## 4 Discussion
However, if growth is not excessive, it is not expected to cause problems, and the results of this study showed no functional occlusal plane (i.e., posterior occlusal plane) changes after molar distalization with TADs. Extrusion of the maxillary incisors can be problematic. A deep anterior overbite and excessive incisor display are functional and esthetic issues. Harmony between condylar guidance and incisal guidance during functional movement of the mandible may be jeopardized with steeper incisal guidance caused by extrusion and upright maxillary incisors. Therefore, clinicians should be aware of maxillary incisor extrusion in patients scheduled to undergo maxillary molar distalization. Compensatory maxillary incisor intrusion combined with careful considerations of the dento-skeletal frame should be considered when formulating the treatment plan. In our preliminary study, group 1 showed no anatomical occlusal plane changes, whereas the other two groups showed an increase in anatomical occlusal plane inclination. This may have occurred because not all features of the patients included in this study were controlled. Both extraction and nonextraction cases were included, the initial anterior overbite was not compared, skeletal vertical patterns were not controlled, and the mechanism of change in the anterior overbite was not calibrated among the groups. This heterogeneity in the sample characteristics may have resulted in different anatomical occlusal plane changes. As there were no functional occlusal plane changes before and after treatment in all three groups, these characteristics may be the reason underlying the anatomical occlusal plane change and not maxillary molar distalization. Furthermore, we found that some of these features were not controlled in previous studies that measured the occlusal plane (the anatomical occlusal plane, to be precise) after maxillary molar distalization. This could be the reason why most studies reported occlusal plane changes after maxillary molar distalization, whereas some of the others did not. The exact reason for the lack of change in the functional occlusal plane is unclear within the scope of limitations of this study; they were biomechanically expected to increase in groups 1 and 3. We postulate four possible mechanisms for this finding. First, we simultaneously used a fixed buccal appliance with a molar distalizer. This could have minimized the distal tipping tendency of molars during distalization. Second, the unchanged mandibular occlusal plane stabilized with a fixed buccal appliance may have played a role in functional homeostasis. Oral function, especially mastication and biting, may have helped maintain the pretreatment value of the maxillary occlusal plane. Third, molar distalization was not as fast as the conventional maxillary molar distalizer (i.e., the pendulum appliance), which provided time for the molars to adapt to a new position (distal to their original position) and maintain their original occlusal plane. Fourth, the amount of distalization in this study may have been too small to trigger significant functional occlusal plane changes. Eventhough measuring sagittal movements in the second molar using two-dimensional lateral cephalography underestimates the real amount of movement, our study sample showed slightly less molar distalization than other maxillary molar distalization studies. The aim of this study was to review previous studies that evaluated occlusal plane changes after maxillary molar distalization with TADs and evaluate and compare occlusal plane changes according to various molar distalization methods. Some critical variables were not appropriately controlled because this was a type of preliminary study. Further clinical studies with properly controlled variables are required. In addition, discriminating between the functional occlusal plane and the anatomical occlusal plane is strongly recommended in future studies.