Study On The Mechanical Properties Of Orthodontic Brackets-enamel Bonded Structure Subjected To Hygrothermal Aging

As people pay more attention to the aesthetics of teeth,the oral orthodontics received more and more attention in recent years.Meanwhile,in order to ensure the effect of orthodontic treatment and reduce the failure rate of brackets in the process of orthodontic treatment which will decrease the frequency of visits effectively,more and more attention has been paid to the bonding effect of brackets.And the stability of brackets bonding is necessary to achieve the satisfactory treatment effect.Most of the existing studies on the bonding strength of brackets are based on experimental testing and finite element static analysis,while the failure of bracket bonding also needs to be taken into account,including the ultimate strength,fracture energy and other material parameters of orthodontic adhesive.In this paper,the related properties of bonded bracket were studied by combining experiments with finite element method.Based on the Cohesive Zone Model,the progressive damage of the adhesive layer was carried out,and the mechanical performance and strength prediction of the bracket-adhesive layer-enamel bonded structure after aging in the hygrothermal environment were further studied.Firstly,according to the requirements of the ASTM testing standards,the elastic modulus,tensile strength,shear modulus,shear strength,mode I/II fracture energy and other relevant material parameters of the orthodontic adhesive were tested through reasonable experiments,so as to determine the reliable parameters of the orthodontic adhesive and provide parameter basis for the subsequent numerical simulation.In order to better simulate the bonding situation of brackets in the process of orthodontic treatment,the human extracted teeth were selected as the substrate of brackets bonding.The failure strength of the bracket bonding structure was measured by quasi-static shear loading.The failure process of the bracket-adhesive-enamel was simulated through Finite Element Modelling by bilinear cohesive element,and the load-displacement curve obtained numerically was compared with the experimental results which verified the reliability of the Cohesive Zone Model.Furthermore,the stress distribution of the adhesive layer and the surface of enamel was analyzed.Considering that the whole process of orthodontic treatment was completed in the oral humid environment,the water immersion test of orthodontic adhesive was further carried out to test the hygroscopic performance parameters of orthodontic adhesive under 37 ℃,such as hygroscopic rate and saturated hygroscopic capacity.The water diffusion process of the adhesive layer in the bonding structure was simulated by ABAQUS user subroutine.In the humid and hot environment,after a period of aging,the properties of orthodontic adhesive will degrade to a certain extent,which leads to the reduction of the bonding strength of the bracket bonding structure.The preliminary parameter degradation rule was obtained by experimental tests on the bonding strength of the aging bond structure of orthodontic adhesive and material parameters of dumbbell type specimens.The residual strength of the bracket bonding structure was predicted by ABAQUS user subroutine combined with the moisture distribution in the previous step.Compared with the experimental results,the validity of the numerical prediction method was proved.Meanwhile,the prediction results also showed that the bonding strength of brackets gradually degrades with the increase of aging time.The failure analysis of the adhesive layer and the hygrothermal aging research carried out in this paper can provide some basis for the therapeutic force applied in clinical treatment and the possible fracture mechanism of the bracket used in daily life,which can also provide some reference for the debonding mechanism of the bonded bracket after clinical treatment.