## 4 Discussion
The result of this study shows that the increase in the width of the arches was better maintained than that of the other treatments using TADs. This may be due to the fact that the upper and lower dentitions during the treatment period are occluded and simultaneously moved backwards to help stabilize the width of the arches. Vertical skeletal parameters have been considered a factor affecting the stability of treatment outcome [39]. However, [PERSON] et al. reported that vertical skeletal parameters have no significant influence on the stability of orthodontic treatment outcome [40]. Our correlation analysis revealed no statistically significant correlation between the mandibular plane angle and facial height ratio at the initial status and the teeth movements during the retention period. However, a negative correlation was found between only the ANB angle at the initial status and the amount of maxillary incisor. First and second molar crown mesial drift during the retention period can possibly be explained by the fact that the anteroposterior skeletal factor is more correlated with relapse than the vertical factor. In this regard, further study and investigation of relapse-related parameters are considered necessary. As regards relapse rate, mean horizontal relapse rates of 16.0%, 21.1%, and 28.8% were exhibited by the incisor crown, first and second molar crowns, respectively. What is worth noting here is the high interpatient variability; for example, the first molar relapse rates ranged between 0.79 and 43.84%, and the amounts of mesial drift between 0.02 and 0.87 mm. Further research is necessary to find out the causes of this high interpatient variability and to identify related indicators, which will greatly contribute to developing personalized orthodontic options to reduce overcorrection and relapse. Previous studies on the stability of orthodontic treatment were primarily carried out with pediatric patients, relying on dental casts and the peer assessment rating index or Little's irregularity index for stability evaluation. Therefore, we could not find any study to directly check the results of our study against. The dental casts and index alone are barely enough to determine the relative movements of maxillary and mandibular teeth, and cannot be used to identify the place of relapse. On this note, the significance of this study lies in the fact that it used lateral cephalometric radiographs along with diagnostic casts for the evaluation of the stability of orthodontic treatment in adult patients. The limitations of this study are small sample size, measurements made at the midpoints of the opposite-side teeth, and image overlap with the maxilla and mandible due to the use of radiographs taken in a closed mouth position. [PERSON] et al. used lateral cephalometric radiographs taken in an open mouth position for easier and clearer identification of individual teeth [41]. Additionally, this study included buccal, midpalatal and retromolar miniscrews, which required different biomechanics to obtain the wanted distalization. Prospective studies with a larger number of subjects need to be carried out as follow-up studies.