|Year : 2013 | Volume
| Issue : 1 | Page : 21-25
Detection of anterior loop and other patterns of entry of mental nerve into the mental foramen: A radiographic study in panoramic images
Asha Raghunandan Iyengar1, Seema Patil1, Kikkeri Seetharamiaha Nagesh1, Sushma Mehkri1, Aastha Manchanda2
1 Department of Oral Medicine and Radiology, D.A. Pandu Memorial R.V. Dental College, Bangalore, India
2 Department of Oral Medicine and Radiology, Inderprastha Dental College, Sahibabad, Ghaziabad, India
|Date of Web Publication||10-May-2013|
G-40, Pushkar Enclave, Paschim Vihar, New Delhi - 110 063
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Purpose: The pattern of entry of mental nerve into the mental foramen after its emergence from the inferior alveolar nerve bundle is an important presurgical landmark in the mandibular premolar region. Various patterns of entry of mental nerve have been identified. As panoramic radiographs are routinely used in presurgical evaluation, the present study was undertaken to evaluate the various entry patterns of mental nerve into the mental foramen.
Materials and Methods: Total 300 panoramic radiographs taken for routine diagnostic purposes were selected for the study. The radiographs were assessed independently by two observers and the position and the entry pattern of the mental nerve on the left and right side were recorded. The entry patterns recorded were categorized as straight, looping, or perpendicular.
Results: The study revealed that the most common pattern of entry of mental nerve was a straight one accounting for a total of 79% on at least one side of the total radiographs examined, followed by the presence of an anterior loop on at least one side in 21% and perpendicular pattern on at least one side in 6% of the total radiographs examined.
Conclusion: Panoramic radiography may not be a very reliable imaging modality for identifying the presence of anterior loop which needs to be determined for preoperative planning of surgical procedures in the mandibular premolar region. Other modalities, such as cone beam CT, can be used for better visualization of the area.
Keywords: Anterior loop, mental nerve, panoramic radiographs, patterns of emergence
|How to cite this article:|
Iyengar AR, Patil S, Nagesh KS, Mehkri S, Manchanda A. Detection of anterior loop and other patterns of entry of mental nerve into the mental foramen: A radiographic study in panoramic images. J Dent Implant 2013;3:21-5
|How to cite this URL:|
Iyengar AR, Patil S, Nagesh KS, Mehkri S, Manchanda A. Detection of anterior loop and other patterns of entry of mental nerve into the mental foramen: A radiographic study in panoramic images. J Dent Implant [serial online] 2013 [cited 2019 Jul 20];3:21-5. Available from: http://www.jdionline.org/text.asp?2013/3/1/21/111678
| Introduction|| |
The inferior alveolar nerve, a branch of mandibular nerve enters the mandibular ramus at the lingual aspect and runs down in the mandibular canal, supplying the mandibular teeth and the associated soft tissue structures. The mental nerve, one of the terminal branches of the inferior alveolar nerve, emerges through the mental foramen to supply the skin and mucous membrane of the buccal vestibule of the lower jaw from the medial border of the masseter muscle to the midline. Though a few studies have addressed the actual pattern of entry of mental neurovascular bundle into the mental foramen, the emphasis of most studies has been on determining the position of the mental foramen. 
Some investigators have described an anterior loop to the nerve before its emergence from the mental foramen [Figure 1]. When inferior alveolar nerve arises from the mandibular canal and runs outward, upward, and backward to open at the mental foramen (MF) it was referred as anterior loop.  A more precise description was reported by Bavitz and Misch who described anterior loop as the structure "where the mental neurovascular bundle crosses inferior and anterior to the mental foramen (MF) then doubles or loops back to exit the mental foramen (MF)".  Kuzmanovic with co-authors have also described the anterior loop of mental nerve.  This anterior looping cannot be appreciated clinically but can be visualized in about 11-60% of panoramic radiographs.  Not much has been described about the other paths of mental nerve before it exits through the mental foramen. Morphometric studies have either broadly divided the pattern into loop type and nonloop types,  or have described various patterns such as a posteriorly directed, an anteriorly directed [Figure 2], and a right angled pattern of emergence [Figure 3]. 
|Figure 2: Schematic representation of straight pattern (anteriorly directed)|
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|Figure 3: Schematic representation of perpendicular pattern (right angle)|
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One of the most difficult but usually inadvertent complications which has been found to occur during any surgical procedure, most importantly implant placement, in mandibular premolar region is neurosensory alterations in the chin and lower lip. This complication can occur if important vital structures such as mental foramen and anterior mental loop are not properly identified and protected.  Consequently, to avoid damage to these vital structures, the exact localization of the anatomical structures should be identified prior to surgery using appropriate radiographic techniques.
Though the entry pattern of mental nerve into the mental foramen has been described in a number of morphometric studies, only a few radiographic studies have been performed in this respect. As panoramic radiographs are routinely used to locate anatomic landmarks for planning these surgical procedures, this study has been undertaken to determine if a panoramic radiograph is sufficient to identify the presence of anterior loop and other patterns of entry of mental nerve or is there a need for using advanced imaging techniques for the same.
| Materials and Methods|| |
Source of data
Total 300 panoramic radiographs obtained from records maintained in the Department of Oral Medicine and Radiology were randomly selected for the study.
Only high-quality radiographs with respect to geometric accuracy and contrast of image were selected for the study.
The exclusion criteria which were taken into account were as follows. Radiographs showing radiolucent or radiopaque lesions in the mental foramen region of mandible and evidence of fracture around the mental foramen and mandibular canal region, radiographs showing supernumerary or unerupted teeth in the mental foramen region and radiographs showing processing or exposure errors and artifacts obscuring visibility of structures in the mandible were excluded from the study.
The radiographs were placed on a well-illuminated view box and viewed with a magnifying lens. The radiographs were evaluated independently by two observers for the pattern of entry of mental nerve into the mental foramen on either sides of the mandible.
Patterns observed in the study other than the presence of an anterior loop were a straight and a perpendicular path of entry of mental nerve into the mental foramen [Figure 4], [Figure 5] and [Figure 6]. The observations thus made were recorded as AL, S, or P and the total numbers of patterns observed on both sides were recorded.
|Figure 5: Radiograph showing straight pattern of entry of mental nerve into the mental foramen|
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|Figure 6: Radiograph showing perpendicular pattern of entry of mental nerve into the mental foramen|
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Interobserver variability was assessed using kappa test. Tabulations for the presence of each pattern were done for both sides and the percentages calculated.
| Results|| |
In the present study, the most commonly observed pattern of entry of mental nerve was the straight pattern accounting for 79% on at least one side of the 300 radiographs examined, followed by the presence of an anterior loop in 21% and a perpendicular pattern in 6% on at least one side of the total radiographs examined [Table 1].
|Table 1: Frequency of the three observed patterns on both sides of the panoramic radiographs|
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The agreement between the recordings made by observers one and two on either sides with respect to the presence of anterior loop, straight, and perpendicular patterns was found to be very strong using Kappa test, k > 0.8.
| Discussion|| |
The intraosseous course of mental nerve in the mental canal after its emergence from the inferior alveolar nerve bundle has for long been a subject of academic interest but varied opinions. A few studies have addressed the transitional part of the inferior alveolar nerve between the mandibular canal and the mental foramen in cadaveric mandibles. ,
Recently, interest in the emergence and location of the mental nerve has been rekindled by the need for accurate presurgical planning, most importantly for placement of endosseous implants in the mandibular premolar region.
Though the various patterns of the nerve beyond the inferior alveolar canal and before its exit through the mental foramen have been recognized in cadaveric studies, the emphasis of most studies has been on determining the presence and extent of the anterior loop of mental nerve.
The presence of an anterior loop of the mental nerve in cadaveric mandibles has been supported by most investigators [Table 2]. ,
|Table 2: Studies for detection of anterior loop and other patterns of entry of mental nerve into mental foramen|
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Solar, et al., based on the presence of this loop, divided their cases simply into loop and nonloop types.  Kieser, et al., studied the actual path of emergence of mental nerve and divided it into posterior, anterior, right angled, or multiple. The path of emergence of the mental nerve in a number of human population groups was also investigated in the skulls wherein different population types should different patterns.  In another study conducted on cadaveric mandibles by Hu, et al., to determine the intraosseous course of mental nerve, the transitional part between inferior alveolar nerve and mental nerve was classified based on its exit morphology into the loop, straight, and vertical patterns. The straight pattern was observed as a slight curve of mental canal opening directly into the mental foramen. The vertical pattern was seen when the mental canal bent perpendicularly into the foramen [Table 2]. 
Though these patterns have been described in cadaveric studies, no radiographic studies have been done so far to determine if such different patterns are visible as such on the radiographs as well. Based on these cadaveric findings, this study was undertaken to determine these three patterns on panoramic radiographs which are routinely used for the purpose of any preoperative surgical planning. The results of the present radiographic study were much in contrary to the above-mentioned cadaveric study, since the percentages of anterior loop observed were much less and that of the straight pattern was much higher than the cadaveric study. The percentage of perpendicular or right angle or vertical pattern observed was also lesser than that observed in the cadaveric study.
Although the various patterns of entry of mental nerve into the mental foramen have not been addressed in radiographic studies, the presence of anterior loop has been a subject of interest in studies comparing cadaveric and radiographic findings. Anterior mental loop assessment using radiographic methods and comparison with morphometric observations has revealed large variations [Table 2]. ,,
Direct radiographic visualization of anterior loops in previous studies has suggested that panoramic radiographs are unreliable in identifying the loop [Table 2]. ,,, In accordance with these studies, the present study also showed a visible anterior loop on at least one side in only 21% of the total radiographs examined.
The mental foramen region assessment can be unreliable based on the panoramic imaging only due to the fact that the tomographic layer may not capture the entire area appropriately. Also, the panoramic technique does not render itself to high level of repeatability due to the fact that the patient positioning can be unreliable. The variability in the radiographic assessment of anterior loop may be explained by accounting for the different criteria used to define the anterior loop, dissimilar diagnostic techniques, and equipment and poor radiographic quality. The most probable explanation seems to be the inability to distinguish it from normal trabecular pattern.
Since, panoramic radiographs give only a 2-dimensional view of the area examined, there are advantages of using CT images in identifying the mandibular canal and anterior loop of mental nerve,  although contradictory results have also been found.  Studies conducted to determine the visibility of anterior loop on 3-dimensional conebeam CT have revealed considerably better results [Table 2]. 
An important point noted in the present study was that in the radiographs where an anterior loop was seen, the mental foramen was located closer to the alveolar crest as compared to the radiographs where no loop was observed. Thus, based on this finding, it may be assumed that in cases where the position of mental foramen is higher up in the vertical plane, the mental nerve has to loop back to enter the foramen. Further studies involving cadaveric and radiographic comparisons of this observation may be required to prove the same.
Lastly, the anatomy of mandibular premolar region has clinical significance in pretreatment planning of surgical procedures due to the presence of the anterior loop. Damage to this nerve bundle may cause neurosensory alterations in the chin and lower lip. Of particular interest has been the placement of endoosseous implants in the anterior interforaminal region. To maximize the distance between implants, the most posterior implant should be placed as close as possible to the mental foramen. The greater the distance between the interforaminal implants, the better the anterior implants can counteract the forces generated on the distal cantilevers of the fixed prosthesis. According to Bavitz, et al., an implant in the mental region is best positioned, so that its distal aspect is 1mm anterior to anterior border of mental foramen. However, some authors have recommended a minimum distance of 6 mm between the mental foramen and the most posterior implant. The reason behind this large safety margin is to prevent damage to the anterior loop of mental nerve that has been documented to extend anteriorly upto 5 mm. 
Thus, it can be concluded that for preoperative planning purposes in mandibular premolar region, the panoramic images available need to be carefully inspected to enable visualization of the anatomic variations in the pattern of entry of mental nerve from inferior alveolar nerve bundle and its intraosseous course before the mental foramen until it exits through the foramen. However, panoramic radiography may not be a sufficient imaging modality for preoperative planning of any surgical procedures in the mandibular premolar region and may be needed to be supplemented with other modalities such as a cone beam CT for better visualization of the area. More studies taking into account, various ethnic groups may be required to actually determine how predictive the conventional as well as advanced imaging modalities prove useful in assessment of exact anatomy of mandibular premolar region, which holds immense importance for planning any surgical procedures in this region.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]
[Table 1], [Table 2]