Abstract:

Objective To discuss the biomechanical effect of the invisible orthodontic appliance in the remote displacement of mandibular molars assisted by the micro-implants in different parts by finite element analysis method， and to identify the optimal scheme of the micro-implant implantation site. Methods The cone beam computed tomography（CBCT） data of an Angle Class Ⅲ adult male patient with malocclusion defermity was obtained， and the Mimics Medical and 3-Matic modeling software were used to establish a three-dimensional finite element model of the remote mandibular molars with the invisible orthodontic appliance. According to whether the microimplants were used， the patients were divided into control group （without microimplants， condition 1）， and three experimental groups ［interroot micro-implant group of the first and second mandibular premolars （condition 2）， interroot micro-implant group of the second and first mandibular premolars （condition 3）， and interroot micro-implant group of the first and second mandibular molars （ condition 4）］. In the Ansys Workbench finite element analysis software， the second molar of mandible of models in various groups was moved at a step of 0.2 mm， and the molar displacement assisted by traction from the micro-implant to the invisible orthodontic appliance was applied with 2 N/side. The tooth displacement trends， deformation characteristics of the invisible orthodontic appliance，and Von Mises equivalent stress nephograms were analyzed. Results The distal and intrusive movement of the teeth to be treated were in the order of condition 4 > condition 3> condition 2> condition 1， and the distal movement of the second mandibular molar in condition 4 was 0.188 mm. In condition 1， the orthodontic teeth showed the displacement trend of mesial and labial movement， while in experimental groups， the orthodontic teeth showed the displacement trend of distal and lingual movement in the order of condition 4> condition 3 > condition 2. The extrusion deformation variable between the first molar and the second molar was the largest， and the stress peak value was 192.15 Mpa. After the stress was released， the stress concentration in control group was still located between the first molar and the second molar of the appliance， while the stress concentration in experimental groups was located on buccal surface of the appliance， and the stress peak value in condition 4 was 56.48 Mpa. Conclusion The use of micro-implant anchorage to assist the distal displacement of mandibular molars can increase the molar displacement and reduce the loss of anterior anchorage. The further back the implant site is， the more obvious the effect of molar displacement is， and the higher the tooth movement efficiency of the invivible orthodontic appliance is.

Key words: Finite element analysis, Invisible orthodontic appliance without brackets, Molar distalization, Micro-implant anchorage