Effect Of Repeated Sintering On The Color/optical Parameters And Microstructure Of Three Dental All-ceramic Systems

Objective: The aim of this study was to compare the effects of repeated sintering on the color(L*,a*,b*), optical parameters(CR,TP,S,K,RI) and microstructure of three dental all-ceramic systems, to evaluate the relationship between change of microstructure and change of color / optical parameters. Material and methods:Specimens were made of three kinds of all-ceramics: IPS EmpressⅡheat-pressed all-ceramic, Vita In-cream alumina all-ceramic, Cercon CAD/CAM zirconia-based all-ceramic.Color parameters(L*,a*,b*) and spectral reflectance data(R0, R) were measured according to the CIELAB color scale relative to the standard illuminant D65 in the reflectance mode over a black background and a white background on Color- Eye 7000A spectrophotometer. The transmission parameters CR and TP were calculated from spectral reflectance data, the optical constants including scattering coefficient(S), absorption coefficient(K) and light reflectivity(RI) were calculated algebraically using Kubelka-Munk equations. Specimens were characterized by using X-ray diffraction, Scanning electron microscopy and Pore master. Color/optical parameters and microstructure were measured again after repeated sintering. Repeated measures ANOVA analysis of variance and LSD-t test were used to analyze the data for significant differences (α=.05), the correlation between pore volumes and Color/optical parameters were analyzed. Results: With the number of sintering increasing, the particles of three brand all-ceramic core specimens slightly increased, glass phase decreased; pore diameter of core and veneering ceramics were decreased. There was no statistically significant difference between pore volume after first and fifth sintering for both IPS EmpressⅡheat-pressed all-ceramic and Vita In-cream alumina all-ceramic(P<0.05). A difference was found in pore volume of different brand core and veneering ceramics (P<0.05). A difference was found in RI and TP after repeated sintering (P<0.05). There was no significant difference in other color/optical parameters after fifth sintering(P>0.05). The correlation coefficients have no regularity between color and optical parameters. Conclusions:With the number of sintering increasing, pore volume of Vita In-cream alumina all-ceramic was affected minimum, followed by the Cercon zirconia-based all-ceramic, finally IPS heat-pressed all-ceramic. At the same number of sintering of different brands of ceramic samples, the pore volume of Vita In-cream alumina all-ceramic was affected minimum, followed by IPS heat-pressed all-ceramic and Cercon zirconia-based all-ceramic. The pore diameter of three brand core and veneering ceramics were decreased, showing the migration from large to small. For the same brand all-ceramic, the color parameters were not affected up to the 5th sintering. With the same number of sintering, maximum color difference was found in Vita In-cream alumina all-ceramic, Cercon zirconia-based all-ceramic was affected minimum. With the same number of Sintering , maximum change of pore volume was found in IPS EmpressⅡheat-pressed all-ceramic. Regression analysis showed that change of pore volume can not explain the change in the color difference and color parameters difference. For the same brand of all-ceramic specimens, the number of sintering has an impact on optical parameters in some degree. With the same number of sintering, brands has impact on optical parameters.