The Finite Element Analysis Of The Effect Of Several Factors On Biomechanics Characters In Intrusion Of Maxillary First Molar Treatment

The study is to investigate the stress and strain of the roots, periodontal ligament(PDL) and alveolar bone in the different periods of intrusion of maxillary first molar treatment and to research the effect of height of periodontal supporting tissue and density of alveolar bone on the stress and strain on the basis of reconstruction of the non-linear three-dimensional finite element analysis models of the maxillary first molar in order to be helpful for clinical practice.Objectives:1 Reconstruct the non-linear three-dimensional finite element analysis models of the maxillary first molar and periodontal supporting tissue.2 To study the stress and strain of the roots, PDL and alveolar bone in the different periods of intrusion of maxillary first molar treatment.3 To study the effect of density of alveolar bone on the stress and strain of the roots, PDL and alveolar bone. 4 To study the effect of height of periodontal supporting tissue on the stress and strain of the roots, PDL and alveolar bone.Methods:1 After the three dimensional sensing system(3DSS) scanning to the standard model of maxillary first molar, the non-linear three-dimensional finite element analysis models of the maxillary first molar was established by Geomagic Studio 8, a reverse engineering software and ANSYS10.0, a three-dimensional finite element analysis software.2 According to force degradation of orthodontic elastomeric chains in vivo, the stress and strain of the roots, PDL and alveolar bone in the different simulated periods of intrusion of maxillary first molar were calculated.3 Different Yang's modulus of alveolar bone were set according to different densities of alveolar bone and the stress and strain of the roots, PDL and alveolar bone were calculated.4 The non-linear three-dimensional finite element analysis models of the maxillary first molar of different heights of periodontal supporting tissue were established and the stress and strain were calculated.Results:1 The non-linear 3-D FEA model of maxillary first molar, PDL, lamina dura, cancellous bone and cortical bone was reconstructed successfully, which had 96 875 10-node-tetrahedral elements and 132 838 nodes in total. Thereinto maxillary first molar had 40 415 elements and 58 605 nodes,and PDL had 10 258 elements and 20 602 nodes,and lamina dura had 12 202 elements and 24 413 nodes,and cortical bone had 4 132 elements and 8 053 nodes,and cancellous bone had 29 868 elements and 44 977 nodes.2 Positions of the stress and strain concentrations of the roots, PDL and alveolar bone were similar on the whole in the different periods of intrusion of maxillary first molar treatment. The stress and strain concentrations of the roots located in the furcation of the maxillary first molar and the middle part of the roots while those of the PDL located in root tip and dental cervix and those of the alveolar bone located in the furcation and root tip of alveolo-fossa. The maximum von-mises stress and strain decreased progressively with loading-time increasing gradually.3 Densities of alveolar bone had no effect on positions and value of the stress and strain concentrations of the roots and PDL and also had no effect on positions of the stress and strain concentrations of alveolar bone.However,the densities had effect on the value of the stress and strain of alveolar bone. The maximum von-mises stress and strain of the alveolar bone increased progressively with the value of the densities decreasing gradually.4 The maximum von-mises stress and strain increased progressively with height of periodontal supporting tissue decreasing. With the height of periodontal supporting tissue decreasing and the furcation of the maxillary first molar exposing, the stress and strain of roots decreased gradually in the furcation and concentrated in the middle of roots finally and the stress and strain concentrations of the PDL located in root tip and dental cervix at all times and increased gradually. The stress and strain of alveolar bone increased first and then decreased gradually and finally concentrated in root tip of alveolo-fossa.Conclusions:1 It is feasible to reconstruct the non-linear three-dimensional finite element analysis models of the maxillary first molar by 3DSS and reverse engineering technique. And the reconstructed finite element analysis model has high geometrical similarity, intact structure and precise mesh, which meets the requirement of finite element analysis.2 The PDL plays a nice buffering part in intrusion of maxillary first molar treatment.It is necessary to control the initial loading force to protect it due to the larger strain of the PDL. Furthermore, the root tip won't bring about visible absorption because the stress and strain concentrations of the roots locate in the furcation and the middle part of the roots, which suggests we research the curative effect and prognosis of the intrusion of maxillary first molar by 3-D measure technique.3 The maximum von-mises stress and strain of the alveolar bone increased progressively with the value of the densities decreasing gradually, which suggests we reduce the initial loading force to protect the stability of anchorage and to benefit the physiological intrusion of maxillary first molar when we give the intrusion of maxillary first molar treatment to the patients whose value of density of alveolar bone are lower(osteoporosis).4 The stress and strain of the roots, PDL and alveolar bone increase progressively and get concentrated with the height of periodontal supporting tissue decreasing, which is harmful to tissues. The writer suggests that we be careful when the height of periodontal supporting tissue is less than nine millimeter(the furcation of the maxillary first molar is about to expose) and consider that the height of periodontal supporting tissue is less than seven millimeter (the furcation of the maxillary first molar have exposed completely) is one of the contraindications.