The distalization of mandibular molar teeth is one of the difficult treatments in orthodontics [4]. The factors such as the dense cortical bone structure, the limitations in the placement of temporary anchor devices, and the anatomical structures that may limit tooth movement, complicate the distalization of mandibular molars [8, 13, 14].
Dang et al. [9] stated that in CBCT, the roots of the lower second molars are in contact with the inner lingual cortex of the mandibular corpus, and the limits of the distalization movement should be investigated in order to prevent this. Lateral cephalograms provide limited information regarding the retromolar area and root contact with the inner lingual cortex [13]. Kim et al. [13] mentioned that the limit of mandibular molar distalization is not the anterior of the ramus, but the lingual cortex of the mandibular corpus, and emphasized the importance of 3D imaging.
In our study, the anatomical limits of molar distalization in the mandibular jaw in individuals with different sagittal and vertical growth patterns were determined. For this purpose, we focused on the bone structures in the retromolar area that may limit the distal movement of the right mandibular second molar tooth. Only hyperdivergent and normodivergent individuals were included in our study because sufficient number of Class III hypodivergent individuals could not be reached in approximately 2500 scanned CBCT images. This may be due to the fact that many individuals with skeletal Class III malocclusion have a vertical growth pattern. Mandibular molar distalization is an orthodontic camouflage treatment method that is generally preferred in skeletal Class I or Class III patients with Angle Class III dental relationship. Therefore, as in similar studies, Class II individuals were not included in our study [13, 15].
Most of the studies evaluating the effects of different vertical and sagittal growth patterns on the retromolar area were performed on 2D radiographs and to observe the eruption path of impacted teeth [16–19]. Choi et al. [15] reported that Class III normodivergent individuals had significantly more retromolar area than Class I normodivergent individuals in their retromolar area measurements only at the furcation level on CBCT axial sections. In our study, it was observed that the retromolar area in normodivergent Class III individuals at most root levels was significantly larger than in normodivergent Class I individuals. Soft tissue distal to the mandibular second molar can restrict the amount of distalization movement. After the distalization movement, the distobuccal surface of the mandibular second molar tooth should be located within the borders of the attached gingiva [20]. Since our study is a retrospective study on CBCT sections, it is impossible to look at soft tissue thickness. So, this situation was not taken account.
Mandible morphology of hyperdivergent Class I and Class III individuals may be different. In our study, the retromolar area in hyperdivergent Class III individuals was higher in length at most levels than hyperdivergent Class I individuals, but this excess was found to be significantly higher only at the CEJ10mm level.
Kim et al. [13] emphasized that different vertical growth patterns may affect the remodeling of the mandibular ramus and thus the retromolar area, and this issue should be taken into account. Zhao et al. [21] reported that class I hyperdivergent patients had the smallest retromolar area in their study, which they performed on CBCT sections in only class I patients. In our study, it was seen that the retromolar area in normodivergent individuals was higher in length than in hyperdivergent individuals in both Class I and Class III individuals at the root level, but this excess did not have a significant impact on the retromolar area. This finding shows that vertical growth pattern does not have a significant impact on the retromolar area. Future studies addressing the 3D mandible morphology of patients with different vertical growth patterns will make significant contributions to this issue.
In this study, intra-group measurements decreased towards the root apex and the lowest values were obtained at the CEJ10mm level in all groups. Kim et al. [13] also stated that in Class I normodivergent individuals, similar to our study, the retromolar area at the root level decreases towards the apex. Choi et al. [15], on the other hand, stated that the retromolar area decreased towards the root apex only in Class III individuals, but there was no such decrease in Class I. The reduction of the retromolar area towards the root apex may be due to the fact that the lingual cortex of the mandibular corpus is more inclined towards the apex in the buccolingual direction than the inclination of the mandibular second molar tooth in the buccolingual direction towards the apex [13]. Although there are studies on the incline of the mandibular corpus [22, 23] in the literature, there are no studies specifically examining the incline of the lingual cortex.
When the tooth root comes into contact with the inner lingual cortex of the mandible, tooth movement slows down and the possibility of root resorption increases [13]. If the movement is continued after contact with the inner lingual cortex, contact with the outer lingual cortex will occur, increasing the possibility of alveolar bone resorption in the region. In severe cases, perforation of the cortex, exposure of the root surface and gingival recession may occur. As a result, the periodontal support of the region is lost [13]. Based on all this information, the limits of molar distalization at the CEJ10mm level in the mandible can be determined.
Choi et al. [15] found that the distance of the distal root of the second molar tooth from the inner lingual cortex in their measurements from the 6 mm apical furcation was found to be greater in Class I than Class III ones, which is not in line with the findings of our study. Kim et al. [13] determined the distance between the root and the inner and outer lingual cortices to be 2.87 mm and 6.73 mm, close to the root apex, in Class I normodivergent individuals, and they stated that the amount of molar distalization to be planned as approximately 3 mm would be reasonable. In our study, these values were determined as 1 mm and 4 mm in Class I normodivergent individuals, respectively. Although Kim et al. [13] stated that the existence of mandibular third molars doesn’t have a significant effect on the retromolar area, there are many studies [17, 18, 24, 25] indicating that the retromolar area is more in individuals whose impacted teeth have erupted into the mouth. Therefore, individuals with impacted teeth were not included in our study in order to ensure standardization and to eliminate the possible effects of impacted teeth. Impacted third molars were not considered as exclusion criteria as they did not affect our measurements. However, it is recommended that these teeth be extracted before the distalization phase from a clinical point of view. The inclusion of individuals with impacted teeth in their study can be considered as the reason why the retromolar area length values in the study of Kim et al. [13] were higher than the values in our study.
It is recommended that in patients with a need for mandibular molar distalization greater than 3 mm, it would be more appropriate to obtain space by premolar tooth extraction [12]. In some individuals, the roots of the molar teeth may be in contact with the inner or outer lingual cortex before starting the distalization movement. In the axial sections we examined, especially in Class I individuals, a high rate of root in contact with the inner lingual cortical bone was detected. It was seen that the highest contact rate in all groups was at the CEJ10mm level. Similar to our study, Kim et al. [13] found the highest contact rate at the root apex level. This can be explained by the reduction of the retromolar area towards the root apex. While the rate of teeth in contact with the inner lingual cortex was high, the outer lingual cortex was found to be very low in all groups. Kim et al. [13] found the incidence of contact between the inner-outer lingual cortex and the root to be 35.3% in Class I normodivergent individuals, and it is thought that the reason why it was seen as 50% in our study may be due to interracial mandibular morphology differences.
It is predicted that the distal root of the mandibular second molar will make contact with the inner cortical bone after an average of 1 mm distalization movement in skeletal Class I individuals and 2.2 mm in skeletal Class III individuals. After this stage, more controlled forces should be given, since the probability of resorption at the root will increase. The mandibular molar distalization limits are approximately 3.4 mm in skeletal Class I individuals and approximately 6 mm in skeletal Class III individuals. If more than these amounts of distalization movement are made, external cortical bone perforation may occur and the tooth root may be exposed. In cases where more distalization movements are planned than these amounts, it is recommended to take CBCT before treatment.