Abstract
Purpose:
The study aims to evaluate the location of mandibular foramen (MF) with respect to the occlusal plane (OP) and its location on the ramus using Anatolian dry mandibles.
Materials and methods:
A total of 115 dry mandibles with mandibular molars were analyzed. The distance between the MF and the OP was examined with a flat metal plate. Group A was above the OP; group L was at level, and group B was below the OP. The distances between the MF and anterior border (A-MF), sigmoid notch (U-MF), posterior border (P-MF), and lower border (L-MF) were measured. The symmetry between the two sides was examined. Pearson chi-square and Student's t-test were performed for statistical analysis.
Results:
According to the analysis, 50.23% of MF was located below the OP (p<0.05). The mean distances of Groups A and B were 3.45 and 4.78 mm, respectively. There was no difference between the left and right in groups (p>0.05). The distance A-MF was 14.71 mm. There was no statistical difference between the distances A-MF and P-MF or U-MF and L-MF.
Conclusion:
Half of the MF (50.23%) was located below the occlusal plane with a mean distance of 4.78 mm. It may be helpful to place the needle 3-4 mm above the OP and 1.5-2 mm back of the anterior border to obtain a successful inferior alveolar nerve block. The MF was located at the center of the medial surface of the ramus.
Keywords: Anatomy, mandible, mandibular foramen, mandibular nerve, occlusal plane
Introduction
Modern dentistry denies painful practices. However, it is not possible to deny the failure rate of the inferior alveolar nerve block (IANB), which has been reported to be between 13% and 57% (1, 2). More dramatically, 90% of the clinicians reported difficulties with obtaining proper anesthesia in dental practice (1). IANB, which is used in the treatment of mandibular posterior teeth, is one of the local anesthesia techniques that require the most diligent and target-oriented application in dental practice. IANB, in other words, the Halsted technique, or ‘mandibular nerve block’, which is a misnomer according to Malamed (3), is based on the deposition of an anesthetic solution in the pterygomandibular space, which includes the inferior alveolar nerve and foramen mandible. The inferior alveolar nerve enters the mandibular canal through the mandibular foramen and provides innervation to the mandibular molars and premolars along its course (1,3). Various reasons lead to the failure of the anesthetic technique including anatomic variations, uncommon physiology, and the presence of infection; however, the prominent reason for the failure is the inability to determine the mandibular foramen location (4). Since the needle penetration depth or location is adjusted according to the surrounding tissues or landmarks, it is substantial to have a fixed reference point that can be observed directly in the mouth while administering anesthesia, instead of hard-toobserve anatomical formations to ensure the proximity of the anesthetic solution to the nerve (5, 6).
For IANB, another important point in addition to the failure of anesthesia is the possible complications including hemorrhage, soft or hard tissue necrosis, or nerve damage, that may occur during the application (6). Therefore, meticulous knowledge about the location of the MF and the topography of the ramus is required. The morphological dimensions of the mandible and the topography of the mandibular foramen relative to the occlusal plane (OP) were evaluated using dry mandible (4,7, 8, 9, 10) or radiography (11, 12, 13, 14, 15) in different populations. Since different methodologies of radiographic measurement may change in determining the location of MF (13,15), dry mandibles provide substantial data as a direct observation method despite having a limited sample size. Altunsoy et al. (12) reported the relative location of MF in children using cone-beam computed tomography (CBCT) in the Turkish population. Oğuz and Bozkır (16) analyzed the morphology of the ramus using Turkish dry mandibles but provided no information about the OP and the relative location of the MF. When considering the effects of racial and methodological factors, more studies are needed to obtain well-established data. The objective of this study is to evaluate the relative location of MF to the OP and to analyze the topographic placement of the MF on the ramus using Anatolian dry mandibles. The null hypothesis of the study was that the MF would not be above the OP.
Materials and methods
Study design
The study was conducted in the anatomy department laboratory of Akdeniz University Faculty of Medicine and Süleyman Demirel University Faculty of Medicine with the obtained permissions. A total of 298 dry mandibles were scanned. Age and gender were unknown. Inclusion criteria were mandibles with intact ramus and with mandibular molars for analyzing the relative location to the OP. Exclusion criteria were ragged or deformed mandibles excessively enough to affect the measurement, mandibular molars in supraocclusion or inclined mandibular molars, and mandibular molars with advanced crown destruction. The supraocclusion was regarded if the distance between the cementoenamel junction and the alveolar bone was more than 1 mm. For the study, 115 dentate dry adult human mandibles were selected.
Cadaveric measurements
We measured the distance between the MF and the most concave point of the anterior border (coronoid notch) (A-MF), posterior border of the ramus (P-MF), sigmoid (mandibular) notch (U-MF), and lower border of the ramus (L-MF), at the left and right sides (Figure 1). The center of the mandibular foramen was taken as a fixed reference point. While determining the occlusal plane, two buccal cups of the second mandibular molar were used according to the basic geometry logic of “a line passes through two points.” A rigid flat metal plate of 0.5 mm in thickness was placed on the second molar and stabilized. In the inner surface of the ramus, the point on the line referred by the metal plate and in the MF alignment was marked with a fine-tipped erasable pen, and the distance between the MF and the mark was measured with a digital caliper.
Figure 1.
Morphometric measurements of the ramus (MF: Mandibular foramen. A-MF: the distance between the MF and the most concave point of the anterior border. P-MF: the distance between the MF and posterior border of the ramus. U-MF: the distance between the MF and sigmoid notch. L-MF: the distance between the MF and lower border of the ramus).
All measurements were performed independently by two observers (DY, a 6-year experienced endodontist, and AMN, a 10-year experienced periodontist) twice to obtain inter-class and intra-class reliability. Prior to measurements, for the calibration of observers, the measurements of 21 dentate mandibles (10% of the total sample size) were performed, and Cohen’s kappa values were obtained (ranging from 0.88 to 0.93). The relative location of the MF to the occlusal plane and morphometric measurements was recorded.
Study groups
The relative location of the MF to the OP was allocated to three groups; Group A: MF was above the OP, Group L: MF was at the level of the OP, and Group B: MF was below the OP (Figure 2). Each measurement was performed three times, and the average value was recorded for statistical analysis. All measurements were recorded in millimeters. A digital caliper (SC-6, Mitutoyo Corporation, Tokyo, Japan) was used for the measurements.
Figure 2.
Representative image of the measurement of the location of the mandibular foramen. A: Group L; at level, B: Group B; below the occlusal plane.
Statistical analysis
(IBM SPSS Inc, Armonk, NY, USA). The normality distribution of the data was analyzed using Kolmogorov-Smirnov. Levene’s test was performed to determine the homogeneity of the data. Pearson chi-square test was applied for the symmetry of the relative location of the MF to the OP. Morphometric measurements were analyzed with Student’s t-test. Interclass correlation coefficient (ICC) was used for the reliability of the observers. Statistical difference was considered significant with a 95% confidence interval at p<0.01 for the ICC, and p<0.05 for other tests.
Results
There was no statistical difference in the intra-observer and inter-observer measurement values. For the morphometric measurements of the MF, ICC values were 0.845 and ICC=0.913. The mean and standard deviations of upper, lower, anterior, and posterior measurements and symmetry of the MF location are presented in Table 1. The distance between the MF and anterior border of the ramus was 14.71 mm. The distance between the MF and posterior border was 12.67 mm. The distance of the MF to the sigmoid notch was 22.61 mm, and the distance to the basis of the ramus was 23.04 mm. According to Student’s t-test, no significance was detected between the distances A-MF and P-MF, and similarly, in the distances U-MF and L-MF. Hence, the MF was located approximately at the center of the ramus. There was also no significant difference between the left and right sides in terms of morphometric measurements. The morphometric distances to the OP, and percentages of Group A, L, and B were shown in Table 2. The mean distance between the MF and OP in the group A was 3.45 mm, while group B was 4.78 mm (Figure 2). The MF was located in an area ranging from 11.04 mm superior to 9.89 mm inferior to the occlusal plane. The frequency of groups A, L, and B were 38.43%, 11.34%, and 50.23%, respectively. The most observed (50.23%) location was Group B, which means it was below the OP (p<0.05). No statistical difference was detected in the symmetry of the level of the MF according to the OP between the left and right sides.
Table 1.
Morphometric measurements (in mm) and symmetry of the location of the mental foramen. No statistical difference was observed between morphometric data or two sides according to the Student’s t-test (p less than 0.05).
| Total | Symmetry | |||
|---|---|---|---|---|
| Right | Left | P value | ||
| Mean (±Std) | Mean (±Std) | Mean (±Std) | ||
| Anterior | 14.71 (±2.24) | 15.77 (±2.51) | 13.65 (±1.69) | 0.074 |
| Posterior | 12.67 (±2.18) | 12.19 (±2.60) | 13.15 (±1.98) | 0.067 |
| P value | 0.19 | 0.07 | 0.34 | |
| Upper | 22.61 (±4.45) | 21.92 (±4.90) | 23.03 (±3.87) | 0.14 |
| Lower | 23.04 (±3.23) | 23.41 (±3.83) | 22.67 (±2.97) | 0.063 |
| P value | 0.24 | 0.16 | 0.21 |
Table 2.
Relative location of the mental foramen to the occlusal plane (Group A: above, Group L: at level, Group B: below the occlusal plane). Bold p values mean statistical difference according to the Pearson chi-square test (p less than 0.05). Minus indicates below the OP and indicates above the OP.
| Total | Right | Left | P value | ||||
|---|---|---|---|---|---|---|---|
| Mean (±Std) | % | Mean (±Std) | % | Mean (±Std) | % | ||
| Group A | +3.45 (±1.29) | 38.43% | +3.12 (±1.27) | 41.01% | +3.78 (±1.35) | 35.85% | 0.16 |
| Group L | N/A | 11.34% | N/A | 12.17% | N/A | 10.51% | 0.27 |
| Group B | -4.78 (±1.28) | 50.23% | -4.56 (±1.34) | 49.25% | -5.01(±1.13) | 51.21% | 0.08 |
| P value | 0.001 | 0.002 | 0.001 |
Discussion
Within the limited sample size of the study, the most important result of the study is that 50.23% of the MF was located below the OP. Endodontic treatment, extraction, and most dental treatments require block anesthesia rather than local infiltration that does not supply proper anesthesia due to the thick compact bone of the mandible (15). The clinician has blind access to the MF as it is covered by soft tissue, but the solution must be injected as close to the MF as possible to rule out the possible complications including hemorrhage, and persistent injury to the nerve, and for successful anesthesia (13). Therefore, for proper anesthesia, a clinical reference point is required. Previous studies indicated that MF was positioned below the OP in the range of 80.4% to 19.87% in African, Thai, Zimbabwean, Brazilian, and East Indian populations as described in Table 3 (5, 7, 9, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26). Different methodologies or ethnicity lead to this wide range of results. One of the methodological differences was the measurement point of the MF. The middle of the superior line of the MF (14), the lingula (7,15), or the center of the MF (4,8) was assumed when analyzing the relative location. In the present study, the center of the MF was accepted as the measurement point. Different reference points in different studies make it difficult to compare in millimeters, and the distance of the area where anesthesia will be applied relative to the occlusal plane should be interpreted carefully. Besides the reference point that is covered with soft tissue cannot be directly observed clinically. Another reason for the differences in MF location may be due to various definitions of the occlusal plane in the studies. The first molar (11, 20, 21, 22), second molar (7,13), or mandibular molars (17-19), the line passing through the canine and molars (23), or the adjusted occlusal plane (14) were considered for the measurements. According to Russ (20), the first molar is the best reference point for IANB. However, the first molar is the first permanent tooth to erupt, it is the most frequently decayed or lost. In the present study, we considered mandibular molars as a reference line using a metal flat line. Taking a single mandibular molar as a reference will affect the measurement, reducing the accuracy of the investigation when considered that the tooth has occlusal wear, restoration, or decay, or that it was in supraocclusion position or pathologic migration. Taking the occlusal plane as a reference by including molars instead of a singlemolar provides a more precise definition of the occlusal plane.
Table 3.
Location of mandibular foremen in previous studies (CBCT: cone-beam computed tomography).
| Study | Ethnicity | Methodology | Sample size | References | Location |
|---|---|---|---|---|---|
| Afsar | Canadian | Panoramic | 79 | First molar | 1.9 mm above |
| Hwang | Chinese | Cephalometric | 112 | Undetermined | 4.16 mm above |
| Feurerstrein | French | CBCT | 260 | Mandibular teeth | 2-3 mm above |
| Altunsoy | Turkish | CBCT | 20 | Mandibular molars | 2.5-3.6 mm above |
| Blatcher | American | CBCT | 203 | Second molar | 9 mm above |
| Al-Shayyab | Jordanian | CBCT | 224 | First molar | 2.5 mm above |
| Jang | CBCT | 125 | Second molar | 8.85 mm above | |
| Bunyarit | Malay | CBCT | 87 | Undetermined | 10 mm above |
| Kang | Korean | CT | 59 | First molar | 3.8 mm above |
| 12.4% above (2.5mm) | |||||
| Zhou | Korean | CBCT | 106 | Mandibular molars | 3.3% at level |
| 84% below (4.5mm) | |||||
| Russa | Tanzania | Cadaveric mandible | 44 | First molar | 10 mm above |
| Monnazzi | Brazilian | Dry mandible | 44 | Mandibular crest | 0.02 below |
| Thangevalu | Indian | Dry mandible | 102 | Mandibular molars | Few mm above |
| Kositbowornchai | Thai | Dry mandible | 23 | First molar | 10 mm above |
| 2.5% above | |||||
| Nicholson | East Indian | Dry mandible | 80 | Second molar | 22.5% at level |
| 75% below | |||||
| 15.1% above | |||||
| Palma | Brazilian | Dry mandible | 82 | Mandibular molars | 0.8% at level |
| 84.1% below | |||||
| Jansisyonont | Thai | Dry mandible | 146 | Mandibular molars | 80.13% above (4.5 mm) |
| 19.87% below (3.1 mm) | |||||
| Mandibular molars | 29.4% above | ||||
| Mbarjorgu | Zimbabwean | Dry mandible | 38 | 47.1% at level | |
| 23.5% below | |||||
| Mandibular molars | 4.7% above | ||||
| Mwaniki | African | Dry mandible | 79 | 30.7% at level | |
| 64.6% below | |||||
| 38.43% above (4.5 mm) | |||||
| Present study | Anatolian | Dry mandible | 105 | Mandibular molars | 11.34% at level |
| 50.23% below (3.1 mm) |
A previous study performed on dry mandibles analyzed the level of the MF according to the coronoid notch (10). However, the coronoid notch is covered by the moveable mucosa and may not present a stable point like molar tubercles. Thus, since it is a difficult point to estimate, its precision in clinical practice is reduced when considering the quantitative data. The coronoid notch properly guides the anteroposterior position of the needle, but its reference characteristic for the vertical level of the MF should be interpreted with caution.
The most important drawback in the evaluation of the occlusal plane is the irregularity of the mandibular teeth. The irregular mandibular teeth can alter the data, consequently, the precise location of the MF in mm cannot be compared properly in different populations. Likewise, various reference points, definitions of the occlusal plane, differences in measurement methodologies, and several measurement media may cause millimetric changes in the distance of MF from the occlusal plane, as well as racial factors. On the other hand, in the quantitative analysis of a dry skull, the measurements are performed on an amorphous surface, and minor angle differences affect the results. Besides, due to the curve of Spee, the buccal tubercle levels of mandibular molars may be divergent (27). Therefore, instead of specifying the numeric data belonging to the distance of the MF, which is not already a clinically observable point, determining the area where the MF is located frequently, such as above, below, or at the level of the occlusion, may create a clinically easier and more applicable approach. Therefore, although we provided quantitative data in the present study, we emphasized the areas where MF was concentrated. Group B (below the OP) was higher. The relative location of MF was similar on each side. This result was congruent with previous studies (11, 12,14,20). Investigation of the dry mandible provides a direct observation. However, for observation on panoramic radiography, a patient’s improper head movement or position may create a false asymmetry between the left and right sides.
We found that the location of the MF was at the center of the ramus, which is consistent with previous studies conducted with Turkish people or other populations (7,16). While measuring the distance of the MF to the anterior border on the ramus, the most concave point, the coronoid notch, was accepted, as in most previous studies (10,14). Topographic measurements of MF localized in the ramus are important for IANB (15). However, during injection, since the posterior border, the sigmoid notch, and the medial aspect of the ramus are not seen totally, clinically observable or palpable points are of greater importance in determining the technique of the anesthesia.
The direct method using skulls or radiographic method was used to determine the relative location of MF with the OP (5, 7, 9, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26). Although the skull provides direct observation, when the investigation is based on an “imaginary line” without systematic methodology, this approach makes such valuable data as direct observation subjective and unrepeatable. Therefore, the accuracy of the measurements would be skeptical. In the present study, it was used a metal line to determine the OP alignment directly, similar to the study of Palma et al. (9) which used plastic to compare the level previously.
The limitation of the study is that it concerns a certain population as it was conducted in dry skulls. In the present study, the effect of age and gender or different skeletal patterns was not examined. Besides, another limitation of the study is that the dry mandibles were obtained from one region of Anatolia and they were limited in number. More clinical and cross-sectional studies on different populations and a wider sample are needed. From a different perspective, future studies should focus on clinical applications of different anesthetic techniques rather than detecting the exact localization of MF that has different variations. The strength of the study is that it is the first study in which the relationship of the occlusal plane with MF was examined in the Anatolian population by direct observation using a dry mandible.
Conclusion
Due to the limitation of this study, it was concluded that the distance of MF to the coronoid notch was 14.71 mm. The mandibular foramen was located at the center of the medial surface of the ramus. The mandibular foramen was located below the occlusal plane, with a rate of 50.23%. The MF located was in an area that was between 4.78 mm below and 3.45 mm above the plane of occlusion. This anatomic data may be useful in the projection of the needle during IANB. The take-home message of the present study is that the highest rate of success can be achieved during anesthesia when the needle is positioned slightly above the occlusal plane (approximately 3.5 - 4 mm) and 1.5 - 2 cm behind the anterior edge, given that the results that showed 50% of MF was located below the occlusal plane and the 14.71 mm distance to the coronoid notch. Besides, the results can be informative for the location of osteotomies during maxillofacial surgeries in terms of the avoidance of permanent damage to vital structures including nerves. Nevertheless, it is crucial to remark that the data of the study demonstrated a benchmark result. In clinical practice, it is essential to perform analysis on a case-by-case basis.
Footnotes
Ethics committee approval: Necessary permissions were obtained from Akdeniz University Faculty of Medicine and Süleyman Demirel University Faculty of Medicine.
Informed consent: Not required.
Peer review: Externally peer-reviewed.
Author contributions: AMN, DY, SA participated in designing the study. AMN, DY, SA participated in generating the data for the study. AMN, DY, SA participated in gathering the data for the study. AMN, DY participated in the analysis of the data. DY wrote the majority of the original draft of the paper. AMN, DY, SA participated in writing the paper. AMN, DY has had access to all of the raw data of the study. AMN, DY, SA has reviewed the pertinent raw data on which the results and conclusions of this study are based. AMN, DY, SA have approved the final version of this paper. AMN, DY, SA guarantees that all individuals who meet the Journal’s authorship criteria are included as authors of this paper.
Conflict of interest:The authors declared that they have no conflict of interest.
Financial disclosureThe authors declared that they have no financial support.
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