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Revista Română de Anatomie funcţională şi clinică, macro- şi microscopică şi de Antropologie

Vol. XIV – Nr. 1 – 2015 ORIGINAL PAPERS

ANATOMICAL VARIATIONS OF THE MENTAL FORAMEN ASSESSED BY CONE-BEAM COMPUTED TOMOGRAPHY

Ana Gabriela Benghiac1, V. Costan2, Alexandra-Maria Dumitrescu3, Anca Sava4, M.S.C. Haba3, Danisia Haba2

“Gr. T. Popa” University of Medicine and Pharmacy, Iaşi1. PhD Student

2. Department of Oral and Maxillofacial Surgery3. Student

4. Department of Morphofunctional Sciences

ANATOMICAL VARIATIONS OF THE MENTAL FORAMEN ASSESSED BY CONE-BEAM COMPUTED TOMOGRAPHY (Abstract): This retrospective study aimed at assessing the prev-alence and location of the anatomical variations of the mental foramen (MF) using partial or full mandible CBCT scans performed with a Planmeca 3D Mid device and Romexis 3.6 software in a group of 188 Romanian patients. All images was analyzed in different anatomical planes (sagittal, axial, coronal) and 3D reconstructions. Only the images with bifid (BMF) or multiple mental foramens were included in the research. The CBCT scans were performed by a licensed radiology technician and read by one examiner who also made the assessment. The normal radio anatomy of the mental nerve and its variations should be known and properly assessed by all dental profes-sionals prior to procedures in this specific area. Anesthetic and surgical success rates are strong-ly correlated to accurate diagnosis and therapy planning. CBCT scans can be useful paraclinic exams for concise tailored surgical treatments, avoiding iatrogenic nerve damage that can lead to unpleasant neurological disturbances and possible malpractice litigations. Key words: CBCT, MENTAL FORAMEN, ANATOMICAL VARIATIONS, PREVALENCE

INTRODUCTIONThe mental foramen (MF) is a circular or oval

opening of the mental canal located on the ante-rior surface of the mandible, generally between or below the two inferior premolars (1). Ra-diologically, it is a well-defined element with internal radiolucency. It allows the passage of the mental nerve (MN) and blood vessels. The MN is one of the terminal branches of the in-ferior alveolar nerve, which is itself a branch of the trigeminal nerve (CN V). It exits the MF and ramificates into several fillets that supply innervation to the corresponding vestibular mu-cosa, hemi-lower lip and mental region (the mental bundle). Due to its strategic location,

the MF is an essential landmark for local anes-thesia, surgical and dental procedures. Data re-garding its anatomical variations such as: loca-tion, size, number, configuration, incidence and prevalence are extremely important not only for surgical and dental procedures performed in this area, but also for anthropologic assessment, medico-legal purposes and for anatomists. Specialty lit-erature reports several anatomical variations of the MF including: multiple (2), bifid (double) foramen (3) and even its rare absence (4,5). There is no clear definition for the accessory mental foramen (AMF), despite the fact it as been constantly reported in the literature (6,7,8,9,10,11). However, researchers say that

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Anatomical Variations of the Mental Foramen Assessed by Cone-Beam Computed Tomography

a bifid or multiple MF is reffered as AMF (12) and that the AMF is a buccal foramen with a smaller diameter than the MF followed by ac-cessory branch of the mandibular canal (7). The MF anatomical variations are oftentimes re-lated to demographic data, mainly ethnicity and sex. Studies show that the prevalence of the AMF varies according to ethnicity from 6.7% to 12.5% in Japan (13). In other ethic groups, the AMF varies largely: 1.4% in American Whites, 1.5% in Russians, 2.6% in French, 3.0% in Hungarians, 3.6% in Egyptians, 5.7% in American Blacks and 9.7% in Melanesians (13). The AMF’s incidence varies according to ethnicity from 1.4% to 9.7% (10). When com-paring the worldwide frequency of the AMF, Central Asian and Subsaharian African studied populations reported the highest frequencies while Europeans, South Asians, East and South-east Asians, (except for the Ainu sample), South Americans and western Oceanians showed a lower frequency (14).

Some studies report no gender differences regarding the AMF presence (9,13).

The MF also varies according to age, being closer to the alveolar margin in children before tooth eruption and descending at the middle of the distance between the upper and basilar margin during the dental eruption and furtherly de-scending to the basilar margin in dentate adults. However, with tooth loss and bone resorption, the MF gets closer to the edentate ridge (15).

The MF anatomical variations can be as-sessed radiologically (2D and 3D), by dissect-ing dry mandibles (4) or by blunt dissection of the MN and its fillets allowing clear and direct visualization of both the nerve and the foramen (16). Due to higher accuracy and reliability, 3D imaging is preferred to conventional 2D radi-ography for surgical treatment planning (1,2) and for the identification of the AMF espe-cially considering the fact that its size is small-er than the MF and oftentimes it is unnoticed on 2D X-rays (17).

The aim of this retrospective study was to evaluate the prevalence and location of the an-atomical variations of the mental foramen (MF) using CBCT scans performed with a Planmeca 3D Mid device and Romexis 3.6 software in a group of 188 Romanian patients. To our knowl-edge, this is the first 3D imaging study that focused on the assessment of the MF and its anatomical variations in Romanian populations.

MATERIAL AND METHODS

Data from CBCT examinations including the MF area in 188 Romanian patients who had been referred to MedImagis, a private radiology and 3D imaging clinic in Iasi, Romania, during February 2014-November 2014 were analyzed retrospectively. These cases were selected from a large database of 1629 patients that underwent various 2D and 3D exams during the time frame

Fig. 1. CBCT panoramic section depicting the IAN trajectory through the mandible.

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mentioned above. CBCT scans that included partial or full mandible were performed in 188 patients. Out of these 188 patients, 53 were referred for cranial scanning (age range: 4-72 years), 35 for hemi-mandible or mandibular section including the area between the canine and the first lower molar (age range: 8-67 years) and 100 patients for full mandible CBCT scan (age range: 7-79 years). The referral di-agnoses of patients scanned during this period included, but were not limited to pre-operative treatment planning in dento-alveolar surgery and implantology. The exclusion criteria were: excessive artefacts, developmental and patho-logical impairments that interfered with MF location and anatomical variations, single MF. No case with MF absence was registered. Only the images with bifid or multiple mental fora-mens were included in the research.

The study was conducted in accordance with the Declaration of Helsinki.

The CBCT scans were performed by Plan-meca 3D Mid (Planmeca, Helsinki, Finland). The acquisition protocol was tailored to include the mandibular anatomy of interest and it is further detailed for each patient. The obtained images were exported in a digital imaging com-munication in medicine (DICOM) file format (.dcm) and imported into a personal computer (Dell Latitude e6320, Dell Inc., TX, USA) using DICOM viewer Romexis 3.6 software (Plan-meca, Helsinki, Finland). The CBCT scans were performed by a licensed radiology technician

and read by one examiner (A.G.B.) who also made the assessment. The images were ana-lyzed in different anatomical planes (sagittal, axial, coronal) and 3D reconstructions.

Of the 188 selected cases with mandibular CBCT scans including the MF area, 3 patients (2 females, 1 male) presented MF anatomical variations. The mean age of this population sample was 28.6 years (range: 23-35years). Among these, one patient presented bilateral anatomical variations as multiple MF, one pa-tient presented bifid MF and the third patient had multiple MF. Images were analyzed in 3D reconstructions and in different planes (sagittal, axial, coronal) to identify the continuity of the mandibular canal to the MF and, thus, to elim-inate from the study any nutrient foramina.

The MF can be projected as far as anteri-orly as the first mandibular premolar and pos-teriorly as the first molar. Thus, its location can be assessed as: (1) in line with the first molar, (2) somewhere between the first molar and the second premolar, (3) in line with the second premolar, (4) between the two premo-lars, (5) anterior to the first mandibular premo-lar (18). Considering that some of our patients presented mandibular edentations in the MF area, we couldn’t evaluate the location of the MF in relation with the neighboring teeth as in a previous study (18), but we related the AMF’s position to the MF. However, despite the fact that the MF and its anatomical variations are commonly assessed radiologically in relation to teeth positions (18,19,20), paleoanthropolo-gists consider that this is not a reliable indica-tor because of the teeth size differences (1).

Fig. 2. Axial view of the right hemimandible.

Fig. 3. 3D reconstruction of the right hemi-man-dible showing the IAN course through the mandi-

ble until its mental branch exists the MF. The AMF is located posterior to the MF, in line with

the second premolar.

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Anatomical Variations of the Mental Foramen Assessed by Cone-Beam Computed Tomography

Case nr. 1C.A., 23 years old, female patient who un-

derwent the CBCT scan using the following acquisition protocol: 401x401x200 mm field of view (FOV), voxel size: 200, kV: 90, mA: 10.0 with an exposure time of 12 seconds. The dose area product (DAP) (mGyxcm2) was 820. AMF was viewed on the right hemi-mandible as a multiple MF. In this patient, the AMF is located posterior to the MF, in line with the second premolar.

Case nr. 2C.V.M., 35 years old, female patient who

underwent CBCT scan using the following acquisition protocol: 901x901x311mm FOV; voxel size: 200; kV: 90; mA: 12 with an exposure time of 18.53 seconds and DAP

(mGyxcm2) 1166.5. The MF presented anatom-ical variations only on the right hemi-mandible as a bifid MF.

Case nr.3Z.D.A, 28 years old, male patient who un-

derwent CBCT scan using the following acqui-sition protocol: 500x500x255mm, voxel size: 400, kV: 90, mA: 10.0 with an exposure time of 13.846 seconds and DAP(mGyxcm2) 1245. The patient presented multiple, bilateral MF situated, on each side, distal to the MF.

DISCUSSIONOur research shows that the Romanian pop-

ulation included in the study has a very low prevalence of the AMF (0.564%). Despite this, one should consider the limited sample size and

Fig. 4. CBCT coronal slices of the right hemi-mandible showing the canal of the IAN, the MF and the AMF.

Fig. 5. 3D reconstruction of the right hemi-mandible depicting partially the IAN and the bifid MF.

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Fig. 6. CBCT panoramic view of the IAN partially outlined, its canal and of the bifid MF.

Fig. 7. CBCT coronal slices of the right hemi-mandible with the IAN and the MF anatomic variations.

Fig. 8. Axial view of the entire mandible.

the fact that all patients come from a relatively homogenous ethnic area of our country. The studied population is not necessarily represent-ative for Romania.

Unlike 2D radiographies, CBCT scans are not reimbursed by the government. Thus, high-tech medical explorations, including 3D imag-ing may not be available or accessible to all individuals residing in our country, which can impair research on the prevalence and ana-tomical variations of the MF.

Further research, including a multiethnic, wider sample size should be performed to have a better understanding of the facial bones’ ana-tomical variations in Romanian individuals.

CONCLUSIONSTo sum up, the normal radio anatomy of the

MF and its variations should be known and

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Anatomical Variations of the Mental Foramen Assessed by Cone-Beam Computed Tomography

Fig. 9. Panoramic view of the entire mandible showing the IAN trajectory until its mental branch exists the MF. Multiple, bilateral AMF can be seen posterior to the MF.

Fig. 10. Frontal view of the 3D reconstruction displaying the bilateral MFs and AMFs.

Fig. 11. 3D reconstruction of the right hemi-man-dible revealing the MF, the IAN course through

the bone and the AMF.

Fig. 12. 3D reconstruction of the left hemi-mandible showing the MF, the AMF and the course of the IAN through the bone.

properly assessed by all dental professionals prior to procedures in this specific area. Anes-thetic and surgical success rates are strongly correlated to accurate diagnosis and therapy planning. CBCT scans can be useful paraclinic exams for concise tailored surgical treatments, avoiding iatrogenic nerve damage that can lead to unpleasant neurological disturbances and possible malpractice litigations.

ACKNOWLEDGEMENTSThis research was co-financed by the Project

“Excellence program for multi-disciplinary doc-toral and postdoctoral research in chronic dis-ease”, Grant No. POSDRU/159/1.5/S/133377 partially supported by the Sectorial Operation-al Programme of Human Resources Develop-ment, financed from the European Social Fund.

CONFLICTS OF INTERESTThe authors declare that they have no con-

flict of interest.

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Corresponding author

Ana-Gabriela Benghiace-mail: dr.gabriela_benghiac@yahoo.com