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Multiple mandibular nerve canals: Radiographic observations and clinical relevance. Report of 6 cases Ajit Auluck, MDS1/Keerthilatha M. Pai, MDS2/ Muralidhar Mupparapu, DMD, MDS3 Variation in the normal anatomic pattern of the inferior alveolar nerve canal such as bifid or trifid mandibular nerve canal is one of the reasons for local anesthetic failure in dental practice. The present article reports 5 cases of bifid mandibular nerve canal and 1 case of trifid mandibular canal and discusses their diagnostic criteria, radiographic features, and clinical implications in dental practice. The objective of this review is to help clinicians identify bifid and trifid mandibular canals on panoramic radiographs and subsequently use the information in the modification of dental treatment planning. Once the multiple canals are identified, the local anesthetic injection technique, prosthetic design, and surgical procedures can be modified to prevent pain and discomfort during treatment procedures. (Quintessence Int 2007;38:781–787)
Key words: bifid mandibular canal, inferior alveolar nerve, local anesthesia, mandibular canal, panoramic radiograph, trifid mandibular canal
The mandibular canal, or the inferior alveolar canal, transmits the inferior alveolar nerve, a branch of the third division of the fifth cranial nerve (trigeminal nerve), and the associated vessels. The canal typically extends from the mandibular foramen to the mental foramen. The terminal dental and incisive branches leave the inferior alveolar nerve within the canal to supply the teeth and adjacent structures. A terminal branch leaves the canal at the mental foramen to become the mental nerve.1 Occurrence of bifid mandibular nerve canals is very rare, and its reported incidence is less than 0.9%.1 Chavez-Lomeli and colleagues2 suggested that during embryon-
1
Assistant Professor, Oral Medicine and Radiology, Manipal
ic development there may be 3 inferior dental nerves innervating 3 groups of mandibular teeth. Later they fuse to form a single nerve. Incomplete fusion of these nerves can explain the presence of bifid or trifid nerve canals3 in some patients. It is important for clinicians to recognize the presence of bifid canals on panoramic radiographs and modify the dental treatment accordingly. This article describes 5 cases of bifid mandibular nerve canal and 1 case of trifid mandibular canal and discusses their radiographic diagnosis and clinical anesthetic, orofacial surgical, and prosthetic implications in dental practice.
CASE REPORTS
College of Dental Sciences, Mangalore, India. 2
Professor and Head, Oral Medicine and Radiology, Manipal College of Dental Sciences, Manipal, India.
3
Associate Professor and Director, Division of Oral and Maxillofacial Radiology, University of Medicine and Dentistry of New Jersey, New Jersey Dental School, Newark, New Jersey.
Reprint requests: Dr M. Mupparapu, Diagnostic Sciences, D860, UMDNJ New Jersey Dental School, 110 Bergen Street, PO Box 1709, Newark, NJ 07101-1709. Fax: 973-972-3164. E-mail: mupparmu@umdnj.edu
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Case 1 A 35-year-old woman was referred for a panoramic radiograph (College of Dental Sciences [CODS], Manipal, India) to evaluate periodontal bone loss. The radiograph revealed generalized loss of crestal cortication along with significant bone loss and a bifid mandibular canal on the left side (Fig 1).
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Fig 1 Cropped panoramic radiograph demonstrating the bifid mandibular canal on the left side. The duplicated canal is toward the posterior and inferior border of the mandibular angle.
Fig 2 Cropped panoramic radiograph demonstrating the bifid mandibular canal on the right side.
Case 2
Case 5
A 27-year-old woman presented (CODS) with a prescription for a panoramic radiograph for pre–implant placement evaluation. The radiograph revealed an impacted maxillary right canine and bifid mandibular canal on the right side (Fig 2).
A 45-year-old man reported to the Oral and Maxillofacial Radiology Clinic (New Jersey Dental School [NJDS], Newark, New Jersey) from the Oral Diagnosis Clinic seeking general dental care. The panoramic radiograph showed multiple missing teeth in the maxilla and mandible, caries, residual roots, and
Case 3 A 30-year-old woman had an overretained maxillary left primary canine, and the permanent canine was missing. A panoramic radiograph, obtained (CODS) to investigate the missing permanent canine, showed an impacted maxillary left permanent canine and bifid mandibular canal on the left side (Fig 3).
multiple restorations, and the mandibular nerve was noticed to be bifid bilaterally (Fig 5). The mesial extent of the inferior portion of the bifid canal could not be traced completely. It is assumed that it either extends beyond the tomographic layer mesially or does not have enough canal space to be captured as radiolucent on the film.
Case 4
Case 6
A 16-year-old girl who had recently undergone orthodontic treatment reported for a prescribed panoramic radiograph (CODS). The radiograph showed apical root resorption of the anterior and premolar teeth as well as a trifid mandibular canal on the left side (Fig 4).
A 29-year-old woman was referred to the orofacial pain clinic (NJDS) for evaluation of chronic paroxysmal pain in relation to the mandibular left third molar region. Despite treatment with anti-inflammatory medication, pain was persistent.
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Fig 3 Cropped panoramic radiograph showing the bifid mandibular canal on the left.
Fig 5
Fig 4 Cropped panoramic radiograph showing the trifid mandibular canal on the left side.
Bilateral bifid mandibular canal. (arrowheads) Primary canal; (arrows) duplicated (bifid) canal.
To assess the proximity of the impacted and inverted third molar to the inferior alveolar nerve canal, a computerized tomographic (CT) scan was obtained. During examination of the hard tissue anatomy of the region in the mandibular coronal views in bone windows, the variation in the mandibular canal was noticed on the left side. The canal
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appeared dumbbell shaped superiorly with an extension inferiorly at its origin, and in the more anterior sections it appeared separate (Figs 6a and 6b). The sections in the region of the mental foramen failed to show additional evidence of this duplication and probably were not captured well because of miniaturization.
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Fig 6a Coronal CT view of the mandible in bone window demonstrating the bifid mandibular canal immediately beneath the impacted and inverted mandibular left third molar.
Fig 6b A more posterior coronal CT section of the mandible in bone window, showing the mandibular canal beneath the impacted left third molar. Note the area of confluence just before the canal separates into 2 different canals.
DISCUSSION
aration of bifid mandibular canal is seen, it is a pathognomonic feature indicating the presence of double canals.1 In cases 1 to 5 where
The term bifid is derived from the Latin word meaning cleft into 2 parts or branches. For the mandibular nerve, this occurs at the level of the mandibular foramen.3 Various studies using panoramic radiographs have depicted the presence of bifid mandibular nerve canals. Its incidence is considered very low and is reported to be 0.08%, 0.4%, and 0.9%4,5 in various studies. It is suggested that bifid mandibular canals occur more frequently in females.1,4 Interestingly, 5 of our 6 cases were in women. Chavez-Lomeli and colleagues2 suggested that during embryonic development 3 separate canals fuse to form a single mandibular canal. The bifid and trifid mandibular canals seen in the present cases possibly represent unfused canals that persisted as separate, distinct mandibular canals. Case 4 is the second reported case in the literature of a trifid mandibular canal.3 Bifid mandibular canals may show multiple patterns, and 2 distinct classifications were proposed by Langlais et al4 and Nortje et al5 (Tables 1 and 2). Each of the nerve canals is surrounded by upper and lower cortical boundaries that are easily visible on radiographs. These nerve canals originate from the mandibular foramen. The cortical boundaries surrounding the nerve canals can fuse to form a triangular area of bone. If, on a panoramic radiograph, a triangle having its vertex at the root of sep-
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panoramic radiographs were used, this triangular area of bone was identified on the radiograph. In case 6, bifid canals were identified incidentally on a CT scan. There may occasionally be a false appearance of multiple canals on panoramic radiographs. This can be due to the imprint of the mylohyoid nerve on the internal mandibular surface where it separates from the inferior alveolar nerve and travels to the floor of the mouth.6,7 The false radiographic appearances can also be due to radiologic osteocondensation caused by the insertion of the mylohyoid muscle into the internal mandibular surface, with a distribution parallel to the dental canal.1 On panoramic radiographs, to detect bifid mandibular canals, clinicians must look for the presence of accessory mandibular foramen or the triangular area of bone formed by the inner cortical boundaries of canals. On a cross-sectional tomogram or a CT scan (reformatted coronal slices), a well-corticated outer ring of the inferior alveolar canal can be identified, and the position can be traced in multiple cross-sectional slices until it extends up to the mental foramen region. The traditional cortical notching technique can also be used to identify the canal on CT studies.8 A DentaScan (GE Healthcare) or a SIM-PLANT (Materialise) software programs can be used to obtain the dental CT reformatted images.8
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Ta b l e 1
Langlais et al4 classification of bifid mandibular canals
Type I
Consists of unilateral or bilateral bifid mandibular canals extending to the third molar or immediate surrounding area. Consists of unilateral or bilateral bifid mandibular canals extending along the course of the main canal and region within the ramus or body of the mandible. Combination of first 2 categories. Consists of a bifid canal extending to the third molar or surrounding area or 1 side, as in type 1, and a bifid canal extending along the course of the major canal regions within the ramus or body of mandible in the other side, as in type 2. Consists of 2 canals, each of which originates from a separate mandibular foramen and then joins to form 1 canal.
Type II Type III
Type IV
Ta b l e 2
Nortje et al5 description of the patterns of duplication of mandibular canals
Type I
(Most common) Duplicate canals originating from a single mandibular foramen; the canals are usually the same size. The lower canal is smaller. The upper canal is the smaller of the 2 canals. A short upper canal extending to the second or third molar areas. (Least common) 2 canals of equal size, arising from separate foramina that join in the molar area. A double canal variation in which the supplemental canals arise from the retromolar pad area and join the main canals in the retromolar areas.
Type Ia Type Ib Type II Type III Type IV
Identification of the second or third canal is an important task for the practicing clinician because of its implications on local anesthetic outcomes. In the authors’ experience, the inferior alveolar nerve can appear in various anatomic positions. The nerve may start out as separate canals within the ramus of the mandible only to fuse at the molar region. From there it runs a common course until the region of the mental foramen. This is the most common and easily identifiable pattern with the triangular bony island between the 2 canals. It is likely that in patients with bifid or trifid canals the branches of the nerve run parallel courses within the body of the mandible until the mental foramen and possibly end up giving rise to a secondary mental foramen. Based on this anatomic variation, an algorithm (Table 3) was developed for a quick identification of these canals on the panoramic radiographs, selection of advanced radiographic procedures, and to help in the decision-making process. One should remember that panoramic radiographs and cross-sectional tomograms should not be prescribed for identification
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purposes of these canals alone. If there is already an indication for obtaining the panoramic radiograph, either to identify the position of the tooth in question or to visualize the pathosis that was not completely seen on an intraoral periapical or occlusal film, then the radiograph can be examined for the presence of the mandibular canals. Inadequate anesthesia is the most common problem encountered in patients with bifid mandibular nerve canals. There is no specific pattern of division or duplication of the nerve canals.9 This makes it difficult to predict the efficacy of inferior alveolar nerve block injections. Inadequate anesthesia of the region supplied by the mental nerve may be a problem because 2 separate mental foramina may exist and the anesthetic solution may have affected only 1 of the 2 neural bundles. In cases of bifid canals, a high inferior alveolar nerve block, such as the Gow-Gates technique, can be used wherein the anesthetic solution is deposited around the trunk of the mandibular nerve. This technique simultaneously anesthetizes both the buccal and lingual nerves along with anomalous or minor divisions of the mandibular nerve.1,6,9,10
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Ta b l e 3
Diagnostic algorithm for multiple mandibular canals with treatmentplanning implications
Mandibular canal
Implication for the general dentist or the implant dentist
Identification features
Procedures to confirm
Single canal
Appearance of the radiolucent, single canal with superior and inferior well-corticated borders bilaterally
None necessary unless a root-form implant is being planned
None
None
Double canals that are fused posteriorly
Two distinct radiolucent canals, one above another, separated usually by an island of medullary bone
A cross-sectional tomogram or CT is recommended if an implant is planned in the region
Usually none if the inferior alveolar block is given at a higher level (before the nerve enters the canal)
Double canals that remain separate until the mental foramen region
Two distinct radiolucent canals separated by a uniform amount of medullary bone
Cross-sectional tomogram/CT recommended
Triple canals that are fused posteriorly or remain separate until the mental foramen region
Three distinct radiolucent canals separated by islands of medullary bone; all canals have corticated boundaries
Cross-sectional tomogram/CT recommended
Use caution while performing dental surgery in the region supplied by the mental nerve; if a mental nerve block is ineffective, an inferior alveolar block is recommended Use caution while performing dental surgery in the region supplied by the mental nerve; if a mental nerve block is ineffective, resorting to a full inferior alveolar nerve block is a better way to achieve optimal local anesthesia
Position of the canals may be somewhat superior; use caution while assessing the position of the canals before implant placement implants Additional crosssectional CT may be necessary
Gow-Gates technique requires maximal mouth opening and hence, in patients with restricted mouth opening, the Akinosi technique can be used to administer local anesthesia.9,10 In both these techniques, since the needle is inserted at a higher level, potential damage to the maxillary artery and the pterygoid plexus exists, and care should be taken to avoid the injury.10 These techniques should be used only when there is definitive radiographic evidence of multiple mandibular canals and when conventional inferior
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Implications for local anesthesia
Use caution while assessing the bone for implant placement; additional cross-sectional CT may be necessary; use caution while treatment planning implants both anterior and posterior to the mental foramen; additional cross sectional CT may be necessary
alveolar nerve block fails to provide adequate anesthesia. Bifid or trifid mandibular canals contain neurovascular bundles.3,11 This fact has important surgical implications. During third molar extraction, extensive care must be taken because the tooth may infringe on or be within the canal itself, increasing the risk of nerve damage. Failure to recognize extra canals can also result in postextraction complications such as traumatic neuroma, paresthesia, and increased postoperative bleeding.6
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Epker12 suggested that in surgical procedures such as mandibular osteotomy, it is necessary to protect the vascularity of the mandible. Hence, the presence of additional neurovascular bundles adds to the complexity of surgical procedures. Furthermore, in cases of trauma, all mandibular fractures should be handled with care to ensure that the neurovascular bundles line up exactly, to avoid impingement when fracture is reduced. Alignment of the fractured fragments becomes more difficult when the second neurovascular bundle is located in a different plane.6 Therefore, while planning the surgical procedures, the dental surgeon must look for radiographic evidence of such an anomaly, determine its location, and modify the surgical techniques accordingly. As alveolar bone resorbs in the proximity of the mental foramen, patients with a mandibular prosthesis experience pain because of the pressure on the neurovascular bundle. In cases of bifid mandibular canal with branches extending to the third molar and retromolar pad areas, such a problem can be encountered.6 Therefore, recognition of this possible anomaly will allow the clinician to modify the prosthetic design and impression techniques. Another important aspect of duplication of the mandibular canals concerns the identification of the neurovascular bundles precisely before placement of any root-form implants. The duplicate canals may or may not be visible on head and neck radiographs; it is possible that the bone in cross section can be evaluated more easily using either a complex motion tomography, cone beam CT, or conventional CT utilizing dental reformation software.13 Awareness and detection of bifid canals is of considerable interest to the general dentist. Careful radiographic observation can help the clinician recognize this anomaly, achieve appropriate local anesthesia, and modify dental treatment accordingly. Identification of the multiple mandibular canals also will help the clinician in the planning and placement of root-form implants within the mandibular body.
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ACKNOWLEDGMENT The authors would like to thank Dr Gary Heir, professor, Department of Diagnostic Sciences, University of Medicine and Dentistry of New Jersey, New Jersey Dental School, Newark, for allowing us to use the material in case 6.
REFERENCES 1. Sanchis JM, Penarrocha M, Soler F. Bifid mandibular canal. J Oral Maxillofac Surg 2003;61:422–424. 2. Chavez-Lomeli ME, Mansilla Lory J, Pompa JA, Kjaer I. The human mandibular canal arises from three separate canals innervating different tooth groups. J Dent Res 1996;75:1540–1544. 3. Auluck A, Pai KM. Trifid mandibular nerve canal. Dentomaxillofac Radiol 2005;34:259. 4. Langlais RP, Broadus R, Glass BJ. Bifid mandibular canals in panoramic radiographs. J Am Dent Assoc 1985;110:923–926. 5. Nortje CJ, Farman AG, Grotepass FW. Variation in the normal anatomy of inferior dental (mandibular) canal: A retrospective study of panoramic radiographs from 3612 routine dental patients. Br J Oral Surg 1977;15(1):55–63. 6. Wilson S, Johns P, Fuller PM.The inferior alveolar and mylohyoid nerves: An anatomic study and relationship to local anaesthesia of the anterior mandibular teeth. J Am Dent Assoc 1984; 108:350-352. 7. Sillanpaa M, Vuori V, Lehtinen R. The mylohyoid nerve and mandibular anesthesia. Int J Oral Maxillofac Surg 1988;17:206–207. 8. Mupparapu M, Beideman R. Imaging for maxillofacial reconstruction and implantology. In: Fonseca RJ (ed). Oral and Maxillofacial Surgery. Vol 7: Reconstructive and implant surgery. Philadelphia: Saunders, 2000:17–34. 9. Desantis JL, Liebow C. Four common mandibular nerve anomalies that lead to local anesthesia failures. J Am Dent Assoc 1996;127:1081–1086. 10. Meechan JG. How to overcome failed local anaesthesia. Br Dent J 1999;186:15–20. 11. Claeys V, Wackens G. Bifid mandibular canal: Literature review and case report. Dentomaxillofac Radiol 2005;34:55–58. 12. Epker BN. Vascular consideration in orthognathic surgery. I. Mandibular osteotomies. Oral Surg Oral Med Oral Pathol 1984;57:467–472. 13. Mupparapu M, Singer SR. Implant imaging for the
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