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The Use of 3-Dimensional Reconstructions to Evaluate the Anatomic Relationship of the Mandibular Canal and Impacted Mandibular Third Molars
J Oral Maxillofac Surg 66:1678-1685, 2008
The Use of 3-Dimensional Reconstructions to Evaluate the Anatomic Relationship of the Mandibular Canal and Impacted Mandibular Third Molars Bernard Friedland, BChD, MSc, JD,* Bruce Donoff, DMD, MD,† and Thomas B. Dodson, DMD, MPH‡ Identifying the position of the mandibular nerve is critical for a number of procedures, including sagittal split osteotomy,1 removal of impacted third molars (M3s),2 and implant placement.3 The nerve can be identified on magnetic resonance imaging (MRI),4 but such imaging is not used routinely in dental practice. Thus, as a practical matter, the nerve is identified indirectly by visualizing the mandibular canal. Locating the canal accurately and predicting the risk of mandibular nerve injury associated with mandibular M3 removal pose challenges to the oral and maxillofacial surgeon. Our concerns involve identifying the anatomic relationship between the mandibular nerve and associated M3 and roots.
Using Panoramic Radiographs to Assess the Relationship Between Impacted M3s and the Mandibular Canal Mandibular nerve injury is a serious complication of M3 surgery. The reported overall frequency of inferior alveolar nerve injury after M3 removal ranges from 0.5% to 8%.5-7 The frequency of permanent manReceived from the Harvard School of Dental Medicine, Boston, MA. *Assistant Professor and Head, Division of Oral & Maxillofacial Radiology. †Walter C. Guralnick Distinguished Professor of Oral and Maxillofacial Surgery and Dean. ‡Associate Professor, Visiting Oral and Maxillofacial Surgeon, and Director, Center for Applied Clinical Investigation, Departments of Oral and Maxillofacial Surgery. Address correspondence and reprint requests to Dr Friedland: Harvard School of Dental Medicine, Oral Medicine, Infection, and Immunity, 188 Longwood Ave, Boston, MA 02115; e-mail: [email protected] © 2008 American Association of Oral and Maxillofacial Surgeons
0278-2391/08/6608-0018$34.00/0 doi:10.1016/j.joms.2007.08.032
dibular nerve injury is reported to be less than 1%.5,8-11 Although commonly used, preoperative periapical films are generally inadequate for imaging M3s.12 In oral and maxillofacial surgery practice, panoramic radiographs are the most widely used technique for evaluating the morphology of an impacted M3, as well as the relationship between the tooth and the mandibular nerve canal.13,14 Using panoramic radiographs to assess the relationship between the canal and an M3 and to predict the risk for injury is problematic, however. As is the case with periapical films, panoramic radiographs simply do not show the buccolingual position of the canal. Further, the buccolingual position of the canal is not significantly consistently located more to one side than the other,15 so that the clinician cannot rely on averages or probabilities. For this and other reasons,16 the clinician must look for certain signs on the panoramic radiograph to assess the precise relationship between the canal and tooth. Just how challenging this can be is demonstrated by the myriad of studies and publications on the topic, including many recent ones.9,17-21 Koong et al13 listed multiple references relating to 9 signs that are used on plain films, including panoramic radiographs, to assess the relationship between the canal and M3s (Table 1). There is not universal agreement as to which of these 9 signs is the best predictor that exposure of the nerve or a clinical complication, such as paresthesias, will be encountered during or as a result of M3 removal. Some studies report that root darkening, loss of the cortical boundaries of the canal, and diversion of the canal are more likely to be associated with nerve injury.9,21,22 On the other hand, whereas Bell23 reported an intimate relationship between the M3 and the inferior alveolar nerve in 51% of cases when darkening of the root was observed, he found such a relationship in only 11% of cases when interruption of the cortical borders of the inferior alveolar canal was observed. Sedaghatfar et al21 found an association between narrowing of the tooth root and an increased
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Table 1. RADIOGRAPHIC SIGNS ASSOCIATED WITH AN INTIMATE ANATOMIC RELATIONSHIP BETWEEN THE MANDIBULAR CANAL AND MANDIBULAR M3S
Radiolucent band Loss of cortex of canal
Change in mandibular canal direction Mandibular canal narrowing Root narrowing Root deviation
Bifid apex Superimposed Contact mandibular canal
Increased radiolucency (radiolucent band) of the root(s) of the mandibular molar where the mandibular canal crosses it Interruption of the radio-opaque lines that represent the superior and inferior borders of the mandibular canal where it crosses the root(s) of the molar Significant change in the direction of the mandibular canal where it is superimposed on or is in contact with the root(s) of the mandibular molar Narrowing of the mandibular canal where it is superimposed on or is in contact with the root(s) of the mandibular molar Narrowing of the root(s) of the mandibular molar where the mandibular canal crosses it Abrupt deviation in form (dilaceration) of the root(s) of the mandibular molar where it is superimposed on or is in contact with the mandibular canal Bifid and dark apex of the root(s) of the mandibular molar where the mandibular canal crosses it Superimposition of the root(s) of the mandibular molar and the mandibular canal Root(s) of the mandibular molar in contact with the superior border of the mandibular canal
Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
chance of visualizing the mandibular nerve after M3 extraction. Notably, there appears to be general agreement that the absence of positive radiographic signs on the panoramic film is more useful than is the presence of such signs.21,24 Without positive signs, the risk of injury to the nerve is considered miniscule, whereas the presence of 1 or more positive signs is not a good predictor of injury to the nerve.
Assessing the Relationship Between Impacted M3s and the Mandibular Canal: Beyond Panoramic Imaging PLAIN FILM COMPUTED TOMOGRAPHY
Given the difficulty of using panoramic films to identify the relationship between the canal and M3, it is not surprising that clinicians have sought out newer technologies as they have become available. Although
computed tomography (CT) has been available since the mid-1970s,25 it was not used routinely in dentistry for some time, due to both an initial lack of access to the machines and concerns about the cost– benefit ratio when the x-ray dose was considered. The introduction of film-based cross-sectional tomography in dentistry in the 1990s allowed dentists to visualize the buccolingual dimension of the jaws. White and Pharoah25 have described the differences between filmbased cross-sectional tomography and CT in detail. Cross-sectional tomography made it possible for dentists to identify the mandibular canal, including its buccolingual position, with greater accuracy than is possible with panoramic films. In addition to the lower cost of a cross-sectional tomography x-ray machine, making it affordable for the dental market, x-ray doses are lower than those received with a CT, at least for a limited examination such as the M3 area only.26,27 But although cross-sectional tomography performs adequately in identifying the canal,28-30 it has not been widely adopted, probably due in part to dentists’ unfamiliarity with the technique and the resultant difficulties in reading and interpreting the images, which are somewhat blurry.31 CONE BEAM COMPUTED TOMOGRAPHY
With the possible exception of MRI (which has drawbacks of high cost and inconsistent accessibility, as well as the claustrophobia it engenders), CT is the best imaging modality for showing the location of the canal in both the superoinferior and buccolingual dimensions.1 The advent of cone beam CT (CBCT) in the early 2000s32 reduced the cost of CT machines and altered the cost– benefit analysis when comparing the radiation dose to the prospective information to be gained. An extensive list of articles on CBCT can be found at http://www.conebeam.com/libraryresources. htm. CBCT has multiple advantages compared with traditional CT. CBCT delivers a significantly lower radiation dose,26 permitting its use in applications that were previously difficult to justify due to concerns about radiation exposure. The increased use of CT in dentistry also has spurred the improvement of existing programs designed primarily for dental use, as well as the development of new programs. For example, with the anticipated use of CBCT in orthodontics, programs have been designed to fully exploit the 3-dimensional (3D) potential of this imaging modality. Such programs include Dolphin 3D (Dolphin Imaging, Chatsworth, CA) and Quick Ceph (Quick Ceph Systems, San Diego, CA). New programs, such as NobelGuide (Nobel Biocare, Yorba Linda, CA), have been introduced to compete with older “implant programs,” such as SimPlant (Materialise, Ann Arbor, MI).
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FIGURE 1. Typical appearance of the cross-sectional, axial, and panoramic views in SimPlant. The canal has been traced on the left side and partially traced on the right side. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
The Use of “Implant Programs” to Assess Impacted M3s Although no dental image processing program has been designed specifically or primarily for use in the evaluation of impacted M3s, all such programs can be readily adapted for such use. Because we are most familiar with SimPlant, our discussion focuses on this program. SimPlant was designed as a tool to aid in implant imaging. The program accepts Digital Imaging and Communications in Medicine (DICOM) images. DICOM is the standard format for distributing and viewing any kind of medical image regardless of its origin. SimPlant is able to accept DICOM images from a wide variety of CT machines, both medical CTs and CBCTs. Once imported into SimPlant, the images can be manipulated in various ways to show the jaw in the axial and coronal planes, and also can display a panoramic reconstruction. The images are a true representation of the jaws (ie, they are 1:1), allowing accurate measurements. Figure 1 shows a typical presentation displaying the 3 views. The mandibular canal is readily viewed on the panoramic reconstruction, as well as in the coronal views, commonly called cross-sections when viewed in the jaws. The cross-
sections display the precise buccolingual position of the canal, whereas its height can be determined from either the panoramic or cross-sectional views. Occasionally, the canal may be difficult to visualize on the cross-sectional view; however, the inability to view the canal on a panoramic reconstruction is quite unusual. With the aid of the “nerve tool,” SimPlant allows the examiner to trace the mandibular canal. This maneuver can be carried out either in the cross-sectional or panoramic view; in our experience, it is almost always far easier to do so on the panoramic view. As the nerve is outlined and displayed on the panoramic view, the software displays it on the crosssectional view as well (Fig 1). The ability to trace the canal on the panoramic view and to have it displayed on the cross-sectional view is no trifling matter, because it often is not seen on the cross-sections with nearly the same clarity as on the panoramic view. The greatest strength of SimPlant and other programs like it lies in its ability to display 3D images (Fig 2). Virtual replicas of the bone, teeth, canal, and other structures can be created. Although the program has been designed for implant planning purposes, the underlying basis on which it works allows it to be
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suggested an increased risk for mandibular nerve injury. The patient was referred for CBCT imaging to better assess the anatomic relationship of the M3 and mandibular canal. Figure 3A shows a cropped image of the partial-thickness panoramic reconstruction based on CT data. Figure 3B is the same view, but with the canal traced. Cross-sections through the area are shown in Figure 3C. The 3D reconstructions in Figures 3D-F show different perspectives of the relationship of the canal to the apices, demonstrating that the root is partially enveloping the nerve. FIGURE 2. 3D reconstruction showing the bone, teeth, and template each in a different mask. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
used to display 3D images not only of all structures of and in the jaws, but also of any other bones in the body. Figure 2 shows the program works by separating tissues by density—in the jaws, bone, and teeth. The clinician can specify the densities to include any 3D reconstruction. Depending on the parameters specified, the bone and teeth can be included in a single mask (a mask corresponds to a color in the 3D picture) (Fig 2). Alternatively, the bone can be done in 1 mask and the teeth in another mask. More sophisticated 3D images (eg, in which the teeth, bone, and a stent are in different masks) are possible (Fig 2). Individual teeth can be separated out as well. The program reaches its full potential, however, with the ability to rotate the image, allowing the clinician to view any structure from any perspective and to hide or separate the masks in any combination. This feature adds a dimension that provides information well beyond that provided by the radiographic part of the CT only. It not only aids the clinician, but also makes informed consent far more meaningful, because the patient is able to see the problem and does not need to try to visualize it. No matter how good the CT scan is, few patients are able to form a picture of the anatomy in their mind, even with extensive explanation by the clinician.
Case Presentations All CT scans were performed on an I-CAT machine (Imaging Sciences International, Hatfield, PA; http:// www.imagingsciences.com). Advanced image reconstruction was performed by 3D Diagnostix, Boston, MA (http://www.3ddx.com). CASE 1
A 24-year-old female presented for evaluation of impacted M3s. The panoramic radiographic findings, deviation of the canal, and darkening of the root
CASE 2
A 26-year-old female was referred for CBCT after M3 removal. Figure 4A shows a cropped digital panoramic radiograph of impacted mandibular left M3. No CT scan was obtained before the attempt to remove the tooth. The oral surgeon reported that during surgery, “tugback” occurred when removal of the remaining root was attempted. On CBCT imaging, the full-thickness panoramic reconstruction (Fig 4B) shows the remaining root. Figure 4C is a partial-thickness panoramic reconstruction showing the canal; Figure 4D is the same view, but with the canal traced. Cross-sectional slices 148 and 150 (Fig 4E) appear to show that the canal is located between 2 roots, buccal and lingual to the canal. The 3D reconstruction (Figs 4F-I) removes all doubt and provides a definitive picture of the anatomy. Clearly, such knowledge before the surgical procedure would have been of inestimable value.
Ancillary Issues: Standard of Care, Insurance, and Radiation Dose There is an ongoing debate in the literature about the need for CT scans before M3 removal, even when 1 or more of the signs is noted on the panoramic image. There are strong feelings on the matter.33,34 Oral and maxillofacial surgeons who are of the opinion that CT scans are not necessary usually point to the fact that panoramic images are the film of choice for oral surgeons in their preoperative assessment of M3, and that they also use this film for informed consent. They also feel that as far as M3s are concerned, “a CT scan. . .is an unnecessary study for most, if not all, practicing oral and maxillofacial surgeons,”14 and also point to the Oral and Maxillofacial Surgery Parameters of Care.35 These surgeons further raise the issue of the higher cost of CT scans. Although there are no hard data to substantiate this fact, it is generally accepted that in the United States, CT scans to evaluate M3s are not covered by health insurance. Meyer, for example, stated that “a CT scan is an expensive study, not generally covered by health care plans for third-
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FIGURE 3. Case 1. A, Cropped CT image of a “partial-thickness” panoramic reconstruction showing impacted mandibular right M3 and the canal. B, Same image, with the canal trace C, Cross-sections through the area. D,E,F, 3D reconstruction viewed from different perspective and showing the root to be partially enveloping the mandibular nerve. In the middle view the mask containing the other teeth has been hidden. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
molar evaluation. This would be an unnecessary burden on the already beleaguered health care dollar.”34 It is also our experience that health insurance generally does not cover the cost of these CT scans. The lack of health insurance coverage does not, of course, address the question of whether a scan is an appropriate procedure for a particular patient. In addition, individuals choose to spend their money in various ways, and some may choose to pay for a CT scan. This is for the patient, not the clinician, to decide. The matter boils down to a question of informed consent, and the clinician should discuss the potential risks and benefits of undertaking M3 removal using a CT scan compared with no CT scan. Although rarely mentioned in the discussions surrounding this issue is the additional x-ray burden to the population, which is a valid concern. No one is
advocating the use of CT scans as a screening film, as panoramic films are, to evaluate M3s. Rather, what is suggested is that clinicians use CT scans selectively on those patients whose panoramic images show 1 or more of the classic signs, or at least give patients the option of a CT scan. In addition, CBCT has significantly reduced the exposure compared with traditional CT, considerably reducing the radiation burden compared with that of traditional CT. In addition to the potential benefit to the patient of reducing the risk of alveolar nerve damage, surgeons who have used the 3D images have reported anecdotally that they approach the surgery with a significantly reduced level of stress in difficult cases, and that operating time may be slightly reduced. A lower stress level may have an added benefit to the patient in reducing injury beyond the
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FIGURE 4. Case 2. A, Cropped preoperative panoramic film showing an impacted mandibular left M3 and the canal. The patient was not referred for a CT prior to attempted removal of the tooth. B, Cropped CT image of a “full-thickness” panoramic reconstruction showing the root of the mandibular left M3 and the canal. C, Cropped CT image of a partial-thickness panoramic reconstruction showing an impacted mandibular left M3 and the canal. D, Same image, with the canal traced. (Figure 4 continued on next page.) Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
direct benefit of the surgeon knowing exactly where the vital structures are. These topics merit further investigation. In conclusion, advanced 3D reconstructions to evaluate impacted M3s provide information beyond that provided by radiographic images. Clinicians should consider the use of CT in selected cases, chiefly when 1 or more of the telltale signs are present on the panoramic image. At a minimum, the
option of obtaining a CT scan and advanced 3D reconstruction should be discussed with the patient as part of informed consent in these cases. Acknowledgments Preparation of this manuscript was supported in part by the Center for Applied Clinical Investigation and the Education and Research Fund of the Massachusetts General Hospital Department of Oral and Maxillofacial Surgery.
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FIGURE 4 (cont’d). E, Cross-sectional views showing the canal to appear to be passing between 2 roots, buccal and lingual to the canal. F, 3D reconstruction in the “transparent mode,”showing the root of the mandibular left M3 and its relationship to the nerve. F, G, H, and I depict 3D reconstructions viewed from different perspectives and showing the nerve to be passing through and surrounded by the root. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
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References 1. Ylikontiola L, Moberg K, Huumonen S, et al: Comparison of three radiographic methods used to locate the mandibular canal in the buccolingual direction before bilateral sagittal split osteotomy. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 93:736, 2002 2. Maegawa H, Sano K, Kitagawa Y, et al: Preoperative assessment of the relationship between the mandibular third molar and the mandibular canal by axial computed tomography with coronal and sagittal reconstruction. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 96:639, 2003 3. Yang J, Cavalcanti MG, Ruprecht A, et al: 2-D and 3-D reconstructions of spiral computed tomography in localization of the inferior alveolar canal for dental implants. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 87:369, 1999 4. Nasel C, Gahleitner A, Breitenseher M, et al: Localization of the mandibular neurovascular bundle using dental magnetic resonance imaging. Dentomaxillofac Radiol 27:305, 1998 5. Bruce RA, Frederickson GC, Small GS: Age of patients and morbidity associated with mandibular third molar surgery. J Am Dent Assoc 101:240, 1980 6. Rood JP: Permanent damage to inferior alveolar and lingual nerves during the removal of impacted mandibular third molars: Comparison of two methods of bone removal. Br Dent J 172:108, 1992 7. Sisk AL, Hammer WB, Shelton DW, et al: Complications following removal of impacted third molars: The role of the experience of the surgeon. J Oral Maxillofac Surg 44:855, 1986 8. Alling CC 3rd: Dysesthesia of the lingual and inferior alveolar nerves following third molar surgery. J Oral Maxillofac Surg 44:454, 1986 9. Blaeser BF, August MA, Donoff RB, et al: Panoramic radiographic risk factors for inferior alveolar nerve injury after third molar extraction. J Oral Maxillofac Surg 61:417, 2003 10. Hochwald DA, Davis WH, Martinoff J: Modified distolingual splitting technique for removal of impacted mandibular third molars: Incidence of postoperative sequelae. Oral Surg Oral Med Oral Pathol 56:9, 1983 11. Wofford DT, Miller RI: Prospective study of dysesthesia following odontectomy of impacted mandibular third molars. J Oral Maxillofac Surg 45:15, 1987 12. Klinge B, Petersson A, Maly P: Location of the mandibular canal: Comparison of macroscopic findings, conventional radiography, and computed tomography. Int J Oral Maxillofac Implants 4:327, 1989 13. Koong B, Pharoah MJ, Bulsara M, et al: Methods of determining the relationship of the mandibular canal and third molars: A survey of Australian oral and maxillofacial surgeons. Aust Dent J 51:64, 2006 14. Miloro M, DaBell J: Response to a letter to the editor by AG Farman. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 102:146, 2006 15. Ohman A, Kivijarvi K, Blomback U, Flygare L: Pre-operative radiographic evaluation of lower third molars with computed tomography. Dentomaxillofac Radiol 35:30, 2006 16. Serman NJ: Pitfalls of panoramic radiology in implant surgery. Ann Dent 48:13, 1989 17. de Melo Albert DG, Gomes AC, do Egito Vasconcelos BC, et al: Comparison of orthopantomographs and conventional tomography images for assessing the relationship between impacted
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lower third molars and the mandibular canal. J Oral Maxillofac Surg 64:1030, 2006 Mahasantipiya PM, Savage NW, Monsour PA, et al: Narrowing of the inferior dental canal in relation to the lower third molars. Dentomaxillofac Radiol 34:154, 2005 Miloro M, DaBell J: Radiographic proximity of the mandibular third molar to the inferior alveolar canal. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 100:545, 2005 Monaco G, Montevecchi M, Bonetti GA, et al: Reliability of panoramic radiography in evaluating the topographic relationship between the mandibular canal and impacted third molars. J Am Dent Assoc 135:312, 2004 Sedaghatfar M, August MA, Dodson TB: Panoramic radiographic findings as predictors of inferior alveolar nerve exposure following third molar extraction. J Oral Maxillofac Surg 63:3, 2005 Rood JP, Shehab BA: The radiological prediction of inferior alveolar nerve injury during third molar surgery. Br J Oral Maxillofac Surg 28:20, 1990 Bell GW: Use of dental panoramic tomographs to predict the relation between mandibular third molar teeth and the inferior alveolar nerve: Radiological and surgical findings and clinical outcome. Br J Oral Maxillofac Surg 42:21, 2004 Valmaseda-Castellon E, Berini-Aytes L, Gay-Escoda C: Inferior alveolar nerve damage after lower third molar surgical extraction: A prospective study of 1117 surgical extractions. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 92:377, 2001 White S, Pharoah M: Oral Radiology: Principles and Interpretation (ed 5). St Louis, MO, Mosby-Year Book, 2004 Clark DE, Danforth RA, Barnes RW, et al: Radiation absorbed from dental implant radiography: A comparison of linear tomography, CT scan, and panoramic and intra-oral techniques. J Oral Implantol 16:156, 1990 Kassebaum DK, Stoller NE, McDavid WD, et al: Absorbed dose determination for tomographic implant site assessment techniques. Oral Surg Oral Med Oral Pathol 73:502, 1992 Lindh C, Petersson A: Radiologic examination for location of the mandibular canal: A comparison between panoramic radiography and conventional tomography. Int J Oral Maxillofac Implants 4:249, 1989 Lindh C, Petersson A, Klinge B: Measurements of distances related to the mandibular canal in radiographs. Clin Oral Implants Res 6:96, 1995 Miller CS, Nummikoski PV, Barnett DA, et al: Cross-sectional tomography: A diagnostic technique for determining the buccolingual relationship of impacted mandibular third molars and the inferior alveolar neurovascular bundle. Oral Surg Oral Med Oral Pathol 70:791, 1990 Todd AD, Gher ME, Quintero G, et al: Interpretation of linear and computed tomograms in the assessment of implant recipient sites. J Periodontol 64:1243, 1993 Sukovic P: Cone beam computed tomography in craniofacial imaging. Orthod Craniofac Res 6(Suppl 1):31, 2003 Farman A: Reproducible precision does not necessarily mean validity. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 102:145, 2006 Meyers R. CT scans. J Am Dent Assoc 135:846, 2004 American Association of Oral and Maxillofacial Surgeons. Parameters and pathways: Clinical practice guidelines for oral and maxillofacial surgery (AAOMS ParPath 01), version 3.0. J Oral Maxillofac Surg (Suppl), 2001
The Use of 3-Dimensional Reconstructions to Evaluate the Anatomic Relationship of the Mandibular Canal and Impacted Mandibular Third Molars Bernard Friedland, BChD, MSc, JD,* Bruce Donoff, DMD, MD,† and Thomas B. Dodson, DMD, MPH‡ Identifying the position of the mandibular nerve is critical for a number of procedures, including sagittal split osteotomy,1 removal of impacted third molars (M3s),2 and implant placement.3 The nerve can be identified on magnetic resonance imaging (MRI),4 but such imaging is not used routinely in dental practice. Thus, as a practical matter, the nerve is identified indirectly by visualizing the mandibular canal. Locating the canal accurately and predicting the risk of mandibular nerve injury associated with mandibular M3 removal pose challenges to the oral and maxillofacial surgeon. Our concerns involve identifying the anatomic relationship between the mandibular nerve and associated M3 and roots.
Using Panoramic Radiographs to Assess the Relationship Between Impacted M3s and the Mandibular Canal Mandibular nerve injury is a serious complication of M3 surgery. The reported overall frequency of inferior alveolar nerve injury after M3 removal ranges from 0.5% to 8%.5-7 The frequency of permanent manReceived from the Harvard School of Dental Medicine, Boston, MA. *Assistant Professor and Head, Division of Oral & Maxillofacial Radiology. †Walter C. Guralnick Distinguished Professor of Oral and Maxillofacial Surgery and Dean. ‡Associate Professor, Visiting Oral and Maxillofacial Surgeon, and Director, Center for Applied Clinical Investigation, Departments of Oral and Maxillofacial Surgery. Address correspondence and reprint requests to Dr Friedland: Harvard School of Dental Medicine, Oral Medicine, Infection, and Immunity, 188 Longwood Ave, Boston, MA 02115; e-mail: [email protected] © 2008 American Association of Oral and Maxillofacial Surgeons
0278-2391/08/6608-0018$34.00/0 doi:10.1016/j.joms.2007.08.032
dibular nerve injury is reported to be less than 1%.5,8-11 Although commonly used, preoperative periapical films are generally inadequate for imaging M3s.12 In oral and maxillofacial surgery practice, panoramic radiographs are the most widely used technique for evaluating the morphology of an impacted M3, as well as the relationship between the tooth and the mandibular nerve canal.13,14 Using panoramic radiographs to assess the relationship between the canal and an M3 and to predict the risk for injury is problematic, however. As is the case with periapical films, panoramic radiographs simply do not show the buccolingual position of the canal. Further, the buccolingual position of the canal is not significantly consistently located more to one side than the other,15 so that the clinician cannot rely on averages or probabilities. For this and other reasons,16 the clinician must look for certain signs on the panoramic radiograph to assess the precise relationship between the canal and tooth. Just how challenging this can be is demonstrated by the myriad of studies and publications on the topic, including many recent ones.9,17-21 Koong et al13 listed multiple references relating to 9 signs that are used on plain films, including panoramic radiographs, to assess the relationship between the canal and M3s (Table 1). There is not universal agreement as to which of these 9 signs is the best predictor that exposure of the nerve or a clinical complication, such as paresthesias, will be encountered during or as a result of M3 removal. Some studies report that root darkening, loss of the cortical boundaries of the canal, and diversion of the canal are more likely to be associated with nerve injury.9,21,22 On the other hand, whereas Bell23 reported an intimate relationship between the M3 and the inferior alveolar nerve in 51% of cases when darkening of the root was observed, he found such a relationship in only 11% of cases when interruption of the cortical borders of the inferior alveolar canal was observed. Sedaghatfar et al21 found an association between narrowing of the tooth root and an increased
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Table 1. RADIOGRAPHIC SIGNS ASSOCIATED WITH AN INTIMATE ANATOMIC RELATIONSHIP BETWEEN THE MANDIBULAR CANAL AND MANDIBULAR M3S
Radiolucent band Loss of cortex of canal
Change in mandibular canal direction Mandibular canal narrowing Root narrowing Root deviation
Bifid apex Superimposed Contact mandibular canal
Increased radiolucency (radiolucent band) of the root(s) of the mandibular molar where the mandibular canal crosses it Interruption of the radio-opaque lines that represent the superior and inferior borders of the mandibular canal where it crosses the root(s) of the molar Significant change in the direction of the mandibular canal where it is superimposed on or is in contact with the root(s) of the mandibular molar Narrowing of the mandibular canal where it is superimposed on or is in contact with the root(s) of the mandibular molar Narrowing of the root(s) of the mandibular molar where the mandibular canal crosses it Abrupt deviation in form (dilaceration) of the root(s) of the mandibular molar where it is superimposed on or is in contact with the mandibular canal Bifid and dark apex of the root(s) of the mandibular molar where the mandibular canal crosses it Superimposition of the root(s) of the mandibular molar and the mandibular canal Root(s) of the mandibular molar in contact with the superior border of the mandibular canal
Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
chance of visualizing the mandibular nerve after M3 extraction. Notably, there appears to be general agreement that the absence of positive radiographic signs on the panoramic film is more useful than is the presence of such signs.21,24 Without positive signs, the risk of injury to the nerve is considered miniscule, whereas the presence of 1 or more positive signs is not a good predictor of injury to the nerve.
Assessing the Relationship Between Impacted M3s and the Mandibular Canal: Beyond Panoramic Imaging PLAIN FILM COMPUTED TOMOGRAPHY
Given the difficulty of using panoramic films to identify the relationship between the canal and M3, it is not surprising that clinicians have sought out newer technologies as they have become available. Although
computed tomography (CT) has been available since the mid-1970s,25 it was not used routinely in dentistry for some time, due to both an initial lack of access to the machines and concerns about the cost– benefit ratio when the x-ray dose was considered. The introduction of film-based cross-sectional tomography in dentistry in the 1990s allowed dentists to visualize the buccolingual dimension of the jaws. White and Pharoah25 have described the differences between filmbased cross-sectional tomography and CT in detail. Cross-sectional tomography made it possible for dentists to identify the mandibular canal, including its buccolingual position, with greater accuracy than is possible with panoramic films. In addition to the lower cost of a cross-sectional tomography x-ray machine, making it affordable for the dental market, x-ray doses are lower than those received with a CT, at least for a limited examination such as the M3 area only.26,27 But although cross-sectional tomography performs adequately in identifying the canal,28-30 it has not been widely adopted, probably due in part to dentists’ unfamiliarity with the technique and the resultant difficulties in reading and interpreting the images, which are somewhat blurry.31 CONE BEAM COMPUTED TOMOGRAPHY
With the possible exception of MRI (which has drawbacks of high cost and inconsistent accessibility, as well as the claustrophobia it engenders), CT is the best imaging modality for showing the location of the canal in both the superoinferior and buccolingual dimensions.1 The advent of cone beam CT (CBCT) in the early 2000s32 reduced the cost of CT machines and altered the cost– benefit analysis when comparing the radiation dose to the prospective information to be gained. An extensive list of articles on CBCT can be found at http://www.conebeam.com/libraryresources. htm. CBCT has multiple advantages compared with traditional CT. CBCT delivers a significantly lower radiation dose,26 permitting its use in applications that were previously difficult to justify due to concerns about radiation exposure. The increased use of CT in dentistry also has spurred the improvement of existing programs designed primarily for dental use, as well as the development of new programs. For example, with the anticipated use of CBCT in orthodontics, programs have been designed to fully exploit the 3-dimensional (3D) potential of this imaging modality. Such programs include Dolphin 3D (Dolphin Imaging, Chatsworth, CA) and Quick Ceph (Quick Ceph Systems, San Diego, CA). New programs, such as NobelGuide (Nobel Biocare, Yorba Linda, CA), have been introduced to compete with older “implant programs,” such as SimPlant (Materialise, Ann Arbor, MI).
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FIGURE 1. Typical appearance of the cross-sectional, axial, and panoramic views in SimPlant. The canal has been traced on the left side and partially traced on the right side. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
The Use of “Implant Programs” to Assess Impacted M3s Although no dental image processing program has been designed specifically or primarily for use in the evaluation of impacted M3s, all such programs can be readily adapted for such use. Because we are most familiar with SimPlant, our discussion focuses on this program. SimPlant was designed as a tool to aid in implant imaging. The program accepts Digital Imaging and Communications in Medicine (DICOM) images. DICOM is the standard format for distributing and viewing any kind of medical image regardless of its origin. SimPlant is able to accept DICOM images from a wide variety of CT machines, both medical CTs and CBCTs. Once imported into SimPlant, the images can be manipulated in various ways to show the jaw in the axial and coronal planes, and also can display a panoramic reconstruction. The images are a true representation of the jaws (ie, they are 1:1), allowing accurate measurements. Figure 1 shows a typical presentation displaying the 3 views. The mandibular canal is readily viewed on the panoramic reconstruction, as well as in the coronal views, commonly called cross-sections when viewed in the jaws. The cross-
sections display the precise buccolingual position of the canal, whereas its height can be determined from either the panoramic or cross-sectional views. Occasionally, the canal may be difficult to visualize on the cross-sectional view; however, the inability to view the canal on a panoramic reconstruction is quite unusual. With the aid of the “nerve tool,” SimPlant allows the examiner to trace the mandibular canal. This maneuver can be carried out either in the cross-sectional or panoramic view; in our experience, it is almost always far easier to do so on the panoramic view. As the nerve is outlined and displayed on the panoramic view, the software displays it on the crosssectional view as well (Fig 1). The ability to trace the canal on the panoramic view and to have it displayed on the cross-sectional view is no trifling matter, because it often is not seen on the cross-sections with nearly the same clarity as on the panoramic view. The greatest strength of SimPlant and other programs like it lies in its ability to display 3D images (Fig 2). Virtual replicas of the bone, teeth, canal, and other structures can be created. Although the program has been designed for implant planning purposes, the underlying basis on which it works allows it to be
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suggested an increased risk for mandibular nerve injury. The patient was referred for CBCT imaging to better assess the anatomic relationship of the M3 and mandibular canal. Figure 3A shows a cropped image of the partial-thickness panoramic reconstruction based on CT data. Figure 3B is the same view, but with the canal traced. Cross-sections through the area are shown in Figure 3C. The 3D reconstructions in Figures 3D-F show different perspectives of the relationship of the canal to the apices, demonstrating that the root is partially enveloping the nerve. FIGURE 2. 3D reconstruction showing the bone, teeth, and template each in a different mask. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
used to display 3D images not only of all structures of and in the jaws, but also of any other bones in the body. Figure 2 shows the program works by separating tissues by density—in the jaws, bone, and teeth. The clinician can specify the densities to include any 3D reconstruction. Depending on the parameters specified, the bone and teeth can be included in a single mask (a mask corresponds to a color in the 3D picture) (Fig 2). Alternatively, the bone can be done in 1 mask and the teeth in another mask. More sophisticated 3D images (eg, in which the teeth, bone, and a stent are in different masks) are possible (Fig 2). Individual teeth can be separated out as well. The program reaches its full potential, however, with the ability to rotate the image, allowing the clinician to view any structure from any perspective and to hide or separate the masks in any combination. This feature adds a dimension that provides information well beyond that provided by the radiographic part of the CT only. It not only aids the clinician, but also makes informed consent far more meaningful, because the patient is able to see the problem and does not need to try to visualize it. No matter how good the CT scan is, few patients are able to form a picture of the anatomy in their mind, even with extensive explanation by the clinician.
Case Presentations All CT scans were performed on an I-CAT machine (Imaging Sciences International, Hatfield, PA; http:// www.imagingsciences.com). Advanced image reconstruction was performed by 3D Diagnostix, Boston, MA (http://www.3ddx.com). CASE 1
A 24-year-old female presented for evaluation of impacted M3s. The panoramic radiographic findings, deviation of the canal, and darkening of the root
CASE 2
A 26-year-old female was referred for CBCT after M3 removal. Figure 4A shows a cropped digital panoramic radiograph of impacted mandibular left M3. No CT scan was obtained before the attempt to remove the tooth. The oral surgeon reported that during surgery, “tugback” occurred when removal of the remaining root was attempted. On CBCT imaging, the full-thickness panoramic reconstruction (Fig 4B) shows the remaining root. Figure 4C is a partial-thickness panoramic reconstruction showing the canal; Figure 4D is the same view, but with the canal traced. Cross-sectional slices 148 and 150 (Fig 4E) appear to show that the canal is located between 2 roots, buccal and lingual to the canal. The 3D reconstruction (Figs 4F-I) removes all doubt and provides a definitive picture of the anatomy. Clearly, such knowledge before the surgical procedure would have been of inestimable value.
Ancillary Issues: Standard of Care, Insurance, and Radiation Dose There is an ongoing debate in the literature about the need for CT scans before M3 removal, even when 1 or more of the signs is noted on the panoramic image. There are strong feelings on the matter.33,34 Oral and maxillofacial surgeons who are of the opinion that CT scans are not necessary usually point to the fact that panoramic images are the film of choice for oral surgeons in their preoperative assessment of M3, and that they also use this film for informed consent. They also feel that as far as M3s are concerned, “a CT scan. . .is an unnecessary study for most, if not all, practicing oral and maxillofacial surgeons,”14 and also point to the Oral and Maxillofacial Surgery Parameters of Care.35 These surgeons further raise the issue of the higher cost of CT scans. Although there are no hard data to substantiate this fact, it is generally accepted that in the United States, CT scans to evaluate M3s are not covered by health insurance. Meyer, for example, stated that “a CT scan is an expensive study, not generally covered by health care plans for third-
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FIGURE 3. Case 1. A, Cropped CT image of a “partial-thickness” panoramic reconstruction showing impacted mandibular right M3 and the canal. B, Same image, with the canal trace C, Cross-sections through the area. D,E,F, 3D reconstruction viewed from different perspective and showing the root to be partially enveloping the mandibular nerve. In the middle view the mask containing the other teeth has been hidden. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
molar evaluation. This would be an unnecessary burden on the already beleaguered health care dollar.”34 It is also our experience that health insurance generally does not cover the cost of these CT scans. The lack of health insurance coverage does not, of course, address the question of whether a scan is an appropriate procedure for a particular patient. In addition, individuals choose to spend their money in various ways, and some may choose to pay for a CT scan. This is for the patient, not the clinician, to decide. The matter boils down to a question of informed consent, and the clinician should discuss the potential risks and benefits of undertaking M3 removal using a CT scan compared with no CT scan. Although rarely mentioned in the discussions surrounding this issue is the additional x-ray burden to the population, which is a valid concern. No one is
advocating the use of CT scans as a screening film, as panoramic films are, to evaluate M3s. Rather, what is suggested is that clinicians use CT scans selectively on those patients whose panoramic images show 1 or more of the classic signs, or at least give patients the option of a CT scan. In addition, CBCT has significantly reduced the exposure compared with traditional CT, considerably reducing the radiation burden compared with that of traditional CT. In addition to the potential benefit to the patient of reducing the risk of alveolar nerve damage, surgeons who have used the 3D images have reported anecdotally that they approach the surgery with a significantly reduced level of stress in difficult cases, and that operating time may be slightly reduced. A lower stress level may have an added benefit to the patient in reducing injury beyond the
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FIGURE 4. Case 2. A, Cropped preoperative panoramic film showing an impacted mandibular left M3 and the canal. The patient was not referred for a CT prior to attempted removal of the tooth. B, Cropped CT image of a “full-thickness” panoramic reconstruction showing the root of the mandibular left M3 and the canal. C, Cropped CT image of a partial-thickness panoramic reconstruction showing an impacted mandibular left M3 and the canal. D, Same image, with the canal traced. (Figure 4 continued on next page.) Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
direct benefit of the surgeon knowing exactly where the vital structures are. These topics merit further investigation. In conclusion, advanced 3D reconstructions to evaluate impacted M3s provide information beyond that provided by radiographic images. Clinicians should consider the use of CT in selected cases, chiefly when 1 or more of the telltale signs are present on the panoramic image. At a minimum, the
option of obtaining a CT scan and advanced 3D reconstruction should be discussed with the patient as part of informed consent in these cases. Acknowledgments Preparation of this manuscript was supported in part by the Center for Applied Clinical Investigation and the Education and Research Fund of the Massachusetts General Hospital Department of Oral and Maxillofacial Surgery.
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FIGURE 4 (cont’d). E, Cross-sectional views showing the canal to appear to be passing between 2 roots, buccal and lingual to the canal. F, 3D reconstruction in the “transparent mode,”showing the root of the mandibular left M3 and its relationship to the nerve. F, G, H, and I depict 3D reconstructions viewed from different perspectives and showing the nerve to be passing through and surrounded by the root. Friedland et al. CT Imaging and Third Molars. J Oral Maxillofac Surg 2008.
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