Effect Of Materials And Thicknesses On Fracture Resistance Of Inlay And Three-Dimensional Finite Element Analysis

There are two kinds of repairing methods for restoration of tooth defect in clinic.One is direct repairing,that is direct filling of cavity with resin and glass ionomer,which is also the most commonly used method at present.The other is indirect repairing,that is indirect restoring with inlay.Although direct filling is relatively fast and economical,in some cases,such as large defect cavity,direct filling treatment can not completely restore the occlusal surface shape of defective teeth,the adjacent surface is prone to suspension,resulting in food impaction and periodontal problems.Compared with the traditional direct filling treatment,inlay repair not only solves the above problems,but also has better aesthetics and edge tightness.The mechanical properties of all-ceramic inlay are much better than that of direct filling material.With the development of CAD/CAM technology,inlays can also achieve one-time treatment,saving patients' time.Therefore,all-ceramic inlays have become the choice of more and more dentists and patients.In inlay restoration,the preparation of teeth and the selection of inlay materials determine the service life of inlays and the health of the remaining hard tissues,which is the most concerned aspect of dentists.OBJECTIVE: In this study,two representative inlay materials,Vita enamic and IPS e.max,were selected to study the effects of different occlusal thickness and material materials on the fracture strength of inlay and stress distribution of hard tissues of inlay and residual teeth from two aspects of in vitro experiments and computer-aided three-dimensional finite element analysis,to provide experimental basis and support clinical guidance.Methods: Thirty standard occlusal inlays of right lower first molar were prepared.The inlay thickness was adjusted to be 1.5mm,2.0mm and 2.5mm respectively.The inlays were randomly divided into two groups.The inlays were made of Vita enamic ceramic block and IPS e.max ceramic block beside the chair.The inlays were bonded with 3M Rely X Unicem resin binder.After bonding,normal saline was bathed in water for 24 hours,and its maximum fracture strength was tested by universal dynamometer.An adult's first right lower molar with normal occlusal relationship and no caries was selected.The digital model was obtained by CBCT scanning technology.Then the computer reverse engineering technology and the combination with finite element method were used to complete the simulation of the inlay model of the adjacent mid-maxillofacial region.The static axial loading was simulated.Eight loading points of normal occlusal contact were selected and a total of 280 N loads were applied.Stress distributions of inlays and hard tissues of the remaining teeth with different thickness of the joint and materials were analyzed.RESULTS: The results showed that there were significant differences among IPS e.max thickness groups.The fracture strength was the largest at 2.0 mm.There were significant differences among Vita enamic thickness groups,The fracture strength was the largest at 2.5 mm.Under the same thickness,IPS e.max has higher fracture strength than Vita enamic at 2.0 mm thickness,and there is no statistical difference between 1.5 mm and 2.5 mm thickness.The results of three-dimensional finite element analysis showed that the peak value of the equivalent stress in the whole normal teeth was distributed in the neck and the loading point.The stress value in the central area was larger,and the stress value was smaller as the teeth moved around.In IPS e.max group,the peak value of the equivalent stress of the inlay increases with the increase of the thickness;in Vita enamic group,the peak value of the equivalent stress is maximum at 2.0 mm;in the same thickness,the equivalent stress of IPS e.max is greater than that of Vita enamic.In IPS e.max group,the equivalent stress of residual enamel was lower than that of intact enamel by 1.06%,0.16%,0.48%;In Vita enamic group,when the thickness was 1.5mm,the equivalent stress of residual enamel decreased by 0.86% compared with intact enamel,and when the thickness was 2.0mm,it increased by 0.29%.The difference between normal teeth is not significant,but at 2.5 mm,the peak stress of residual enamel increases 16.3%.Under the same thickness,the peak equivalent stress of residual enamel of IPS e.max is less than that of Vita enamic.The peak equivalent force of dentin restored with IPS e.max increased by 15.37%,15.04%,7.42% and Vita enamic increased by 15.23%,14.79% and 7.46% respectively,but the peak equivalent force of residual dentin decreased with the increase of thickness.CONCLUSION: The fracture strength of IPS e.max group was the highest when the thickness was 2.0 mm,and that of Vita enamic group increased with the increase of thickness.When the inlay thickness is 2.0 mm,the fracture strength of IPS e.max group is higher.Under the same thickness,IPS e.max bears more stress than Vita enamic inlay,which reduces the stress on the remaining tooth tissue.Moreover,the greater the inlay thickness in IPS e.max group,the smaller the stress on the tooth tissue,which is beneficial to the preservation of the remaining tooth tissue.