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Journal of Dental Sciences (2016) xx, 1e7

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ORIGINAL ARTICLE

Anatomical and radiographic study of themandibular retromolar canal

Min-Kyu Park a, Wonsug Jung b, Jung-Hee Bae c,Hyun-Ho Kwak a*

a Department of Oral Anatomy, BK21 PLUS Project, School of Dentistry, Pusan National University,Yangsan, South Korea

b Department of Anatomy, School of Medicine, Gachon University, Inchon, South Koreac Division in Anatomy and Developmental Biology, Department of Oral Biology, College of Dentistry,Yonsei University, Seoul, South Korea

Received 23 March 2016; Final revision received 8 April 2016Available online - - -

KEYWORDSmandible;retromolar canal;retromolar foramen;retromolar triangle

* Corresponding author. Departmentro, Mulgeum-eup, Yangsan, Gyeongsan

E-mail address: kwakhh@pusan.ac

Please cite this article in press as: PDental Sciences (2016), http://dx.do

http://dx.doi.org/10.1016/j.jds.2016.01991-7902/Copyrightª 2016, Associatioarticle under the CC BY-NC-ND license (

Abstract Background/purpose: As the demand for surgical procedure in the retromolar areaof the mandible has been increasing, the identification of the retromolar foramen (RMF) andcanal involving the retromolar triangle (RMT) has become an issue of clinical concern. Weexamined the shape of the RMT, incidence of the RMF, and intraosseous trajectory of the retro-molar canal (RMC).Materials and methods: A total of 118 sides of dry mandibles, 22 sides of mandibles of 13 ca-davers, and cone-beam computed tomography (CT) images of 100 patients were examined.Micro-CT data of 13 cadavers were reconstructed using imaging analysis software for the pres-ence of an RMC. RMCs were classified into three types according to the courses. The width andlocation of the RMCs were evaluated.Results: The shape of the RMTwas classified into three categories, with the most common typebeing the triangular type (81.4%). Forty-seven retromolar foramina (33.6%) were observed in140 sides of mandibles. The horizontal distances from the RMF to the second and third molarswere 12.1� 3.3 mm and 5.8� 3.6 mm (mean� standard deviation), respectively, and the dis-tance from the mandibular foramen to the arising point of the RMC and the vertical distancefrom the RMF to the mandibular canal were 21.5� 11.2 mm and 15.3� 4.6 mm, respectively.Conclusion: This study used various methods to obtain precise anatomical data on the RMT,

of Oral Anatomy, BK21 PLUS Project, School of Dentistry, Pusan National University, 49 Busandaehak-gnam-do 626-870, South Korea..kr (H.-H. Kwak).

ark M-K, et al., Anatomical and radiographic study of the mandibular retromolar canal, Journal ofi.org/10.1016/j.jds.2016.04.002

4.002n for Dental Sciences of the Republic of China. Published by Elsevier Taiwan LLC. This is an open accesshttp://creativecommons.org/licenses/by-nc-nd/4.0/).

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Please cite this article in press as: PDental Sciences (2016), http://dx.do

foramen, and canal in Koreans. The reported findings may be helpful for the clinical manage-ment of patients.Copyright ª 2016, Association for Dental Sciences of the Republic of China. Published by Else-vier Taiwan LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

The retromolar triangle (RMT) is a depressed area formedby the bifurcation of the mandibular temporal crest and theposterior edge of the last mandibular molar. The grossanatomy of the RMT, which is an area of great importancein dental practice, varies greatly according to the presenceof the mandibular third molar and alveolar bone resorp-tion.1 The retromolar canal (RMC) normally arises from themandibular canal behind the third molar, and travelsanterosuperiorly to the retromolar foramen (RMF), which islocated within or around the RMT.2 Several studies on theRMC and RMF have reported considerable diversity with afrequency from 1.7% to 72% depending on the study designand the race of the participants.3e5 However, the preciselocation of these structures relative to each other does notappear to have been determined.

Knowledge of the location of the RMC and of itsanatomical variations can facilitate surgical interventionand protect the patient from the potential complicationsthereof. The RMT is relevant to surgical procedures, such asextraction of an impacted third molar, dental implanttreatment, bone harvesting as a donor site for bone graftsurgery, and sagittal split ramus osteotomy.6 Moreover, thecontent of the RMC is an issue of clinical concern in surgicalprocedures involving the RMT. The neurovascular content ofthe RMC has been evaluated in cadavers5,7 and in clinicalbiopsy specimens.8e11 According to these reports, theneurovascular bundle within the RMC contains predomi-nantly thin nerve fascicles of myelinated nerve fibersbranching from the inferior alveolar nerve, small arteries,and numerous venules accompanying those arteries.Therefore, damage to the RMC contents could lead to un-expected bleeding and iatrogenic nerve damage, poten-tially resulting in paresthesia.

In clinical practice, anatomical variations, such as sup-plemental or accessory canals and foramina, can only bedetected by radiologic methods. However, conventionaltwo-dimensional (2D) radiographs such as panoramic im-ages are insufficient for detecting all anatomical struc-tures, and in particular the presence of an RMC.12 Sectionalimaging, such as computed tomography (CT) and cone-beam CT (CBCT), has been used successfully indentistry,13 and CBCT systems are available for routineinvestigation in dental clinics. However, studies that haveevaluated the location of the RMCs using CBCT images havefound incidences ranging from 14.6% to 47.4%, with noagreement between the various reports.6,12,14,15 Kawaiet al6 investigated the occurrence of the RMC in Japanesecadaver mandibles using CBCT images prior to dissectionand macroscopic observation, and reported that the

ark M-K, et al., Anatomical and ri.org/10.1016/j.jds.2016.04.002

subsequent cadaver dissection revealed some RMCs thathad not been identified in the CBCT images. Therefore, amore precise description of the anatomy of the RMC re-quires a high-resolution imaging modality and no movementof objects during imaging. Over the past few years, micro-CT systems have been used for the evaluation of the bonycanal morphology because of their high resolution.16 Thesystem can also be used to assess the characteristics of theRMC/RMF both qualitatively and quantitatively.

The present study (1) carried out an anatomical andmorphometric analysis of the RMT/RMF, (2) investigated theincidence of the RMF in Korean cadaver mandibles, (3)described the microanatomy of the RMC using three-dimensional (3D) reconstruction of micro-CT images, and (4)evaluated how frequently the RMC is visible on CBCT images.

Materials and methods

A total of 118 sides of dry, adult Korean mandibles from 59cadavers (i.e., 59 right sides and 59 left sides) of unspecifiedsex and age were obtained from the Department of Anatomyof Pusan National University (Yangsan, Korea). The single in-clusion criterion was that the first permanent molars or theiridentifiable socket needed to be present on each mandiblebilaterally. Twenty-two sidesofmandibles from13embalmedKorean adult cadavers (11 males and 2 females, with a meanage at death of 65.3 years) were also evaluated.

The RMT was identified and demarcated on 140 mandiblesides. The boundaries of the RMT were defined by the bifur-cation of the mandibular temporal crest and the posterioredge of the last mandibular molar. The maximum height andwidth (sagittal and transverse distances) were measuredusing a digital caliperwith a sensitivity of 0.01mm (Model CD-15CP; Mitutoyo, Kawasaki, Japan). All measurements weremadeby the sameperson in triplicate, and themeanvalues ofthese three measurements are reported. The RMTs wereclassified into three categories based on their shape, ac-cording to the classification of Suazo et al.17

The RMF was identified macroscopically; its diameterwas determined using round stainless-steel orthodonticwires with a diameter of 0.5 mm. The wires should enterthe RMF without excessive force and exhibit minimal play.Any foramina having a diameter of less than 0.5 mm werenot considered as the RMF.

Micro-CT images and 3D reconstruction

Twenty-two sides of mandible from 13 cadavers werescanned using a micro-CT system (SkyScan 1076 in-vivo X-ray microtomograph; SkyScan, Kontich, Belgium) with a

adiographic study of the mandibular retromolar canal, Journal of

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slice thickness of 35 mm. The serially obtained 2D imageswere saved in JPEG format for easy image control, andwere three-dimensionally reconstructed using two types ofimaging software: OnDemand3D (Cybermed, Seoul, Korea)and Reconstruct (http://synapses.clm.utexas.edu/tools/reconstruct/reconstruct.stm).18

Figure 2 Linear measurements made of the retromolar canal(RMC): 1, distance from the mandibular foramen to the arisingpoint of the RMC; 2, height of the RMC; and 3, horizontal dis-tance from the midpoint of the retromolar foramen to the

CBCT images

One hundred Korean patients (30 males and 70 females;mean age, 52 years) who underwent preoperative CBCTimaging for dental implant treatment at Pusan NationalUniversity Dental Hospital (PNUDH) between August 2011and April 2012 were enrolled with approval from the Insti-tutional Review Board of PNUDH (E-2012019). Images wereobtained using a CBCT scanner (Pax-Zenith 3D, Vatech,Gyeonggi-do, Korea) with the following settings: field ofview, 20 cm� 19 cm; tube voltage, 90 kVp; tube current,4.0 mA; scan time, 24 seconds; and isotropic voxel size,0.30 mm. The DICOM files were reformatted to 3D imagesusing 3D imaging software (OnDemand3D; Cybermed).

second or third molar.

Image evaluation

The CBCTandmicro-CT images were evaluated in a darkenedroom.The longitudinal plane and themandibular canal on theretromolar area and perpendicular plane were reconstructedfrom volumetric CBCT/micro-CT data using image-analysissoftware (OnDemand3D; Cybermed). The presence and buc-colingual and mesiodistal locations of the RMF/RMC wereconfirmed by these images. The existence of the RMC wasdefined when it branched off from the mandibular canal andalso exited as the RMF on the retromolar surface.

The RMCs were classified into three categories based ontheir course and morphology (Figure 1), according to theclassification of von Arx et al12: Type A, a vertical course;Type B, a curved course; and Type C, a horizontal course. Inaddition, the following linear measurements were made onsagittal CBCT and micro-CT images (Figure 2): the distancefrom themandibular foramen to the arising point of the RMC,the vertical distance (height) from themidpoint of the RMF tothe upper border of themandibular canal, and the horizontal

Figure 1 Schematic illustrations of the three types of the retromand Type C, a horizontal course.

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distances from the midpoint of the RMF to the distal edge ofthe last tooth or socket of the second and third molars.

Histological evaluation

A histological study was performed to confirm the contentsof the RMC. Twenty-two hemimandibles were dissected,and the RMFs clearly visible in one of several specimenswere stained. The bone fragment including the RMC wasfixed with neutral-buffered formalin, decalcified with 10%EDTA for 2 months, sectioned at a thickness of 5 mm using amicrotome, and stained with hematoxylineeosin.

Statistical analyses

The collected values were analyzed statistically using theStatistical Package for Social Science software version 18.0(SPSS, Chicago, IL, USA). The data are presented as

olar canal. Type A, a vertical course; Type B, a curved course;

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mean � standard deviation values, and the threshold forstatistical significance was set at P< 0.05.

Results

Morphometry of the RMT

The measured transverse and sagittal lengths of the RMTwere found to be 7.6� 1.8 mm and 10.8� 2.4 mm,respectively. The RMT was classified into three types ac-cording to its shape (drop, tapering, and triangular forms),as partially described by Suazo et al.17 Of the 140mandibular sides, the drop form was found in 12, thetapering form in 14, and the triangular form in 114(Figure 3).

Frequency and location of the RMF

In total, 47 RMFs (33.6%: 17 right, 18 left, and 12 bilateral)were detected in 140 mandibular sides (Figure 4). The RMFswere located on the right side of the mandible in 23 cases(32.9%) and on the left side in 24 cases (34.3%). Thirty-twoof the 47 RMFs (68.1%) were found within the RMT. Based ona midsagittal line through the RMT, the RMF tended to be onthe buccal side (nZ 26) more than on the lingual side(nZ 21), but the difference was not statistically signifi-cant. Meanwhile, of the 200 mandibular sides examined onCBCT, an RMC was present in 23 (11.5%).

Morphometry and histological evaluation of theRMC

The linear measurements taken from the CBCT and micro-CT images are summarized in Table 1. The distance fromthe mandibular foramen to the arising point of the RMC was21.5� 11.2 mm (range, 9.6e35.2 mm), the vertical

Figure 3 Classification of the retromolar triangle according totriangular form.

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distance (height of the RMC) from the midpoint of the RMFto the upper border of the mandibular canal was15.3� 4.6 mm (range, 9.6e21.7 mm), and the distancesbetween the RMF and the distal edge of the last tooth (orsocket) were 12.1� 3.3 mm and 5.8� 3.6 mm for the sec-ond and third molars, respectively.

The RMCs were divided into three categories, TypesAeC, according to their direction and course. The canal hada curved course (Type B, 62.7%) in most RMCs, and a ver-tical course (Type A) in 37.3%; Type C, a horizontal course,was not identified in any of the images (Figure 5).

The 3D reconstruction from micro-CT images revealedthe part of mandible containing the RMC (Figures 6 and 7).The RMC arose anterosuperiorly from the mandibular canalbehind the third molar and then curved posterosuperiorlyto the RMF, which is located on the lingual side of the RMT.

The neurovascular bundle was well-defined in thedecalcified histological sections (Figure 8). The artery withits thick wall and the veins with thin walls were readilydistinguished. There were numerous nerves in the RMC, andthe nerve or its branches were located mainly in the pe-riphery of the canal.

Discussion

Anatomical variations in the form of the accessory foraminaand canals of the mandible can cause surgical complica-tions if not properly identified. The RMC is a rare anatom-ical variation found in the RMT, which is a small, triangular-shaped region posterior to the third molar tooth in themandible.8 The neurovascular content of the RMC is anissue of clinical concern in surgical procedures involving theretromolar area.

The morphometric characteristics of the RMT aredescribed herein. The RMT was classified into three types(tapering, drop, and triangular forms) according to its

its shape (green): (A) drop form, (B) tapering form, and (C)

adiographic study of the mandibular retromolar canal, Journal of

Figure 4 Photographs showing the retromolar foramen (RMF, red arrowheads). (A) RMF located in the retromolar foramen (RMT)and on the lingual side of the midsagittal line of the RMT. (B) RMF located in the RMT and on the midsagittal line of the RMT. (C) RMFlocated outside the RMT and on the buccal side of the midsagittal line of the RMT.

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shape, as described previously in part by Suazo et al.17 Theauthors of that study reported that the most frequent type,the triangular form, had a prevalence of 80%17; in closeagreement with that study, the prevalence of the triangularform in our study was 81.4%. The shape of the RMT variedgreatly according to the presence (or not) of the posterior

Table 1 Measured distances between the retromolar fo-ramen and adjacent structures.

Measurement Mean � standarddeviation (mm)

Range (mm)

Distance from themandibular foramento the arising point ofthe RMC

21.5 � 11.2 9.6e35.2

Vertical distance fromthe midpoint of theRMF to the upperborder of themandibular canal

15.3 � 4.6 9.6e21.7

Horizontal distance fromthe midpoint of theRMF to the distal edgeof the second molar

12.1 � 3.3 6.5e19.0

Horizontal distance fromthe midpoint of theRMF to the distal edgeof the third molar

5.8 � 3.6 3.0e18.5

RMC Z retromolar canal; RMF Z retromolar foramen.

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teeth of the mandible. When the third or second molarswere present, the RMT tended to have a triangular form.

The incidence of the RMF has been reported to vary from7.7% to 66% depending on the study design.19,20 The inci-dence of the RMF in this study was 33.6% (47 of 140mandibular sides), which is close to that reported by bothHu et al21 and Kawai et al6; all of these results were ob-tained from Northeast Asians. It appears that the incidenceof the RMF varies hugely between races, and Ossenberg3

reported that the rate in a North American populationwas lower than in other populations, such as those fromNortheast Asia, Europe, and Africa.

The most common 3D diagnostic imaging modality usedto study mandibular anatomy is CBCT.22 Naitoh et al23

observed a bifid mandibular canal in the mandibularramus region in 65% of CBCT images, while Kuribayashiet al2 reported that 47 of 301 (15.6%) patients studied withCBCT had bifid mandibular canals. In CBCT images, a bifidmandibular canal was observed on 18 of 38 sides and anRMC was seen in three canals (7.9%) by Naitoh et al.22 Infact, the criteria for a precise radiological classification ofa bifid canal such as an RMC are not clear.14 However,three recent studies focused on the radiological detectionand incidence of the RMC by CBCT. von Arx et al12 provideda detailed description of the courses, morphologies, anddimensions of 31 RMCs on 121 mandibular sides (25.6%),Kawai et al6 described the frequency, location, and con-tent of 34 RMFs on 90 mandibular sides (37%), and Lizioet al14 reported that 34 RMCs were detected on 233 CBCTimages (14.6%). In this study, the RMC was present in 23 of200 CBCT images (11.5%). The low incidence of the RMC inthis study compared with previous studies may be

adiographic study of the mandibular retromolar canal, Journal of

Figure 5 Classification and examples of the retromolar canal: (A) Type A, vertical course; and (B) Type B, curved course.

Figure 6 Three-dimensional (3D) reconstruction of a retromolar canal (RMC) on the right side of a mandible created usingOnDemand3D. The RMC (red) arose anterosuperiorly from the mandibular canal (blue) behind the third molar and then curvedposterosuperiorly to the retromolar foramen, which is located within the retromolar triangle. (A) Lingual view, (B) superior view.

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explained by restrictions associated with the inclusioncriterion applied in this present study (only retromolarradiolucent images with a diameter �0.5 mm) and otherradiological variables such as a bigger voxel size (0.3 mm;

Figure 7 Three-dimensional reconstruction of the retromolar csoftware. (A) Lingual view, (B) superior view, (C) buccal view.

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(compared with voxel sizes of 0.08e0.125 mm in previousstudies).

In this study, the incidence of an RMF was 33.6% ac-cording to macroscopic examination of cadaver mandibles,

anal on the left side of the mandible using the Reconstruct

adiographic study of the mandibular retromolar canal, Journal of

Figure 8 Histology of the retromolar canal. A Z artery witha thick wall; N Z nerves, which could be readily distinguished;V Z veins with thin walls.

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whereas the incidence of an RMC/RMF on CBCT was 11.5%.This finding demonstrates that the ability to detect an RMCusing CBCT is limited compared with direct anatomicalobservation. However, we agree that CBCT is the bestmethod for identifying this anatomical variant in the clinic,and may be useful when a preliminary radiographic surveyshows no clear relationship between the inferior alveolarnerve and the other anatomical structures in the mandib-ular molar area.

This study used various methods to establish preciseanatomical data for the RMT, RMF, and RMC in Koreans. Theresults of this study provide useful information for surgicalinterventions and may protect patients from complications.The presence of an RMC may warn clinicians about thepossibility of inadequate anesthesia and bleeding in theretromolar region.

Conflicts of interest

The authors have no conflicts of interest relevant to thisarticle.

Acknowledgments

This study was supported by a grant of the Korean HealthTechnology R&D Project, Ministry of Health & Welfare,Republic of Korea (Grant No. A111442).

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