Objective
To explore the three-dimensional displacement patterns of maxillary molar distalization using clear aligners (CA) under four anchorage designs, and to provide guidelines for selecting traction methods that ensure optimal outcomes in clinical practice.
Materials and Methods
A three-dimensional finite element model was constructed based on CBCT and digital model from an adult patient who needed maxillary molar distalization. The model included cortical bone, cancellous bone, periodontal ligament, teeth, CA and mini-screws. Four working conditions of anchorage designs were set when distally moving bilateral maxillary second molars, first molars and second premolars in sequence, they were (a) intramaxillary anchorage group; (b) intermaxillary anchorage group; (c) buccal mini-screw anchorage group; (d) palatal mini-screw anchorage group, respectively. The displacement patterns of anterior teeth and molars were analyzed and calculated with ANSYS software, and the results were compared with actual displacement of the patient’s maxillary dentition.
Results
The actual displacement pattern of most teeth closely mirrored the predicted results from the finite element simulation, with the sole exception being the central incisors. That is, the central incisors exhibited distal tipping in three-dimensional finite element simulation, but its actual movement was mesial tipping. Among the finite element simulation of teeth sequence movement, when only distally moving bilateral maxillary second molars, the maxillary anterior teeth in all groups exhibited labial tipping and extrusion, among which Group (a) and Group (b) exhibited a greater degree of labial tipping; the second molars in all groups exhibited distal tipping, buccal tipping and intrusion; the first molars of all groups exhibited buccal tipping and extrusion, and exhibited distal tipping in Group (c) and Group (d) while mesial tipping in Group (a) and Group (b). When simultaneously distally moving bilateral maxillary second and first molars, the displacement patterns of anterior teeth were slightly different from that of only moving bilateral second molars, that is, the right canines in Group (c) and Group (d) exhibited palatal tipping and intrusion; the first and second molars in all groups exhibited distal tipping and intrusion; the second molars exhibited palatal tipping while the first molars exhibited buccal tipping. When initiating the distal movement of bilateral second premolars, the anterior teeth in Group (c) and Group (d) exhibited a lower degree of labial tipping and extrusion, even exhibited palatal tipping and intrusion; the second molars in Group (a) and Group (b) exhibited mesial tipping, buccal tipping and extrusion, whereas the first molars in all groups and the second molars in Group (c) and Group (d) exhibited distal tipping, palatal tipping and intrusion. In addition, there was no significant difference in the torque and vertical control of anterior teeth and molars between the buccal and palatal mini-screw anchorage group.
Conclusions
Compared to intramaxillary or intermaxillary anchorage, the combination of mini-screws anchorage has a better effect on the torque control of anterior teeth, which also helps to promote the distal movement of molars. There is no significant difference in the torque and vertical control of anterior teeth and molars between the buccal and palatal mini-screws. Additionally, the displacement patterns of bilateral maxillary were not always symmetrical.
