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Introduction

 The infraorbital nerve, a branch of the maxillary nerve descends along the anterior surface of the maxilla, exits through the infraorbital foramen, and innervates to the skin and mucosa of the facial region such as the lower eyelid, conjunctiva, lateral surface of external nose, and upper lip. The mental nerve, a branch of the inferior alveolar nerve runs through the mandibular canal, exits through the mental foramen located below the second premolar, and innervates to the skin and mucosa of the lower lip, jaw and the gingiva of incisor region.

The infraorbital and mental foramina are important anatomical structures in the dental, plastic, and maxillofacial surgery procedures. The infraorbital and mental nerves that emerge from these foramina are blocked at the each foramen to induce local anesthesia during oromaxillofacial and plastic surgery [1-4]. In addition, during facial wound closure, biopsies, and cosmetic facial surgery, injury of neurovascular bundles results in massive bleeding and numbness [5]. Therefore, to prevent injury of neurovascular bundles, it is important to know the exact locations of the foramina in clinical situations requiring a local nerve block. 

Numerous investigators examined locations of the infra­orbital and mental foramina by various methods. Agthong et al. [6] assessed locations of the foramina based on the infraorbital margin, anterior nasal spine, and inferior rim of the mandible on skulls. On the hypothesis that the supraorbital, infraorbital, and mental foramen are being on the same sagittal plane, Chung et al. [7] examined the relative locations of the infraorbital and mental foramina on the sagittal plane that passes through the supraorbital foramen on front photographs. The location of the mental foramen has been determined primarily according to the lower teeth [8], and measured by using clinical images such as panorama, computerized tomography (CT), and radiography [9-11].

Most previous studies are based on the hard-tissue landmarks and lower teeth on skulls, whereas studies of cadavers are comparatively rare [12-16]. The results from such studies have made it possible to predict locations of the infraorbital and mental foramina with a considerable degree of accuracy. However, these hard-tissue landmarks are not easily accessible in living subjects, and especially the mental foramen might encounter problems determining the reference point in case of lossing lower teeth. Therefore, it is required using soft-tissue landmarks as reference points to supplement the weakness of using only hard-tissue landmarks and to explain locations of these foramina more accurately.

 The purpose of the present study was to determine locations of the infraorbital and mental foramina in relation to the soft-tissue landmarks on cadavers, such as the alare and cheilion. These data may provide a useful anatomical information to clinical procedures such as dental, maxillofacial, and facial plastic surgery. 

Materials and Methods

Materials

Fourteen embalmed cadavers (28 sides; 10 male and 4 female cadavers) that were donated to Department of Anatomy, Chosun University School of Medicine for educa­tional purposes were examined. The age of the cadavers at death ranged from 37 to 91 years, with a mean of 64.2 years. To expose the infraorbital and mental foramina, the overlying soft tissue was dissected away and the periosteum was removed. And the front photographs of the subjects including a scale bar were obtained by a digital camera (D90; Nikon, Tokyo, Japan). 

Measuerment

 We used the alare (ala of nose; alare) and cheilion (mouth of corner; cheilion) as the soft-tissue landmarks that the most commonly used on the facial region. The distances between the same structures bilaterally (infraorbital foramen, alare, cheilion, mental foramen) were measured directly on each cadaver using a digital vernier caliper (CD-15CP, Mitutoyo Co., Kawasaki, Japan) at a 0.01 mm level. The locations of the infraorbital and mental foramen relative to the alare and cheilion were measured indirectly on the digital photographs using the Adobe Photoshop CS5 ver 12 (Adobe, CA, USA). These measurements were utilized to calculate the distance and angle between the alare and infraorbital foramen, and the cheilion and mental foramen (Fig. 1). All measurements were made based on the center of each foramen.

Fig. 1. The parameters of the infraorbital foramen and mental foramen in relation to the alare and cheilion. IOF, infraorbital foramen; MF, mental foramen; AL, alare (ala of the nose); CH, cheilion (mouth corner); P1, cross point between alare vertical line and infraorbital foramen horizontal line; P2, cross point between cheilion vertical line and mental foramen horizontal line; Angle 1, vertical angle from alare to infraorbital foramen; Angle 2, vertical angle from cheilion to mental foramen.

Statistical analysis

 All statistical analysis was performed using SPSS 12.0 (Chicago, IL, USA). The data were analyzed the difference of inter-observer, and between the right and left side of each parameter using one-way ANOVA. Inter-observer differences were not significant (P = 0.942), and thus the average of the measurement values obtained by four investigators was used as the final measurement data. In addition, the right and left side of each parameter also did not show significant dif­ferences, and thus they were considered as a same group. The distances between the contralateral foramen and soft-tissue landmarks were analyzed using one-way ANOVA with post-hoc comparsion. The significant differences in gender and age were not considered. All measurements were evaluated at mean ± SD, and the significance level was set at P < 0.05.

Results

 The distance between the bilateral infraorbital foramina 58.09 ± 4.04 mm) was longer than the distance between the bilateral mental foramina (50.32 ± 1.93 mm). The distance between the bilateral alares and cheilions were 41.22 ± 3.44 mm and 58.43 ± 6.62 mm, respectively (Table 1). The infraorbital foramen was located more laterally than the mental foramen and alare (P < 0.05), and in the same vertical line as that of cheilion (P = 0.998). The mental foramen was located medial to the vertical line of the cheilion (P < 0.05) (Table 1).

 The infraorbital foramen was located 12.92 ± 3.75 mm superior and 7.88 ± 2.56 mm lateral to the alare. The distance between the alare and infraorbital foramen was 15.42 ± 3.54 mm, and the vertical angle (Angle 1) between these structures was 31.67 ± 13.36^o superolaterally (Table 2 and Fig. 2).

The mental foramen was located 21.83 ± 3.26 mm inferior and 5.56 ± 3.37 mm medial to the cheilion. The distance between the cheilion and mental foramen was 22.74 ± 2.96 mm, and the vertical angle (Angle 2) between these structures was 14.05 ± 10.12^o inferomedially (Table 3 and Fig. 2).­ 

Table 1. Distances between the contralateral foramen and soft-tissue landmarks

Table 2. Statistical analysis of infraorbital foramen regarding its location

Table 3. Statistical analysis of mental foramen regarding its location

Fig. 2. Diagram showing the mean value of the foramen from the soft-tissue landmarks.

Discussion

 The infraorbital foramen, the continuity of the infraorbital canal, is situated just below the infraorbital rim on the anterior surface of the body of maxilla. The infraorbital nerve emerges through the infraorbital foramen and innervates to the skin and mucosa of the lower eyelid, upper lip, and skin of lateral part of the nose, anterior region of the nasal septum. The mental foramen is located halfway between the lower border of the alveolar part and the upper border of the base of mandble. The mental nerve exits through mental foramen and innervates to the skin and mucosa of the jaws, lower lip and gingiva of the lower incisiors, canines [17]. Likewise, these nerves are innervated widely in the facial region, and thus they may be injured during local anesthesia for dental, ophthalmological, and maxillofacial surgery resulting in side effects such as paraesthesia, anesthesia. Therefore, an information on locations of the infraorbital and mental foramina is useful to dentists as well as to surgeons in the clinical procedures such as diagnose, facial plastic, and orthognathic surgery [15].

Numerous studies have investigated the locations of the infraorbital and mental foramina in relation to the hard-tissue landmarks and teeth. However, the results of such studies are difficult to apply clinically [11,12,14]. To resolve such weakness, in our study, the location was measured and analyzed by the application of the alare and cheilion as the soft-tissue landmarks that have been the most commonly used ones in the facial region.

The infraorbital margin has been used frequently to measure the locaions of the infraorbital foramen, and the distance between them is 4-10 mm, and it varies between studies and races [13,18-20]. The distance from the infraorbital foramen to anterior nasal spine was 34.3 mm on the left and 34.1 mm on the right [6]. The mean distance between the infraorbital foramen and piriform aperture was 15.56 mm [21].

The infraorbital foramen was located 1.6 ± 2.7 mm lateral, 14.1 ± 2.8 mm superior, and 64.1 ± 9.9^o laterosuperiorly to the alare [22]. In our study that applied same landmarker, the distances between the alare and P1 (cross point between alare vertical line and infraorbital foramen horizontal line), and P1 and infraorbital foramen were 12.92 ± 3.75 mm and 7.88 ± 2.56 mm, respectively. The distance between the alare and infraorbital foramen was 15.42 ± 3.54 mm, and the vertical angle (Angle 1) between these structures was 31.67 ± 13.36^o. In summary, the infraorbital foramen was located 12.92 ± 3.75 mm superior, 7.88 ± 2.56 mm lateral, and 31.67 ± 13.36^o superolaterally to the alare which was similar to the results reported by Song et al. [22].

Although numerous studies that measured the location of the mental foramen using the mandible and clinical images have been conducted [11,15,23], they have weaknesses that it is difficult to visually distinguish the mental foramen accurately on radiographs, and there being no absolute anatomical landmarks for this foramen. Therefore, location of the mental foramen was assessed primarily based on the distance from the surrounding anatomical reference points and the spatial relation with teeth [12,24,25].
    

 The mental foramen was located 13.2 mm superior to the mandibular inferior margin and 24.4 mm lateral to the facial midline [7,19]. With reference to the teeth, the mental foramen was generally located inferior to the lower second premolar [17]. But, the mental foramen was located in between the lower first and second premolars in 37.75% and in the lower second premolar in 27.5% [26].

 Because the location of the mental foramen vary between races and individuals, it is difficult to predict the location of this structure using intraoral landmarks such as teeth, especially in endentulous patients or oligodontia patients [2,15]. Therefore, to avoid these errors, it is required to get the research data on the location of mental foramen using softtissue landmarks.

The mental foramen was located 20.4 ± 3.9 mm inferior, 3.3 ± 2.9 mm medial, and 9.2 ± 8.1^o  inferomedially to the cheilion [22]. In our study that applied same landmarker, the distances between the cheilion and P2 (cross point between cheilion vertical line and mental foramen horizontal line), and P2 and mental foramen were 21.83 ± 3.26 mm and 5.56 ± 3.37 mm, respectively. The distance between the cheilion and mental foramen was 22.74 ± 2.96 mm, and the vertical angle (Angle 2) between these structures was 14.05 ± 10.12^o. In summary, the mental foramen was located 21.83 ± 3.26 mm inferior, 5.56 ± 3.37 mm medial, and 14.05 ± 10.12^oinferomedially to the cheilion. Overall, our results showed slightly higher values in comparison with the results reported by Song et al. [22]. The mental foramen was located mostly medial to the vertical line of the cheilion, however, in three samples, it was observed to be located more laterally than the cheilion.
 

Based on the relative location of the major facial foramen in Koreans, the infraorbital foramen was located more laterally than the supraorbital and mental foramina. This might be because of the infraorbital foramen, which is part of the maxilla, was influenced by the lateral growth of the zygomatic arch [7]. As such, in our study, the mean distances between the bilateral infraorbital foramina and bilateral mental foramina were 58.09 ± 4.04 mm and 50.32 ± 1.93 mm, respectively, and the infraorbital foramen was located more laterally than the mental foramen. In addition, the mean distances between the bilateral alares and bilateral cheilions were 41.22 ± 3.44 mm and 58.43 ± 6.62 mm, respectively. These results could imply that the infraorbital foramen was located lateral to the alare and on the vertical line of cheilion. However, the cheilion layed far from the infraorbital foramen and thus did not constitute a better landmark than the alare.
 

 The results of this study provide more accurate data on the locations of the infraorbital and mental foramina during ophthalmological, plastic, maxillofacial surgery and local anesthesia procedures in the facial region, when used together with the hard-tissue landmarks. Especially, the results on the location of mental foramen will be useful anatomical data for dental surgery in endentulous patients or patients with missing teeth in the mandibular premolar region.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (KRF-2008-313-E00549).