to the IAN. However, structures also seen to anatamose with the incisive smaller than 3 mm may show up as canal at, or adjacent to, the symphyseal artifact or be indistinguishable from the midline (Fig. 7). The sub-mental artery is surrounding bone. There have been an- a branch of the facial artery and is con- ecdotal reports of accessory foramina sidered to be the main arterial blood anterior to the mental foramen, varying supply to the floor of the mouth and in number, size, and location.27 Utilizing mandibular lingual gingiva. This vessel focused cone beam imaging in the 6” runs medial to the mandible and may field of view to produce the most fa- insert into the mandibular symphysis at vorable voxel size (3-dimensional pixel) Figure 10: Multiple perforating vessels on the inferior border. Transection of the and the lowest axial slice thickness (.08 the lingual surface of the anterior mandible. (A: lingual artery, B: sublingual artery, C: submental artery requires deep dissec- mm), these accessory foramina can submental artery, D: mylohyoid artery, E: tion in the floor of the mouth and liga- be clearly discerned. The authors have secondary mylohyoid artery). tion of the facial artery. The submental found that, in approximately 80% of artery also supplies the submandibular scans, additional anterior foramina can lymph nodes, submandibular salivary be located and their anastamoses with Figure 11: Cross gland, mylohyoid and digastric muscles, the incisive canal traced. Some of these sectional of superior and the skin of the chin. The submental foramina can have a diameter of up to genial foramen at the vein drains the tissues of the chin as well 2 mm, indicating a substantial neurovas- cuspid position. as the submandibular region.26 cular component exiting to supply the chin (Figs. 11, 12). Mylohyoid Artery Reports of substantial bleeding in Branches of the facial artery run the symphysis after raising flaps may be anteromedially below the mandible and attributable to these larger vessels.28 superficial to the mylohyoid muscle. They These bleeding points have previously give off some perforating branches to been described as “nutrient canals”, the overlying platysma and the mylohy- indicating that they had no sensory oid branch to the underlying mylohyoid component. Following block graft har- muscle during its course. The terminal vesting, parasthesia of the midline chin branches continue toward the midline, area has been reported and may be crossing the anterior belly of the digas- the result of transecting these anterior tric muscle either superficially or deep, neurovascular components.29 Injury to and end at the mental region in general. these vessels can be avoided by limit- Some perforating arteries from the ter- ing the apical extension of flaps during Figure 12: Reconstructed 3D image showing minal branches supply the anterior belly implant placement and harvesting block positions and relative sizes of the superior of the digastric muscle. The mylohyoid grafts closer to the midline. genial and mental foramena.

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Bifurcated Mental Foramen damage sublingual blood vessels, result- Cases of double or bifurcated mental ing in a significant bleeding episode.33 foramina are described in the literature, adding to the variability of the anatomy Bone Density in this area (Figs. 13, 14).30 The second- Pre-operative evaluation of bone ary, smaller foramen may be viewed as density is best accomplished with artifact on a periapical or panaoramic computerized tomography. Each pixel film. This neurovascular component may of an image has a CT number known be inadvertently injured or transected as a Hounsfield unit. The higher the during surgery, leading to parathesia of Hounsfield units, the greater the the lip and chin on that side. density of bone. In general, the denser the trabecular pattern, the greater Figure 13: Distal bifurcated branch of the mental nerve. Orientation of Ridge Position chance of initial implant stability and Another advantage of focused cone consequently implant osseointegration. beam imaging is the ability to create The symphysis is usually composed of virtual study models. In addition to mea- D1 or D2 bone (Misch Classification) suring the cross-sectional volume of the which has been correlated by Misch implant site, we can now measure the and Kirkos to measure 850 to greater trajectory of the planned implant rela- than 1,250 Hounsfield units. Misch also tive to the opposing dentition.31 Focused reports failed sites in the mandible have cone beam imaging has recently been higher than usual Hounsfield units.34 T h i s taken to a new level with the report may be a result of denser cortical bone

Figure 14: Axial tomography tracing the of navigated insertion of implants using overheating during osteotomy prepara- path of a bifurcated mental nerve. software originally developed for spinal tion and a lack of vascularity at the site. surgery.32 When a radio-opaque model of the final prosthesis is included in the Navigated Surgery scan, the position and trajectory of the Implant interactive software (i.e. Figure 15: Cross sectional tomograph implant can be planned for optimal Simplant, Materialise and Easy Guide, demonstrating severe emergence and biomechanics, taking Keystone) has been developed that facial resorption. into account the available bone and allows us to trace the neurovascular anatomic structures. In this way, implant components of the intraforaminal region. dentistry becomes a more prosthetically Through the use of a radiographic pros- driven discipline. The symphysis tends thetic scan appliance, we can determine to resorb in a lingual direction, altering the ideal prosthetic positioning of our the trajectory of the symphysis to a final reconstruction. Virtual implants can pronounced lingual angulation (Fig. 9). be visualized within the existing bone In an attempt to create a more ideal volume and, utilizing a passive navigation emergence profile, surgeons often place device, implants can be placed in posi- implants with a more facial inclination. tions that avoid damage to these critical The available bone height may also be neurovascular structures (Fig. 16). overestimated when looking at a panoramic X-ray view due to this lingual CONCLUSIONS angulation. The osteotomy path may The utilization of 12-, 14-, and 16-bit prematurely come in contact with the focused cone beam volumetric scanners Figure 16: Virtual implant placement that bypasses critical structures (Easy Guide, lingual plate, and perforation into the gives us unparalleled imaging of fine struc- Keystone USA). floor of the mouth may occur. This may tures in the oral and maxillofacial region.

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The knowledge of intricate anatomy and the mental region for reduction of bleeding of the mouth during immediate implant complications during implant surgery in the placement in the mandibular canine region. our ability to avoid iatrogenic damage interforaminal region. Int J Oral Maxillofac J Periodontol. 2000;71:1893-5. to critical structures in the mandibular Implants 1999:14;379-383. 25. Mardinger D, Manor Y, Mijiritsky E, Hirshberg symphysis will enhance patient care and 12. Garg AK. Dental implant imaging:TeraRecon’s A. Lingual perimandibular vessels associated dental 3D cone beam computed tomography with life-threatening bleeding: An anatomic safety. It will allow surgeons to preplan system. Dent Implantol Update. 2007;18:41-5. study. Int J Oral Maxillofac Implants 2007 ideal implant placement and to make 13. Moon, HS, Won, Y Y, Kim, K D, Ruprecht, A, 22:127-31. Kim, H J, Kook, HK, Chung, MK. The three 26. Atamaz Pinar Y, Govsa F, Bilge O. The anatomi- critical decisions prior to surgery. This dimensional microstructure of trabecular cal features and surgical use of the submental enhancement to our understanding of bone in the mandible. Surg Radiol Anat. artery. Surg Radiol Anat. 2005;27:201-5. maxillofacial anatomy will also enable us 2004;26:466-473. 27. Yoshida S, Kawai T, Okutsu K, Yosue T, 14. Mraiwa N, Jacobs R, Moerman P, et al. Presence Takamori H, Sunohara M, Sato I. The appear- to move closer to fulfilling the concept and course of the incisive canal in the human ance of foramen in the internal aspect of the of minimally invasive surgery as well as mandibular interforaminal region: two-dimen- mental region of mandible from Japanese sional imaging versus anatomical observations. cadavers and dry skulls under macroscopic achieving more ideal patient outcomes. Surg Radiol Anat. 2003;25:416-23. observation and three-dimensional CT im- 15. Jacobs R, Mraiwa N, vanSteenberghe D, et al. ages. 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