A comparative study on the location of the mandibular foramen in CBCT of normal occlusion and skeletal class II and III malocclusion

The mandibular foramen and the mandibular canal form during the process of intramembranous ossification of the mandibular ramus and the body of mandible. During the 24th week of the embryonic stage, a groove forms that contains the nerves and blood vessels, and the shapes of the mandibular foramen and canal completed as ossification progresses [12]. Starting from the mandibular foramen within the ramus, the mandibular canal containing the inferior alveolar nerve and blood vessels descends in antero-inferior direction and then runs horizontally once it reaches the molar area of the mandible body. At this point the canal splits into the incisive canal, which runs from the premolar to the anterior mandible, and the mental canal, which runs in a postero-superior direction and opens below the apical root of the second premolar, becoming the mental foramen [13].

Although inferior alveolar nerve block is frequently used as a local anesthetic method for restorative treatment and surgical treatment of mandibular molars [14, 15], Malamed et al. [16] reported that this method is associated with a high clinical failure rate of up to 15 to 20 %. This can be explained by the fact that the positions of the mandibular ramus and foramen vary widely from person to person [14]. In addition, if surgeon fails to identify the exact anatomical position of the mandibular foramen during orthognathic surgery, complications may ensue, such as damage to the inferior alveolar nerve or blood vessels. Thus, the position of the foramen serves as a critical anatomical reference point for reducing the risk of complications and for the success of inferior alveolar nerve block anesthesia [3].

The position of the mandibular foramen is known to vary with age. According to Hwang et al. [17], the mandibular foramen is located below the occlusal plane during the deciduous dentition stage and is positioned at 4.14 mm above the occlusal plane in adults. Kanno et al. [18] have reported that the mandibular lingula can be seen at 6 mm above the occlusal plane in children ages 7 to 8 and at 10 mm above the occlusal plane in children ages 9 to 10. In addition, during deciduous dentition the anterior-posterior position of the foramen is in center of the ramal surface but will then move slightly toward the back [19]. To avoid the possible effect of this positional change, in this study, only patients who were 18 to 31 years of age were included.

It is critically important to accurately identify the position of the mandibular foramen in the clinical setting. Radiography makes this possible in a non-invasive manner, and panoramic radiography is the most commonly used technique, because it enables the practitioner to simultaneously observe the teeth, jaw and temporomandibular joint [20–22]. Although panoramic radiography is cost-effective and easy to handle, magnification varies depending on camera type [1], which may result in distorted or deformed images if the patient’s jaw bone is not positioned correctly in the focal trough. CT enables a three-dimensional evaluation by the reconstruction of images, resulting in better precision. With the images obtained by means of panoramic radiography, the probability of identifying the incisive canal in the mandible in two different studies was 2.7 % [23] to about 15 % [20]. In comparison, three studies evaluated CT images and reported corresponding values to be 83 % [24] to 100 % [25, 26]. Although CT offers many advantages, it is expensive and difficult to use in the clinical setting and is therefore employed less often. In contrast, when compared with conventional CT, CBCT requires a lower dose of radiation, is easier to control, and minimizes metal artifact and is therefore used more frequently in clinical setting. For these reasons, this study relied on the more accessible CBCT to analyze the position of the mandibular foramen.

In their study comparing the anterior-posterior position of the mandibular foramen, da Fontoura et al. [9] noted that the foramen is positioned in the middle third of ramus. In addition, Trost et al. [10] found that the mandibular foramen did not exist in the region of the upper and posterior third of the ramus. In their study of the distance from the anterior edge of the ramus to the foramen in a horizontal relationship, Afsar et al. [27] analyzed radiographs and found this distance to be an average of 20.20 mm. In their analysis of 40 Taiwanese patients, Yu et al. [28] reported this distance to average 18.00 mm in the women and 19.30 mm in the men. In addition, Kaffe et al. [1] measured this distance as an average of 20.26 mm using panoramic radiography of dry mandibles.

In contrast, Seo et al. [11] in their examination of panoramic radiographs from Korean patients diagnosed with either normal occlusion or prognathism, compared the distance from the anterior edge of the ramus to the mandibular foramen and found it to be average of 24.48 mm in those with normal occlusion and 24.535 mm in those with prognathism. In this study using CBCT, however, this distance was an average of 19.41 mm, 19.01 mm, and 19.85 mm in the normal occlusion group, the skeletal class II malocclusion group, and the skeletal class III malocclusion group, respectively. Although these results were lower than those in the study by Seo et al. [11] and there was no significant difference among the three groups, when this distance compared in the male and female patients in the skeletal class III malocclusion group, it averaged 20.47 mm in the men and 19.04 mm in the women–that is, 1.43 mm longer in the men–and the difference was statistically significant (p <0.05). With regard to the horizontal width of ramus, the average values were 31.99 mm in the men and 30.16 mm in the women–that is, 1.83 mm longer in the men.

Lima et al. [29] compared the vertical position of the mandibular foramen and reported that the distance from mandibular notch to the foramen served as a critical reference point for identifying the position of the mandibular foramen during orthognathic surgery and thus gave it clinical importance. They reported an average distance of 27.70 mm, whereas Gutierrez-Ventura et al. [30] reported an average distance of 17.44 mm. In a report by Yu et al. [28], these values were 22.70 mm in their male patients and 20.50 mm in their female patients. In this study, the distance from the mandibular notch to the mandibular foramen was 21.59 mm in the normal occlusion group, 20.49 mm in the skeletal class II malocclusion group, and 18.77 mm in the skeletal class III malocclusion group. These values were similar to those reported in other studies, although the value was significantly lower in the skeletal class III malocclusion group.

In an analysis of the occlusal plane, the position of mandibular foramen was 0.10 mm below this plane in the normal occlusion group, 0.03 mm below this plane in the skeletal class II malocclusion group, and, significantly, 2.79 mm above the occlusal plane in the skeletal class III malocclusion group. These results were the same as those in a study conducted by Seo et al. [11].

The average vertical length of the mandibular ramus at about 2.89 mm above the occlusal plane. The average vertical length of the mandibular ramus was 48.38 mm in the normal occlusion group, 44.47 mm in the skeletal class II malocclusion group, and 52.94 mm in the skeletal class III malocclusion group, in whom it was the longest. This difference appears to be the reason for the difference in the vertical position of the mandibular foramen among the groups studied.

In addition, there were significant differences in the distance from the occlusal plane to the mandibular foramen in the intergender comparisons. Specifically, this distance averaged 1.89 and 0.04 mm in the men and women, respectively, or 1.85 mm greater in the male patients. The average vertical length of the ramus was 52.6 mm in the men and 45.88 mm in the women, or about 6.76 mm longer in the male patients. These results were consistent with those of a study by Indira et al. [31] and indicate that variations in the vertical length are due to differences in the length of the mandible ramus.

Until now, no other study has compared the position of the mandibular foramen among patients with normal occlusion, skeletal class II and III malocclusion. Here, CBCT images determined that the position of the foramen varies from person to person and is higher in patients with skeletal class III malocclusion. The intergender comparisons showed that the mandibular foramen is higher in men than in women. Thus, CBCT can most likely be used effectively to evaluate the position of the mandibular foramen prior to inferior alveolar nerve block anesthesia or orthognathic surgery. In addition, it will be useful to reconstruct images of the mandible obtained with CBCT to create three-dimensional images and then to compare the position of the mandibular foramen with the results of this study.