Characterization of the Mental Foramen and Mental Loop Measures in Fresh Frozen Cadavers

The mental foramen is situated in the anterolateral region of the jaw body. The artery, vein, and mental nerve, which is responsible for the innervation of the skin of the chin, vestibular gum, and lower lip, pass through this foramen. Normally, the mental foramen is located between the apexes of the roots of the lower premolars, but variations can be found in its shape and position.¹  Thus, the anterolateral region must be studied to prevent accidents and complications, such as neurosensory disorders, which may occur during surgery.^2–4 

Until recently, the area between the mental foramens was considered completely safe for implant placement and the removal of bone grafts. However, a detailed study of the anterior segment of the mandible demonstrates that, occasionally, an anatomical structure called the anterior loop of the mental nerve may exist in this region.³  The mental nerve loop originates from the mandibular canal and crosses anteriorly to the mental foramen and back toward the foramen itself.⁴  Several studies have been conducted to detect the prevalence and location of the mental nerve loop as well as to propose safe guidelines for the placement of implants in the interforaminal area.^5–7  However, the prevalence of the anterior loop was reported to range from 28%⁸  to 96%,¹  which hinders surgical planning. The size of the loop often varies among individuals, ranging from 0.89 to 7.61 mm.⁹ 

Another point to be considered is that the position, width, and length of the mental foramen and mental loop, and how they correlate with the sex of individuals, have not been described in the current literature. Thus, the objectives of this study were to evaluate the position of the mental foramen and to verify the presence of and evaluate the mental loop in fresh cadavers. In addition, we aim to investigate any association between our findings and the sex of individuals.

This study involved 39 adult male and female human jaws from fresh cadavers. These cadavers were from people who lived in the United States. The selection of the anatomical specimens occurred according to the objective of the research in order to maximize their use. Dissections were performed on both sides to expose the bone surface of the mandible in the region of the mental foramen.

The body donation form was in accordance with the Anatomical Gift Act. The Miami Anatomical Research Center (MARC) received the right to handle the specimens for research, training, and education from tissue providers. The donors signed a form authorizing their bodies to be used for dissection, research, training, and education. MARC is responsible for handling, storing, and protecting the donor's identity. To be able to work on the specimens, we signed a document informing us of all the legal and ethical terms associated with proper handling, including protecting the donor's identity.

The relationship between the mental foramen location and teeth was investigated by using, as reference, 2 lines parallel to each other drawn perpendicular to the occlusal plane. The teeth that were evaluated were the first and second premolars. Mental foramen measurements were performed bilaterally with a digital caliper and millimeter rule using the mental foramen as reference. The width of the mental foramen was determined as the distance from the mesial end of the foramen to the distal end, following an imaginary line parallel to the occlusal plane. The height of the foramen was determined as the distance from the upper extremity to the base of the foramen, following an imaginary line perpendicular to the occlusal plane. The distance from the foramen to the base of the mandible was measured along a line perpendicular to the base of the mandible using a digital caliper. These measurements can be seen in Figure 1.

After performing these measurements, an osteotomy was performed to expose the mental nerve loop and confirm its presence or absence. When the mental loop was present, the distance from the loop to the base of the mandible was measured. To do this, the distance from the mesial portion of the border of the mental foramen to the most anterior part of the mental nerve loop was measured (Figure 2). The distance between the foramens was also measured with the caliper, by placing each end of the equipment in contact with both foramens, as can be seen in Figure 3.

All measurements were made by the same examiners using the same instrument. Statistical analyses were reviewed by an independent statistician (C.C.G.). Qualitative variables were expressed as frequencies and percentages. Numerical variables were first tested using the Shapiro-Wilk test. In addition, the association between sex and the measurements was evaluated using either Student t test or the Mann-Whitney test. The level of significance was set at P < .05. Data were analyzed with SPSS software (SPSS, v.23, IBM, Armonk, NY).

All descriptive results are shown in Table 1. There was no difference when measurements on the right and left sides of the mandible were compared (P > .05). In Table 2, the comparison between measurements and sex are shown.

The accurate knowledge of variations in the position and size of the mental foramen and the presence of the mental loop is important for successful surgical procedures on the mandible, such as dental implants, root canal treatments, and orthognathic surgeries.¹⁰  In our study, we evaluated the mental foramen location on both sides of the mandible for comparison. We observed that the most common position of the mental nerve on the left side of the mandible was between the apexes of the roots of the premolars and on right side was toward the second premolar. These data demonstrate that the mental foramen location changes in the same mandible when we switch sides, which is inconsistent with previous studies that reported that the most common position of the mental foramen on the right side is along the longitudinal axis of the second premolar and on the left side is along the longitudinal axis between the first and second premolars.^10–13 

The size of the mental foramen has been described, and its width and height range from 2.57 mm to 3.14 mm and from 2.38 mm to 2.64 mm, respectively.³  In our study, the median sizes for the width and height were 4 mm and 3 mm, respectively, on the right side, and the median size for the width and height on the left side was 3 mm. The prevalence of the anterior loop of the mental nerve varies greatly. The assessment of the anterior loop using cone beam computerized tomography identified the mental loop 85.2%,¹⁴  28.5%,¹⁵  and 32.8%¹⁶  of the time. Our study showed that 66% of the mandibles contained the mental loop. This difference in prevalence may be attributable to the use of different techniques for evaluation (ie, the type of tomography) and the freshness of the mandibles. It is also known that the likelihood of identifying the anterior loop is affected by the degree of calcification of the cortex. This calcification appears as a radiopaque object in radiographic images.^4,17  Our study differs from other studies in the method used for data collection. In addition, our study involved the use of fresh cadavers.

Different studies have reported different maximum widths of the mental loop, which suggests that maintaining a minimum distance (1 to 5.5 mm) between the mental foramen and implants is important. In the present study, we obtained the following measures: right mental loop, mean = 6.42 mm, and left mental loop, mean = 6.16 mm. We could suggest that a safe margin for surgical procedures would be 7 mm instead of 4 mm, but since the dimension of the mental loop varies, it should be evaluated individually in each case.

Even though previous studies reported that there were no significant differences in the width of the mental loop based on sex,¹⁰  we have shown that the distance from the loop to the base of the mandible on the right side is smaller in women. The distance between the foramen and mandible base on the left side is also smaller in women. This may be because women are generally smaller than men, resulting in women having smaller faces.¹⁸  These data can influence surgical procedures and mental nerve block prior to procedures. Clinicians must pay attention to these values.

We believe that one of the highlights of our work is that our research was performed in fresh cadavers, which may be the reason for the differences in measurement values when compared with values obtained using imaging techniques reported in the literature. In future studies, it will be important to evaluate the ethnic background of the specimens because it may affect the values obtained after measurements. It will also be important to consider the height of the specimens during the study.

The authors would like to thank the Miami Anatomical Research Center (Doral, Fla) for their contribution. Statistical review was performed by Dr Carla Gonzaga, Assistant Professor of Biostatistics at the School of Health and Biological Sciences at Universidade Positivo, Curitiba, Paraná, Brazil.

The authors declare that they have no conflict of interest. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sector.