Branching Pattern of the Extraosseous Mental Nerve in a Kenyan Population

Abstract

Knowledge of the branching pattern of the mental nerve is an important consideration during placement of tooth implants and reconstructive plates. It is known to display population variations and data for the same is scarce form the sub-Saharan region. With the recent increase in surgical interventions in the mandibular region in Kenya, a detailed description of mental nerve is warranted. A total of 64 mental nerves were dissected and branching pattern was noted. Single mental nerve was present in 60 (93.7%) cadavers while double mental nerves occurred in 3 (4.7%) and accessory in 1 (1.6%). Most common pattern was bifurcation (39%) followed by trifurcation (34%), single (19%), and quadrification (8%). The double and triple branches were seen to further divide into two to three subbranches with diverse patterns. Side symmetry in branching was seen in four (6.25%). These are important anatomical considerations during placement of reconstructive plates for mandibular trauma and administration of mental nerve blocks.

The mental nerve (MN), terminal branch of the inferior alveolar nerve, exits the mental foramen (MF) [1] and divides into three branches to innervate the skin of chin, lower lip, and gingivae [2]. The number of MNs on the ipsilateral side varies from none to four with the most common variation being two [3,4,5,6,7]. Branches of the MN vary from one to four [8,9], with the most common being four branches, namely medial, lateral inferior labial, angular, and mental [10]. These data are, however, mainly in form of case reports, hence the true incidence of such occurrences is difficult to establish. A detailed description regarding the exact branching pattern is important for effective presurgical evaluations in implant surgery, administration of MN block anesthesia, and for osteotomy procedures [11]. Surgical interventions involving the mandibular region have increased in the past decade [12]. The branching pattern displays population differences [9]. There is however no data for the black Kenyan population. The aim of this study was, therefore, to clearly elucidate the branching pattern of the MN in Kenyans to minimize iatro- genic injuries during surgical maneuvers.

Materials and Methods

A total of 64 MNs were dissected at the Department of Human Anatomy, University of Nairobi. All the specimens were from adult black Kenyan. The MN was exposed by three skin incisions and lateral reflection of the skin flap. A midline incision extended from the midpoint of the superior border of the lower lip to the midpoint of the inferior border of the mental symphysis (MS). A lateral incision extended approxi- mately 7 cm from the angle of the lip on either side. An inferior incision was made along the lower border of the mandible from the MS till the lateral extent of the lateral incision. Using blunt dissection, the flap was detached from the bone and reflected laterally until the MN was seen to exit from the MF. The MN was carefully dissected and the branching pattern was carefully displayed. Data collected included the number of MNs, initial branching pattern, terminal branching pattern, and symmetri- cal branching patterns. A branching pattern was classified as high or low depending on whether the MN branched at the level of the MF or at a distance from the MF, respectively. The data collected was analyzed using Microsoft Excel 2007 (Micro- soft Corporation, Redmond, WA). The frequencies and percen- tages of the various branching patterns were determined. The variations noted were photographed using a high resolution digital camera and managed using Photoscape version 3.5 (MOO11 Tech, Korea).

Results

The MN was present in all cadavers that were dissected. The pattern of branching of the MN after its exit from the MF displayed diverse set of variations in the number and branch- ing pattern. A single MN was present in 60 cadavers (93.7%) while double MNs occurred in 3 (4.7%) (Figure 1A,B) and accessory in 1 (1.6%) (Figure 1C). The accessory MN emerged in the region of the second premolar, 4 mm from the MN. When the inferior alveolar nerve branched into two terminal branches which were of similar sizes, the pattern of branching was described as double MNs. Accessory MNs were smaller branches from the inferior alveolar nerve in addition to the MN that had the same area of innervations as the MN.

Initial Branching Pattern

The MN displayed a varied branching pattern with the number of branches varying from one to four on the ipsilat- eral side. The most common pattern of branching was divi- sion of the MN into two branches (39%) which displayed a high level division, that is, branching at the level of the MF in 11% (Figure 2A) while in 28%, the branching was seen at a distance from the MF hence a low division branching pattern (Figure 2B). The MN split into three branches in 34% (Figure 2C), continued its course as a single nerve without splitting into any branches in 19% (Figure 2D). In 8% of the cadavers, a quadruple division was observed (Figure 2E).

Terminal Branching Pattern

The double and triple branches of the MN further exhibit a diverse pattern of division into subbranches. For the double branches, the most commonly observed redivision pattern was seen in 12.7%, where each of the two main branches of the MN further divided into two smaller branches (Figure 3A), hence a total of four subbranches arising from the two branches of the MN. In 7.8%, one of the two main branches of the MN further redivided into two smaller subbranches (Figure 3B). In 1.56%, five subbranches were seen to arise from the two branches of the MN (Figure 3C). About 9.1% of the triple branching patterns, divided further into two subbranches (Figure 3D) while in 1.56%, four subbranches were observed (Figure 3E).

Symmetry

Significantly high levels of side asymmetry were observed in the same cadaver. Symmetrical branching pattern was ob- served in only 6.25% of the cadavers (Figure 4A,B). In 3.125%, the MN continued as a single branch on both the right and left side on the same cadaver (Figure 4A). Similarly, for another 3.125% of the cases, it split into the triple branch presentation on both the right and the left sides of the same cadaver (Figure 4B).

Discussion

The MN was present in all the 64 hemifaces, which is at variance with previous studies that have reported unilateral and bilateral absences [13]. The prevalence of double MNs in the current study (4.7%) falls in the range of 1 to 10.6% reported in the previous studies [4,14]. It is, nonetheless higher than that reported for the Thai population but lower than in Greeks, Indian, and Japanese population (

Table 1. Frequency of double mental nerves in different populations.

Study (n)	Study population	Method	Prevalence (%)
Current study (64)	Kenyan	Cadaveric	4.7
Zografos and Mutzuri (800) [25]	Greek	Osteological	6.68
Agthong et al. (110) [24]	Thai	Osteological	1.8
Naitoh et al. 2009 (157) [4]	Japanese	Osteological	7
Sankar et al. 2011 [26] (90)	Indian	Osteological	8.9

). This suggests wide population differences. Separation of the MN earlier than the formation of the MF is a likely explanation for the presence of variations in the number of nerves [4]. The presence of additional MNs has been associated with failed mandibular anesthesia [15]. There are currently no studies done on the failure rates of complete mandibular anesthesia in the sub- Saharan region, but these results suggest a lower rate for Kenyans compared with the Greek, Indian, and Japanese population (Table 1).

Knowledge of the prevalence of accessory MNs is impor- tant during the placement reconstruction plates for mandib- ular fractures and in screwing tooth implants as accessory MNs may be injured during these procedures [11]. The 1.6%, prevalence of accessory MN is comparable to that seen in the Russian and White American population (

Table 2. Frequency of the accessory mental nerves in different populations.

Study (n)	Study Population	Method	Prevalence (%)
Current study (64)	Kenyan	Cadaveric	1.6
Montagu 1954 [16] (800)	French	Osteological	2.6
Montagu 1954 [16] (2400)	Russian	Osteological	1.5
Riesenfeld 1956 [17] (430)	Polynesian	Osteological	12.5
Riesenfeld 1956 [17] (484)	Melanesian	Osteological	9.7
Naitoh et al., 2009 [4] (157)	Japanese	Cone-beam computed tomography	7
Sawyer et al., 1998 [18] (510)	White American	Osteological	1.4
Sawyer et al., 1998 [18] (332)	Black American	Osteological	5.7

). It is however, lower than that reported for Polynesian, Melanesian, Japanese, and Black American population. This suggests wide population differences. The discrepancies observed could also be attributed to the differing sample sizes and to the different study methods whereby dry mandibles [16,17,18] or cone beam computed tomography [4] and cadaveric dissection (current study) were used.

The current results confirmed the previous descriptions of the MN dividing into one to four branches (

Table 3. Prevalence of the different branching patterns of the mental nerve in different populations.

Study (n)	Population	Number of branches
		1	2	3	4
Current study, 2012 (64)	Kenyan	19	39	34	8
Hu et al., 2007 [10] (62)	Korean	–	38.8	61.3	–
Alsaad et al. 2003 [20] (26)	Ireland	–	44.4	55.6	–
Alantar et al. 2000 [19] (32)	French	22	28	28	22

). The classic description of division into three branches was only seen in 34% of the cadavers. The most commonly observed branching pattern was division into two branches. These results are at variance with those reported by previous studies where the most commonly observed pattern is divi- sion into three main branches [10,19,20]. The MN divided into four branches in 8% and continued as a single branch 19%. This is lower when compared with the prevalence reported by a French study, at 22% for each branding pattern [19].

It is important to know the branching pattern of the MN since accessory branches can be used for facial nerve grafting. This diverse pattern of branching needs to be considered before biopsy of minor salivary glands, excision of nodules symphyseal bone procedures through the labial route. Dam- age to the branches of the MN can lead to labiomental sensory disturbances which produce severe discomfort [19]. Knowledge of variant branching patterns could be of help in explaining failed MN anesthesia [21] and also referred pain to teeth during cases of injury to any of the cutaneous branches. The branch- ing pattern of the MN becomes an important surgical consid- erate in sliding horizontal osteotomy procedures of the mentum during genioplasties. The MN is also vulnerable to iatrogenic injury during procedures involving placement of alloplastic implants, in horizontal advancement osteotomy also during bone grafting to correct microgenic mandible [22].

The branching pattern further becomes relevant during MN transpositioning and repositioning in mandible endoste- al reconstruction implant surgeries. This is a difficult surgical procedure where the anatomy of the MN often challenges the surgeon during replacement of removable prosthetics, stabi- lizing the anterior residual dentition, and reducing alveolar ride atrophy [23].

In the current study, only 6.25% of the MNs exhibited the same branching pattern bilaterally compared with a French study where 56% homogeneity in branching pattern was observed. This finding has important implications during surgical interventions where side consideration becomes an important factor in avoiding nerve injury. The branching pattern on the right and left side should not be treated in the same way during surgical maneuvers in the area.

Conclusion

Double and accessory MNs were present in 6.3%. The diverse hierarchical pattern of branching of the MN and the high level of asymmetrical branching are important anatomical consid- erations during placement of reconstructive plates for man- dibular trauma and administration of MN blocks.