Drs. Winter, Baradarian, Sivitz

Dental 3D Cone Beam CT Imaging: Part III
Bifid Canals (and other deviations) of Inferior Alveoloar Nerve

Dental cone beam 3D CT scans enable dentists to view anatomic structures not easily seen on 2D dental x-rays, including bifid mandibular canals, which is a common variation of the mandibular canal. The mandibular canal may split along different positions of the mandibular nerve; one branch may be smaller than the other (1-2). Langlais et al reported a 0.95% prevalence of bifid mandibular canals (3) while Sanchis (4) reported an incidence of bifid nerves as 0.4%, finding 7 out of 2012 mandibles that were studied. Many authors have investigated the incidence of bifid canals using panoramic or CT 3D images or both, and have concluded that while uncommon, they need to be identified when surgical procedures, such as removal of impacted third molars, insertion of dental implants, and osteotomies, are to be performed (5 - 9). Once multiple (bifid) canals are identified, the local anesthetic injection technique, prosthetic design, and surgical procedures can need to be modified to prevent pain and discomfort during treatment procedures (10) in order to insure better outcomes.

The purpose of this study was to identify the incidence of bifid and/or multiple branches emanating from the inferior alveolar canal in 500 consecutive patients using cone beam 3D CT imaging.

Methods and Materials
CT scans of the dental arches from five hundred (500) consecutive patients taken in nine (9) centers located in three (3) states were uploaded to the main processing center of a single dental radiological practice (i-dontics, llc., New York, N.Y.), which is limited to taking and processing 3D CT images for the dental community. Scans were taken on either i-CAT scanners (8 centers) or on a (1) NewTom 3G scanner. All studies were converted to SimPlant™ (Materialise, Glen Burnie, MD). When not specified, the data was converted to SimPlant™ version 10.

In Part I of the cone beam 3D CT study, the following parameters were recorded for each patient: age, gender, reason for the CT scan, which dental arch was to be studied, the format for the delivery of the data, and whether or not a radiographic guide was used. These results were published in Part I of the study. Parameters relative to the prevalence, location, and diameter of the lingual artery were measured and reported in Part II. The value and relevance of 3D imaging was also discusse din this paper.

In this study, Part III, the incidence of bifid nerves of the inferior alveolar canal were recorded by viewing images from a 3D dental cone beam CT scan. The position of the second canal was noted and listed as posterior to the teeth, within the body of the mandible but posterior to the mental foramen, coincident with the mental foramen, or anterior to the mental foramen. Multiple branches (more than two) were identified and recorded. All CT studies were made into 1.0 mm slides and viewed both in the coronal and transaxial planes. To be counted as a bifid canal, each offshoot had to be continuous with the main inferior alveolar canal in each slice. For consistency, all CT studies were examined for bifid or multiple branches that were offshoots of the inferior alveolar canal by one examiner.

Two hundred and ninety-six (296) mandibles were included in this 3D CT dental cone beam study. Of these, 186 patients or nearly sixty-three percent (62.84%) did not demonstrate evidence of a bifid canal. In contrast, 110 patients or more than thirty-seven percent (37.16%) had one or more bifid canals.

Figure 1. Nearly 63% of the mandibles studied did not have evidence of a bifid canal. However, 37,16% of the patients had one or more bifid canals.

Of the 110 patients demonstrating bifid canals, 56 or 50.9% had one bifid canal. Two bifid canals, as noted on CT scans, were demonstrated in 37 or 33.6% of the mandibles and 17 or 15.45% had three or more canals.

Figure 2. Of the mandibles demonstrating a bifid canal, more than half (50.9%) had one canal, while 33.6% had two canals and 15.45% had three or more canals.

Fifty-five (55.45%) of bifid canals were unilateral. Two thirds (67%) of the unilateral bifid canals were on the right side of the mandible; one third (33%) of the unilateral bifid canals were on the left side of the mandible. Nearly 46% (45.55) of the bifid canals were bilateral. These findings were determined by viewing 3D CT images.

Figure 3. Fifty-five percent of the bifid canals were unilateral while nearly 46% were identified bilaterally.

In addition to identifying if a bifid canal was present, if it was in only the right or left side of the mandible or if they were bilateral, the location of each bifid canal was noted in the following manner: did it end at the mental foramen, posterior to mental foramen, or continue anterior to the mental foramen. Nine (9) bifid canals (8.18%) ended at the mental foramen, 94 or 85.45% ended posterior to the mental foramen and 7 or 6.36% continued anterior to the mental foramen.

Figure 4. The majority of the bifid canals (85%) ended posterior to the mental foramen, with 8 percent ending at the mental foramen and 6% extending beyond (anterior) the mental foramen.

The relative incidence of bifid canals has been reported as less than 1% (3,4) while it has been shown that the split of the mandibular nerve may be of unequal sizes (1,2). Regardless of the frequency of identifying bifid canals, various authors have identified the surgical risks and complications that may be experienced when they are encountered, including an inability to obtain profound anesthesia using a local anesthetic (5-9), injury from dental implant insertions, removing impacted wisdom teeth, and more.

In order to achieve standardization and consistency, the authors agreed as to what constitutes a bifid canal as identified on the 3D image: any branch that appeared as a continuous radiolucent canal extending from the inferior alveolar nerve. All 3D CT slices were 1mm in thickness and all bifid canals were viewed and appeared to emanate from the IAN in three planes: axial, coronal, and sagittal. Once the parameters were defined, one researcher examined and identified all of the bifid canals noted in this study, which were then verified by a second author.

Based on these parameters, the incidence of identifying bifid canals in this study was greater than in previously reported studies: 37%. The concept of bi- means “two,” and bifid means forked or cleft. While the purpose of this CT cone beam study was to identify the incidence of bifid canals, more than two canals of the IAN were identified in 17 patients or 15.45% of the cases. In most instances, 3 branches were identified; in one case, 8 branches were identified.

A relative few bifid canals ended at the mental foramen or extended anterior to it: 16 patients in total, or 14.54%. More than 85% of the bifid nerves identified in this study, as determined by 3D CT cone beam images, ended posterior to the mental foramen.

The significance of the findings in this study matters relative to the size and location of the bifid canals, and what clinical procedure is anticipated being performed. When it comes to operative dentistry, it has been postulated that bifid nerves may explain why anesthesia is not as profound as it should be when employing a local anesthetic. When encountered, infiltration of the local anesthetic to anesthetize these extra branches of the IAN may help achieve greater local anesthesia.

When planning dental implant surgery, it is helpful to identify if bifid canals exist in the surgical site. Encountering these extra canals may not only contribute to unwanted local paresthesias of the gingival that these aberrant nerve branches may serve, but may explain unusual bleeding that emanates from the alveolar bone (10-11) during periodontal osseous or dental implant surgeries.

Figures 5 and 6 illustrate an example of multiple canals as they were identified in this study. While the widest branch, which is anterior to tooth #18, is evident on the panoramic slice, smaller canals are highlighted in Figure 6. Note the arrow in Figure 5 that highlights another bifid canal. Careful inspection will note additional canals emanating from the right IAN.

Figure 5. Arrow indicates a small bifid canal that starts and ends distal to tooth #31. A larger canal can be seen anterior to tooth #18.

Figure 6. The canal is highlighted in red, illustrating 3 bifid canals.

Mention must be made of the value of 3D images identifying normal and abnormal structures when compared to 2D images. Figure 7 is a panoramic image (formatted in a 15 mm trough) taken on a patient that was referred to the CT lab after an implant was inserted that resulted in paresthesia in the patient.

Figure 7. Patient presented after an implanted was inserted in the #30 site resulting in paresthesia.

Figure 8 highlights a bifid branch of the IAN that was traumatized by the implant. This aberrant branch was not evident in the panoramic view due to the dense cortical bone. Traditional 2D imaging - both panoramic or periapical film - is limited in revealing key anatomic structures that are obscured by thick buccal and/or lingual bone. In this example, using 3D imaging prior to implant insertion would have identified the bifid (aberrant) branch and altered the surgical site.

8. A bifid nerve rises from the IAN and was traumatized by the implant insertion.

It is suggested that more studies be undertaken to identify bifid canals and their clinical significance.

Utilizing 3D cone beam CT scanning images, this study identified bifid canals in 110 out of 296 patients. The incidence (37%) was greater than reported in other studies. The clinical implications of bifid canals were discussed, as well as an appreciation for the value of utilizing 3D CT cone beam scanners.

Acknowledgements: Support for this study was generously given by Nobel Biocare AB Gothenberg, Sweden (Grant 2006-492) and Imaging Sciences Inc., Hatfield, PA.

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Tags: 3D cone imaging, sinus grafts, periodontics, dental implants, laser gum surgery, dentist, dental practice, gum treatment. LANAP, bleeding gums, periodontal disease

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