Anthropometrics of mental foramen in dry dentate and edentulous mandibles in Coastal Andhra population of Andhra Pradesh State.

AIM: The aim of this study is to determine the morphological features and morphometrics of mental foramen with reference to surrounding anatomical landmarks in Coastal Andhra population of Andhra Pradesh State.
MATERIALS AND METHODS: Two-hundred and nineteen dry dentate and edentulous mandibles are examined in this study. Out of these 127 were dentate and 92 were edentulous. Various morphological and morphometrical parameters were measured by using digital Vernier caliper, metallic wire and metallic scale on both the right and left sides.
RESULTS: In the present study, the distance between most anterior margin of mental foramen and posterior border of ramus of the mandible is [MF-PR], MF-PR is 69.61 ± 6.03 mm on the right side and is 69.17 ± 6. 0 mm on left side in dentate mandible. In edentulous type, MF-PR is 68.39 ±6.4 mm on right side and 68.81 ± 6.55 mm on left side. In the present study, the distance between symphysis menti and most anterior margin of mental foramen [MF-SM] in dentate mandible is 28.24 ± 5.09 mm on right side and is 27.45 ± 3.7 mm on left side. In edentulous mandible (MF-SM) is 28.51 ± 4.5 mm on right side and on left side is 27.99 ± 4.50 mm.
CONCLUSION: Acquiring the knowledge and importance of anatomy of mental foramen is helpful in avoiding neurovascular complications, during regional anesthesia, peri apical surgeries, nerve repositioning and dental implant placement.

INTRODUCTION

The mandible is the largest and strongest bone of the face. It develops from the first pharyngeal arch. It has a horse shoe shaped body which lodges the teeth and a pair of rami which project upwards from the posterior ends of the body and provide attachment to muscles. Each half of the mandible ossifies from only one center which appears at about the 6th week of intrauterine life in the mesenchymal sheath of meckels cartilage near the future mental foramen. Meckels cartilage is the skeletal element of first pharyngeal arch. At birth, the mental foramen opens below the sockets for the two deciduous molar teeth near the lower border, this is because the bone is made up of only the alveolar part with sockets.[1]

The mandibular canal runs near the lower border. The foramen and canal gradually shift upwards. In adults, the mental foramen opens mid way between the upper and lower borders because the alveolar and sub alveolar parts of the bone are equally developed. The mandibular canal runs parallel with the mylohyoid line. In edentulous mandible the alveolar border is resorbed, so that the height of the body is markedly reduced. The mental foramen and the mandibular canal are close to the alveolar border.[1]

The mental foramen is an important anatomical structure situated in antero lateral aspect of the body of the mandible which transmits mental nerve, artery and vein. Mental nerve is a terminal branch of the inferior alveolar nerve which supplies sensory innervations to lower lip, buccal vestibule and gingiva mesial to the first mandibular molar.

Mental foramen serves as an important anatomical landmark, the orientation and position of which facilitate local anesthetic, surgical and other invasive procedures for oral and maxillofacial surgeries.[2] Its location and the possibility that an anterior loop of the mental nerve may be present mesial to the mental foramen needs to be considered before any surgery in the foramina area in order to avoid injuring of the neurovascular bundles passing through these foramina and notches.[3] The foramen may be occasionally misdiagnosed with a radiolucent lesion in the apical area of the mandibular premolar teeth. So identification of accurate anatomical position of mental foramen is very important in periodontal surgery especially during flap surgery in lower teeth, apical curettage of mandibular premolars, retrograde amalgam fillings and surgical orthodontics.

Replacement of missing teeth by dental implant and the increasing frequency of orthognathic surgery have increased the possibility of surgical procedures near the mental foramen. Local anesthesia of the terminal incisive branches of the inferior alveolar and mental nerves can be obtained effectively if the mental foramen is correctly identified.[4] Localization of mental foramen radiographically is difficult due to lack of consistent anatomical landmarks for reference and the foramen cannot be clinically visualized or palpated. Therefore, variable anatomical positions of the foramen have been described.[5]

The location of the mental foramen had been studied by means of direct measurement on dry mandibles or by using radiographs of dry mandibles in patients.[6] As the bone density increases the mental foramen becomes more difficult to identify on radiographs. Knowledge of the most common position of the mental foramen in the population may give additional information in the mental nerve blocks and related mandibular surgeries. So the present study had been undertaken to determine the morphological features and morphometrics of mental foramen with reference to surrounding anatomical landmarks in both dentate and edentulous mandibles in coastal Andhra population of Andhra Pradesh state.

MATERIALS AND METHODS

A total of 219 dry dentate and edentulous mandibles are examined in this study, out of these 127 were dentulous and 92 were edentulous. Various parameters were measured by using Digital Vernier caliper, metallic wire and metallic scale on both the right and left sides. Before measuring, the mandibles were placed on a standard horizontal plane to which the lower border of the mandible get its most contact when vertical pressure is applied to the molar region.

The parameters assessed were

[PR-MF] Distance between posterior border of ramus [PR] of mandible and most anterior margin of mental foramen [MF] [Figure 1]

Figure 1

[PR-MF] Distance between posterior border of ramus of mandible and most anterior margin of mental foramen

[SM-MF] Distance between symphysis menti [SM] and most anterior margin of mental foramen [Figure 2]

Figure 2

[SM-MF] Distance between symphysis menti and most anterior margin of mental foramen

[MF-IFB] Distance between inferior margin of mental foramen and inferior border of Mandible. [IFB] [Figure 3]

Figure 3

[MF-IFB] Distance between inferior margin of mental foramen and inferior border of Mandible

[AC-MF] Distance between alveolar crest and inferior margin of mental foramen [Figure 4]

Figure 4

[AC-MF] Distance between alveolar crest and inferior margin of mental foramen

[A1] Distance between alveolar crest to upper most point of mental foramen [Figure 5]

Figure 5

[A1] Distance between alveolar crest to upper most point of mental foramen

[A2] Distance between alveolar margins to the apex of second premolar socket [Figure 6]

Figure 6

[A2] Distance between alveolar margins to the apex of second premolar socket

[A1-A2] Distance between alveolar crest to upper most point of mental foramen-Distance between alveolar margins to the apex of second premolar socket [Figure 7]

Figure 7

[A1-A2] Distance between alveolar crest to upper most point of mental foramen-Distance between alveolar margins to the apex of second premolar socket

Incidence of mental foramen in relation to first premolar, second premolar and first molar on right side in dentulous mandibles [Figure 8]

Figure 8

Incidence of mental foramen in relation to first premolar, second premolar and first molar-Right side of dentulous mandibles

Incidence of mental foramen in relation to first premolar, second premolar and first molar on left side in dentulous mandibles [Figure 9]

Figure 9

Incidence of mental foramen in relation to first premolar, second premolar and first molar-Left side of dentulous mandibles

Edentulous mandible showing accessory mental foramen [Figure 10].

Figure 10

Edentulous mandible showing accessory mental foramen

Statistical analysis

The mean values are expressed as mean ± standard deviation with confidence interval of 95%. Mean values of right and left side were compared using student “t” test.

RESULTS

Morphometric features of 219 mandibles revealed the following results [Table 1].

Table 1

Depicting the values of all parameters

Distance between anterior margin of mental foramen to the posterior border of ramus of mandible [MF-PR] in dentate mandibles is 69.61 ± 6.02 mm on right side and 69.17 ± 6 mm on left side, whereas for edentulous mandibles the values have slightly decreased and it is 68.39 ± 6.4 mm on right side, 68.82 ± 6.55 mm on left side.

The distance between symphysis menti and most anterior margin of mental foramen [MF-SM], appears to be the same for both dentate and edentulous mandibles. MF-SM in dentate mandibles on right side is 28.24 ± 5.09 mm and on left side it is 27.45 ± 3.7 mm whereas for edentulous mandibles it is 28.51 ± 4.53 mm on right side and is 27.99 ± 4.5 mm on left side.

Distance between alveolar crest and inferior margin of mental foramen [AC-MF] in dentate mandibles is 15.29 ± 3.71 mm on right side and is 14.38 ± 3.75 mm on left side whereas for edentulous mandibles it is 15.50 ± 3.71 mm on right side and on left side it is 14.70 ± 4.43 mm which also appears to be the same for dentate and edentulous mandibles.

Distance between inferior margin of mental foramen and inferior border of mandible [MF-IFB] in dentate mandibles on right side is 12 ± 2.37 mm and on left side is 12.39 ± 2.74 mm whereas for edentulous mandibles on right side it is 13.40 ± 4 mm and on left side it is 13.56 ± 4.04 mm which shows that the distance between these parameters is increased in edentulous mandibles compared to dentulous mandibles.

Distance between alveolar crest to upper most point of mental foramen [A1] in dentate mandibles is 13.18 ± 2.32 mm on right side and is 12.56 ± 2.40 mm on left side, whereas for edentulous mandibles on right side it is 11.09 ± 3.41 mm and on left side is 10.01 ± 2.68 mm which shows the distance has significantly reduced in edentulous mandibles.

Distance between alveolar margins to the apex of second premolar socket [A2] in dentate mandibles is 8.93 ± 2.33 mm on right side and is 8.75 ± 2.34 mm on left side whereas for edentulous mandibles it is 6.16 ± 2.60 mm on right side and is 5.38 ± 2.42 mm on left side which reflects a decrease of values for edentulous mandibles.

Distance between alveolar crest to upper most point of mental foramen-distance between alveolar margins to the apex of second premolar socket [A1-A2], in dentate mandibles on right side is 4.18 ± 1.71 mm and on left side it is 3.85 ± 1.85 mm and for edentulous mandibles on right side it is 4.80 ± 2.10 mm and on left side is 5.18 ± 1.92 mm.

The shape of mental foramen was visually observed and analyzed in all the 219 mandibles and it was found that in 87 cases it is round and 40 cases it is oval in dentate mandibles and in 56 cases it is round and in 36 cases it is oval in edentulous mandibles on right side. On left side, it is round in 99 cases and oval in 28 cases in dentate mandibles and in edentulous it is round in 57 cases and oval in 35 cases.

The position of mental foramen on right side and left side in both dentate and edentulous mandibles in maximum cases is in between the first and second premolars [40-50%].

DISCUSSION

The mental foramen is usually a single opening on the antero lateral surface of the mandible, 13-15 mm superior to the inferior border of the body of mandible which is generally seen to be circular or oval in shape.[7] Through this foramen, the mental neurovascular bundles exits. It remains incomplete till the 12th gestational week until the mental nerve separates into several fasciculi at that site. Identifying the precise location of mental nerve is critical while performing peri apical surgery, cyst enucleation, endosseous implant, periodontal surgery and surgical orthodontic treatments such as a mandibular body osteotomy, in order to prevent damage to the mental nerve.[3] In addition, knowing the site of the mental foramen allows for the accurate delivery of local anesthesia during dental procedures.[8] There can be neuro sensory disturbances encountered if this important landmark is ignored while doing any invasive treatment in this region.

The interval between the two premolars is the most common site of the mental foramen and the apex of second premolar is the second most common site.[39] The results of the present study conclude the same. In 42.5% mandibles on right side and 40.9% on left side of dentate mandibles the common site is the interval between the two premolars and in 18.1% of the mandibles it is at the apex of second premolar. Santini and Land et al.,[10] reported that in Chinese the mental foramen was in line with second premolar and in British between first and second premolars. In edentulous mandibles, 50% of the mandibles on right side and 40.9% of the mandibles on left side, the mental foramen was located between first and second premolars. The second most common site is apex of second premolar which is seen in 25.2% of the mandibles, which is in accordance with the study conducted by Shankland, Wang, Green.[23] The position of the mental foramen varies depending on various factors like symmetry of mental triangle, microscopic and macroscopic morphology and maturity of the human mandible, bone remodeling activity and anthropologic features of the facial skeleton in different populations.

According to the present study the distance between the apex of lower second premolar socket to superior border of mental foramen (A1-A2) is 3.8 mm in dentate mandibles and 4.5 mm in edentulous mandibles. The apices of premolars are found to be very close to the mental foramen. Thus, when endodontic treatment is planned for these teeth, over obturation of root canals can lead to impingement and irritation to the mental nerve. Wang[3] reported that the average distance between the apex of the lower second premolar socket and superior border of mental foramen is 2.5 mm. Such a short distance can harm the mental nerve if the root canal is overfilled during root canal treatment procedures.

In the present study, the distance between inferior margin of mental foramen to the inferior border of mandible (MF-IFB) is 12.39 mm in dentate mandibles whereas 13.5 mm in edentulous mandibles which is in correlation with Chung et al.,[11] study. They reported that the average distance from the inferior border of the mandible to the center of mental foramen was 15.5 mm for Korean males and 14.0 mm for Korean females. Gershchenson et al.,[12] reported that the location of the mental foramen in relation to the border of the mandible and teeth depended on age, tooth condition and the degree of resorption. In adults as age advances, mental foramen shifts toward the superior border of mandible. This can be attributed to tooth loss followed by bone resorption. With the loss of teeth, there is change in the relative position from midlevel toward the upper border of the mandible. In severe cases of alveolar resorption the bone loss is so much that the incisive and inferior alveolar nerves are exposed out of bone and lie just below the mucosa which may cause pain on denture wearing. Accordingly, the results of the present study also showed that the values decreased in edentulous mandibles.

Soikkonen et al.,[13] reported that the mental foramen moves toward the lower cortex of the mandible as a result of alveolar atrophy and was situated on average 3.8 mm lower in edentulous jaws than in dentate jaws. In our present study, the distance between alveolar crest and inferior margin of mental foramen [AC-MF] in dentate mandibles is around 3.71 mm and in edentulous mandibles it slightly increased to 4.43 mm.

The distance between the posterior border of ramus of mandible and most anterior margin of mental foramen is very important, as nerve and vessels pass through it and care is taken to avoid complications during inferior alveolar nerve blocks, peri apical surgeries, apical curettage and implant placement and nerve repositioning.

In the present study, in dentate mandible the distance between most anterior margin of mental foramen and posterior border of ramus of the mandible (MF-PR) on right side is 69.61 ± 6.03 mm, left side is 69.17 ± 6.0 mm and in edentulous mandibles (MF-PR) on right side is 68.39 ± 6.4 mm, left side is 68.81 ± 6.55 mm. These results are similar to the study conducted by Shankar et al.[14]

The distance between symphysis menti and most anterior margin of mental foramen (MF-SM) in dentulous mandible on right side is 28.24 ± 5.09 mm, and on left side is 27.45 ± 3.7 mm and in edentulous mandible (MF-SM) on right side is 28.51 ± 4.53 mm, and on left side are 27.99 ± 4.50 mm. These results are similar to the above study conducted Shankar et al.[14] Clinically, there may be instances where the mental foramen cannot be localized without a fixed reference point. In such cases, it can be localized if the distance from the symphysis menti is known.

Accessory mental foramen is a rare anatomical variation. Even so, in order to avoid neurovascular complications, particular attention should be paid to the possible occurrence of one or more accessory mental foramen during surgical procedures. Utmost care to the accessory mental foramen and nerve is essential during dental implant surgery and any surgical procedure involving the mandibular molar and premolar region. This care may reduce the rate of paralysis and hemorrhage in mental region, lower lip and gingiva from the mental foramen to the midline of the ipsilateral side.[15]

In the present study out of 127 dentulous mandibles 13.97% accessory mental foramens were found whereas out of 92 edentulous mandibles 2.76% of accessory mental foramen were found. Out of these 219 mandibles none of them presented with more than two accessory mental foraminae. Gershchenson et al.,[12] reported 7.51% of double foramina in Indian and Israeli mandibles. Suresh et al.,[16] and Zografos and Mutzuri[17] studied the incidence of double foramina in 11.48% of North Indians and 6.68% of Greek population, respectively.

In the present study, 68.5% of dentulous mandibles on right side and 78% on left side, the foramen is round in shape and oval in 31.5 and 22%, respectively. In edentulous mandibles, it is found that 60.9% is round on right side and 62% is round on left side, whereas, it is oval in 39.5 and 38%, respectively. These values are similar to study reported by Mbajiorgu et al.,[18] and Junior et al.[19] According to Al-khateeb et al.,[20] majority of foraminae were round in shape which is similar to the present study.

The present analysis revealed variations in position, shape and size of the mental foramen. The knowledge of exact position and various distances of mental foramen are necessary for its anesthetic and surgical intervention. Thus, it is important to know the anatomical variations of mental foramina in order to avoid nerve damage in connection with surgical procedures. Since this study had been conducted in the local population, the ethnic and racial variations are not highlighted. Further studies focusing on racial and ethnic variations in different population groups need to be conducted on a large scale to substantiate this study.

CONCLUSION

The knowledge of the position of mental foramen is important for day-to-day clinical practice. This variability of mental foramen should alert the dental surgeons while performing periodontal or endodontic surgery. Clinicians should not rely on panoramic radiographs for identifying the position of mental foramen. The knowledge of the relationship of the foramen, in surgical procedures, in relation to anatomical landmarks can be of assistance in the localization of important maxillofacial neurological structures. This study highlights and adds to the current literature of the mental foramen.