Influence Of The Firing Times On Compressive Strength Of Zirconia All-ceramic Crown

Objectives：Through studying the compressive strength on zirconiaall-ceramic crown.To discusse influenc of the firing times on strength ofzirconia all-ceramie crown. The purpose is to provide the theoretical basis forthe clinical application of zirconia all-ceramic crown.Methods:1Models preparation: A metal model of simulating crown core of theideal premolar, was processed by precision instrument.The diameter of thecrown core was5mm, and the height was6mm, and there was no sharpedges.The base of metal model was50mm, the height was20mm.Used thecomputer aided design and computer aided manufacture (CAD-CAM),puttingthe metal model into the scanister to get the three-dimensional data.While thecomputer used the data to design the basement crown. Then the computerregulatted the turning-lathe to make25zirconia all-ceramic basement crown.The height of the basement crown core was6.5mm, the diameter was6mm,and the thick was0.5mm. After sand blasting,cleaning,heat treatment,allthe basement crowns were firing masking-porcelain two times.The thicness ofthe basement crown was between0.2mm~0.3mm.After firing bodyporcelain,the thickness of all zirconia all-ceramic crowns was ensuered at1.5mm.Randomed all the models into5groups,5in a group,named A-E. Thefiring times of body porcelain for eaeh model from group A to E were1,3,5,7,9respectively.Finally all zirconia all-ceramic crowns was self-glazingone time.2Located these specimens in mental model within Instron testingmachine. Loaded on occlusal surface of each model vertically by a6mmpressure head,at the speed of1mm per min,and then performed destructivetest to determine the compressive strength,to test the most strongly fracture force values,and record.3Statistical analysis was conducted on all values of the compressivestrength by SPSS13.0software. Testing normality and homogeneity ofvariance. If the material was normal distribution, used mean and standarddeviation to describe the results. Then testing analysis of variance(ANOVA).Testing multiple comparison when P<0.05. Inspection standarda=0.05,P<0.05means statistical significance.4Eaeh model’s strueture in fraeture section was observed with SEM.Results:1The destructive values in all crowns of different groups were asfollows:Group A:(2401±184) N,Group B:(2381±143) N,Group C:(2284±95) N,Group D:(1655±132) N,Group E:(1603±95) N.2Analysis of variance(ANOVA) results indieate that there are significantdifference of destruetion test values on models with different firing times(P<0.01)，multiple comparison results indicate that significant difference indestruction test values exist between group A and D,A and E,B and D,B andE,C and D,C and E (P<0.01):no significant difference between any other twogroups (P>0.05).3The fraeture sections strueture:the bonding surface of zirconiaporcelain: Group A:smooth,thightness,a little spilehole; Group B:smooth,thightness,a little spilehole;Group C:flat,pores of different sizescattered;Group D: uneven,big pores scattered,long and irregular crack;GroupE: uneven,big pores scattered,long and irregular crack. porcelain layer:GroupA:flat and smooth hyaloid structure, a little spilehole;Group B：flat and smoothhyaloid structure, a little spilehole,a little microcrack;Group C: the surfacewas uneven, lots of rising as blade and ridge with glassy structure shown inbetween, pores of different size scattered,more microcrack;Group D: thesurface was uneven, lots of rising as blade and ridge with glassy structureshown in between, big pores scattered,long and irregular crack;Group E: thesurface was uneven, lots of rising as blade and ridge with glassy structureshown in between, big pores scattered,more big pores near the surface,more microcrack.zirconia layer:GroupA:alignment of crystal was regular andthight,a little aggregate; Group B:alignment of crystal was regular and thight,alittle aggregate;Group C:alignment of crystal was thight,moreaggregate.;Group D: alignment of crystal was thight,ridge tubes,moreaggregat, the edge of aggregat was irregular;Group E:alignment of crystal wasthight,ridge tubes,more aggregat,the edge of aggregat was irregular.Conclusion：1.When certain firing times is exeeeded,the strength of zirconiaall-ceramic crown become weaker.2.With premise of satisfying clinical requirements,the firing times ofzirconia all-ceramic crown should be as minimum as possible,the firing timesof body porcelain should be controlled not more than5.