[Effect of different root control attachments on the mesially movement of upper molar with plastic aligner]

Three-dimensional (3D) finite element analysis (FEA) was used to explore the mechanical effect of the upper molar with different root control attachment in plastic aligner-based orthodontic technology.

A three-dimensional(3D) finite element model was derived from a cone-beam CT(CBCT) scan data and then built in the software of Mimics, Geomagic, Solidworks and Ansys. A model with an upper left first molar and its periodontal apparatus, different types of light-cured composite attachments and thermoformed plastic aligners were constructed. Four models were constructed: model A without composite attachments, model B with vertical rectangular attachment, model C with one optimized semiterete attachment, model D with two optimized semiterete attachments. Four FE models were used to analyze the effects of initial force system and displacement pattern of the molar in plastic aligner-mediated simulating mesial movement with different attachments. The initial tooth displacement, tension-compression pattern at the PDL and Von Mises stress in mesial alveolar fossa of different FE models were compared.

In model D with two optmisied semiterete attachments, the compression stress of mesial periodontal ligament and the stress of mesial alveolar fossa were most widely distributed. The rotation axis was closest to the apical foramen, the root control effect of the molar was the best. In model B with buccal vertical rectangular, the compression stress of mesial periodontal ligament closer to buccal side was obviously wider than that of the palate. The rotation center of the buccal side was obviously near the apical foramen than the palatal side. The effect of buccal root control was better than that of palatal side. In model C with one semiterete optimized attachment, the stress pattern of the periodontal ligament in the mesial surface and the displacement patterns were not significantly different from those of model A with no attachments, but the maximum displacement of model C was greater than that of model A.

The addition of attachments increased the molar's movement efficiency. Two semiterete optimized attachments on the buccal side had the best root control effect during mesial movement of the molar, but all attachments achieved tipping movement of the molar.