## 4 Discussion
The maxillary second molar underwent less distal tipping than the first molar, presumably because second molars are often distally impacted in the untreated state and undergo mesial tipping during teeth leveling. Of 28 patients, sixteen showed post-treatment mesial tipping of the second molars. The mandibular molars underwent more distal tipping than maxillary molars, especially the mandibular second molars. The anatomical structure of the mandible, which includes the lingual cortex, can be an obstacle for distalizing mandibular molars, which causes distal tipping rather than bodily movement [32]. [PERSON] and [PERSON] reported that distalization using a distal jet resulted in the distal tipping of the first and second molars (8.36\({}^{\circ}\) and 11.99\({}^{\circ}\), respectively), noting that the molar relationship could be corrected but its retention stability was questioned [14]. [PERSON] et al. predicted high-stability treatment for total arch distalization on the ground of the posterior movement close to bodily movement [26]. As shown in this example, the degree of tipping movement of the posterior region is generally believed to be correlated with the stability of treatment outcome. The results of this study also verify the correlation (\(r=-0.634\), \(p<0.01\)) between the degrees of distal tipping of the mandibular second molar during the treatment period and its mesial tipping during the retention period as a result of the correlation analysis of the changes in the molar angulation during the treatment and retention periods. In other words, the larger the distal tipping during the treatment period, the larger the mesial tipping during the retention period, which underlines the need to induce molar bodily movement to ensure a stable treatment outcome. The upper and lower lips relative to the E-line moved distally after distal retraction of the anterior teeth by 0.89 and 1.06 mm, respectively. Although mesial drift of the maxillary and mandibular incisor was observed during the retention period, the upper and lower lips did not change significantly, which means that the distalization effect was stable from the viewpoint of the soft tissue profile. [PERSON] et al. reported a change in the soft tissue profile between the ages of 15 and 25, with the upper and lower lips relative to the E-line moving distally, and the same tendency between 25 and 45 years of age, so that such soft tissue change could affect the determination of the extraction and non-extraction [33]. Therefore, considering the fact that the lips move distally with increasing age, it is efficient to resolve crowding and improve the facial profile using the TADs without extraction, which will provide a better facial profile in the long term. The results of careful observation of post-treatment changes in both anterior and posterior segments of distalization suggest that a method of distal movement of the entire dentition using TADs is advantageous over a method using intraoral distalization appliances such as a pendulum or distal jet, because skeletal anchorage prevents anterior anchorage loss. Even in case of anterior arch crowding, distalization of the buccal segment can provide alignment space for the anterior segment so that arch distalization can be performed after anterior alignment, thus preventing round tipping of the anterior segment. [PERSON] et al. reported that total arch distalization can shorten the treatment period compared with distalization using an intraoral appliance despite slower movement of each individual tooth, because all teeth are simultaneously distalized [26]. The mean treatment period in this study was 24.5 \(\pm\) 9.6 months, similar to 20.0 \(\pm\) 4.9 months reported by [PERSON] et al. In [PERSON] et al., who performed maxillary arch distalization using the similar method, posterior intrusion and anterior extrusion were observed [27]. In our study, however, both anterior and posterior sections showed intrusion.