Journal of Indian Academy of Oral Medicine and Radiology

ORIGINAL ARTICLE
Year
: 2018  |  Volume : 30  |  Issue : 3  |  Page : 216--222

Prevalence of retromolar canal and foramen: A cone beam computed tomography study


Suruchi Bajaj Ahuja1, Neelkamal Sharda Bhardwaj1, Vishal Dang1, Ravjot Ahuja2, Hemant Mathur3,  
1 Department of Oral Medicine and Radiology, Manav Rachna Dental College, Faridabad, Haryana, India
2 Department of Conservative Dentistry and Endodontics, Manav Rachna Dental College, Faridabad, Haryana, India
3 Department of Oral Medicine and Radiology, Pacific Dental College, Udaipur, Rajasthan, India

Correspondence Address:
Dr. Suruchi Bajaj Ahuja
527, Sector 17, Faridabad, Haryana
India

Abstract

Introduction: The retromolar fossa is a triangular area posterior to the third mandibular tooth. An opening called the retromolar foramen (RMF) and a canal called retromolar canal (RMC) are known to be present in this area. The RMF is an anatomical structure on the alveolar surface of the retromolar triangle. The contents of the RMC have been reported to consist of branches of the inferior alveolar vessels and nerves. Retromolar area is related to surgical procedures that may lead to unexpected bleeding or paresthesia. Aims and Objectives: (1) To determine the prevalence of RMF and RMC in the mandible using cone-beam computed tomography (CBCT). (2) To observe variations, if any, in the appearance of the same. (3) To determine the length of the RMC, diameter of RMF, and proximity (distance) of the RMF from the cementoenamel junction (CEJ) of the mandibular molars. (4) To propose a classification for the presentation of the RMC. Materials and Methods: A retrospective analysis was carried out for the presence and characteristics of the RMC and RMF. The retromolar region of the mandible on both sides (right and left) was examined in 80 CBCT scans (N = 80). Results: The RMCs traversed in different directions – horizontal, vertical, and angular. Based on a subjective assessment, each of these canals was further subclassified into either a straight or curved canal. In the present study, RMF was found in 3 cases unilaterally (3.8%), out of which 2 were males (2.5%) and 1 was female (1.2%), and distance from the midpoint of the same to the CEJ of first, second, and third molars were determined. Conclusion: The prevalence of RMC and RMF showed that all RMC may or may not end into RMF. The three different varieties of RMC observed were classified into horizontal, vertical, and angular typeswhich were further subclassified based on a subjective assessment of each of these into either a straight or a curved canal.



How to cite this article:
Ahuja SB, Bhardwaj NS, Dang V, Ahuja R, Mathur H. Prevalence of retromolar canal and foramen: A cone beam computed tomography study.J Indian Acad Oral Med Radiol 2018;30:216-222


How to cite this URL:
Ahuja SB, Bhardwaj NS, Dang V, Ahuja R, Mathur H. Prevalence of retromolar canal and foramen: A cone beam computed tomography study. J Indian Acad Oral Med Radiol [serial online] 2018 [cited 2021 Nov 30 ];30:216-222
Available from: https://www.jiaomr.in/text.asp?2018/30/3/216/243666


Full Text



 Introduction



Mandible, the largest and strongest bone of the face is a complex bone both in terms of its development and anatomy. It provides anchorage to the lower teeth and contains channels for nerves and vessels.[1]

The retromolar fossa (or retromolar triangle) is a triangular area posterior to the third molar tooth in the mandible. The retromolar foramen (RMF) is an anatomical structure occasionally identified on the alveolar surface of the retromolar triangle. This foramen is the termination of the retromolar canal (RMC), which diverges from the mandibular canal. The contents of the RMC have been reported to consist of branches of the inferior alveolar vessels and nerves.[2]

Although the presence of these anatomic structures is acknowledged, some anatomy textbooks do not mention their occurrence in anatomical details.[3] The retromolar region in the mandible is also important while performing surgical interventions such as third molar extraction, implant placement, osteotomies, harvesting bone grafts.

Hence, it is important to know about the presence and location of both RMF and RMC as unexpected bleeding or paraesthesia and even permanent loss of sensation may occur via damage to the vessels and the nerves passing through the RMC during surgical procedures.[4]

In the past different methods have been used to assess the RMC/RMF. Previous studies have reported considerable variation in the prevalence of the RMF and RMC (0–72%).[5],[6] The results were influenced by the population studied, sample size, and methodology used for evaluation [cadavers, dried mandibles, panoramic radiography, CBCT etc. A majority of these earlier studies have evaluated the prevalence of RMF and RMC postmortem, i.e., on dried mandibles and on cadavers, but relatively few studies have been conducted in living subjects [using panoramic radiographs, computed tomography (CT)and CBCT].[3],[4],[7],[8],[9],[10] There has been a considerable interest in the study of these structures (RMC/RMF) in living subjects in the recent years using CBCT. Imaging is an important adjunct to study human anatomy. Certain imaging techniques (CT and CBCT) because of their image quality, three-dimensional (3D), view and interactive display modes are considered the “Third eye” to reveal the hidden mysteries of the human body.[11]

Though several studies have been conducted (using cadaver section, panoramic radiography, and CBCT) to assess the presence of RMF and RMC, they have reported wide variations in the prevalence of these structures.[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22]

Cross-sectional imaging techniques, such as CT and CBCTcan overcome these limitations of panoramic radiography. It is extremely important for the oral radiologist to identify and report all anatomic variations of clinical importance. A literature search revealed that few studies have been carried out to assess the prevalence of RMF/RMC in Indian populations. Moreover, the descriptive characteristics of these structures find limited mention in the literature. Hencethis study was designed and a retrospective analysis of 80 CBCT scans was performed to assess the prevalence of RMC/RMF in a regional Indian population. The descriptive characteristics of these structures were also evaluated in this study.

Aims and objectives

To detect the presence or absence and determine the prevalence of RMF and RMC in the mandible using CBCTTo observe variations, if any, in appearance of the sameTo determine the length of the RMC, diameter of RMF, and proximity (distance) of the RMF from the CEJ of the mandibular molarsTo propose a classification for the presentation of the RMC.

 Materials and Methods



This retrospective analysis of CBCT scans was performed in the Department of Oral Medicine and Radiology, Manav Rachna Dental College, Faridabad, Haryana to detect the presence and characteristics of the RMC and RMF.

Materials

The retromolar region of the mandible on both sides (right and left) was examined in 80 CBCT scans (N = 80). A computer monitor with display resolution of 1366× 768 pixels and screen size of 15 inches was used, and all three sections sagittal, coronal, and axial showing retromolar region were reconstructed.

Inclusion criteria

A large field-ofview (FOV) scan showing the entire retromolar region of the mandible bilaterally.

Exclusion criteria

Radiologic evidence of any bony injury (e.g., discontinuity in bone etc.)Any surgical intervention (e.g., surgical defect following third molar extraction and after harvesting the bone graft, sagittal split osteotomy, etc.) noted radiographically in the retromolar region of the mandiblePresence of any obvious jaw bone anomaly/pathology in the retromolar region, excluding the presence of an impacted third molar.

Methods

Variations with radiographic appearance of these anatomical structures, if present, were noted. The scans were also evaluated on the basis of age group, gender, and site on which RMC/RMF were present was mentioned. Data obtained were carefully examined by two examiners for the presence of these structures.

Allthree sectionssagittal, coronal, and axial were examined using the software with slice thickness of 1 mm. The orthographic views were sequentially examinedfrom buccal to lingual cortex in sagittal section, superior to inferior in axial section, anddistal to mesial in coronal section. In 3D module, various sections were reconstructed by moving thecenter buccolingually and anteroposteriorly by degrees to detect RMC and RMF.

Length of canal was measured using tapeline (angular lines) for curved canals and ruler (straight lines) for straight canals. The variations in length, direction (vertical, angular, horizontal, or any other variation), number of canals and anatomy (bifid and trifid canals) of the retromolar canal were observed.

The retromolar canal was evaluated for the following variables:

Number of canal(s)Branches of canal(s), if anyLength of canal (from its point of origin to exit, if visualized)Course of canals

Horizontal canals[INSIDE:1]Straight/CurvedAngular canals[INSIDE:2]Straight/CurvedVertical canals[INSIDE:3] Straight/Curved

The variations in the diameter and shape of RMF were observed using axial view. In addition, the distance was measured from the midpoint of the RMF to the distal cementoenamel junction (CEJ) of the first, second, and the third molar using sagittal CBCT images. The linear measurements were made along the crest of the mandible. In case of multiple foramens, distance was individually measured from each of the foramen.

Statistical analysis

SPSS software (Version 9.0) was used for the statistical analysis. Mean and standard deviations were calculated to determine the distance between the midpoint of RMF to the distal CEJ of first, second, and third molars, as well as to determine the length of the RMC. The difference between the frequencies was assessed using chi-square test. For all the tests P value of 0.05 or less was considered for statistical significance.

 Results



The RMC was identified in 16 of the 80 scans (20%). [Table 1] shows the prevalence of RMC and its gender distribution in the study sample (N =80). RMC was identified in 6 (19.35%) of the 31 CBCT scans or 7.5% of the sample in females and 10 (20.4%) of 49 CBCT scans or 12.5% of the sample in males. (P-value = 0.91 [P >0.05 which is nonsignificant]). [Table 2] shows the side distribution (unilateral/bilateral) of RMC.{Table 1}{Table 2}

Descriptive characteristics of retromolar canal

The direction, course, and length of the RMC was noted in all cases. RMC traversed in different directions – horizontal, vertical, and angular. Based on a subjective assessment, each of these canals was further subclassified into either a straight or curved canal.

Horizontal

A canal was considered to be horizontal if it was approximately parallel to mandibular canal ±10°. According to its course, it was further divided into a horizontal straight (HS) [Figure 1] or a horizontal curved (HC) canal [Figure 2].{Figure 1}{Figure 2}

Vertical

A canal was considered to be vertical if it was approximately perpendicular to mandibular canal ±10°. It was further divided into a vertical straight (VS) [Figure 3] or a vertical curved (VC) canal [Figure 4] depending on its course.{Figure 3}{Figure 4}

Angular

A canal was consideredangular if it formed an acute angle with the mandibular canal [not covered in criteria (B)]. These canals were further divided into angular straight (AS) [Figure 5] or angular curved (AC) [Figure 6] depending on their course.{Figure 5}{Figure 6}

[Table 3] shows the descriptive characteristics of RMC in the study.{Table 3}

VS and AC canals presented in 5 cases (7.2%). Four cases showed HS direction (5%), followed by 3 cases (3.8%) in HC direction and two cases with VC and AS direction.

RMC may present as a single canal or multiple canals. In this study, three cases presented with multiple canals. In case of multiple canals, length of longest canal was determined.

[Table 4] shows the mean length of the RMCs on right and left side.{Table 4}

The mean length of 10 RMC on right side was 9.292, and was 9.3136 for 11 canals on left side. RMF was found in 3 cases unilaterally (3.8%), out of which 2 were males (2.5%) and 1 was female (1.2%).

[Table 5] shows the prevalence of the RMF and its gender distribution in the sample (N =80).{Table 5}

Total number of foramens were 5 (6.3%), 4 on the right (5%) and 1 on the left side (1.3%). In one of the cases (case number 4), a peculiar pattern of 3 foramens was noted on the right side. Diameter of foramens measured were 4.10 mm, 3.24 mm, 2.06 mm, 1.57 mm, and 1.16 mm.

[Table 6] shows the distance from the midpoint of RMF to the CEJ of first, second, and third molars. The mean distance calculated was 28.07 mm from the first molar, 21.19 mm from the second molar, and 14.19 mm from the third molar.{Table 6}

 Discussion



“Variation is the sign of life, and it must be there.”

“It is variety that is the source of life, the sign of life. Why should we be afraid of it?”

– Swami Vivekananda

The theory of natural selection propounded by Charles Darwin highlights that variations exist bothat the individual and species level.[23] Mandibular canal presents considerable variation in its anatomy. It may present as a bifid or trifid structure. Branching of the inferior alveolar nerve canal is common, and many anatomical variations have been reported (e.g., forward canal, dental canal, buccolingual canal, RMC, etc.).

There is a triangular area posterior to the third mandibular molar, overlying the junction of ascending and horizontal ramus called retromolar triangle. Retromolar triangle constitutes both soft tissue and hard tissues. Retromolar trigone is a pear-shaped pad of soft tissue.[24] In this area, bone is perforated by a variable number of foramina of different calibre. One such accessory foramen in the retromolar region that has been infrequently described in literature is the RMF. RMF is an anatomical structure occasionally identified on the alveolar surface of the retromolar triangle. This foramen is the termination to the RMC, which diverges from the mandibular canal.[2]

Earlier studies in India have been carried out on regional population using dry mandibles/cadavers to describe these structures. The paucity of data, especially pertaining to North Indian population, prompted this research. This retrospective study included an analysis of 80 CBCT scans of the mandible to detect the prevalence of RMF and RMC and elucidate their descriptive characteristics. CBCT permits antemortem evaluation in living subjects with results comparing favorably to postmortem studies on cadavers and CBCT.[3],[4],[7],[8],[9],[10]

All sections, i.e., axial [Figure 7], coronal [Figure 8], and sagittal [Figure 9] were evaluated for the presence of RMC and RMF. The 80 scans studied included 31 females and 49 males. The RMC was identified in 16 (20%) CBCT scans (Males =10 and Females =6). Out of the 16 scans that showed the presence of RMC, 20.4% were males and 19.35% were females; however, this difference was not statistically significant (P = 0.91), which matches the results of earlier studies.{Figure 7}{Figure 8}{Figure 9}

Visualized in axial section

Previous studies have reported a prevalence range of 0–72% for RMC.[5],[6] This large variation can be explained by the method of evaluation (CBCT, panoramic, dry mandible, and cadavers) and the population studied.

Out of 16 RMCs identified, 11 were unilateral and 5 were bilateral. This difference highlights the fact that the two sides of the mandible are not identical as far as the prevalence of RMC is concerned. 13.8% of unilateral RMCs were identified compared to 6.2% of bilateral RMCs. Several previous studies have reported a higher occurrence of unilateral RMC compared to bilateral (P =0.134).[3],[4],[5] In terms of side (right or left), no significant difference was noted in this study.

The direction, course, and length of the RMC were noted in all cases. The RMC traversed in different directions – horizontal, vertical, and angular. Based on a subjective assessment, each of these canals was further subclassified into either a straight or curved canal.

Horizontal canal was approximately parallel to mandibular canal ±10°. According to its course, it was further divided into an HS or HC canalA canal was considered to be vertical if it was approximately perpendicular to mandibular canal ±10°. It was further divided into a VC or VS canal, depending on its courseA canal was considered angular if it formed an acute angle with the mandibular canal [not covered in criteria (2)]. These canals were further divided into AS or AC canals, depending on their course.

VS and AC (6.2%) were most often followed course by the canal followed by HS (5%) and HC (3.8%). The rarely followed course was VC and AS (2.5%). Thomas Von Arx (2011)[4] classified RMC on the basis of course and morphology – vertical course of RMC (type A1), vertical course of RMC with additional horizontal branch (type A2), curved course of RMC (type B1), curved course of RMC with additional horizontal branch (type B2), and horizontal course of RMC (type C). Most canals had a vertical course (type A1, 41.9% and A2, 16.1%) followed by curved course (type B1, 29% and B2, 12.9%). Type C, i.e. horizontal course was never identified. However, the path of the canal – straight or curved – was not clearly described in their study.

In the present study, it was found that RMC may present as a single canal or multiple canals. In case of multiple canals, length of longest canal was determined. In the present study various tools of CBCT were used to determine the length of the canal; for straight canal the ruler tool was used and for curved canals the tapeline tool was used. The mean length of 10 RMC on the right side was 9.292 mm and was 9.3136 mm for 11 canals on the left side. The literature has little information on RMC length; therefore, the data collected from this study can benefit \ better understanding of the anatomy of this structure.

The study also assessed the prevalence of the RMF and its gender distribution in the sample (N =80). In the present study, RMF was found in 3 cases unilaterally (3.8%), out of which 2 were males (2.5%) and 1 was female (1.2%). In one of the cases (case number 4), a peculiar pattern of 3 foramens was noted on the right side [Table 5]. Previous studies on Indian population have reported a prevalence ranging from 3.48–21.9% for the RMF.[9],[25] The prevalence of RMF (3.8%) in the present study falls within this range.

The distance from the midpoint of RMF to the CEJ of the first, second, and third molars were determined. The mean distance calculated was 28.07 mm from the first molar, 21.19 mm from the second molar, and 14.19 mm from the third molar. Kawai et al. (2012)[3] reported less mean distance from the first and second molars. The difference in the results may be influenced by the position of the teeth and jaw size [Table 6].

The prevalence of RMC and RMF in the present study showed that all RMC may or may not end into RMF [Graph 1].[INLINE:1]

CBCT scans are being increasingly prescribed in dental practice today. The oral and maxillofacial radiologist has an opportunity to carefully examine and assess the retromolar region wherever it falls within the FOV. The onus of reporting all anatomical variations of clinical interest lies with the oral and maxillofacial radiologist. The RMC and RMF are infrequently reported structures which can be well studied on CBCT scans. A descriptive analysis of the myriad of presentations of the RMC and RMF would require further research and study.

 Conclusion



From the present study, it was inferred that all RMC may or may not end into RMF, and unilateral presence of RMC is more common than bilateral. CBCT is an excellent tool for studying the osseous structures of the maxillofacial region. The increasing popularity of this imaging modality also places a greater responsibility on the oral and maxillofacial radiologist to carefully examine the complete image dataset, both within and outside the region of interest.

Variations in the anatomy of the retromolar region are frequently encountered, as evidenced by the results of this study. The high percentage of RMC observed in this study (16 cases, 20%) justifies a routine search for the same in all CBCT scans covering this region. The oral and maxillofacial radiologist should carefully evaluate the retromolar region and report on these structures, if identified.

In the present study, the three different varieties of RMC observed were classified into horizontal, vertical, and angular, which were further subclassified based on a subjective assessment of each of these into either a straight or a curved canal. All the three types of observed canals had an equal distribution in the sample studied.

The retromolar region in the mandible is also of considerable importance while performing surgical interventions such as third molar extraction, implant placement, osteotomies, and harvesting bone grafts. Hence, it is important for the dentist to know the presence and location of both RMF and RMC as unexpected bleeding or paraesthesia and even permanent loss of sensation may occur via damage to the vessels and the nerves passing through the RMC during surgical procedures.[4]

CBCT scans are being increasingly prescribed in dental practice today. The oral and maxillofacial radiologist has an opportunity to carefully examine and assess the retromolar region wherever it falls within the FOV. The onus of reporting all anatomical variations of clinical interest lies with oral and maxillofacial radiologist. The RMC and RMF are infrequently reported structures which can be well studied on CBCT scans. A descriptive analysis of the myriad of presentations of the RMC and RMF would require further research with a large sample of population is warranted.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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