Three-dimensional finite element analysis of en masse retraction with integration of maxillary anterior teeth

Background: As a novel force system to safely and efficiently retract the maxillary anterior teeth in orthodontic treatment, we applied an en masse traction method with integration of the maxillary anterior teeth (EMTI) and quantitatively evaluated tooth movement using a three-dimensional (3D) finite element analysis to examine the validity of EMTI. Methods: A 3D finite element model (FEM) of the six teeth of the maxillary dentition, periodontal ligament, and alveolar bone was created. A two-tooth model of the bilateral central incisors, four-tooth model of the bilateral central and lateral incisors, and six-tooth model of the bilateral central and lateral incisors and canines were reconstructed as EMTI models. Each tooth was splinted to the other with a palatally-attached wire, and the moment arms were attached to this wire and extended apically. Traction points were set along the moment arms at 6, 8, and 10 mm from the level of the palatal wire. FEM analysis was performed by applying a traction force of 1.0 N in the palatal direction to the traction point of each moment arm. For the maxillary central incisors of each model, the angular changes in the tooth axis and center of rotation (C-Rot) were investigated. In addition, the 3D displacements of the crown and apex of each tooth were analyzed for each EMTI model. Results: In the two-tooth and four-tooth models, traction of 6 mm showed palatal inclination of the tooth axis, and the C-Rot was located near the root apex. Traction by 8 mm showed bodily tooth movement, and traction of 10 mm demonstrated labial inclination of the tooth axis, in which the crown was displaced labially and the root was displaced palatally. In the six-tooth model, traction by 10 mm exhibited en masse palatal movement of all six teeth without inducing a bowing effect at the canines. Conclusions: FEM analysis of the EMTI technique provided useful information for understanding the 3D movement of the root in clinical practice.