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Table of Contents
ORIGINAL ARTICLE
Year : 2022  |  Volume : 71  |  Issue : 1  |  Page : 47-53

Retromolar canals and mandibular third molar position: Is there a possible connection?


1 Bolu Abant İzzet Baysal University, Faculty of Dentistry, Department of Dentomaxillofacial Radiology, BOLU, Turkey
2 Department of Dentomaxillofacial Radiology, Faculty of Dentistry, İstanbul Medipol University, Fatih, İstanbul, Turkey

Date of Submission23-Sep-2020
Date of Acceptance25-Nov-2021
Date of Web Publication17-Mar-2022

Correspondence Address:
Dr. Oguzhan Demire
Department of Dentomaxillofacial Radiology, School of Dental Medicine, Bahçesehir University, Sahrayi Cedit Mahallesi, Batman Sk. No: 66, Kadiköy 34734, Istanbul
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jasi.jasi_194_20

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  Abstract 


Introduction: The retromolar canal (RMC) is an anatomical variation of the posterior part of the mandibular canal. It is thought that this variation is related to histological vestiges of the gubernacular canal and RMC presence may be associated with mandibular third molar (MTM) malposition. This study aims to investigate the relationship between MTM position and RMC existence. Material and Methods: Patients who had undergone cone-beam computed tomography examination for various purposes were included in the study. All of the patients had unilateral or bilateral MTM teeth. MTM impaction patterns were classified according to Winter and Pell-Gregory classifications. RMC existence, type, retromolar foramen (RMF) position, RMF dimensions, and distance from RMF to second and third molars were recorded. Results: Three hundred and forty-six retromolar areas with MTM of 244 patients were evaluated. RMCs were present in 11.5% of the patients. No statistically significant relationship between RMC and MTM impaction patterns was observed. A1-type RMC was found to be the most prevalent. The mean distance from RMF to the third molar was 6.09 ± 4.20 mm and was found to be higher in male patients. The mean distance from RMF to the second molar was 15.28 ± 4.73 mm. The average dimensions of the RMF were 1.4 ± 0.47 mm. Discussion and Conclusion: RMC is a relatively common anatomical variation of the mandibular canal. Although no correlation was observed between MTM impaction pattern and RMC existence, further research including more samples may be helpful to explain a possible correlation.

Keywords: Impaction, mandibular third molar, retromolar canal, retromolar foramen


How to cite this article:
Demire O, Akbulut A. Retromolar canals and mandibular third molar position: Is there a possible connection?. J Anat Soc India 2022;71:47-53

How to cite this URL:
Demire O, Akbulut A. Retromolar canals and mandibular third molar position: Is there a possible connection?. J Anat Soc India [serial online] 2022 [cited 2022 May 24];71:47-53. Available from: https://www.jasi.org.in/text.asp?2022/71/1/47/339878




  Introduction Top


The retromolar canal (RMC) can be defined as an anatomic variant that branches from the mandibular canal and exits at retromolar foramen (RMF) in the retromolar area.[1]

The canal contains a thinly myelinated nerve with tiny arteries and venules, and the nerve is associated with innervation of the retromolar pad and buccal gingiva of two molar teeth anterior to the foramen.[2] Therefore, clinically it is of importance to define this canal before any surgical procedures involving the posterior part of the mandible. They may be related to extensive hemorrhage during surgical procedures, postsurgical sensory deficiencies, development of traumatic neuroma, and anesthetic failure. Furthermore, in elderly people using prosthetic appliances, following resorption of the alveolar crest, discomfort may be seen due to pressure on the nerve.[3]

The prevalence of the RMC ranges between 8% and 75.4%.[3],[4] The wide range of observed frequency is associated with racial differences, investigation methods, and the criteria used to define the RMC existence.[5] A study using cone-beam computed tomography (CBCT) and panoramic radiography concluded with a prevalence of 24.6% when evaluated with CBCT and with a prevalence of 6.6% when the evaluation was made using panoramic radiography. Of the detected 18 RMCs on CBCT, only 4 were detected on panoramic radiographs.[6] The results of the study indicate the importance of assessment using three-dimensional methods to avoid limitations of two-dimensional techniques such as superimpositions and low diagnostic success.

Moreno Rabie et al. stated that the RMC may be a histological vestige of the gubernacular canal – the canal that contains the gubernacular cord which connects tooth in formation with gums and takes a role in tooth eruption. In their study to investigate the possible connection between two structures, they hypothesized that if there was a connection between the gubernacular canal and RMC, RMCs should have been more common in dental malpositions.[1]

Winter classification and Pell-Gregory classifications are widely used to categorize mandibular third molar (MTM) impaction patterns.[7] Winter classification uses the third molar angulation according to the long axis of the second molar. Third molars are classified as vertical (−10°–10°), mesioangular (11°–79°), distoangular (−11° to −79°), horizontal (80°–100°), and others (101 to −80°).[8],[9]

In Pell-Gregory classification, the definition of depth of third molars is made according to the relationship between occlusal levels of third molars and second molars: the third molar occlusal level is at the same level with second molar (A), the third molar occlusal level is between occlusal level and cervical level of the second molar (B), and the third molar occlusal level is below cervical level of second molar (C). Third molar-ramus relation is classified as: MTM is fully anterior to ramus (1), the third molar is partially inside ramus (2), and the third molar is fully inside ramus (3).[10]

In this study, the initial purpose is to define the relationship between RMCs and the impaction pattern of the MTMs using CBCT; also, it is aimed to evaluate the types of RMCs, RMF positions, dimensions, and their distance to second and third molars.


  Material and Methods Top


Ethical approval was obtained from İstanbul Medipol University Non-Interventional Clinical Research Ethics Committee (no. 10840098-604.01.01-E.65173).

Patient selection

CBCT scans of patients who had undergone CBCT examination for various purposes were included in the study. All the patients had unilateral or bilateral MTM teeth. Exclusion criteria were as follows: any radiographic sign of surgical procedures, trauma or pathologic defects in the retromolar area, and CBCT artifacts that may influence the diagnostic quality of the radiographic image. In the case of unilateral MTM existence, only the side with the third molar was evaluated.

Image acquisition

CBCT scans were taken using iCAT (Imaging Sciences International, Hatfield, PA, USA) with exposure parameters 80 kVp, 5–7 mA, and 14.7–17.8 s with a 0.25-mm voxel size. Vision (Imaging Sciences International, Hatfield, PA, USA) software is used for assessment. Multiplanar reconstructions were made for the evaluation of third molars and RMCs.

Assessment of retromolar canal existence

RMC existence was determined with the consensus of two dentomaxillofacial radiologists. RMC was defined as a radiolucent canal with radiopaque borders, leaving the bone at the retromolar area. RMCs were categorized according to the classification that Patil et al.[3] described:

  • Type A1: Canal branches from the mandibular canal, moves posterosuperiorly, and opens into the retromolar fossa
  • Type A2: Canal branches from the mandibular canal; continues anteriorly, turns posterosuperiorly, and opens into the retromolar fossa
  • Type B: Canal originates from the apical portion of the third molar with no radiographic evidence of a connection with the mandibular canal, runs posterosuperiorly, and opens into the retromolar fossa.
  • Type C: Canal branches from mandibular foramen, moves anteriorly, and opens into retromolar fossa [Figure 1] and [Figure 2]
Figure 1: Retromolar canal types: (a) Type A1, (b) Type A2, (c) Type B, (d) Type C

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Figure 2: Retromolar canals on cone-beam computed tomography images: (a) Type A1, (b) Type A2, (c) Type B, (d) Type C

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Evaluation of mandibular third molar position

MTMs were classified as mesioangular, distoangular, horizontal, vertical, and others according to their angular relationship with mandibular second molar [Figure 3].
Figure 3: Winter classification of mandibular third molars: (a) Vertical, (b) Mesioangular, (c) Distoangular, (d) Horizontal

Click here to view


Pell-Gregory (P-G) depth classification was used for the classification of the inferosuperior position of the third molar according to the cervical level of the mandibular second molar. Third molars were categorized as levels A, B, and C.

Pell-Gregory ramus classification which determines the mesiodistal orientation of MTM according to mandibular ramus was used. MTMs were classified as classes I, II, and III [Figure 4].
Figure 4: Pell-Gregory classification of mandibular third molars: (1a) Level A, (1b) Level B, (1c) Level C, (2a) Class I, (2b) Class II, (2c) Class III

Click here to view


Assessment of retromolar foramen

Buccolingual positions of RMFs were classified depending on their position in the retromolar area as buccal, buccolingual, and lingual.

On panoramic reconstructions, the distance from the RMF to the distal end of the second molar crown and the distal end of the third molar crown was measured. Mesiodistal dimensions of RMFs were measured on panoramic reconstructions.

Statistical analysis

For quantitative measures, analysis results were presented as average ± standard deviation, and for categorical data, frequency and percentages were presented. Statistical analysis of nonparametric data was performed with Chi-square test. Normal distribution of parametric data was evaluated with Kolmogorov–Smirnov test, and homogeneity of variance was evaluated with Levene's test. For the analysis of parametric data, two-group comparisons were performed with Student's t-test and z-test. One-way ANOVA test was used for comparisons between RMC types. Statistical significance was set at P < 0.05.


  Results Top


Demographics

CBCT scans of 244 patients and a total of 346 sides with MTMs were evaluated. While unilateral MTMs were evident in 142 patients, 102 patients had bilateral third molars.

One hundred and forty (57.4%) of the patients were female and 104 (42.6%) were male. The mean age of the evaluated patients was 30.94 ± 10.02 (28.93 ± 8.97 females and 33.65 ± 10.79 males). The average age of male patients was significantly higher than females (z = 3.85; P < 0.01).

Retromolar canal existence

RMCs were evident in 28 (11.5%) of the 244 patients and 29 (8.4%) of the evaluated 346 sides with MTMs. RMCs were seen in 17 (12.1%) female patients and 11 (10.6%) male patients. No statistical significance was observed between genders in terms of RMC existence (P = 0.94, P > 0.05). The average age of the 28 patients with RMCs was 34.61 ± 10.71, and the average age of the patients without RMCs was 30.47 ± 9.88. The average age of the patients with RMCs was significantly higher than patients without (z = 2.10; P = 0.035, P < 0.05).

Third molar position

According to Winter classification, 169 (48.8%) of the evaluated 346 MTMs were in mesioangular position, followed by 76 (22%) – horizontal position, 56 (16.2%) – vertical position, 35 (10.1%) – distoangular position, and 10 (2.9%) – other positions. The frequency of RMC existence in hemi-mandibles with third molars positioned horizontal, vertical, mesioangular, distoangular, and others was 13.2%, 8.9%, 7.1%, 5.7%, and 0%, respectively. No statistically significant relationship between the positions and RMC existence was observed (χ2 = 2.860; P = 0.413, P > 0.05).

Pell-Gregory depth frequencies were 11.9%, 39%, and 49.1% for A, B, and C levels, respectively. RMCs were observed in 6 (14.6%) of the hemi-mandibles with A level, 12 (8.9%) of the hemi-mandibles with B level, and 11 (6.5%) of the hemi-mandibles with C level MTMs. No statistically significant relationship was observed between levels of depth and RMC existence (χ2 = 2.94; P = 0.229, P > 0.05).

In the context of Pell-Gregory ramus classification of MTMs, Class III was most frequent (52%), followed by Class I (29.8%) and Class II (18.2%). While 6.1% of the hemi-mandibles with Class III third molar showed RMCs, 7.8% and 18.9% of the hemi-mandibles with Class I and Class II third molars showed RMCs, respectively. No statistically significant relationship was observed between the third molar-ramus relation and RMCs (χ2 = 5.86; P = 0.053, P > 0.05).

Retromolar canal types

[Table 1] demonstrates the frequency and distribution of RMC types and their relationship with the third molar position.
Table 1: Third molar positions and retromolar canal types

Click here to view


A1-type RMC was found to be the most frequent. Ten (34.5%) of the observed 29 RMCs was type A1, followed by 7 (24.1%) – type C and 6 (20.7%) – type A2 and 6 (20.7%) – type B.

Buccolingual position of retromolar foramen

Eighteen (62%) of the evaluated 29 RMFs were located on the lingual aspect of the retromolar area. Nine (31.1%) were positioned buccolingually and only 2 (6.9%) were on the buccal aspect.

[Table 2] demonstrates the location of RMFs and their relationship with RMC types.
Table 2: Retromolar foramen location and retromolar canal types

Click here to view


Mesiodistal position of retromolar foramen

The mean distance from RMF to the third molar was 6.09 ± 4.20 mm. This value was observed as 4.35 ± 2.08 mm for females and 8.54 ± 5.24 mm for males. The average distance was significantly higher in male patients compared to all patients' averages (z = 2.017; P = 0.021, P < 0.05).

The mean distance from RMF to the second molar was 15.28 ± 4.73 mm. Females showed an average distance of 13.49 ± 4.12 mm, and males showed an average distance of 17.82 ± 4.51 mm. The average distance was significantly higher in male patients compared to all patients' averages (z = 1.857; P = 0.031, P < 0.05).

[Table 3] demonstrates the relationship between RMC type and the distance between RMF and third molar and the distance between RMF and second molar.
Table 3: Retromolar canal type and distance from retromolar foramen to third molar and second molar

Click here to view


Retromolar foramen dimensions

The average mesiodistal dimension of RMF was 1.4 ± 0.47 mm. The average dimensions were 1.41 ± 0.44 mm for female patients and 1.37 ± 0.52 mm for male patients. No statistically significant difference between genders was observed (P = 0.824, P > 0.05).

The average mesiodistal dimension of RMF located on the buccal aspect was 1.35 ± 0.32 mm, located buccolingually was 1.66 ± 0.62 mm, and located on the lingual aspect was 1.27 ± 0.34 mm. Dimensions of RMF located buccolingually were significantly higher than the RMF located on the lingual aspect (t = 2.116; P = 0.044, P < 0.05).

Average RMF mesiodistal dimensions were 1.51 ± 0.53 mm, 1.62 ± 0.67 mm, 1.20 ± 0.24 mm, and 1.21 ± 0.23 mm for A1-, A2-, B-, and C-type RMCs, respectively. In terms of foramen dimensions, there was no statistically significant difference between canal types (P = 0.267, P > 0.05).


  Discussion Top


Variations of the mandibular canal may be seen at various levels along the course of the canal.[11] The RMC is an anatomic variation that arises from the mandibular canal and opens to the retromolar area with a foramen called “retromolar foramen.”[12],[13]

Histological studies showed that the canal contains myelinated nerves, small arteries, and venules; therefore, contents of the canal are of clinical importance which may be associated with complications such as extensive bleeding during surgical procedures to the posterior mandible, insufficient anesthesia, and postsurgical discomfort.[14],[15],[16]

It has been stated that there may be a link between the RMC and gubernaculum chord, and if such a connection exists, RMCs should have been seen more commonly with dental malpositions.[1]

The primary goal of the present study was to define the relationship between MTM positions and RMC existence. Prevalence and characteristics of the RMC and the RMF were also investigated.

Previous reports about the prevalence of RMC and foramen range between 8% and 75.4%.[3],[4] Two major methods were used in previous studies: retrospective CBCT analysis and macro-anatomical assessment using dry mandibles. In CBCT studies, although the vast majority of reported prevalence of the RMCs were around 25%, percentages vary between 8.5% and 75.4%.[1],[3],[6],[11],[12],[13],[14],[16],[17] Lizio et al.'s CBCT assessment excluded RMCs with a RMF below 1 mm, and the observed prevalence was 14.6%.[16] Similarly, in Han and Hwang's research, 0.5 mm was the lower limit for the RMF diameter for the RMC inclusion criteria, and a prevalence of 8.5% was observed.[17] RMF was investigated in macro-anatomical studies, and the frequency is reported between 8% and 71.4%.[1],[2],[4],[5],[14],[18] Inclusion criteria were performed in one study, which reported the lowest prevalence. The insertion of a nonbeveled needle with 1.0 mm in diameter to the foramen was defined for inclusion criteria.[4] In a study conducted by Capote et al., 500 panoramic radiographs were evaluated and a frequency of 8.8% is reported.[15] Kim et al.[5] and Motamedi et al.[2] in their studies reported 27.3% and 40.4% prevalence, respectively. Both studies used histologic investigations to define the characteristics of RMF and canal.

In this study, a prevalence of 11.5% is observed. Differences between observed values among CBCT studies may be the consequence of varying patient selection criteria, exposure parameters, and associated radiographic artifacts caused by patient movement or metallic objects.

This study group consisted of patients who had at least one MTM, and the presence of a dense object in the imaging field may obstruct the visibility of anatomic structures below 1 mm as a result of stripe-like artifacts.[19] Similarly, Patil et al.'s study included patients who had undergone CBCT examination for MTM impaction, and in the study, a prevalence of 75.4% is reported. The difference between results can be explained with imaging parameters they used, such as a smaller field of view and smaller voxel size which allows visualization of smaller subjects.[3] Another factor that may influence the visibility of these anatomic structures may be the metal and the motion artifacts. A recent study, performed by Rabie et al., investigated 89 dry mandibles with third molars. They defined retromolar foramina in 73 of the mandibles in macroscopic evaluation; also, in CBCT assessment, RMCs were evident in 64 of the mandibles.[1] At this point, motion artifacts may be responsible for differing results compared to our study. Motion artifacts are reported as high as 48.2% of the CBCT scans.[20] Therefore, scanning a dry mandible, rather than scanning a living person, may result in fewer motion artifacts. We excluded CBCT scans that have artifacts decreasing the overall image quality; however, as such an anatomic structure with submillimeter dimensions is investigated, our exclusion criteria may be inadequate. RMF dimensions are also investigated in our study, and the smallest foramen was around 0.9 mm. RMCs and RMFs below this value may be unseen due to the abovementioned reasons. As we consider the studies, which used dimensional exclusion criteria, consistency with our results can be seen.[16],[17]

Patient age was not investigated in the vast majority of previous reports, except Patil et al.'s, which reported no significant relationship between age groups.[3] In this study, the average age of the patients with RMCs was found to be significantly higher than the patients without RMCs.

Various classifications are made for RMCs according to their course inside the posterior mandible.[3],[5],[12],[13],[14],[17] We used the classification that Patil et al. described, which adds one more type of RMC. Type B RMC courses between the RMF and apical portion of the third molar, and has no connection with the mandibular canal on CBCT images. They found this type of canal in the majority of their study group, followed by type A1 and type A2. Type C canal was observed in only one subject.[3] In another study, type B RMC, corresponding to type A2 in our study, was found to be the most frequent canal type.[14] In our study, of the observed 29 canals, 10 were type A1, 7 were type C, 6 were type A2, and 6 were type B.

Despite the significant number of studies focused on the prevalence and properties of RMCs and foramina, only one study investigated their relationship with third molars. Moreno Rabie et al. hypothesized that, if RMCs are the histological vestiges of the gubernacular canal, which is a structure involved in tooth eruption, they should be more common with dental malpositions. They investigated 112 hemi-mandibles with third molars, and 58 of the hemi-mandibles showed RMCs. They found no correlation between the third molar position and the presence of RMCs.[1]

In this study, a more comprehensive categorization of third molars is made according to Winter and Pell-Gregory classifications. We investigated the angular position, depth, and ramus relation of MTMs and evaluated the connection between these positions and RMC presence. No statistically significant relationship between the pattern of impaction and RMC presence was observed. However, although no statistical assessment was made because of the small sample group, all type B RMCs, which course between the RMF and apical portion of the third molar, were evident in hemi-mandibles with level B and C third molars. No type B RMCs were seen in hemi-mandibles with level A third molars. A similar situation can be mentioned for type A1 RMCs. Although our results cannot indicate a relationship between RMC presence and third molar impaction pattern, the link between type B RMCs and impaction depth of the third molar should be considered. As described before, these types of canals do not have a radiographic connection with the mandibular canal and they course between the apical portion of the third molar and the RMF. Therefore, rather than being an anatomical variant of the mandibular canal, this type of canal may be a histological vestige of the gubernacular canal. Further research, including more samples, should be performed to clarify the possible relationship.

The buccolingual position of the RMF is of importance to avoid complications. In one study, it was found that more than half of the retromolar foramina were located in the middle of the retromolar triangle, followed by the lingual aspect and buccal aspect.[5] Park et al. stated that retromolar foramina were more common on the buccal side than the lingual.[14] In our study, the majority of the foramina were located on the lingual side, followed by buccolingual foramina which were located in the middle of the retromolar fossa. Only two foramina were observed on the buccal aspect, and both were the foramina of type C canals.

The distance to RMF from the distal end of the third molar and second molar was investigated in previous reports. Patil et al. reported average distances for type A, type B, and type C canal foramina from the distal end of the third molar as 7.0 mm, 7.1 mm, and 14.3 mm, respectively.[3] Park et al. defined the average distance from RMF to the second molar as 12.1 mm, and to the third molar as 5.8 mm.[14] These values were 16.8 mm and 10.5 mm for second and third molars, respectively, in Gamieldien et al.'s study.[4] Another study reported the average distance from the second molar to foramen as 14.08 mm with no significant difference between genders.[17] Von Arx et al. stated that the distance from the distal end of the second molar to RMF is higher in younger age patients.[13] In only one research, a statistically significant difference between genders was defined for the distance between the third molar and RMF.[12] For the second molar-RMF distance and third molar-RMF distance, we observed a statistically significant difference between genders. These measurements were higher in male patients. Another important finding of these measurements was that, when the canal types are considered, the distance from foramina of C type canals to the third molar was significantly higher, compared to other types. Although no statistical significance was observed, foramina of B-type canals had the lowest average distance from third molars (3.28 ± 0.35 mm). This finding should be considered for our hypothesis that type B canals might be associated with the gubernacular canal and should be evaluated in further research.

RMF dimensions were assessed in previous reports. Han and Hwang measured the width of the RMC on sagittal sections, 3 mm below RMF. They found the mean width of RMC 1.13 mm.[17] Kikuta et al., in their research performed on 50 CBCT scans, found the average diameter of RMF as 1.1 mm.[12] The average mesiodistal width of RMCs was found to be 1.05 mm in another research.[6] Von Arx et al. measured RMC 3 mm below RMF, and the mean width of the RMC was 0.99 mm.[13] Motamedi et al. in their cadaveric study defined the average width of RMF as 1.7 mm.[2] In this study, the mean mesiodistal dimension of foramen was 1.4 mm and it was found that the mean dimensions of foramina located on buccolingual position are significantly higher than the foramina located on the lingual position. No statistically significant relationship was observed between canal types in terms of average canal dimensions. However, despite the lack of statistical significance, B-type canals showed the lowest average dimensions (1.20 ± 0.24 mm). Solely based on this finding, it is impossible to proclaim a connection between B-type canals and gubernacular canals.

The limitation of this study is the inadequacy of collected RMC samples, and this may be related to the exclusion criteria we used. This research was a retrospective CBCT analysis, and only the images which were considered to be “diagnostically insufficient” were excluded. CBCT scans were not performed to determine the RMC existence. RMCs, as discussed before, may have submillimeter dimensions, and the artifacts on CBCT images, seen as a result of various reasons, may obstruct their visibility.


  Conclusion Top


In this study, the prevalence of the RMC was 11.5%. No statistically significant relationship between RMC existence and the MTM position was observed. A1-type RMCs, which have a vertical course from the mandibular canal, were the most frequent, followed by C-type RMC, which branches from mandibular foramen. Although no statistically significant finding suggests a relationship between MTM position and RMC existence, type B RMCs were only observed with MTMs below the occlusal level of second molars. There may be a possible relationship between type B RMCs and the position of MTMs. Furthermore, the radiographic properties of type B RMCs support a possible connection between the gubernacular canal and type B RMCs. Further radiographic and anatomic studies will be helpful to determine the relationship.

Ethics statement

Ethical approval was taken from İstanbul Medipol Non-Interventional Clinical Research Ethics Committee (no. 10840098-604.01.01-E.65173).

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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Abstract
Introduction
Material and Methods
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