| Objective Three-dimensional finite element method and in vitro fracture test were used to investigate the effects of occlusal veneer and fiber post of different preparation designs on the stress distribution,fracture load and failure mode of maxillary premolars of pulppiercing NCCLs after RCT.Methods(1)based on the CBCT data of a complete maxillary first premolar,two three-dimensional finite element models of pulp-piercing NCCLs after RCT were generated by finite element software,including glass fiber post + resin core restoration model A and resin core restoration model B.Occlusal veneer(G2),buccal-occlusal veneer(G3)and buccal-proximal-occlusal veneer(G4)were applied to each model.At the same time,the models using only composite resin fillers(G1)and full crowns(G5)were set as negative and positive controls,respectively.The vertical load and oblique 45 °load were applied,and the force was 100 N,which acted on the central fossa of the teeth and the buccal slope of the tongue tip,respectively.The Von Mises stress and maximum principal stress of residual tooth tissue,restoration and composite resin were recorded,and the results were statistically analyzed by Grap Pad Prism8.2.1.(2)60 maxillary first premolars were collected and randomly divided into group A and group B after RCT and severe defects.Group A was filled with glass fiber post and resin core,and group B was filled with resin core only.Then the occlusal veneer(G2),buccal-occlusal veneer(G3)and buccal-proximal-occlusal veneer(G4)were applied to the isolated teeth.The teeth filled with composite resin(G1)and full crown(G5)were set as negative and positive control respectively,with 6 samples in each group.Use IPSe.max CAD to make all the restorations.After the bonding of the restoration was completed,the restoration was stored in distilled water at 37 ℃ for one month,followed by 5000 cycles of hot and cold aging at 5 ℃-55 ℃.After embedding,it was aged for 20000 times at the speed of 1mm/min and 100 N vertical load on the in vitro anti-folding machine.Finally,all the samples were on the same anti-bending machine and loaded with static load until the sample broke at the speed of 0.5mm/min.The fracture load and failure mode of each sample were recorded,and the optical microscope photos were taken.The results were analyzed by LSD with SPSS,and the significance level was set to 0.05.Results(1)No matter whether the fiber post is used or not,the stress distribution of residual tooth tissue,restoration and composite resin of the model shows a similar trend under different loading modes.Only the stress of G1 and G2 are concentrated at the tip of the defect,while the stress distribution of G3-G5 around the defect is relatively small.For the residual tooth tissue,the stress was arranged from small to large as follows: G3<G4≈G2<G1<G5.Compared with the enamel of G1,the G2-G5 restoration can withstand higher stress,and the stress value of the restoration is as follows: G5 < G3 < G4 < G2.For composite resin fillings,under vertical load,the stress value of G3-G5 is very close,and the maximum stress value is significantly lower than that of the former two groups.However,under oblique load,the maximum principal stress of G3-G5 shows a significant difference: the maximum stress of G3 and G4 is about 2 times and 1.5 times of that of G5,respectively.Under the same repair method and loading mode,group A always showed a slightly lower stress than group B,but the difference was not significant.(2)The average fracture load of each group is as follows: group A: G1:1453.962 ±411.596 N,G2:2606.225±203.048 N,G3:2628.134±308.259 N,G4:2072.413 ±147.159 N,G5:2707.824±267.599N;Group B: G1:1221.981 ±147.829 N,G2:2196.737 ±238.686 N,G3:2228.540±156.435 N,G4:1660.238 ±217.726 N,G5:2750.045 ±171.108 N.The order of fracture load in fiber post group and resin group was G5 > G3 > G2 > G4 > G1,and the fracture load after restoration(G2-G5)was significantly higher than that in unrepaired group(G1).Compared with group A and group B,only G4 showed significant difference in fracture load(P < 0.05).83% of the samples in G5 were pattern IV fracture and could not be repaired again,while most of the samples of pattern I and II existed in G3 and G4,and only one sample in G3 was pattern I.Conclusion(1)In the finite element model,three kinds of occlusal veneer can improve the stress distribution of NCCLs filling,and the buccal-occlusal veneer is more beneficial to the preservation of dental tissue.Whether glass fiber post should be used as a routine application remains to be further studied.(2)The maximum fracture load of three kinds of occlusal veneers and full crown all exceeded the maximum bite force of maxillary premolars,and the fracture load of buccal-occlusal veneer was second only to the largest full crown group,and the fracture mode was better.The fracture load of fiber post group is similar to that of resin group,and the fracture mode of fiber post group is slightly better than that of resin group. |
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