The introduction of TADs in orthodontic treatment enables predictable molar distalization with minimal patient compliance . However, little is known about the posterior limit for the mandibular arch. Another issue that has received little attention to date is the limitation to the alveolar bone housing for posterior teeth cause by the inner and outer lingual cortices of the mandibular body. Currently, CBCT is the most complete and efficient imaging tool for diagnosis and planning of orthodontic treatment . The purpose of this study was to investigate the spatial limitation associated with cortical contact with the mandibular second molar during mandibular arch distalization analyzed through CBCT.
Our analysis revealed that the roots of the mandibular second molar were in contact with the cortical bone in 33 out of 67 subjects (49.3%). In four cases, the contact point appeared in the axial view but not in the coronal view. In four other cases, the contact point appeared in the coronal view but not in the axial view. Thus, when contact was assessed only in a specific plane, the contact ratio was underestimated. Therefore, assessment should perform in at least two dimensions. When we evaluated the contact condition only in sagittal view, the contact ratio was 37.3% (25/67), which was similar to the contact ratio for cases of skeletal Class I with normodivergent facial profile as per a study (35.3%) . In the serial studies conducted by Emes et al. and Aktop et al., the contact ratio of lingual soft tissue with the roots of mandibular third molars was 26–34.4% [16,17].
The total ridge width, alveolar housing width, and available distalization distance demonstrated no significant difference according to the side of the mandible assessed or to malocclusion category. The total ridge width was wider in men than in women (p < 0.01), similar to the findings of Zhang et al . In the root-cortex contact group, the alveolar housing width was smaller at the subcrest 12-mm level, which was similar to the total ridge width. However, contact most frequently occurred at the subcrest 8- and 10-mm levels. Actually, the total ridge and alveolar housing were wide enough to contain the molar root, and the buccolingual width of the ridge only contributed partly to the contact. The most superior point of the submandibular fossa was 5.0–10.75 mm. Consequently, not only the size and position of the submandibular fossa but also the concavity of the fossa had an effect on the contact condition. Moreover, the distal limit and the position of the molar root must both be taken into consideration.
Prior to any tooth movement, the contact ratio was 49.3%. In the non-contact group, the least average space available for safe distalization was 1.81 mm. To safely distalize the whole arch, we recommend CBCT-based space assessments in advance of treatment planning. In the group for which wisdom teeth were present, the space for distalization was larger (Table 5), with a significant difference at the subcrest 2- and 4-mm levels (p < 0.01). In the group with wisdom teeth, the lingual side spaces available for distalization were 7.51, 7.69, and 6.57 mm. However, in the group without wisdom teeth, the corresponding spaces were 2.93, 3.99, and 3.93 mm. Regarding the maximum effect in the distal space, dental practicians can distalize the entire arch immediately after extraction of the wisdom teeth, which has the additional advantage of the regional acceleratory phenomenon . The available distalization distance was greater in cases with Class II malocclusion, although no significant differences were observed among the three types of malocclusion (Table 5). Cases with Class I and Class III malocclusion required molar distalization, but the available space was smaller (p > 0.01).
In cases where whole-arch distalization is planned instead of extraction, the patient should undergo root-cortex contact scanning first. For patients whose roots are in contact with the cortical bone, iatrogenic damage can occur if dental practicians distalize the molar unintentionally. For example, resorption of the root and periodontal tissue could occur when the root invades the cortical bone. In our study, the root had already invaded the cortical bone in two patients prior to any movement (2.99%), and the tip of the mandibular second molar root was found to be exposed in one of these cases (Fig. 5). As in other studies, dehiscence was also seen when the mandibular width was surveyed . Therefore, before performing distalization, dental practicians should consider the available distance. If the most posterior mandibular teeth move too far, there may be no antagonist maxillary tooth.
In orthodontic treatment, thinner ridges tended to exhibit greater resorption . Moreover, a thin ridge indicates a thin cortical bone . Thin cortical bone is sensitive to forces and prone to microfracture, leading to vertical bony destruction. However no changes occurred in alveolar bone ridge height at the maxillary lateral incisors with space opening in patients with congenitally missing maxillary lateral incisors . Ridge preservation through orthodontic treatment could maintain the height at the lateral incisor regions . Other factors can nevertheless influence these changes, for example, the direction of movement, the original position of the tooth, and the root-cortex distance . The identification of more factors in future studies will provide dental practicians with a comprehensive overview of the interaction between tooth movement and alveolar height.
This study had some limitations. First, a priori power analysis was not used for the determination of the minimum required sample size. A power analysis should be considered during the design of the study to prevent drawing conclusions that are outside the study’s level of sensitivity. Statistical significance level was set at 1 percent and not at 5 percent in this study. A smaller p value as a means presents more significant findings. Second, soft tissue distal to the mandibular second molar was not taken into account. Clinically, there is thick soft tissue overlying the retromolar pad area that can result in considerably mandibular second molars being partially covered by the soft tissue. On the other hand, the lack of attached gingiva can be a limiting factor for molar distalization. An adequate amount of attached gingiva should be present around the retracted mandibular second molar to maintain periodontal health.