Determination of Optimal Loading Conditions Using Lever Arm and Mini- Implant System for En-masse Retraction in Lingual Orthodontics – A FEM Study

Abstract

Aim: To evaluate the stress distribution along the periodontal ligament and alveolar bone by various combinations of mini-implants and lever arm used during en-masse retraction of anterior teeth using finite element method.Materialsand Methods: Four three-dimensional finite element models of the bilateral maxillary first premolar extraction casewas constructed. Lingual brackets were (0.018” slot) positioned over the center of the clinical crown. In all four models 150 g of retraction force with the help of the NiTi closed coil springs was applied with different combinations of mini-implants and lever arm. A finite element analysis was then performed to evaluate stress distribution, the principle stress, von Mises stress and displacement of the anterior teeth using ANSYS 12.1 software.Result: In this study the constructed models along with the load application were imported into ANSYS (version 12.1) software for analyzing the displacement and stress distribution corresponding to the force application. The maximum tensile stress of periodontal ligament were observed at distal root apex area of canine in all four models. Variable amount of displacements like lingual crown tipping, lingual root tipping and extrusion were observed in all the models suggesting different combinations of lever arm and mini-implants affect the direction of the tooth displacements in lingual mechanics.Conclusion: In lingual orthodontics tipping movement of crown decreases as the height of lever arm increases. It was also found that when mini-implants were placed at higher position the amount of tipping movement decreases with minimum amount of extrusion seen in central incisor and lateral incisor.