The bifid mandibular canal in three-dimensional radiography: morphologic and quantitative characteristics

The mandibular canal is a prominent anatomical structure and it is of great clinical importance since it contains the inferior alveolar nerve. The clinician is advised to proceed cautiously in the vicinity of the mandibular canal to avoid any damage to its neurovascular content. Based on observations in dry mandibles, in panoramic radiographs, and recently in three-dimensional radiography, various anatomical variations of the mandibular canals have been described. One such variant is the so-called bifid mandibular canal (BMC). Embryologically, multiple canals develop and subsequently fuse to form a single mandibular canal; however, occasionally fusion fails or is incomplete resulting in one or multiple BMCs. Clinically relevant issues with regard to the BMCs include hemorrhagic or neurological disorders following damage to these aberrant canals. This literature review presents morphological and quantitative data about BMCs from studies using three-dimensional radiography, i.e. CT and/or CBCT. The reported frequencies of BMCs per patient ranged from 9.8 - 66.5% and per mandibular side from 7.7 - 46.5%. Gender, age or side predilection is currently inconclusive with regard to the occurrence of BMCs. Various types of BMCs have been described in the literature, such as retromolar, dental, forward, or buccolingual canals. BMCs may originate from the mandibular canal along its entire course, but bifurcation appears to be more frequent in the posterior compared to the anterior canal portions. Mean BMC length was reported to range from 10.2 - 16.9 mm, and mean BMC diameter from 0.9 - 2.3 mm. In conclusion, the presence of a BMC must be taken into consideration for treatment planning and anesthetic, endodontic, or surgical interventions in the mandible.


Introduction
The mandibular canal (MC), also called inferior dental (nerve) canal or inferior alveolar (nerve) canal, is a prominent anatomical structure in the mandible ( 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. 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 4 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-LOMELI 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 socalled retromolar canal, located posterior to the 2 nd or 3 rd molar, is another accessory 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 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.

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 6 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. 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

Angle of BMCs
Some radiographic studies have assessed the inferior and/or superior angles of bifurcation of BMCs from the main canal (Fig. 4) Distance from BMCs to root apices 9 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 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.

Schematic illustration of new BMC classification based on L-Or-Di-Co-T (see also
Tab. VIII).

Fig. 6
Referral of a 47-year-old female with severe pain and sensitivity loss in the right lower lip and chin areas following implant insertion in the position of the lower right second molar (47). Panoramic radiograph by private dentist shows good distance from implant tip to mandibular canal (A); tooth 48 was subsequently removed by the referring dentist hoping that the sensitivity would improve but it didn't.