Effect of dehydration time on tooth colour measurement in vitro.

OBJECTIVE: To investigate the effect of dehydration time on tooth colour measurement in vitro using colourimetry. METHODS: Ten extracted human maxillary central incisors were used. L*a*b* values at the centre of the labial surface of each tooth were measured using a colourimeter. The first measurement was performed after removal of excess water. The second and third measurements were performed after the teeth had dried at room temperature for 2 h and 4 h, respectively. The colour differences (ΔE) between dehydration time points were calculated. Repeated oneway ANOVA was performed for colour values at different dehydration time points. Pairwise comparison of group means (Student t test) was used to examine the differences between ΔE and 1.5ΔE (α = 0.05). RESULTS: There were statistically significant changes in the L* values after 2 h and 4 h dehydration as well as between each ΔE and 1.5ΔE. Neither 2 h nor 4h dehydration resulted in change of a* value or b* value. CONCLUSION: Dehydration time affected colour measurement using a colourimeter; the teeth become lighter after dehydration for 2 h or longer.

Effects of background, direction and intensity of ambient light, measuring position, and adjacent teeth, on anterior tooth colour measurement in vitro.

OBJECTIVE: to investigate the effects of different background colours (black, white or pink), direction and intensity of ambient light, measuring position, and the adjacent teeth, on the in vitro colour measurement of maxillary anterior teeth, using the Minolta CR-321 colorimeter. METHODS: ten extracted human maxillary central incisors were selected. A fibre-optic light MI-150 was used as the ambient illuminant. Teeth were irradiated from a 3- or 12-o'clock direction. L*a*b* values of seven sites on the labial surfaces were obtained by means of the Minolta CR-321 colorimeter, using three background colours, with or without the adjacent teeth. The recorded data were analysed with two-tailed Student t tests and analysis of variance (α = 0.05). RESULTS: the ambient light did not affect the colour measurement of anterior teeth, regardless of the presence or absence of the adjacent teeth. There were no statistically significant differences in L*a*b* values at the same position under different background colours, except ΔE12 (colour difference between site 1 and site 2) between black and white backgrounds. ΔE12 (under black background), ΔE13 and ΔE15 were greater than 1.5, while the others were lower than 1.5. CONCLUSION: the background, ambient light and the presence of adjacent teeth did not affect the colour measurement of anterior teeth using the Minolta CR-321 colorimeter in vitro. The inherent disadvantages of using the naked eye during clinical visual shade assessment may be overcome by the colorimeter.

[Influence of MgO and TiO2 on mechanical properties of zirconia toughened alumina ceramics formed by gel-casting technique].

OBJECTIVE: The objective of this study is to investigate the influence of mechanical properties and sintering performance by adding 5% weight percentage aids to nano-compound zirconia toughened alumina (ZTA) ceramics. METHODS: Micrometer Al2O3 and nanometer ZrO2 (quality ratio 4:1) were used to get 55% volume percentage slurry. Magnesium oxide and titanium oxide were taken as aids which were 5% weight percentage of the Al2O3 and ZrO2 powder. Five groups (number 0, 1, 2, 3, 4 group) were divided according to different proportion of aids. After gel-casting, the porcelain pieces were sintered at 1150, 1200, 1300, 1400, 1450, 1500, 1600 degrees C for 2 hours. Static three-point flexure strength, line shrinkage, relative density were measured and scanning electron microscopy (SEM) was used to observe section. RESULTS: Number 1 (MgO 1%, TiO2 4%) group had the highest bending strength. It was (401.78+/-19.50) MPa after sintering at 1600 degrees C for 2 hours and was higher than 0 group (380.64+/-44.50) MPa. Bending strength became lower than 0 group when MgO was more than 2% or more than that weight percentage of ZTA powder. When MgO content was higher than 2% or more than that weight percentage, there was no difference in relative density raising rate between each sintering assistants groups. When the sintering temperature was higher than 1200 degrees C, all groups showed obvious line-shrinkage and the groups which contained sintering assistants were all was higher than 0 group. CONCLUSION: Adding MgO and TiO2 aids from 1% to 4% weight percentage of ZTA will promote fritting and increase ZTA nano-compound ceramics mechanical properties. Adding 2% MgO aids or more than that weight percent will has no obvious help to increase the relative density raising rate of ZTA nano-compound ceramics and will degrade the mechanical properties of ZTA nano-compound ceramics.