3D Finite Element Study of En masse Retraction of Maxillary Anterior Teeth in Two Typical Force Directions

Objective: To investigate the displacement and stress distribution of maxillary anterior teeth during retraction using the finite element method. Methods: A three-dimensional finite element model of six maxillary anterior teeth with a straight wire appliance on intermaxillary bone was established in ANSYS 8.1. Retractions in two typical force directions were simulated: case 1 simulated routine anterior tooth retraction by a force of 150 g applied between the anterior hook of 2.0 mm and the first molar tube; case 2 simulated anterior tooth retraction by a force of 150 g between the anterior hook of 4.0 mm and the implant 10.0 mm towards the gingiva from the first molar tube. Results: This finite element model was highly realistic for the simulation of orthodontic tooth movement. In case 1, lateral incisors showed controlled lingual crown tipping movement, while central incisors and canines showed lingual crown tipping movement. Stress concentration was found close to the cervix and root apex of each tooth. In case 2, the displacement and stress level was much higher Central incisors and canines still showed lingual crown tipping movement while lateral incisors showed bodily retraction and intrusion. In the coronal direction, the lateral incisor root tipped mesially while the canine root tipped buccally. Stress distribution on central incisors and canines was similar to that in case 1, but the maximum principal stress on labial and lingual surfaces of lateral incisors was mostly compressive stress. Conclusion: During routine anterior tooth retraction by sliding mechanics, the crown of anterior teeth tends to tip lingually Retraction tbrce passing near the centre of resistance of six anterior teeth makes their displacement and stress distribution more uniform, but their translation is still undetectable. Loading on lateral incisors is greater than that on other teeth and stress concentration at its lingual apex should be considered.