Imaging Modalities of the Mandibular Canal and the ...

Georges Aad et al - Imaging Modalities of the Mandibular Canal and the Inferior Alveolar Nerve: an Overview

34 Int J Biomed Healthc. 2019; 7(1): 34-38

Imaging Modalities of the Mandibular Canal and the Inferior Alveolar Nerve: an Overview

Georges Aad, Alexandre Khairallah, Georges AounDepartment of Oral Medicine and Maxillofacial Radiology, Faculty of Dental Medicine,

Lebanese University

Corresponding author: Professor Georges Aoun, Head of Department of Oral Medicine and Maxillofacial Radiology, Faculty of Dental Medicine, Lebanese University, Beirut, Lebanon. E-mail: [email protected].

ORCID ID: http://orcid.org/0000-0001-5073-6882.

Background: The mandibular canal, located within the mandible, carries the inferior alveolar nerve and the inferior alveolar vessels. This neurovascular bundle is at risk during mandibular surgical procedures. An adequate preoperatively visualization of the mandibular canal and its content could yield a more predictable treatment features with less postoperative complications. Objective: The aim of this study was to review the present visualization techniques of the mandibular canal and the inferior alveolar neurovascular bundle for better pre-operative planning in dentistry. Methods: Six different visualization methods (periapical, panoramic and three-dimensional radiographs, ultrasonography, endoscopy and magnetic resonance imag-ing), their advantages and disadvantages, are hereby reviewed with a stress on their clinical applicability in the dentist’s everyday practice. Discussion: Panoramic radiography and cone-beam computed tomography technology are considered very useful in the assessment of the mandibular canal. However, in some advanced cases, where the inferior alveolar neurovascular bundle must be identified, ultrasonography, endoscopy and magnetic resonance imaging can be used. Conclusion: All techniques reviewed in this paper except the periapical radiography can be useful in the visualization of either the mandibular canal or the inferior

alveolar nerve.

Keywords: mandibular canal, inferior alveolar nerve, panoramic radiograph, cone-beam computed tomography, ultrasonography, endoscopy, magnetic resonance imaging.

1. BACKGROUNDThe mandibular canal (MC), located within the man-

dible, carries the inferior alveolar nerve (IAN), which is a branch of the mandibular nerve the third division of the trigeminal nerve, and the inferior alveolar vessels (artery and vein) (1, 2). The IAN supplies sensation to the mandib-ular teeth and gingivae and gives off: a) the mental nerve which exits the MC through the mental foramen sup-plying sensory innervations to the chin and lower lip and b) the mylohyoid nerve providing motor innervations to the mylohyoid muscle (2, 3).

According to its location and path, the IAN is at risk during mandibular surgical procedures (4, 5). Any ag-gression to the nervous bundle or ramifications may lead to a temporary/permanent loss of tactile sensation of the lower lip and chin (4). In a study with shocking re-sults performed in 2005, Robert et al. stated that 94.5% of surveyed California Oral and Maxillofacial Surgeons reported instances of injury to the IAN during mandib-ular surgeries in a 12-month period (4). Dimensions and courses of the MC are important parameters which deci-sively contribute to designing a correct treatment plan. Thus, an adequate preoperatively visualization of the

MC/IAN, could yield a more predictable treatment fea-tures with less postoperative complications (6, 7).

In a study investigating the vertical positioning of the IAN in 39 edentulous human cadaveric mandibles, Kieser et al. found 30.7% (12 out of 39) of IAN located in the supe-rior part of the body of the mandible, and 69.2% (27 out of 39) half‐way or closer to the inferior border of the man-dible (8).

On the other hand, Kane et al. who assessed the bucco-lingual position of the MC in 20 patients found that the IAN and accompanying vessels are situated more or less at 4.7mm from the buccal aspect and at 1.8mm from the lingual side at the level of the first mandibular molar (9).

The buccolingual position of the MC and the topog-raphy of the IAN and vessels were investigated using three-dimensional reconstruction by Kim et al. on sixty two mandible sides. The researchers conclude that 70% of the canals followed the lingual aspect at the ramus and the mandibular body, 15% were located at the middle of the ramus behind the second molar and lingually when passing through the second and first molars, and the last 15% followed the middle or the lingual third of the man-dible from the ramus to the body. On the other side and

Review, Received: Jun 15, 2019, Accepted: Jun 30, 2019, doi: 10.5455/ijbh.2019.7.34-38, Int J Biomed Healthc. 2019; 7(1): 34-38

Int J Biomed Healthc. 2019; 7(1): 34-38 35


also according to Kim et al., the inferior alveolar vessels were above the IAN in 80% and in 20% lateral to it (10).

Usually the MC is unique but sometimes it may be bifid (6, 11, 12) and rarely trifid (13). For Nasseh and Aoun the MC, even if considered an uncommon anatomical varia-tion, it can be found in every patient and must be assessed effectively (6).

Bifurcation of the MC was investigated by many au-thors via different radiographic techniques. Panoramic radiographs were used by Nortje et al. (11) and Langlais et al. (12) who found, respectively, a prevalence of 0.91% (33 out of 3612) and 0.95% (57 out of 6000) of bifid MC.

Other authors used other imaging technology such as computerized tomography (CT) and cone beam computed tomography (CBCT) etc. (14, 15)

Usually the MC exits the mandible buccally at the mental foramen located at the apical region of the premo-lars (16-23).

2. OBJECTIVEThe aim of this article was to review the different vi-

sualization methods of the MC and the inferior alveolar neurovascular bundle, their advantages and disadvan-tages, as well as their clinical applicability in the pre- and perioperative nerve injury risk assessment in the den-tist’s everyday practice.

3. METHODSIn daily dental practice, the radiographic evaluation of

the MC is mostly performed on periapical, panoramic and CBCT images with a percentage of visibility of 28%, 32% and 98% respectively (24). On conventional 2D imaging, the MC appears as a radiolucent image, with two well-de-fined radiopaque borders, inferior to the mandibular mo-lars and premolars roots (25). This typical appearance is mainly due to the principle of the radiographic lines for-mation. “A radiopaque radiographic line is visible when-ever the primary X-ray beam is perpendicular to the sur-face of separation of two different densities”. In the case of the MC, the two different densities are due to the trabec-ular bone and the inferior neurovascular bundle.

Periapical radiographDue to their small size and short coverage, periapical

radiographs, although having the best two-dimensional (2D) image resolution, are not advised for MC evaluation (25).

Panoramic radiographUnlike periapical radiographs, the panoramic 2D

X-ray captures the entire mouth and among many other structures the MC is clearly visible. Liu et al. had classi-fied its path into four categories of curves: a) linear, b) arc-elliptic, c) spoon-shaped, and d) turning (26) (Figure 1).

However, the MC visibility decreases when its borders become undetectable due to a poor bone density or a non-perpendicularity between the canal and the principal beam (27). Lesser resolution, elevated distortion and the risk of phantom images are also main disadvantage of this technique (28).

Three-dimensional radiographCBCT has been referred to as the “gold standard” for

maxillofacial imaging. This technology exposes the MC image more accurately. De Oliveira-Santos et al. con-cluded that among 41 % of the MCs not detectable on 2D imaging a large majority was visible on CBCT (29).

On CBCT exams, the MC can be seen and traced manu-ally on thin Panorex. The operator must be careful in mapping the reconstruction and following the MC path (Figure 2).

Kim et al. developed a new automatic technique to iso-late the MC with no intervention from the user. In early experimental results by means of 10 clinical Dicom files, this technique could exactly recognize the MC. This tech-nique has, additionally, the utmost segmentation preci-sion (30).

In a study conducted on fifty-six dry mandibles, Basha et al. evaluated, on crossectional cuts CT scans and real anatomical vertical cuts, the average thickness of the MC border and found them not hard enough to resist the ac-tion of drilling; consequently, the operator must care-fully approach the canal (31).

The location and the anatomical variations of the MC (bifid canal, double and accessory mental foramina, the incidence of an anterior loop, etc.) as noticed on CBCT have been largely assessed in the literature (5-8, 11-15). The 3D representation of the MC path and the inferior al-veolar neurovascular bundle can be generated using spe-cial features in the 3D imaging software (Figures 3 and 4).

UltrasonographyIn 2013, Chanpong et al. develop a practical method for

the ultrasonographic visualization of the IAN by means

Figure 1. Panoramic X-ray materializing a turned curve shaped path of the MC

Figure 2. Accurate Panorex mapping following the path of the MC



of a novel hockey stick-shaped 8- to 15-MHz transducer in volunteers followed by a simulation of IAN scanning and injection in cadavers. The ultrasonographic probe is po-sitioned parallel to the occlusal plane of the mandible op-posed to the pterygomandibular raphe and is rotated lat-erally until identification of the ramus. The probe is then moved slowly towards a higher level until appearance of the IAN which is recognized and followed before en-tering the foramen (32) (Figure 5).

The major advantage of this IAN ultrasound-guided vi-sualization is its applicability in the nerve block thus im-proving the success rates for IAN injection and reducing intraneural injections and vascular accidents (32).

On the other hand, the most important disadvantage of this technique is the lack of visibility of the nerve within the MC as bone and teeth greatly affect the echogenic signal. Therefore, this technique is not accurate when planning oral and maxillofacial interventions (33).

EndoscopyIn oral and maxillofacial surgery, 2 types of endoscopy

of the MC can be done: a) support endoscopy and b) im-mersion endoscopy.

The support endoscopy is performed by an endoscope used in a support-and-irrigation sheath; it is indicated in minimally invasive surgeries. It presents the advantage of providing a stable view of the surgical site without the need to immobilize the patient.

The immersion endoscopy performed under contin-uous irrigation is indicated for the non-accessible sites and/or the ones contaminated by blood, debris, etc.

Finally, when combined together, the two types of en-doscopy permit to the operator to get an excellent and de-tailed surgical site view (34) (Figure 6).

By the inferior alveolar neurovascular bundle endo-scopic examination, the practitioner is able to expose many details such as the path, the microstructure, the vascularization and the alveolar wall imperfections (35).

In dental practice, endoscopy may help in: a) IAN visu-alization in per- and post third mandibular molar extrac-tion, b) detection of accidental lingual nerve exposure, c) evaluating of implant sites for possible immediate im-plant placement (36).

Some of the disadvantages of this technique are the re-quirement of specific and expensive equipment and the prolonged duration of the surgical interventions (35).

Based on a CBCT exam, the intra bony course of the MC is easily shown by a 3D virtual endoscopy. This novel technique can be done by using the camera dedicated to airways endoscopy. This faculty is present in certain 3D software like the OnDemand3D (Cybermed Inc., Seoul Korea). The camera direction is carefully adjusted to enter the MC threw the mental foramen. A series of an-

Figure 4. 3D segmentation of the MC

Figure 3. 3D segmentation of the MC

Figure 5. Ultrasonographic view of the mandibular foramen

Figure 6. Endoscopic view of the MC

Int J Biomed Healthc. 2019; 7(1): 34-38 37


gular adjustments are necessary to follow the concavity of the canal.

The virtual endoscopy can have some advantages in checking the integrity after an inadvertent violation of the canal during endodontic or implant treatment or to see the relation between a lesion or an impacted tooth and the MC (Figure 7).

Magnetic resonance imagingMoreover, there are clinical situations where the IAN

visualization is required and the ionizing radiation is unable to make this possible. For that, the magnetic res-onance imaging, MRI 3Tesla (3T) can be used. In fact yet with no contrast agent, this tool is precise and permits a highly defined visualization of the IAN. The protocol ap-plicable is the non-contrast-enhanced fat-saturated T1 and T2-weighted (T1w/T2w) (37).

The path of the IAN can be followed with high-resolu-tion T1w sequences and post-contrast fat-suppressed T1w images (37) (Figure 8).

The MRI technique offers excellent visualization of the entire path of the IAN. It shows, in detail, the relationship and distances between the IAN and the adjacent anatom-ical entities. The major inconvenience of this technique is the prolonged duration of the procedure that can affect the imaging quality caused by the movement artifacts. Yet, in dental MRI the process time is relatively short

(around 5 minutes) (37).It is important to note that intra-oral ferromagnetic

metallic materials such as dental amalgam, implants, etc. can alter the images by causing distortion of the magnetic field (37).

4. DISCUSSIONPanoramic radiography is commonly used in daily

dental practice because it offers a full teeth/oral struc-tures overview. Concerning the MC, in the majority of cases, it can be detectable without difficulty allowing the practitioner to evaluate the risk of IAN injury during in-vasive interventions. However, this 2D technology lacks 3D information and do not visualize the IAN itself (24).

Although CBCT is of great importance in the daily oral and maxillofacial practice, it can only show the MC and not the nerve itself; yet CBCT is the best technique for pre-surgery radiographic planning such as implants place-ment and surgical extraction of the third molar, etc. Being able to delimit the MC can assist the surgeon and prevent potential surgical problems.

Virtual CBCT endoscopy is a promising technique to evaluate the MC path but needs further adjustments in order to be more accurate (38).

Ultrasonography and endoscopy are two techniques that can directly visualize the IAN (32, 33). They are mostly used in an experimental setting; in the future, they could gain more importance. Ultrasonography cannot be used for the preoperative implant planning and risk as-sessment due to the bone and teeth interference with the echogenic signal (32); however, it can be used as a precise guidance for IAN injection.

High-resolution MRI is the most accurate method for direct visualization of the IAN. Dental MRI can reliably show the content of the MC but more researches in this field are needed. Nevertheless, a specific application of MRI to visualize the IAN during surgery would be useful and could gain importance in the preoperative risk as-sessment (37).

5. CONCLUSIONThe neurovascular bundle located in the MC is at risk

during invasive surgical interventions in the mandib-ular regions. Thus, a thorough assessment before any procedure is essential.

All techniques reviewed in this paper except the peri-apical radiography can be useful in the visualization of ei-ther the mandibular canal or the inferior alveolar nerve.• Author’s contribution: All authors were involved in prepa-

ration this article. Final proofreading was made by the first author.

• Conflict of interest: None declared.• Financial support and sponsorship: Nil.

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