Introduction
The mandibular canal (MC), also called inferior dental (nerve) canal or inferior alveolar (nerve) canal, is a prominent anatomical structure in the mandible (VON ARX & LOZANOFF 2017). The MC is of great clinical importance since it conveys neurovascular components to all mandibular teeth, to the body of the mandible, as well as to the lower lip, chin, and vestibular soft tissues anterior to the mental foramen. Any (inadvertent) damage to the MC may result in sensitivity changes or bleeding complications possibly leading to legal actions. Therefore, the clinician should have a sound knowledge of the anatomy and must proceed cautiously in the vicinity of the MC.
Panoramic radiography remains the standard imaging method for visualization of the MC. However, in many cases radiographic detection of the canal is difficult in its entirety or also partially. Several radiographic studies have documented the poor visibility of the MC, in particular in its middle and anterior sections (ANGELOPOULOS ET AL. 2008, KAMRUN ET AL. 2013, POLITIS ET AL. 2013). In a study of 500 panoramic radiographs, the superior border of the MC was visible only in 37% (PRIA ET AL. 2011). Even when using three-dimensional (3D) radiography, the MC failed to be detected in 18% of cross-sectional cone beam computed tomography (CBCT) images at the level of the first molar (DE OLIVEIRA ET AL. 2012). The same authors also showed that corticalization of the MC (as seen on CBCT images) was significantly associated with the degree of bone trabeculation within the mandibular body.
The MC extends from the mandibular foramen to the mental foramen, thus traverses the mandibular body obliquely from its medial inner aspect at the ramus to the lateral outer aspect in the premolar region. In the sagittal plane, the course of the MC shows a high degree of variability: linear, spoon-shape or elliptic (LIU et al. 2009). Consequently, the MC may be close to or distant to the root apices (CARTER & KEEN 1971, KOVISTO ET AL. 2011, BÜRKLEIN ET AL. 2015).
With regard to the development of the MC, the so-called Meckel’s cartilage forms the skeleton of the first (mandibular) branchial arch. Later, the major portion of that cartilage disappears, but some parts transform into the sphenomandibular ligament. The bilateral bodies of the mandible develop from ossification centers located lateral to the Meckel’s cartilage and its accompanying neurovascular bundle. The presence of the inferior alveolar nerve has been postulated as being necessary to induce osteogenesis. The prior presence of the neurovascular bundle ensures the formation of the actual MC (SPERBER 1981).
A more detailed insight into the development of the human MC has been presented by Chavez-Lomeli et al. (1996). In a unique study of 302 hemimandibles from the second half of the prenatal period, it was shown that the configuration of the developing MC reflected the pattern of innervation to the dentition. Initially, a canal appeared to the primary incisors, followed by a canal to the primary molars, and lastly by one or more canals to the first permanent molars. Each analyzed hemimandible of the most mature group always presented three canals that originated from separate openings on the lingual surface of the ramus and they were directed to the different tooth groups. The authors concluded that the MC develops from at least three separate canals (CHAVEZ-LOMELL ET AL. 1996). Rapid bone remodeling usually results in fusion of the originally distinct canals, but failure of coalescence would result in a bifid MC (BMC) or trifid MC (TMC) (Fig. 1-3). The so-called retromolar canal, located posterior to the 2nd or 3rd molar, is another accessory bone canal in the mandible (VON ARX & LOZANOFF 2017).
Historic classifications of BMCs have been provided by Nortje et al. (1977) and by Langlais et al. (1985) (Tab. I). Both reports described distinctive varieties of BMCs large enough to be detected with panoramic radiographs. A newer classification was reported by Naitoh et al. (2009) based on CBCT data. The most recent classification of BMCs using CBCT was suggested by Luangchana et al. (2019) (Tab. II).
The clinical relevance of the BMC is its presence but difficulty to be detected with two-dimensional radiography. Due to the neurovascular content, the BMC poses a risk for the patient as well as a challenge to the clinician. Several case reports have described intra- or postoperative complications related to damage of the BMC (MAQBOOL ET AL. 2013, ALJUNID ET AL. 2016, VEREA LINARES ET AL. (2016).
The objective of this article is to provide morphological and quantitative data of BMCs based on studies using 3D radiography (i.e. CT and/or CBCT).
Material and methods
The database “PubMed” (https://www.ncbi.nlm.nih.gov/pubmed) was searched for articles pertinent to BMCs. Furthermore, reference lists of papers about BMCs were screened for additional articles.
Some authors claim that the retromolar canal is a variant of the BMC (NAITOH ET AL. 2009, ORHAN ET AL. 2011), while others consider this a separate entity since it has its own opening, i.e. the retromolar foramen. The retromolar canal might also have a different embryological background (HAAS ET AL. 2016). For these reasons, articles limited to the retromolar canal were excluded for this evaluation.
CASTRO ET AL. (2015) provided a literature review about different classifications of BMCs. They concluded that 3D radiographic examinations appear to be the best method for identification of MC branching. Therefore, for the final analysis, only studies that provided morphological and/or quantitative data of the BMC based on 3D radiography (i.e. CT and/or CBCT) were included.
The following data with regard to BMCs were extracted:
- Classification of BMCs
- Frequency of BMCs per patient and/or per side (hemimandible)
- Number of BMCs
- Influence of gender, age or side on BMC frequency
- Location of BMC origin and its distance from the mandibular foramen
- Course of BMCs
- Length, diameter and angle of BMCs
- Distance from BMCs to root apices
- Corticalization of BMCs
Results
Classification of BMCs
The majority of 3D radiographic studies have used the BMC classification established by NAITOH ET AL. (2009) (Tab. II). Only few studies have applied the classifications based on panoramic views by NORTJE ET AL. (1977) or by LANGLAIS ET AL. (1985) even though the image analysis was done with CBCT scans.
Frequency of BMCs
A recent meta-analysis including only study samples > 300 has evaluated the frequency of BMCs including different evaluation techniques: in situ (dry mandibles), panoramic radiography and 3D-radiography (CT, CBCT) (HAAS ET AL. 2016). The calculated mean frequencies were 6.5% (in situ), 4.2% (panoramic radiography), and 16.3% (CT, CBCT). However, the included in situ studies were limited to the retromolar canals.
Several 3D radiography studies have assessed the frequency of BMCs (Tab. II). Data are either presented per patient or per hemimandible. The reported frequencies of BMCs per patient ranged from 9.8% – 66.5% and per mandibular side from 7.7% – 46.5%. A study limited to impacted third molars even found a frequency of 94.6% of BMCs per side in that region (YAMADA ET AL. 2011).
Some differences were noted for BMC detection rates when categorizing the results per geo regions (Tab. III). Studies from Asia commonly reported higher BMC frequencies compared to studies from Europe or the Americas.
CT versus CBCT
Only one study has compared CT and CBCT of the same patients with regard to the radiographic identification of BMCs (NAITOH ET AL. 2010). The depiction rate per side was higher in CBCT (32.1%) than in CT (25.0%). However, no significant difference was reported. In another analysis by SHEN ET AL. (2016), markedly higher BMC detection rates (p < 0.001) were reported for CBCT compared to CT (42.2% vs. 18.7% for hemimandibles and 58.4% vs. 30.6% for patients), but the two study samples were not identical.
Number of BMCs
AFSA & RAHMATI (2017) reported mainly one BMC (31%) or two BMCs (6.9%) per evaluated side; however, in one patient, they found five BMCs on one side.
Influence of gender, age, or side
With regard to gender, FU ET AL. (2014) found significantly more BMCs (per hemimandible) in males (26.7%, p = 0.028) compared to females (14.4%). Also, LUANGCHANA ET AL. (2019) reported significantly more BMCs (per hemimandible) in males (52.5%, p = 0.020) than in females (37.5%). However, the majority of studies reported no significant gender difference for BMC frequency (KANG ET AL. 2014, RASHSUREN ET AL. 2014, LIMA ET AL. 2016, YOON ET AL. 2018, ZHANG ET AL. 2018, OKUMUS & DUMLU 2019, ZHOU ET AL. 2020).
Regarding age, patients ? 20 years of age had a significantly lower BMC frequency (4.2%, p < 0.05) compared to other age groups (15.2 – 22.8%) (ZHANG ET AL. 2018). Similarly, OKUMUS & DUMLU (2019) noted a significantly lower BMC frequency (14.5%, p = 0.006) in subjects younger than 25 years of age compared to older age groups (26.5 – 30.5%). In contrast, RASHSUREN ET AL. (2014) as well as KANG ET AL. (2014) reported no significant differences for BMC frequency among various age groups. YOON ET AL. (2018) stated that ethnicity was not a significant predictor for BMC prevalence.
Studies comparing right and left hemimandibles found no difference for side predilection of BMCs (LIMA ET AL. 2016, LUANGCHANA ET AL. 2019, ZHOU ET AL. 2020).
Location of BMC origin
According to SHEN ET AL. (2014), 27.7% of BMCs originated in the ramus, 40% in the retromolar area, 17.1% in the molar region, 11.8% in the premolar region, and 3.5% anterior to the mental foramen. AFSA & RAHMATI (2017) described 25.4% of BMCs bifurcating in the ramus, also 25.4% in the retromolar area, and 49.2% in the molar region. YANG ET AL. (2017) reported in 38.3% a BMC origin in the ramus, in 27.1% in the retromolar region, and in 34.6% in the molar area.
Distance from mandibular foramen to BMC bifurcation
ZHANG ET AL. (2018) measured a mean distance of 8.1 ±5.6 mm from the mandibular foramen to the BMC origin. No significant differences were noted among the various BMC subtypes.
Course of BMCs
Several authors provided details with regard to dental canals and their extensions to adjacent root apices (Tab. IV). Overall, the majority of dental canals reached the 3rd molars. Regarding the so-called forward canals, some of these BMCs rejoined the main canal anteriorly with confluence rates of 7.4 to 43.5% (Tab. V).
ORHAN ET AL. (2011) identified 40 buccolingual BMCs (23 buccal, 17 lingual). All were located in the ramus region. SHEN ET AL. (2014) detected 170 BMCs of which 95.9% originated from the superior MC wall. In 16.5% of identified BMCs, an accessory foramen was formed in the mandibular cortex by the accessory canal.
SHAH ET AL. (2018) assessed the direction of BMCs in the vicinity of third molars. The majority had a superior (65%) or superolateral (27%) course; only one BMC (1%) showed an inferior direction. The remaining BMCs ran either buccally or lingually to the main canal. ZHANG ET AL. (2018) described a BMC originating from the lower part of the mandibular foramen, coursing antero-inferiorly, and terminating at a foramen on the lingual cortex of the ramus.
Length of BMCs
Several research groups assessed the mean length of BMCs overall and per canal type (Tab. VI). For all canals, the pooled mean lengths ranged from 10.2 to 16.9 mm. Marked differences of canal length were described for the various BMC types. The shortest mean length was 1.6 mm for buccolingual canals, and the longest mean length was 20.1 mm for forward canals.
Diameter of BMCs
The mean diameter of BMCs ranged between 0.9 and 2.3 mm (Tab. VII). Some authors also assessed the diameter of the main canal with mean values of 2.9 to 5.0 mm (KURIBAYASHI ET AL. 2010, KANG ET AL. 2014, RASHSUREN ET AL. 2014, ZHOU ET AL. 2020). While KANG ET AL. (2014) found no significant differences for mean canal diameters among the various BMC types, other authors reported significant differences (AFSA & RAHMATI 2017, ZHANG ET AL. 2018).
Angle of BMCs
Some radiographic studies have assessed the inferior and/or superior angles of bifurcation of BMCs from the main canal (Fig. 4). ORHAN ET AL. (2011) reported mean angles for right and left sides of 139° and 141° for superior angles, and of 38° and 32° for inferior angles. Side and gender had no significant influence on the calculated values. RASHSUREN ET AL. (2014) observed a mean angle of 149.2 ±22.7° for superior angles and 37.7 ±24.1° for inferior angles. Superior angles did not significantly differ among the various canal types. However, retromolar canals had a significantly greater mean inferior angle (44.1 ±26.1°) than dental (24.6 ±11.7°, p < 0.05) or trifid canals (25.2 ±10.1°, p < 0.05). Also, ZHANG ET AL. (2018) reported significant differences for mean inferior angles comparing the various BMCs: 52.4° for retromolar canals, 23.8° for dental canals, and 2.4° for forward canals.
Distance from BMCs to root apices
YOON ET AL. (2018) measured the greatest distance between the superior border of a BMC and the apex of the closest root when applicable. The average distance was 3.45 mm on left sides and 4.85 mm on right sides.
Corticalization of BMCs
One research group published two papers addressing the corticalization of BMCs (SHEN ET AL. 2014 and 2016). In the first study, SHEN ET AL. (2014) calculated the degree of corticalization of 170 BMCs using either multislice CT or CBCT. Degree of corticalization was complete (>85%) in 44.7%, moderate (50-84%) in 23.5%, mild (16-49%) in 10%, and minimal/none (<15%) in 21.8% of the assessed BMCs.
In the second study by SHEN ET AL. (2016) including 194 BMCs, figures of corticalization degree were similar to those mentioned above (40.7%, 23.7%, 12.4%, 23.2%). In addition, the authors reported a mean thickness of BMC corticalization of 0.48 mm in CBCT and 0.65 mm in CT (p < 0.001). Cortex thickness of BMCs and degree of corticalization were significantly correlated (R2 = 0.530, p < 0.001).
Discussion
This literature review collected and presented morphological and quantitative data of BMCs as identified by means of 3D radiography. The fact that studies using CT and/or CBCT have reported higher rates of BMCs compared to those using panoramic radiography is comprehensible. ZHANG ET AL. (2018) assessed concomitant panoramic radiographs of patients with BMCs detected in CBCT. Only 7.1% of BMCs were visible in panoramic views. The authors concluded that the latter is an unreliable imaging method for detection of BMCs. Similarly, SHAH ET AL. (2018) demonstrated that the sensitivity of panoramic radiography to identify BMCs was poor (11%).
However, for cost and radiation issues, CBCT should not be used routinely for planning dental or surgical interventions in the mandible. Nevertheless, the limitations of panoramic views are known, and caution must be exercised when relying on the assessment of variations of the mandibular canal based on two-dimensional radiography alone (HAAS ET AL. 2016, ZHANG ET AL. 2018). Therefore, when in doubt, CBCT is suggested for a detailed, three-dimensional evaluation and identification of potential bifid mandibular canals before surgical procedures to avoid peri-operative complications (DE CASTRO ET AL. 2018, ZHOU ET AL. 2020). Furthermore, in case a CBCT has already been taken, the clinician is advised to carefully search for the presence of a BMC to avoid adverse effects.
The great discrepancy of BMC occurrences among the assessed CBCT studies is surprising (Tab. II). According to ZHANG ET AL. (2018), these variations of results may be explained by the following factors:
- Different study populations: ethnicity, sample size, patient characteristics
- Imaging technique issues: resolution, voxel size, field of view, quality of images (artefacts, motion, noise)
- Radiographic assessment: experience of observer, criteria of BMC definition, interpretation of anatomical structures
Historically, BMCs have been described and categorized using panoramic radiographs (NORTJE ET AL. 1977, LANGLAIS ET AL. 1985) (Tab. I). However, due to the inherent limitations of two-dimensional radiography for correct representation of anatomical structures, such classifications should no longer be used. Newer classifications based on CBCT imaging (NAITOH ET AL. 2009, LUANGCHANA ET AL. 2009) are more accurate but still do not cover all possible BMC variations. In this context, SHEN ET AL. (2014) reported that 38% of detected BMCs did not fit into the classification of NAITOH ET AL. (2009). In addition to the four BMC types categorized by NAITOH ET AL. (2009), a so-called V-type canal was described by YANG ET AL. (2017), and trifid canals were reported by OKUMUS & DUMLU (2019) (Tab. II). Therefore, we suggest a new BMC classification based on five parameters: i.e. location, origin, direction, configuration and termination (Tab. VIII and Fig. 5).
Failure to accurately locate a BMC may result in damage to the canal with subsequent complications including traumatic neuroma, sensory disturbances, intra- or postoperative bleeding and hematoma formation (SHAH ET AL. 2018, ZHANG ET AL. 2018). Case reports have documented severe and persistent pain following implant placement in the posterior mandible despite optimal anatomical host sites in panoramic radiography (MAQBOOL ET AL. 2013, ALJUNID ET AL. 2016). However, subsequent CBCT imaging demonstrated that the implants impinged on BMCs. After the removal of those implants breaching the accessory canals, significant pain reduction was observed (Fig. 6).
A severe bleeding complication related to a BMC was reported by VEREA LINARES ET AL. (2016). A partially erupted 3rd molar closely associated with a BMC was treated with coronectomy. The patient subsequently needed emergency hemorrhage control under general anesthesia due to failure of local measures to arrest bleeding. The authors suggested cross-sectional imaging for adequate surgery planning when a BMC is suspected. Furthermore, they recommended complete removal of the tooth instead of coronectomy to enable direct access to the bleeding source (Fig. 7).
Other clinically relevant complications include over-instrumentation and/or extrusion of filling materials during endodontic treatment. The latter may injure the neurovascular structures of the MC directly or indirectly via a BMC (Fig. 8). This will result in severe complications including throbbing pain, alterations or loss of sensitivity, and/or necrosis of skin or mucosa (Nicolau syndrome) (LINDGREN ET AL. 2002, SHARMA ET AL. 2008, WILBRAND ET AL. 2011).
The presence of BMCs may also be associated with increased difficulty in obtaining mandibular anesthesia with a conventional block of the inferior alveolar nerve (IAN) (LEW & TOWNSEND 2006). This would be most likely the case when an accessory canal originates from a duplicate foramen, often located superior to the mandibular foramen. To overcome this problem, it was suggested to apply the Gow-Gates or Akinosi-Varizani techniques for “high” IAN blocks (LEW & TOWNSEND 2006, OKUMUS & DUMLU 2019).
Commonly, the BMC is considered a duplication or division of the MC. BMCs were also described as tubular bony structures branching from the MC (YAMADA ET AL. 2011). However, radiographically it is not possible to know whether these tubular bony structures contain branches from the inferior alveolar nerve and/or artery. A histologic analysis of the neurovascular content of the BMC has so far only been done with regard to retromolar canals, but not with regard to forward canals (VON ARX ET AL. 2011, FUKAMI ET AL. 2012).
Radiographic interpretation of tubular bony structures is a challenge, even with 3D-imaging techniques. Superimposition of other anatomical structures, i.e. the mylohyoid groove or canal, as well as the internal oblique line that serves for the attachment of the mylohyoid muscle, may result in misdiagnosis, in particular with panoramic radiography (AULUCK & PAI 2005, SERMAN 2012, NEVES ET AL. 2014). Dense trabecular structures may also give the illusion of a BMC in radiographs. KIM ET AL. (2011) documented a dry mandible with radiographic suspicion of a BMC (panoramic radiography, CBCT, micro CT). However, stereoscopic and histologic examinations of cross-sections showed that only the superior canal (round shape) contained neurovascular bundles while the inferior canal (elliptical shape) contained marrow tissue. In the future, high resolution MRI with identification of blood vessels and nerves might be a hopeful and promising step for enhanced diagnosis of BMCs (KRASNY ET AL. 2012a,b).
In conclusion, the presence of a BMC must be taken into consideration in dental medicine, and specifically for treatment planning and anesthetic, endodontic, or surgical interventions in the posterior mandible. Any situation with inexplicable sensitivity disturbances or hemorrhage/hematoma formation in the mandible might be associated with a (radiographically invisible) BMC containing neurovascular structures. The clinician is then advised to obtain a 3D image, preferably a CBCT.
Acknowledgement
The authors thank Bernadette Rawyler, Medical Illustrator, and Ines Badertscher, Media Designer, School of Dental Medicine, University of Bern, Bern, Switzerland, for the illustrations and the preparation of the figures.
Zusammenfassung
Der Mandibularkanal ist eine prominente anatomische Struktur im Unterkiefer. Wegen seines neurovaskulären Inhaltes hat der Mandibularkanal grosse klinische Bedeutung. Eine Schädigung des Mandibularkanals kann zu schwerwiegenden Sensibilitätsstörungen oder Blutungskomplikationen führen. Die embryologische Entwicklung des Mandibularkanals ist streng mit der neurovaskulären Versorgung der pränatalen Zahnanlagen verbunden. Eine einmalige Studie von 302 Unterkieferhälften von Foeten zeigte, dass sich zuerst ein Kanal zu den Milchinzisiven bildet, dann zu den Milchmolaren, und später auch zu den bleibenden Molaren. In der Regel kommt es dann in der Weiterentwicklung des Unterkiefers zur Fusion dieser drei Kanäle zu einem Mandibularkanal. Bei fehlender oder unregelmässiger Verschmelzung kommt es jedoch zu anatomischen Variationen wie Canalis bifidus (oder trifidus) mandibulae.
Ziel dieser Literaturübersicht ist die Präsentation von morphologischen und quantitativen Daten basierend auf 3D-Röntgen bezüglich des Canalis bifidus. Dazu erfolgte eine Literatursuche in «PubMed» (https://www.ncbi.nlm.nih.gov/pubmed) bezüglich relevanter radiologischer Studien. Ausgenommen wurden Arbeiten, die sich ausschliesslich mit dem Retromolarkanal befassten. Auch Studien, die rein auf Panoramaschichtaufnahmen basierten, wurden ausgeschlossen.
Die Angaben in den analysierten Studien bezüglich der Häufigkeiten des Canalis bifidus variieren stark: 9.8 – 66.5% pro Patienten und 7.7 – 46.5% pro Unterkiefer-Seiten. Gründe können unterschiedliche Studienpopulationen sein, Limitationen in der Bildgebung, oder Schwierigkeiten bei der Interpretation anatomischer Strukturen. Die meisten Studien berichteten über keine Unterschiede in den Häufigkeiten bei Männern und Frauen. Einzelne Arbeiten fanden eine statistisch signifikant geringere Häufigkeit des Canalis bifidus bis 20 bzw. 25 Jahre im Vergleich zu den älteren Patientengruppen.
Ein Canalis bifidus kann im gesamten Unterkiefer beobachtet werden, also vom Ramus bis zur Region anterior des Foramen mentale. Morphologische Variationen und Kanalverläufe sind äusserst vielfältig. Ausgangspunkt des Canalis bifidus ist in der Regel der Mandibularkanal, seltener ein akzessorisches Foramen im Ramus oder in der Retromolarregion. Die Kanäle ziehen dann meistens nach oben zur Retromolarregion, schräg nach oben zu den Wurzelspitzen der Molaren, oder horizontal nach vorne unterhalb der Wurzelspitzen der (Prä-) Molaren. Letztere Kanäle verschmelzen in 7.4 – 43.5% wieder mit dem Mandibularkanal.
Quantitative Daten ergeben für den Canalis bifidus durchschnittliche Längen von 10.2 – 16.9 mm und Durchmesser von 0.9 – 2.2 mm. Einzelne Autoren haben auch die durchschnittlichen Bifurkationswinkel beim Abgang des Canalis bifidus vom Mandibularkanal gemessen (139 - 149° für den oberen Winkel und 32 – 38° für den unteren Winkel). Zwei Studien haben den Grad der Kortikalisierung des Canalis bifidus untersucht. Dabei zeigten beide Studien eine maximale Kortikalisierung in 45% bzw. 41% als häufigste Resultate.
Historisch erfolgte die Einteilung des Canalis bifidus auf den Klassifikationen von NORTJE ET AL. (1977) sowie LANGLAIS ET AL. (1985), die aber beide auf Panoramaschichtaufnahmen basierten. Erst über 20 Jahre später erschien dann eine neue und auf der Digitalen Volumentomographie abstützende Einteilung durch NAITOH ET AL. (2009). Wegen der äusserst vielseitigen Variationen des Canalis bifidus erlaubt aber auch diese Klassifikation keine abschliessende Einteilung aller akzessorischer Kanäle. Wir schlagen deshalb eine umfassende Klassifikation des Canalis bifidus vor, welche auf fünf Parameter beruht: Lokalisation, Ursprung, Richtung, Konfiguration und Ende des Canalis bifidus.
In der Literatur finden sich im Zusammenhang mit einem Canalis bifidus Hinweise für Schwierigkeiten bei der Leitungsanästhesie, Komplikationen nach endodontologischer oder implantologischer Therapie, sowie schwere Nachblutungen nach Weisheitszahnentfernung. Als Schlussfolgerung muss die Möglichkeit eines Canalis bifidus für die Planung und Durchführung von Anästhesien, Wurzelkanalbehandlungen, sowie chirurgischen Eingriffen im Unterkiefer unbedingt berücksichtig werden.
Résumé
Le canal mandibulaire est une structure anatomique proéminente de la mâchoire inférieure. Il a une grande importance clinique, car il conduit entre autres le pédicule alvéolaire inférieur. Un dommage au canal mandibulaire peut provoquer de sérieux troubles de la sensibilité ou des hémorrhagies. Le développement embryologique du canal mandibulaire est lié à l’alimentation neuro-vasculaire prénatale des germes dentaires. Une étude exceptionnelle de 302 hémi-mandibules de fœtus a montré qu’un premier canal se forme vers les incisives de lait, un deuxième vers les molaires de lait, et un troisième vers les molaires permanentes. En règle générale, ces trois canaux fusionnent et forment le canal mandibulaire lors du développement ultérieur de la mâchoire inférieure. Les variations anatomiques du Canalis bifidus (ou trifidus) mandibulae surviennent lors de fusion incomplète ou manquante.
Le but de cet aperçu de la littérature est la présentation des données morphologiques et quantitatives du Canalis bifidus visualisé sur des radiographies tridimensionelles.
Pour cela, une recherche de la littérature a été réalisée sur «PubMed» (https://www.ncbi.nlm.nih.gov/pubmed) en quête d’études radiologiques pertinentes. Les études qui n’analysent que le canal rétro-molaire ont été exclues. Les études qui n’analysent que les radiographies panoramiques ont également été exclues. L’incidence du Canalis bifidus varie fortement selon les études analysées: 9.8 – 66.5% des patients et 7.7 – 46.5% des hémi-mandibules. Cela peut être expliqué par les différentes populations analysées, les limitations des radiographies, et les difficultés d’interprétation des structures anatomiques. La plupart des études n’ont pas trouvé de différence d’incidence entre les hommes et les femmes. Quelques études ont montré un abaissement de l’incidence statistiquement significatif du Canalis bifidus chez les personnes jusqu’à l’âge de 20 ou respectivement 25 ans en comparaison aux patients plus âgés.
Un Canalis bifidus peut être visible sur toute la longueur de la mandibule, c’est à dire de la branche mandibulaire au foramen mentonnier. La morphologie et l’orientation des canaux sont extrêmement variables. L’origine du Canalis bifidus est en règle générale le canal mandibulaire, moins fréquemment un foramen supplémentaire dans la branche mandibulaire ou dans la région rétro-molaire. La plupart des canaux montrent un cheminement vers le haut, vers la région rétro-molaire, courent obliquement en direction des pointes des racines des molaires, ou se dirigent horizontalement vers l’avant sous les racines des (pré) molaires. Ces derniers montrent dans 7.4 – 43.5% des cas une confluence avec le canal mandibulaire.
Les données quantitatives concernent la longueur moyenne (10.2 – 16.9 mm) et le diamètre moyen (0.9 – 2.2 mm) du Canalis bifidus. Quelques auteurs mesurent aussi les angles de bifurcation à l’origine du Canalis bifidus du canal mandibulaire (139 - 149° pour l’angle supérieure et 32 – 38° pour l’angle inférieure). Deux études évaluent le degré de corticalisation du Canalis bifidus et montrent le plus fréquemment une corticalisation maximale de 45%, respectivement 41%.
Historiquement, la classification du Canalis bifidus a été faite par NORTJE ET AL. (1977) et LANGLAIS ET AL. (1985), qui se basaient les deux sur des radiographies panoramiques. Vingt ans plus tard, une nouvelle classification a été proposée par NAITOH ET AL. (2009), cette fois basée sur la tomographie volumétrique numérisée. Mais même cette classification ne permet pas une différentiation complète des canaux supplémentaires en raison de variations multiples. Nous proposons donc une classification différentiée du Canalis bifidus selon cinq paramètres: localisation, origine, direction, configuration, et terminaison du Canalis bifidus.
Dans la littérature, on trouve une corrélation entre le Canalis bifidus et des difficultés lors d’anesthésie tronculaire, des complications après des interventions endodontiques ou implantaires, ainsi que des saignements sérieux après l’ostéotomie des dents de sagesse. En conclusion, il faut tenir compte d’un possible Canalis bifidus lors de la planification et de l’exécution des anesthésies, des traitements de racine, ainsi que des interventions chirurgicales dans la mandibule.