DOI: 10.14457/tu.the.2015.1547
title: Effect of force directions in maxillary anterior teeth retraction with skeletal anchorage: finite element analysis

description: The aim of the study was to compare the force directions to allow bodilylike parallel retraction of maxillary anterior teeth. To calculate the tooth elements,surface model of the tooth was made based on a dental study model (i21D-400C;Nissin Dental Products, Kyoto, Japan). This procedure consisted of 3 steps: Firstly,sectional images of the dental study model were taken by using dental cone-beamcomputed tomography (i-CAT, U.S.A.). Secondly, the finite element model wasdeveloped using MSC Patran (MSC Software, Inc.,USA.). The thickness ofperiodontal ligament (PDL) was considered to be uniform (0.25 millimeters). Thealveolar bone was constructed to follow the curve of cementoenamel junction. Thethickness of cortical bone was 0.5 millimeters. The bracket slot was 0.018x0.025 inch(Gemini, 3M Unitek, U.S.A.), in which the 0.016x0.022 inch stainless steel archwirewas inserted. Brackets of the anterior teeth were ligated firmly to the archwire. Onthese teeth, the archwire and the brackets were moved as an united body.Alternatively, buccal tubes of the posterior teeth were loosely engaged to the archwireso that they could be slided along the archwire, but sagittal rotation of the archwire was restricted by bracket slots. The frictional coefficient was ignored. Mini-screws orfixed points were placed in alveolar bone between the second premolars and the firstmolars in position of 3.0, 5.0, 7.0 and 9.0 millimeters from the archwire bilaterally tosimulate the en masse retraction of the anterior teeth. Power arms which were madefrom stainless steel wire were bonded to the archwire between the lateral incisors andthe canines. Thirdly, orthodontic forces were applied from the mini-screws to thepower arms. A line joining the mini-screw with the power arm was the line of actionof the force. According to clinical cases, the magnitude of the orthodontic force wasassumed to be 2.0 N (200 grams). In order to change the force direction, the length ofthe power arms was varied from 3.0, 5.0, 7.0 and 9.0 millimeters. The incisal edgeand apex changes of maxillary central incisor were calculated and compared to eachforce direction. The results were divided into two parts: non-adjusted base of bracketsmodel or non-straight wire model and adjusted base of brackets model which causedarchwire be straightened. In non-adjusted base of brackets model or non-straight wiremodel, wire was not straight and did not afford for teeth movement. High stressoccurred in cervical of maxillary lateral incisor, maxillary first molar and buccal ofmaxillary of second molar. Crown of maxillary lateral incisor displaced more than theother teeth. In periodontal ligament, the highest strain occurred nearly the cervical ofmaxillary lateral incisor. In adjusted base of brackets model or straight wire model,wire was straight and afforded for teeth movement like the teeth which have beenleveled and aligned already. High stress occurred in cervical of maxillary lateralincisor, maxillary first molar, maxillary canine, maxillary second premolar and buccalof maxillary of second molar. Crown of maxillary lateral incisor displaced more thanthe other teeth. In periodontal ligament, the highest strain occurred nearly the cervicalof maxillary lateral incisor.The incisal edge and apex changes of maxillary central incisor werecalculated and compared for each force direction. In two simulated models, 3.0millimeters of mini-screws and 9.0 millimeters of power arm showed the leastdifferential change or nearly zero of the incisal edge and apex. However, the differential change of the incisal edge and apex in adjusted base of brackets model orstraight wire model was only minus value. That means while incisal edge of maxillarycentral incisor moved linqually in upper anterior teeth retraction, apex of maxillarycentral incisor moved more labially. The next order which had the least change ornearly zero in adjusted base of bracket model was 9.0 millimeters of mini-screws and9.0 millimeters of lever arm in adjusted base of bracket model or straight wire model.So during the simulation, 9.0 millimeters mini-screw and 9.0 millimeters lever armwas the force direction which bodily movement nearly to be occurred. The maxillarylateral incisor was protruded and more protruded in adjusted base of brackets modelor straight wire model than non-adjusted base of brackets model or non-straight wiremodel. It might be hardly movement of adjacent teeth that limited maxillary lateralincisor movement of non-adjusted base of brackets model or non-straight wire model.In clinical application, less crowding and fair aligned teeth can be applied anteriorretraction force, and there is less protrude of maxillary lateral incisor than wellaligned teeth. However, periodontal strain in adjusted base of brackets model orstraight wire model is greater than non-adjusted base of brackets model or nonstraight wire model, which risk or induce to occur root resorbed in these area.