Influences Of Veneering Porcelain Thickness On The Microstructure And Strength Of Zirconia All-ceramic Crowns

Objective: To observe the microstructure of zirconia all-ceramic crownswith different veneering porcelain thickness and through compression tests onthem, influences of veneering porcelain thickness on the microstructure andstrength of zirconia all-ceramic crowns are studied, which provide theoreticalbasis for technical manufacture of zirconia all-ceramic crowns.Method:1Models preparation:15metal models were produced by precisioninstruments, of which the base was cylindrical with the diameter of50mm andthe height of20mm; and in the central of the base there was a metal crowncore, simulating the premolar preparation, diameter of5mm and height of6mm, smooth and rounded. Selecting one model from the15randomly forbackup, through the computer-aided-design and computer-aided-manufacture(CAD-CAM) and the3shape scanner, to get the three-dimensional data of themodel, which was used to design the basement crowns and25bodies ofzirconia basement crown were produced by the precision lathe. The bodieswere put into the porcelain furnace, being sintered into basement crowns withdiameter of6mm and height of6.5mm and thickness of0.5mm. All thebasement crowns were sand-blasted, cleaned and heated; and then they wererandomly divided into5groups, they were group A-E with different veneeringporcelain thickness of0.5mm,1.0mm,1.5mm,2.0mm,2.5mm, and weresintered according to the procedures specified by the manufacture. Finally allthe specimens were auto-glazed once.2Compression test：Cementing these specimens to the metal models withthe glass-ionomer cement and leaving them at room temperature for24h,located them on the universal testing machine successively. The test wasperformed with a spherical loading material, of which the diameter is6mm, at the speed of0.5mm/min, one the central of the specimen, until the porcelaincollapsed, and recorded the compression values.3Statistical analysis of those values was performed by software ofSPSS13.0. First, the values were used to test normality and homogeneity ofvariance, if they followed, to apply one-factor analysis of variance, if P＜0.05,to apply multiple comparison. If these values didn’t follow a normaldistribution or heterogeneity of variance, the rank test was used. Significancelevel（also as size of a test）：α＜0.05; P＜0.05means statistical significance.4One specimen was randomly selected from each group after thecompression test to observe the porcelain section with the scanning electronmicroscope (SEM).Results:1Values obtained from compression tests are as follows (unit: N): groupA:1279.96±42.85; group B:2235.44±50.14; group C:2216.38±48.97; groupD:2169.22±60.40; group E:2028.70±47.37.2Results of analysis of variance (ANOVA) indicate that there aresignificant difference of compression test values on zirconia all-ceramiccrowns with different veneering porcelain thickness (P<0.01), results ofmultiple comparison indicate that significant difference in compression testvalues exists between group A and any other groups, group E and any othergroups; no significant difference between any other tow groups (P>0.05).3Microstructure of zirconia all-ceramic crowns’ section:The bonding interface: group A, loose, a few tiny pores with diameter of20-40μm scattered, long and irregular micro-cracks with length of20-100μmshown; group B, smooth, tightness, no obvious cracks, few tiny pores found;group C, smooth, no obvious cracks, few tiny pores found; group D, a few tinycracks shown, a few tiny pores found, larger than that of group C; in group E,cracks with length of about30μm scattered, more and longer; pores withdiameter of6-25μm appeared, more and larger.The veneering porcelain section: in group A, tiny micro-cracks withlength of10-30μm widely distributed, tiny pores with diameter of10-25μm appeared; in group B, no obvious cracks, a few tiny pores found; In group C,no obvious cracks, a few tiny pores found, larger than that of group B; ingroup D, a few tiny cracks shown, a few tiny pores found, larger than that ofgroup C; in group E, cracks with length of25μm scattered, more and longer;pores with diameter of5-25μm appeared, more and larger.Conclusion:1When base crown thickness is certain, veneering porcelain thicknesshas important influences on the microstructure and strength of zirconiaall-ceramic crowns.2When veneering porcelain thickness is1.0mm,1.5mm,2.0mm, thecracks and spores in the microstructure appeared fewer and smaller and have amore sparely distribution; when the veneering porcelain thickness is0.5mmand2.5mm, the cracks and spores in the microstructure appeared more andlarger and have a more intensive distribution.3In this test, the zirconia all-ceramic crowns’ strength first increases andthen decreases with the increasing of veneering porcelain thickness; when theveneering porcelain thickness is1.0mm, the strength is the maximum; whenthe veneering porcelain thickness is0.5mm and2.5mm, the strength decreasesa lot; in other words, when the veneering porcelain is too thick or too thin, thezirconia all-ceramic crowns’ strength will decreases significantly.