Should we be concerned about accessory mandibular foramina and canals? A cone-beam computed tomography study

Objective: Analyze the prevalence of mandibular accessory foramina and canals using cone-beam computed tomography (CBCT). Methodology: 136 mandibles divided into 10 predetermined areas were analyzed through CBCT looking for accessory foramina and canals. The Chi-square and Wilcoxon tests were used. Results: We found 1.316 accessory foramina, which 486 were accompanied by canals. 70.3% of accessory foramina were on the internal mandibular surface, most below the mylohyoid line and genial tubercles. The M1 area had the highest number of foramina, especially in the internal surface. The right mandibular side revealed a significantly greater number of foramina when compared to the left side. The mean diameter of accessory foramina analyzed was 0.85mm. Most of the accessory canals were on the internal mandibular surface, with a longer average length when compared to external surface canals. Conclusion: Our study showed that more detailed studies of accessory mandibular foramina and canals should be carried out, since a high prevalence of these structures and they have not named or classified yet. Furthermore, procedures that reach the internal mandibular surface, especially the anterior region, may be more subject to complications, as well as failure of anesthetic blocks on the right side of the mandible.


Introduction
Accessory foramina and canals in mandibles are all apertures in this bone, except the tooth sockets, mental and mandibular foramen (Sutton, 1974). These foramina are usually not observed with conventional radiographic techniques (Sisman, 2012), better detected in cone beam computed tomography (CBCT) (Katakami et al., 2018). The unintentional injury of its contents (vessels and nerves) can lead to complications such as hemorrhages, sensory losses, and development of neuromas (Przystanska & Bruska, 2012;Mendonza et al., 2004). These foramina can serve as a route for tumor metastasis (Murlimanju et al., 2011) and invasion of tumor cells on the mandibular surface (Fanibunda & Matthews, 1999).
This study aims to evaluate the prevalence of accessory foramina and canals in human mandibles through predetermined areas and measure their diameter and length.

Mandible Division
The mandible division was based in Muley et al. (2022) study. The mandible was divided into 10 areas (Figure 1).
The division of these areas were identical in internal and external surface (5 areas/surface). The distal of the lower lateral incisors, the mental foramen and the ascending branch were used as anatomical parameters. Anterior mandible region corresponding to the M1 area. The M2 area comprises the parasymphysis region (distal of the lateral incisor to the mental foramen). The M3 area was determined as the posterior region to the mental foramen. Furthermore, we divided these areas into external (buccal) and internal (lingual) surface, and M2 and M3 areas into right (R) and left (L) sides. Mandibular area: M3R -right posterior region to the mental foramen; M2R -right parasymphysis region (distal of the lateral incisor to the mental foramen); M1 -anterior mandible region; M2L -left parasymphysis region (distal of the lateral incisor to the mental foramen); M3Lleft posterior region to the mental foramen. These areas were analyzed both on the external (buccal) and internal (lingual) surfaces. Source: Authors.

Mandible Analysis
The mandibles were analyzed using CBCT obtained from a Specialized Radiological Center in Varginha, Minas Gerais, Brazil. Images with distortion or overlapping, patients with a history of mandibular trauma, craniofacial malformations or syndromes were excluded. Patients >18 y.o., male/female, with all inferior teeth were included. The CBTC images were obtained at 87kV and 8 mA, with a scan time within an interval of 8,01 to 8,655 seconds, with a voxel size of 180 μm and slice thickness of 0,180 mm. The sections obtained in the sagittal, axial and coronal planes, and each multiplanar data measuring 180 × 180 × 180 μm pixels at 16 bits were stored in the computer. Detailed analysis of the foramina and accessory canals was performed using CS 3D software. The curved slicing mode was used for the evaluation and inspection of the images, being employed the cross-sectional view, using as a guideline the sagittal section where these structures are in evidence. The contrast and density of the CT scans were modified for better visualization of the canals and accessory foramina. The location, quantity, and diameter (or length) of the accessory foramina and canals in each area were noted. The diameter of the mental foramina was also analyzed, as well as the accessory foramina in this region, considered as accessory mental foramina. In addition, the foramina were considered above or below the genial tubercles and mylohyoid line. This study is laboratorial quantitative research.

Statistical Analysis
Descriptive measures such as minimum, maximum, median, interquartile range, mean and standard deviation (s.d.) and percentages were presented as measures to describe variable results. The Chi-square test was used to compare the mandibular areas by accessory foramina and canals prevalence. The Wilcoxon test was used to compare two measurements performed in the same sample unit. A p value <.05 was considered statistically significant for all analyses, which were performed with SPSS v21.0 software (IBM Corporation, New York, NY, USA).

Results
One thousand three hundred and sixteen accessory foramina were found in 136 mandibles. Table 1 represents the average of foramina by mandibular area. The internal surface showed 925 (70.3%) foramina, the majority (n = 743, 80.3%) below the mylohyoid line and genial tubercles. Furthermore, 61.0% (n = 802) of the mandibles had at least 3 accessory foramina at the internal surface. The M1 area showed a significant number of foramina (n = 529, 40.2%) compared to the other areas (p <0.001). Besides, the M1 internal area showed the highest number of foramina (n= 401, 30.5%), being significantly compared to the other areas (p <0.001). Still on the internal surface, there was a significant difference between the M2R and M3L areas (p <0.001), with greater number of foramina on the right side (n = 231) ( Table 2).

Mandibular Area Accessory Foramina External Surface
Internal Surface Total M3R Min 0.9 ± 1.2 2.9 ± 1.4 3.9 ± 1.  Mean ± s.d.* 0.5 ± 0.8 Mean ± s.d. 0.9 ± 1.2 2.9 ± 1.4 3.9 ± 1. In total, regarding mandibular right and left sides, the right side showed more foramina (n = 437, 33.2%) and was significant (p <0.001) compared to the left side (n = 350, 26.5%) (Table 3). Specifically, the M3R external and M2R internal areas showed a prevalence of foramina. Significant differences were identified on the external surface (p = 0.011), with a higher number of foramina in the M3R area (n = 71) compared to M3L area (n = 44). On the internal surface, the M2R area showed a greater number of foramina (n = 166) compared to the M3L area (n = 100). All mandibular areas analyzed had a greater number of foramina on the internal surface when compared to the external surface, even with significant results, except for the M2L area (p =0.241) ( Table 4).

Foramina Diameters
Concerning the accessory foramina diameter, the average was 0.9mm on the internal surface and 0.8mm on the external surface. The accessory mental foramina had the largest diameters, with an average of 3.5mm and 3.4mm on the right and left sides, respectively. No significant results were found about the accessory foramina, in contrast to the diameter of the mental foramen, which showed significant results, being greater on the right side.

Accessory canals
Regarding 1.316 foramina studied, 486 (37.0%) were accompanied by accessory canals, with 354 on the internal mandibular surface, representing most canals. The length (mean) of these canals on the internal surface was statistically superior compared to the canals on the external surface. The areas M1 and M3 at the internal surface showed a statistically greater length of accessory canals when compared to the same areas externally (Figure 2).

Discussion
In our study, we divided the mandible into ten main areas, including the symphysis, parasymphysis and the mandibular body, evaluating the external and internal surfaces. Concerning the 136 mandibles analyzed using CBCT, we found 1.316 accessory foramina (9.7 foramen/mandible). Muley et al. (2022) also analyzed the body and mandibular symphysis looking for accessory foramina using CBCT and showed 245 accessory foramina in 50 mandibles (4.9 foramen/mandible). In addition, the studies differ about the areas analyzed. Sutton (1974) and Mathews (1999 and2000) analyzed the entire mandibular extension. Haveman and Tebo (1976) however, excluded an important part of the mandible in their experiment, studying only the regions posterior to the second premolar. In our study, like Muley et al. (2022), only ramus was not analyzed, giving greater importance to the toothed areas. 95.5% of the mandibles had foramina belonging to this region, with most of the foramina also above these structures.
Particularly, the M1 area of our study (corresponding to the mandibular symphysis), showed the highest number of foramina (n = 529, 40.2%), especially in the internal surface, revealing 401 foramina, representing 30.5% of the total found. However, unlike the authors, we found most foramina below the genial tubercles (TAB). Muley et al. (2022) found similar results, which 89.0% of the foramina found were in the anterior region of the mandible, the majority (76.0%) on the lingual surface and below the genial tubercles. Failures in the anesthetic blockade in these areas of the mandible can be explained by the presence of these foramina, and through them nervous vascular bundles pass, such as branches of the mylohyoid nerve, often innervating the lower teeth (Madeira et al., 1978;Stein et al., 2007).
Regarding the molars and premolars region, Muley et al. (2022) found more foramina on the right side compared to the left side. Similar manner in our study revealed significantly more foramina on the right side in areas M2 and M3. In addition, the M2 areas showed a greater number of foramina on the internal surface, as well as the results published by these authors. However, analyzing only the left side, these researchers found more foramina on the external mandibular surface, diverging from our study. Narayana and Prashanthi (2003) mentioned the presence of an accessory mandibular foramen with 10.0mm in diameter, nevertheless accessory foramina with this diameter are not routinely found in the literature (Sutton, 1974;Haveman and Tebo, 1976;Soto et al., 2012). In our research, the areas analyzed also did not show large diameter foramina, and despite the non-significant results, the mental accessory foramina had the largest diameters.

Conclusion
Our study showed that more detailed studies of accessory mandibular foramina and canals should be carried out, since a high prevalence of these structures and they have not named or classified yet. Furthermore, procedures that reach the internal mandibular surface, especially the anterior region, may be more subject to complications, as well as failure of anesthetic blocks on the right side of the mandible.