Anatomical Variations of the Bifid Mandibular Canal on Panoramic Radiographs in Citizens from Zagreb, Croatia.

BACKGROUND: The bifid mandibular canal (BMC) is an anatomical variation with reported prevalence ranging from 0.08 to 65%. Identifying anatomical variations of mandibular canal is very important in order to prevent possible complications during oral surgical and other dental procedures.
OBJECTIVES: The aim of this study was to determine the prevalence and to classify the morphology of BMCs using digital panoramic radiographs.
MATERIAL AND METHODS: A retrospective study was conducted that included 1008 digital panoramic radiographs (412 female and 596 male) used to identify the type of BMC. Panoramic radiographs were analyzed by three oral surgeons and one dentist, and BMCs were classified into six different types, 4 types according to Langlais et al. (types 1-4), and two new types (types 5 and 6) described by authors.
RESULTS: The prevalence of BMC was 4.66% (n=47), with no significant differences in gender between BMC types (P=0.947; χ2=0.74). The prevalence of type 1 BMC was 0.79% (n=8), type 2 2.08% (n=21), type 3 0.30% (n=3), type 4 0% (n=0), type 5 0.89% (n=9) and type 6 0.60% (n=6).
CONCLUSION: This study revealed a relatively high prevalence of BMCs among Zagreb citizens. Furthermore, two new types of BMCs were described. These results stress the importance of a careful and thorough radiographic analysis prior to each invasive procedure in the mandible.

Introduction

In 1973, Patterson and Funke (), and Kiersch and Jordan () first used the term bifid mandibular canal (BMC). Until then, the mandibular canal had been described as a single structure, with the possibility of multiple smaller branches along the main trunk. Furthermore, it has been mentioned that some of these branches could be large enough to form a BMC or trifid mandibular canals (TMCs) (). The bifid mandibular canal is an anatomical variation located in the body or ramus of the mandible, where the mandibular canal is divided into two branches. The mandibular nerve is divided into the anterior branch, containing motor branches, and one sensitive buccal nerve, and the posterior branch containing sensitive branches. The mandibular canal enters the mandible and extends bilaterally from mandibular foramen to the mental foramen and it ends as an incisive nerve. In radiographs, the mandibular canal appears as a dark, linear shadow between two radiopaque thin borders. It contains a bundle with several important structures: the inferior alveolar nerve (IAN), artery and vein (). In the development of the mandibular canal, Meckel's cartilage forms the skeleton of the first branchial arch (mandibular). The bilateral bodies of mandible evolve from ossification centers located lateral to Meckel’s cartilage. The inferior alveolar nerve has been assumed significant in inducing osteogenesis. A study by Chavez-Lomeli et al. () on 302 hemimandibles from the second half of the prenatal period aimed to investigate the development of the mandibular canal. First, the canal appears in the area of the primary incisors, afterwards in the area of primary molars and finally in the permanent molars region. The authors concluded that the mandibular canal develops from at least three independent canals. Further development commonly results in fusion of the originally distinct canals, but fusion failure can result in a BMC or TMC ().

The prevalence of the BMC has been reported in many studies, ranging from 0.08% to 8.3% when it was assessed using panoramic radiographs, and ranging from 10-65% when they were evaluated using cone-beam computed tomography (CBCT) (-). A panoramic radiograph is one of the most used diagnostic tools in dentistry. To a clinician, a panoramic image provides information on the location of mental foramen, IAN and other important anatomical structures in the maxilla and mandible ().

Various types of BMCs have been described and classified according to anatomical location and based on panoramic radiographs or CBCTs. There are several classifications of BMC in the literature in the past 50 years: BMC classification by Nortje et al. (1977), Langlais et al. (1985), Kieser et al. (2005), Naitoh et al. (2009), Kuribayashi et al (2010), Rashid et al (2011) and Andrade et al. (2015) (, , , , -). All these classifications divide BMCs into four different types, except the classification by Rashid et al. () which divides them into three types. The most commonly used BMC classification in the literature is the one by Langlais et al. () from 1985 (, , ). In the Langlais et al. classification, type 1 (Figure 1) refers to a unilateral or bilateral bifurcation extending to the region of the third molar or surrounding area. Type 2 (Figure 2) refers to a unilateral or bilateral bifurcation extending along the main canal and then merging in the mandibular ramus or body. Type 3 (Figure 3) refers to a combination of type 1 and 2, and type 4 (Figure 4) refers to BMC originating from two distinct mandibular apertures which afterwards merge into a single, broad mandibular canal ().

Figure 1

Type 1 bifid mandibular canal according to Langlais et al.

Figure 2

Type 2 bifid mandibular canal according to Langlais et al.

Figure 3

Type 3 bifid mandibular canal according to Langlais et al.

Figure 4

Type 4 bifid mandibular canal according to Langlais et al.

It is needless to stress the importance of preoperative determining of the course and position of the mandibular canal and identifying possible anatomical variations. After 1973, when the first case was reported, clinicians and researchers began to pay more attention to the potential existence of BMC (). Third molar extraction, dental implant placement, enucleations of pathologies, peri-apical surgery and sagittal ramus split osteotomy are just some of the procedures in which determining the position of the mandibular canal is of the utmost importance (). Furthermore, BMCs are often associated with the failure to obtain an adequate anesthesia on IAN ().

The aim of this study was to determine the prevalence of BMC variations following the Langlais’s et al. classification (), by analyzing digital panoramic radiographs taken at the University Hospital Dubrava, Zagreb, Croatia.

Materials and methods

A retrospective study was carried out that included 1008 randomly selected digital panoramic radiographs taken from the University Hospital Dubrava database, in Zagreb. All radiographs were taken between January 2019 and August 2020.

Patients of both genders, older than 18 years of age were included in this study. Apart from a person's age, the exclusion criteria included a low-quality panoramic radiograph. Digital panoramic radiographs were obtained using the Ortopantomograph Soredex (Nahkelantie 160, Tuusula, Finland), adjusted at 66 kVp, 8.0mA, and acquisition time of 16.4 s. All of the radiographs were projected and analyzed on a monitor screen by four experienced clinicians. The identification of BMC was confirmed if two radiolucent lines were present in the area of IAN, if at least 3 radiopaque borders were clearly visible on the panoramic radiograph and if the research team (one dentist and three oral surgeons) agreed with the finding. According to the criteria set by Langlais et al., BMCs were classified into four different types (Figure 1-4) ().

Categorical data are shown by frequency and relative frequency and compared using the χ2 test. Data were collected and stored in the database in MS Excel. Statistical data processing was done using the MedCalc ver. 16. 2. 1. (MedCalc Software, Ostend, Belgium). The level of statistical significance was set at 5% (P <0.05).

Results

The total prevalence of BMC was 4.66% (n=47). As expected, there were more single mandibular canals registered than BMCs (χ2 =22.183, P<0.0001). No significant difference was found between male and female patients (χ2 =0.736, P=0.947) although more BMCs were detected among the male population (76.60% of the total BMCs identified) (Table 1). No statistically significant differences were found between the types of BMCs (χ2 =6.979, P=0.137) (Table 2). Type 4 BMC according to Langlais et al. was not registered in this study. In addition, we identified two new types that are not described in the Langlais et al. classification. We named them type 5 and type 6 (Figure 5-6) BMC. The description of BMC types is presented in Table 3.

Table 1

Prevalence of bifid mandibular canal by gender

	Total sample	Male	Female
Total number of x-rays	1008	596	412
Bifid mandibular canal (N, %)	47 (4.66)	36 (6.04)	11 (2.67)
Statistics	χ2 =0.736; P=0.947

Table 2

Prevalence of different types of bifid mandibular canals

	Total	Female	Male	Unilateral	Bilateral
Type 1	8 (17.02%)	2	6	8	0
Type 2	21 (44.68%)	4	17	11	10
Type 3	3 (6.38%)	1	2	0	3
Type 4	0 (0.00%)	0	0	0	0
Type 5	9 (19.15%)	2	7	7	2
Type 6	6 (12.77%)	2	4	4	2
	47	11 (23.4%)	36 (76.6%)	30(63.8%)	17 (36.2%)

Figure 5

Bifid mandibular canal type 5

Figure 6

Bifid mandibular canal type 6

Table 3

Types of bifid mandibular canals (BMCs)

Type of BMC	Description
Type 1	Unilaterally/bilaterally start as a mandibular foramen and immediately divides into two separate canals. One canal ends in the third molar region and the other as a mental foramen
Type 2	Unilaterally/bilaterally starts as a mandibular foramen, immediately divides in two separate canals. The canals rejoin in the ramus or corpus of the mandible and end as a single canal as a mental foramen.
Type 3	Combination of Types 1 on one side and 2 on the other
Type 4	Unilaterally/bilaterally starts as two separate mandibular foramina and then merge into a single, broad mandibular canal
Type 5	Unilaterally/bilaterally starts as a mandibular foramen and ends with two separate mental foramina
Type 6	Unilaterally/bilaterally starts as a mandibular foramen; one branch separates from the main canal and ends in the angle of the mandible, while the second branch ends with a mental foramen

The prevalence of BMC types is presented in Table 2. Type 2 BMC was the most common BMC type, followed by type 5 and type 1. The unilateral BMC (63.8%, N=30) was more common than the bilateral BMC (36.2%, N=17).

Discussion

Knowledge of the head and jaws anatomy is an imperative in dentistry (). The bifid mandibular canal is one of the most common anatomical variations of the mandibular canal although it is usually ignored and misdiagnosed in dental practice (). Although panoramic radiographs are commonly used as a preoperative examination tool for dental and oral surgical procedures, they have limitations in detecting some uncommon anatomical and morphological BMC variations. This study revealed a prevalence of 4.66%, which is comparable to several studies on the topic (, , ), however the reported prevalence in the available literature varies from 0.08 to 8.3%, depending on the used radiological diagnostic tool (-, -). The prevalence of 4.66% in this study is much higher than the prevalence of 0.35% reported by Sanchis et al. (), 0.9% reported by Nortje et al. () and the prevalence of 0.95% reported by Langlais et al (). These differences in values may be explained by the quality of the examination technique used to detect BMC and the observers’ experience. The studies that reported low prevalence of BMCs (less than 1%), used conventional panoramic radiographs. Digital radiographs, which were used as a diagnostic tool in this study, definitely enabled better precision and clarity of the image (, , ). The studies that reported a prevalence similar to the one obtained in this study, also used digital radiographs, e.g. study by Haas et al. (), Schilling et al. () and Andrade et al. () who reported the prevalence of 4.2%, 5.0% and 5.3%, respectively. However, the studies with CBCT imaging and BMC identification showed a much higher prevalence (from 10 to 65%) (-). The utilization of digital panoramic radiographs in order to detect BMCs has some objective disadvantages compared to CBCT scans. However, by using the most commonly used diagnostic tool in dentistry, we aimed to assess the possibilities of diagnosing BMCs on digital panoramic radiographs.

No statistically significant difference was found between males and females and BMC prevalence in this and several other studies with a sample size ranging from several hundred up to several thousand examined panoramic radiographs (-).

Types 1 and 2, according to Langlais et al. classification (), were the most common BMC type in several studies. Fuentes et al. () in Chilean, Kuczynski et al. () and Correr et al. () in Brazilian population reported that the most prevalent variation in their studies was type 1 (67%, 83% and 72.6%, respectively) (, , ). Type 2 was the most prevalent variation of BMC in this study (44.68%); furthermore, type 2 was also most frequent in a study by Langlais et al. () (USA) (54%) and in the study by Kalantar Motamendi et al. () (Iran) (82%). With respect to the above-mentioned studies, it should be noted that, apart from the differences in sample size, ethnical diversity may affect the results. Overall, the results obtained in this study resembled those of Langlais et al. () who studied the North American population (44.68 and 54%).

Our study revealed two types of BMCs not described in the BMC classifications available in the literature. We named them type 5 and type 6. Type 5 was referred to as unilateral or bilateral BMC that started from the mandibular foramen and ended with two separate mental foramina. Mahnaz et al. () published the case of a 47-year-old male patient who had experienced a persistent dull pain after the root canal treatment. He observed two separate mandibular foramina and canals, which merged into a single canal anteriorly and ended as a single mental foramen on the right side (type 4 according to Langlais et al. () classification). On the left side, he observed two separate mandibular foramina and canals which ended as two separate mental foramina, which corresponds to the type 5 described in this study. Along with the type 1 and 2, type 5 was also not uncommon (N=9; 0.89%).

Type 6 referred to as unilateral or bilateral canal that started from the common mandibular foramen, after which one branch separates from the main canal and ends in the angle of the mandible, while the second branch ends a mental foramen. To the best of our knowledge, type 6 was not previously described in the literature. We have not registered type 4 BMC in our study. Kuczynski et al. also did not register type 4 BMC in their study on 3024 panoramic radiographs (). An extremely rare anatomical variation of a trifid mandibular canal was not observed in this study. Data on trifid mandibular canals are limited to several case reports (, ).

Accessory canals have been implicated in failure to achieve complete anesthesia since 1960, when variations of mandibular canal were identified (). Based on the available literature, a success rate of the IAN block anesthesia has ranged from 80-92%. Kaufman et al. have registered a success rate of the IAN block ranging from 80 to 85%, while Keetley and Moles recorded a higher success rate of 91.9%, which was lower than other nerve blocks in the maxilla (, ). There are several possibilities for the ineffectiveness of IAN blocks, such as the presence of an acute infection, poor technique and the presence of a morphological and anatomical variation (-). Lew and Townsen () discussed the failure of obtaining an adequate anesthesia in their case report due to a BMC with two separate mandibular foramina, i.e. the type 4 BMC according to Langlais et al. classification (). Apart from the type 4, all other BMC types could present an obstacle for an effective IAN block (, ). Based on the findings of this and similar studies, it would be interesting to assess the connection between the anatomical variation of BMC and the efficiency of IAN blocks.

Types 1 and 2 by Langlais et al. () and type 5 described in this study, probably have the highest risk of IAN injury during tooth extraction, dental implant placement, apicoectomy or removal of pathologies in the molar region. The reported frequency of IAN injuries associated with extraction of third molar ranges from 0.4-9.4%, but the risk of permanent IAN injury was reported, which was less than 1% (-). Moreover, types 1, 2 and 5 were the most common types in our study. De Toledo Telles-Araújo et al. () did not find CBCT superior to panoramic radiograph in avoiding nerve injuries i.e. neurosensory disturbances following the removal of the mandibular third molar.

Conclusions

This study revealed a relatively high prevalence rate of BMCs. No significant difference was found between males and females with BMC. Type 2 BMC, according to Langlais et al. classification (), was the most common BMC type. We identified and described two new BMC types, Type 5 and Type 6, previously not described in the available literature. The results obtained in this study point to the importance of a thorough radiological planning prior to surgical interventions in the mandible in order to avoid potential complications.