| Objective: To analysis the effect of the thickness of opaque porcelain on the color of the nano-ceramic and the traditional ceramic. The results were expected to provide effective experimental basis reference to application of the nano-ceramic in clinic.Methods:1 Specimens preparation:Making 10mm diameter circle of wax using finished medical wax with the thickness of 0.5mm by special tools.Then obtain 48 metal base specimens by casting process, which were divided into 8 groups at random, 6 metal base specimens each group. A3 color of Refle (Weiland, Germany) for nano -ceramic was coated on group A-D and A3 color of Vintage (shofu, Japan) for traditional ceramic was coated on group A′-D′. the opaque thickness of 0.1mm was coated on group A,A′, 0.2mm was coated on group B,B′, 0.3mm was coated on group C,C′, and 0.4mm was coated on group D,D′.1.0mm dentin porcelain was coated on all specimens. The two kinds of Ceramic were fired in the same porcelain furnace following the respective manufacturers'recommendations, and the fired numbers of all samples were uniform. Then, every was burnished by the same dental grinding machine at the same speed, then flattened every surfaces along the same direction by grinding on 240#, 400#, 600#, 800#, 1000# and 1500# water sand papers. The thickness of the specimens was controlled by An electronic sliding caliper. Error was controlled at±0.01mm. Finally, they were glazed by itself and uitrasonic following the respective manufacturers'recommendations, and cleaned for five minutes by distilled water, dried and spared.2 Measurement of surface roughness: A surface roughness tester was used to measure the value of surface roughness (Ra) of every specimen. Three points were respectively selected to measure the value of surface roughness every sample. The mean of the three roughness values was considered as the surface roughness value of the specimen.3 Measurement of the color of the specimens: A Spectrascan PR-650 spectrophotometer was used to measure the color of the specimens, The measure is in a dark room, black backgroud, with a standard lightness of D65, the illumination was 2000lx, 45°/0°geometry, at a 45 degree angle. Then adjust the length between body glasses and the specimens, make the light bot of body glasses is 5.0mm Before evaluation session, the spectrophotometer was calibrated by a white working standard(L﹡=100,a﹡=0.00,b﹡=0.00). After half an hour of warm-up, the center point of each specimen were measured three times, take the mean as the result of each specimen. Colormetric data were recorded as CIE1976-L*a*b*values. All values were analyzed by SPSS 13.0 to compare the difference from each group for the L*,a*,b*values.Results:1 Surface roughness value of each group: the mean of surface roughness of the nano-ceramic group was 0.090±0.035μm; the mean of surface roughness of the traditional ceramic group was 0.129±0.050μm. There was significant difference between the nano-ceramia group and the traditional ceramic group (P<0.01).2 The Colormetric data of the nano-ceramic groups and the traditional groups with different thickness of opaque porcelain(1) The nano-ceramic groups: the values of L* were: group A 63.37±0.42,group B 64.81±0.32,group C 64.79±0.35,group D 64.92±0.42; the values of a* were: group A 3.91±0.16,group B 4.31±0.23,group C 4.41±0.17,group D 4.35±0.06;the values of b* were: group A 22.12±0.32,group B 21.95±0.43,group C 22.50±0.19,group D 22.30±0.26;the values of chroma (C﹡ab) were: group A 22.46±0.33,group B 22.37±0.45,group C 22.39±0.20,group D 22.72±0.11;the values of color difference(△E) were: group A and B 0.95±0.28,group A and C 0.94±0.58,group A and D 0.92±0.45,group B and C 0.70±0.30, group B and D 0.69±0.38,group C and D 0.49±0.11.(2) the traditional ceramic groups: the values of L* were: group A' 63.63±0.42,group B' 65.30±0.44,group C' 64.94±0.66,group D' 65.51±0.57; the values of a* were: group A' 2.27±0.14,group B' 3.48±0.04,group C' 3.63±0.10,group D' 3.65±0.07; the values of b* were: group A' 18.90±0.33,group B' 20.95±0.16,group C' 21.09±0.40,group D' 21.04±0.07; the values of chroma (C﹡ab) were: group A' 19.04±0.34,group B' 21.24±0.16,group C' 21.40±0.40,group D' 21.36±0.08; the values of color difference(△E) were: group A' and B' 2.95±0.76,group A' and C' 3.17±0.88,group A' and D' 3.20±0.70, group B' and C' 0.98±0.37,group B' and D' 0.74±0.24, group C' and D' 0.84±0.66.3 Comparison of the groups in the nano-ceramic group and the traditional ceramic group with the different thickness of opaque porcelain.(1) The results of the nano-ceramic group:The values of L* were increased with the thickness of opaque porcelain becoming thicker. There was significant difference between the group A and group D; The values of a* were increased with the thickness of opaque porcelain becoming thicker; There was significant difference between the group A and groups B,C,D(P<0.01), the mean of a* group C was the biggest; The mean of b* increases firstly, then decreases. There were significant difference between the group C and groups A,B(P<0.05). There were significant difference for the value of L*,a*,b* between the group A' and groups B',C',D' (P<0.01), and the mean of values of a* was increased with the thickness of opaque porcelain becoming thicker, there were difference between the group B' and groups C',D'(P<0.05).(2) The results of chroma show that there were significant difference between the groups A,B and group C, the mean of values of chroma of group C was the biggest one, and there were significant difference between the group A' and the groups B',C',D', the mean of chroma of group A' is the smallest.(3) The results of△E show that there was significant difference between the△E of groupA and group B and the△E of group C and group D, the△E of groupA and group B,group B and group C,group C and group D reduced with the thickness of opaque porcelain becoming thicker; the△E of group A' and group B',group B' and group C',group C' and group D' reduced with the thickness of opaque porcelain becoming thicker, and there were significant difference between the△E of groupA' and group B' and the△E of group B' and group C' and the△E of group C' and group D' with the thickness of opaque porcelain becoming thicker.4 The results of△E nano-ceramic groups and traditional ceramic groups were compared with the same thickness variation of opaque porcelain.With the thickness of opaque porcelain increasing from 0.1mm, there were significant difference between the△E of group A and group B and the△E of group A' and group B'(P<0.01), and the△E of group A and group B was much smaller than that groupA' and group B'. There were not difference between the△E of group B and group C and the△E of group B' and group C'(P>0.05) and between the△E of group C and group D and the△E of group C' and group D'(P>0.05).Conclusion:1 After glazing by themselves, the surface of the nano-ceramic was smoother than the traditional ceramic.2 The thickness of opaque porcelain effected the color of the nano-ceramic and the traditional ceramic.3 With the thickness of opaque porcelain becoming thicker, the color differences of the nano-ceramic groups and the traditional ceramic groups were reduced, the color differences of traditional ceramic was significant and could be discreminated by eyes; the color of samples trended to yellow and red, and the chromas trended to increased.4 The nano-ceramic could cover the color of metal alloy with 0.1mm opaque ceramic, while the traditional ceramic need 0.2mm opaque ceramic.
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