| Objectives:1 To reconstruct the three-dimensional finite element analysis models of miniscrew-assisted upright and intrusion of mandibular second molar and to optimize the parameters of periodontal ligament(PDL)and the meshes density.2 To study the stress and strain of the roots,PDL and alveolar bone in the different periods of miniscrew-assisted upright and intrusion of mandibular second molar.3 To study the stress and strain of the roots.PDL and alveolar bone during miniscrew-assisted upright and intrusion of mandibular second molar with various lingual inclination.4 To study the effect of density of alveolar bone on the stress and strain of the roots,PDL and alveolar bone during upright and intrusion of mandibular second molar.5 To study the effect of height of periodontal supporting tissue on the stress and strain of the roots,PDL and alveolar bone during upright and intrusion of mandibular second molar.Methods:1 The three-dimensional data of mandibular second molar were optimized by Geomagic Studio 10.0,a reverse engineering software.And the three-dimensional finite element models(3D-FEMs)of miniscrew-assisted upright and intrusion of mandibular second molar were established by ANSYS 15.0,a three-dimensional finite element analysis software.The tests were divided into six working conditions according to the parameters of PDL(linear or non-linear)and the meshes density(more,medium or less meshes)to study the stress and strain of the roots,PDL and alveolar bone.2 The 3D-FEMs of mandibular second molar with lingual inclination of 40° were established.The force applied to.molar in 0,1,7,14,21 and 28 days were 100g(1N),75g(0.75N),59g(0.59N),43.75g(0.4375N),32.5g(0.325N)and 22.5g(0.225N)according to force degradation of 3M orthodontic elastomeric chains in vivo.The different periods of upright and intrusion of mandibular second molar were simulated by using the following force.And the stress and strain of the roots,PDL and alveolar bone of mandibular second molar were calculated.3 The 3D-FEMs of mandibular second molar with lingual inclination of 400°,30°,20°,10° and 0° were established.And the stress and strain of the roots,PDL and alveolar bone of mandibular second molar were calculated.4 The 3D-FEMs of mandibular second molar with lingual inclination of 40° were established.Four parameters 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.5 The 3D-FEMs of mandibular second molar with lingual inclination of 40° were established.The heights of periodontal supporting tissue were decreased by 1 mm in sequence and the FEMs with heights of periodontal supporting tissue of-1mm,-2mm,－3mm,-4mm,-5mm,-6mm and-7mm were established and the stress and strain of the roots,PDL and alveolar bone were calculated.Results:1 The 3D-FEMs including mandibular second molar,PDL,lamina dura,cancellous bone,cortical bone and miniscrew were reconstructed successfully,which had high geometric similarity,complete structure and precise meshes.The meshes density in the experiment had no significant effect on intensity and distribution of stress and strain of the roots,PDL and alveolar bone.The parameters of PDL had significant effect on intensity and distribution of stress and strain of the roots and PDL.However,the parameters of PDL had significant effect on intensity of stress and strain of alveolar bone,but not distribution of that.2 During the several periods of upright and intrusion of mandibular second molar,the distribution of stress and strain of the roots,PDL and alveolar bone were similar.The stress and strain concentrations of the roots located on the middle 1/3 of buccal root,buccal neck and middle 1/3 of lingual root while those of the PDL located on the lingual root tip and buccal neck,and those of the alveolar bone located on the buccal furcation and neck of four line angles of alveolo-fossa.The maximum von-mises stress and strain of the roots,PDL and alveolar bone decreased progressively with loading-time increasing gradually,and those decreased fastest at the first day.The maximum von-mises strain of PDL was far more than that of roots and alveolar bone.3 During upright and intrusion of mandibular second molar,with lingual inclination decreasing,the stress and strain concentrations of lingual root and buccal neck of PDL were increased gradually,and those of buccal alveolar bone decreased(minimum at 20°lingual inclination)while those of lingual alveolar bone increased(maximum at 20°lingual inclination),and were greater than buccal alveolar bone.The maximum von-mises stress and strain of the roots and PDL increased progressively,and those of alveolar bone increased and were maximum at 20°lingual inclination.The tendency of intrusion was more than that of upright.4 During upright and intrusion of mandibular second molar,the bone density had no significant effect on intensity and distribution of stress and strain in the roots and PDL.With bone density decreasing,the stress concentrations were increasingly significant and the strain concentrations were shifted from lamina dura to cancellous bone in the buccal furcation of alveolar bone.With bone density decreasing,the maximum von-mises stress of alveolar bone increased gradually,mainly in the lamina dura,And the maximum von-mises strain of alveolar bone also increased gradually,and the alveolar bone strain of A and B bone densities increased mainly in the lamina dura and that of C and D bone densities increased in the cancellous bone.5 During upright and intrusion of mandibular second molar,with height of periodontal supporting tissue decreasing,the stress and strain concentrations of the roots,PDL and alveolar bone moved to root tip gradually,and the maximum von-mises stress and strain increased gradually.The stress and strain of the roots,PDL and alveolar bone concentrated mainly in the root tip and the maximum von-mises stress and strain increased significantly with the furcation exposing.Conclusions:1 The PDL of nonlinear parameter model had more obvious cushioning effect on force and more realistic stress distribution.In a subsequent experiment,PDL should be defined as hyperelastic nonlinear model,and meshes should be defined according to the medium meshes density,which not only have accuracy assurance,but also help to improve the efficiency.2 Due to stress concentrated distribution of root didn’t locate in the root furcation and root tip,there were not obvious absorption in the root furcation and root tip during upright and intrusion of mandibular second molar.There were stress concentrated in both buccal and lingual alveolar bone,which revealed there were resorption risk in both buccal and lingual alveolar bone during upright and intrusion of mandibular second molar.The initial force should be regulated to protect PDL due to larger strain of PDL,and it is appropriate for 100 g of initial force.3 The stress-strain distribution and intensity in the lingual alveolar bone increase with molar upright and the tendency of molar intrusion was more than that of upright,therefore,the resorption risk in the lingual alveolar bone should be notable.With molars lingual inclination decreasing,the loading force should be reduced,and the minimum force should be applied at 20° lingual inclination.4 During upright and intrusion of mandibular second molar,the patients with lower bone density were inclined to indirect absorption and should reduce the initial loading force,which not only protect the stability of the implant anchorage,but also prevent pathological absorption of alveolar bone.5 During upright and intrusion of mandibular second molar with periodontal problems,the force should be reduced(<100 g),and the furcation exposion was the contraindication of molar upright and intrusion. |
PDF Full Text Request