The Effect Of Different Occlusal Thickness On The Compressive Strength Of Monolithic Mutilayer At Zirconia Crowns

Objective: Under the condition of CAD/CAM techniques,this study compared the compressive strength of monolithic Mutilayer AT zirconia crowns with monolithic SHT zirconia crowns and Porcelain-veneered Y-TZP crown which are often used in clinical,and evaluated the effect of different occlusal thickness on the compressive strength of monolithic AT zirconia crowns to find the lest thickness that fulfill the maximum bite force,and guided for the tooth preparation and the fabrication of the AT crown.Method:1 The fabrication of the metal die Prepared the standard first molar of the left mandibular,referenced the principle of the all-ceramic crown abutment design in the seventh edition of prosthodontics.The principle was:functional cusp reduced 1.5mm,unfunctional cusp reduced 1.0mm,the axial degree was 8,the width of chamfer was 0.8mm.A polyvinyl siloxane impression was made of the master die,30 dies were cast with special wax which were machined according to the ratio of 1:1,and then embedded the wax moldings,casted them,polished and sand blasted the metal dies routinely.2.1 The design of monolithic zirconia crown and zirconia core model Each metal die was scanned by the 3shape optical impression scanner,and the result was transmitted to the dental designer software,and then the system generated the standard first molar crown of the left mandibular.According to the need of the in vitro study,adjusted the thickness of the monolithic zirconia models,occlusal surface to 1.0mm,1.5mm,2.0mm,and the width of all the chamfers to 0.8mm by the virtual knife.And then the zirconia core with 0.5mm was finished with the same method.2.2 The fabrication of the monolithic crownTransmitted the data of the different thickness of the monolithic crowns to the CAD cutting machine,machined the pro-rate magnified AT zirconia crown with the thickness of 1.0mm,1.5mm,2.0mm and machined the magnified SHT zirconia crown with the thickness of 1.0mm according to the manufactures,instructions.Each group of the crowns were 6,sintered them completely,and then glazed each crown.Each thickness of the AT crowns and the SHT crowns were named group A,group B,group C,group D,respectively.2.3 The fabrication of the porcelain-veneered Y-TZP crown Transmitted the data of the zirconia core to the CAD cutting machine,machined the magnified SHT zirconia cores with the thickness of 0.5mm,sintered them,overlaid a uniform thickness of the veneer ceramic,sintered completely,and finished 6 porcelain-veneered Y-TZP crowns with the thickness of 2.0mm.3 The bonding of the specimens All the crowns were bonded to the metal dies using 3M glass ionomer cement(GIC),press the crowns for 5 mins with the strength of 20 N by fingers,moved away the exceeded cement,put the 30 full ceramic crown in the37?constant temperature box with distilled water for 24 h.4 The compressive strength test The bonded all-ceramic crowns were sequentially placed on a universal testing machine for compression test.The loading head was loaded at a constant speed of 0.5 mm / min on the functional cusp of the crowns until the all-ceramic crown was damaged.5 Statistical analysis Used SPSS 21.0 software to analysis the data.Use Shapiro-Wilk to check whether the data conform to the normal distribution(?=0.1).Using the Levenes test to test the variance homogeneity of the data(?=0.1).The experimental data could meet the normality and variance homogeneity test(P>0.1).The experimental data could be expressed with mean ± standard deviation(SX ±).The compressive strength of AT zirconia crowns with different thicknesses were analyzed by one-way ANOVA.The compressive strength of AT zirconia crowns,SHT zirconia crowns and porcelain-veneered Y-TZP crowns were analyzed using the same method.If there were differences between groups,then Student-Newman-Keuls was used to compare the two groups.P <0.05 was considered statistically significant.Results:1 The compressive strength of the crowns of each group Group A:(1202.02 ± 51.81)N,Group B:(2244.59 ± 51.19)N,Group C:(4723.61± 126.21)N,D:(2383.96± 96.40)N,E:(2148.14± 103.7)N.2 Statistical analysis2.1 The results of the compressive strength of the AT zirconia crowns,the SHT zirconia crowns and the porcelain-veneered Y-TZP crowns were analyzed statistically.Each group of data was consistent with the normality and variance homogeneity,and the overall average of the different thickness of the AT zirconia crowns were different.There was no significant difference between the AT zirconia crown group with a thickness of 1.5mm,the SHT zirconia crown group with a thickness of 1.0mm and the porcelain-veneered Y-TZP crown group(P>0.05);The compressive strength of the AT zirconia crown with a thickness of 1.0 mm was lower than that of SHT zirconia with a thickness of 1.0 mm,the difference was statistically significant(P<0.05);The compressive strength of the AT zirconia crown with a thickness of 2.0 mm was higher than the porcelain-veneered Y-TZP crowns with thickness of2.0mm,and the difference was statistically significant(P<0.05).2.2 The result of the different thickness of the AT zirconia crowns were analyzed statistically.Each group of data was consistent with the normality and variance homogeneity,and the overall average of the different thickness of the AT zirconia crowns were different.As the thickness increased,the compressive strength increased.Comparison between groups displayed,there was significant difference in the compressive strength of 1.0mm group,1.5mm group and2.0mm group(P <0.05).2.3 The fracture pattern The fracture pattern of the single-layer AT zirconia crowns were the same as that of the SHT zirconia crowns,which were the completely damage of the whole layer.Under the optical microscope,the functional cusp under the loading head is a tapered crack,part of the crack and the surface of the fissure or triangular ridge overlapped.The greater the thickness was,the more obvious the semi-plastic damage at the loading point.The fracture of the porcelain-veneered Y-TZP crowns showed the exfoliation of the veneers.Conclusions:Within the limitations of this study,the following conclusions were drawn:1 After Compared the compressive strength with the SHT zirconia crown with a thickness of 1.0mm and the porcelain-veneered Y-TZP crown with a thickness of 2.0mm,when the AT zirconia crown is used in clinical,the thickness of the anterior tooth and premolar areas can be reduced to 1.0mm,if there is no enough area to prepare.while the molar area is no less than 1.5mm.2 When the thickness is the same,the compressive strength of the AT zirconia crown exhibit better than that of the SHT porcelain-veneered Y-TZP crown,but is inferior to the commercially available SHT zirconia crown.3 The thickness of the AT zirconia crown has a great influence on the compressive strength of the AT zirconia crown,and the compressive strength of the zirconia crown increases with the increase of the thickness.