## 4 Discussion
From the biomechanical perspective, no changes were expected in the occlusal plane in group 2 because the distalizing force passes near the center of resistance of the maxillary molars, which may not cause distal tipping of molars. A clinical study using the same maxillary molar distalizer as that was used in group 2 showed no distal tipping of the maxillary molars after distalization [10]. Similarly, changes in the occlusal plane were expected in groups 1 and 3. For group 1, the distalization force vector passes occlusal to the center of resistance of the maxillary dentition, thereby causing a clockwise rotation of the maxillary dentition that results in a steepening of the occlusal plane. For group 3, it is well-known that pendulum arms significantly tip the maxillary molars distally, and this tipping moment might also tip the maxillary occlusal plane to a steeper cant [10,19]. The results of the current study showed an unexpected increase in the anatomical occlusal plane in group 2 and no changes in group 1. As expected, group 3 showed steepening of the anatomical occlusal plane. A finite element method (FEM) study investigated the changes in the maxillary dentition in response to whole arch distalizing forces according to various force directions measured from the maxillary occlusal plane [35]. The authors found that all angulated distalization forces from \(-30^{\circ}\) to \(+30^{\circ}\) caused lingual and distal tipping of the incisors and molars. This movement pattern created a clockwise rotation of the occlusal plane, which included extrusion of the incisors and intrusion of the second molars. This FEM study design was very similar to that of group 1. Another clinical study investigating dental and skeletal changes after maxillary molar distalization and long-term follow-up using the same set-up as group 1 reported an increase in the anatomical occlusal plane after treatment, which was stable after two years of follow-up [16]. Unlike the previous FEM and clinical studies, in the current study, we found no significant changes in either the anatomical or functional occlusal plane. According to a systematic review of miniscrew-supported maxillary molar distalization, maxillary molars distally tipped when distalized, and this distal tipping was less when force was applied from the palatal side [13]. This systematic review also stated that the reason for a lesser degree of distal tipping with palatal distalizing forces is that the force can be located close to the center of resistance of the molar. This situation is very similar to that of group 2 in this study, where it was expected that the occlusal plane changes would not occur due to less distal tipping. Two clinical studies conducted by the same research groups utilized a palatal plate as an anchor for maxillary molar distalization, a design similar to that of group 2 in this study. They found an increase in the occlusal plane (mean 2.81\({}^{\circ}\) and 3.77\({}^{\circ}\)) after molar distalization [14; 17] and stated that the reason for this occlusal plane change was the extrusion of the maxillary incisors and intrusion of the maxillary molars [17]. These study results are consistent with the current result: the anatomical occlusal plane increased after maxillary molar distalization. After reviewing the studies published on maxillary molar distalization with TADs, it seems logical to expect that the anatomical occlusal plane would become steeper after maxillary molar distalization, irrespective of the location of the TADs and the appliance design. There are some discrepancies among the studies, but a majority of them have reported the intrusion of the maxillary molars and extrusion of the maxillary incisors. The intrusion of the maxillary molars and resultant occlusal plane change should be considered with functional problems. Several studies found a possible correlation between occlusal plane cant and jaw growth direction; therefore, this should be considered when treating growing individuals with maxillary molar distalization [25; 36].