| Objective:To investigate the effect of restoration thicknesses and firing times on the fatigue properties and failure modes of posterior occlusal veneers made of lithium disilicate glass ceramics.Methods:(1)Different thicknesses:IPS e.max CAD(IC)and IPS e.max Press(IP)posterior occlusal veneer restoration(n=30)with a thickness of 0.6mm,0.9mm and 1.2mm(n=10)were cemented to the maxillary molars composite resin dies.Five specimens from each group were subjected to thermo-mechanical fatigue(TMFT)(5?-55?,5000 cycles,30-300 N,10Hz,sinusoidal wave,500000cycles)test.After fatigue testing,the unfailure specimens and control groups were subjected to compressive loading in which the maximum force at fracture were recorded.And through the findings of stereomicroscope and scanning electron microscope(SEM),the failure modes were analyzed.(2)Different firing times: according to the number of times of firing(1,3,5,7 times)(n = 5),the IPS e.max CAD(IC)and IPS e.max Press(IP)occlusal veneer restorations with a thickness of 1.2mm were prepared,and then were cemented to the maxillary molars composite resin dies.Each group was subjected to thermo-mechanical fatigue(TMFT)(5?-55?,5000 cycles,30-300 N,10Hz,sinusoidal wave,500000cycles)test.After fatigue testing,the unfailure veneers were subjected to compressive loading in which the maximum force at fracture were recorded.And through the findings of stereomicroscope and scanning electron microscope(SEM),Energy dispersive x-ray spectroscopy(EDX)and X-ray diffraction(XRD),the failure modes and microstructural characterization were analyzed.(3)Non-parametric test and two-way ANOVA were carried out by employing SPSS20.0 software.Results:(1)Different thicknesses: the fracture load of each group: In the IC control group,the corresponding average fracture load of 0.6mm,0.9mm,and 1.2mm were(1191.98±127.74)N,(1296.35±261.97)N,(1593.59±241.13)N;and in the IP control group were(1164.83±191.31)N,(1290.84±145.82)N,(1511.86±129.01)N;in the IC experimental group,the average fracture load of 0.9 mm and 1.2mm were(1305.22±68.76)N,(1546.73±192.85)N;in the IP experimental group were(1320.01±141.9)N,(1498.8±206.35)N(Table 1-2).When the material and thickness were consistent,there was no statistical significance between the control group and the experimental group(P>0.05).The average load value of the 1.2mm was significantly much higher than that of the 0.9mm in the IC/IP group.As the thickness increased,the fracture load also increased.However,in the experimental group,the 0.6mm group showed a crack after TMFT.SEM showed that the bulk fracture originated under the contact area and cone and radial cracks appeared,which was the primary mode of failure modes.(2)Different firing times: the fracture load of each group: The average loads(N)at fracture for the IC groups with the firing times of 1,3,5,7 times were(1546.73±192.85)N,(1516.70±239.44)N,(1382±227.59)N,(1476.07±215.51)N;in the IP group,the average loads were(1498.8±206.35)N,(1504.46±138.56)N,(1430.94±173.93)N,(1183.17 ± 72.89)N(Table 2-3).there was no significant difference in the fracture load with different firing times for IC/IP(P >0.05).There were higher Si and O elements and Zr,Al,K,Ce,P and other elements in two groups of materials according to EDX.The XRD analysis mainly consists of lithium disilicate,lithium metasilicate and lithium orthophosphate.After repeated firing,there was no new diffraction peaks appearing.SEM showed that the size of crystal was shorter and the porosity increased after multiple firing.Conclusion:(1)Fatigue loading has no effect on the fracture load of the posterior occlusal veneers made of lithium disilicate glass ceramics,while the thickness has a positive effect on it,where the fracture load increases with the increase of thickness.(2)After repeated firing,the fatigue loading of lithium disilicate glass ceramics has a decreasing trend of fracture load value,but there is no statistical defference,and it meets the clinical requirements. |
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