Stress Analysis And Movement Prediction Of Periodontal Ligament In Orthodontic Treatment

Dental malformation is a common oral disease,which is mainly caused by the discordant spatial relations among teeth,Periodontal ligament(PDL),alveolar bone and other periodontal tissues,and its incidence is between 50% and 70%.The PDL is a kind of connective tissue.On the one hand,it can connect the teeth with the PDL.On the other hand,the PDL can transfer the bite force from the teeth to the mandible.As a kind of connective tissue,the composition of PDL is very complex,including but not limited to matrix,cell,fiber and capillary.Orthodontic treatment mainly relies on the stimulation of the PDL under external forces to perform osteotomy and bone reconstruction of the mandible to achieve tooth movement.In view of the material properties of PDLs,the properties of linear elasticity,bilinear,hyperelasticity,viscoelasticity and hyper-viscoelasticity have been studied academically.But so far there is no recognized constitutive equation to describe the properties of PDL.In this study,two kinds of in vitro experiments were conducted on PDL specimens: compressive stress relaxation test and static tensile test.The viscoelastic model and the hyperelastic model were fitted with the stress relaxation experiment results and the static tensile experiment results,and the model parameters were obtained,which provided the basis for the finite element simulation of PDL.Three dimensional finite element models of mandible,left and right central incisors,left and right lateral incisors,left and right incisors and their PDL were established.One group treated the PDL as a whole,and its material properties were set as hyperelastic properties(as defined by experimental data using statically drawn tests).In the other group,the PDL was divided into five layers along the axial direction of the tooth,and each layer was assigned a different hyper-viscoelastic property(defined jointly by the experimental data of statics tensile and shear stress relaxation).Braces is taken into account in PDL the layered model assembly to the teeth of the upper,middle and lower influence on orthodontic,respectively in three assembly way around the teeth,left and right lateral incisor and monitored cutting teeth on vertical direction,straight down,and your breath three different direction of orthodontic force,direction of three kinds of orthodontic force for both for 0.5 N,N,1.5 N 1 and 2 N.Through finite element calculation,it can be concluded that in the same type of dental hoop assembly,When applying orthodontic forces in different directions and sizes to different teeth in the same type of hoop assembly,the stress values of the periodontal membrane of different teeth all conform to the law of the central incisors’ > lateral incisors’ > incisors.The stress value of the periodontal membrane of the same tooth conforms to the law of from the lip to the tongue>Vertical upward direction> Vertical downward direction.According to the above conclusions,in the periodontal membrane layered model,the braces are assembled to the upper part of the tooth,and the orthodontic force is applied consistent with that of the previous model.The rule of(1)is the same as that of conclusion by calculation.When orthodontic forces were applied to the central incisors,lateral incisors and incisors in accordance with the previous model,the stress value of the periodontal membrane decreased first and then increased along the axial direction of the tooth,and the minimum value appeared in the middle of the periodontal membrane.In the two models of unstratified PDL and stratified PDL,the stress value of central incisors under the condition of applying orthodontic force is relatively large,so the orthodontic force applied on central incisors should be reduced appropriately in clinical treatment.In the unstratified model,when the same orthodontic force is applied to the same tooth,the maximum stress value is generated by the PDL when the braces are assembled to the upper part of the tooth.Therefore,under the premise of not affecting the masticatory function of the tooth,the frontal orthodontic effect generated by the braces on the upper part of the tooth is better.