Cerium oxide as a silver decolorizer in dental porcelains.

OBJECTIVES: Silver-base alloys are known to produce an esthetically unpleasant yellow-green tint in the dental porcelain when making PFM restorations. Cerium oxide is used in dental porcelains to simulate the natural fluorescence found in human dental enamel, and has also been used in the glass industry as a decolorizer. The purpose of this study was to determine the effect of CeO2 additions on the resistance of dental porcelain to staining from silver contamination. METHODS: Five batches of porcelain were prepared according to Weinstein et al. (1962) with 0.00, 0.05, 0.10, 0.15 and 0.20 wt% additions of CeO2. To determine the resistance of these porcelains to silver staining, 0.10 wt% additions of the silver oxides were triturated into the prepared CeO2 porcelains prior to sample fabrication. This procedure provided a more quantitative method of staining than firing directly on silver alloys. Silver oxide was added in two valence states as Ag2O and AgO to test for any possible effects on staining. Samples were pressed into a 17 mm diameter mold, and fired to 960 degrees C under vacuum. Three additional samples were prepared from the non-cerium porcelain frit to produce a non-stained control group. Color measurements were made with a spectrophotometer on the ten experimental groups and the control group. The CIE L*a*b* color difference, delta E*, was calculated between the control and the experimental groups. RESULTS: There was a significant decrease in the silver staining of dental porcelains when CeO2 additions of 0.10 wt% or greater were used. SIGNIFICANCE: Cerium oxide additions in the range of 0.10 to 0.20 wt% caused a three-fold reduction in the staining of dental porcelain samples which had been doped with 0.10 wt% of AgO or Ag2O.

Color distribution of three regions of extracted human teeth.

OBJECTIVES: Knowledge of human tooth color and its distribution are critical to the understanding of shade matching in esthetic dentistry. The color of human teeth shows a gradation from the gingival to the incisal region. There have been many reports in the literature on the distribution of color in teeth, but not in the CIE 1976 L*a*b* system. This study was conducted to determine the color distribution in three regions in a sample of human teeth and express the results in Munsell notation, CIE 1976 L*a*b* and CIE delta E* color differences. The hypothesis of this research was that it is possible to detect significant differences in the color parameters of the three distinct regions in teeth. METHODS: All of the teeth used in this study were extracted, cleaned and stored in artificial saliva. Prior to measurement, the teeth were removed from the solution and mounted in a holder to ensure consistent measurements. Spectral data were collected using a GE recording spectrophotometer, CIE chromaticity coordinates calculated using CIE illuminant C and 1931 observer data, and conversions made to L*, a*, b* and Munsell notation. The results were analyzed by ANOVA and Scheffé's multiple comparisons test. RESULTS: The mean L*, a*, b*s were 72.6, 1.5, 18.4 for gingival, 72.4, 1.2, 16.2 for middle, and 71.4, 0.9, 12.8 for incisal. Average Munsell parameters were 1.2 Y 7.1/2.7 for gingival, 1.3 Y 7.1/2.4 for middle, and 1.4 Y 7.0/1.9 for incisal. The mean CIE delta E* between the gingival and incisal regions of the 95 teeth showed a clinically significant difference of 8.2. SIGNIFICANCE: The distribution of color was identified for three regions of the tooth. A statistical analysis determined that there are statistically significant color differences between the regions, and these differences are also clinically significant. This information is beneficial when esthetic restorations are required.

Evaluation of some properties of an opaque porcelain fired simultaneously with the body porcelain.

Recently, a porcelain-fused-to-metal opaque porcelain was introduced that does not require a separate firing before application of the body porcelain. The objective of this study was to determine the properties of this new opaque porcelain and its ability to bond to metal. The properties studied included flexural strength, linear firing shrinkage, coefficient of thermal expansion, powder particle size, and ability to bond to body porcelain and dental alloys. Sintering of this opaque porcelain was complete when fired at 1760 degrees F (960 degrees C) with a linear firing shrinkage of 13.1% +/- 0.2%. No boundary between the opaque and body porcelains could be found with a scanning electron microscope after firing at 1760 degrees F (960 degrees C). The mean flexural strengths were 99 +/- 7 and 101 +/- 8 MPa respectively, for this opaque porcelain and a conventional opaque porcelain, and were not significantly different as assessed with Student's t-test (p = 0.548). The coefficients of thermal expansion for this opaque porcelain was 13.3 +/- 0.2 x 10(-6)/degrees C. Particle size analysis showed a 63% increase in the particles below 5 microns for this opaque porcelain and bonding to two alloys was adequate as indicated by its cohesive failure. Simultaneous firing of this special opaque porcelain and body porcelain produced satisfactory sintering, strength, and bonding to metal.

Leucite content of selected dental porcelains.

Leucite is a major crystalline component of dental porcelains. The presence of tetragonal leucite in dental porcelains increases their coefficients of thermal expansion due to its high coefficient of thermal expansion (20-25 x 10(-6)/degrees C). This is particularly useful for those porcelains designed for bonding to precious metals and nickel alloys. The purpose of this study was to determine the leucite content of selected commercial dental porcelains in relation to their coefficient of thermal expansion values. The weight fraction of leucite was determined with quantitative x-ray diffraction using copper as an internal standard. Coefficient of thermal expansion values were determined using a thermal dilatometric analyzer. Five commercial body porcelains were studied. Leucite was not detected in samples of Vitadur N and Duceram LFC. An ANOVA showed that there was a significant difference in the weight fraction of leucite for Silhouette, Ceramco II, and Optec HSP porcelains. Linear regression revealed a correlation (R = 0.91) between weight fraction of leucite and the coefficient of thermal expansion for those samples containing leucite. Duceram LFC, which is recommended by the manufacturer for use with metals and leucite-containing porcelains, had no detectable leucite although the coefficient of thermal expansion was found to be 13.2 +/- 0.4 x 10(-6)/degrees C at 25-472 degrees C. A low glass transition temperature contributed to the high average coefficient of thermal expansion value.

The strengthening mechanism of a magnesia core ceramic.

A high-expansion core material containing magnesia and forsterite may be used to make all-ceramic dental crowns with porcelain-fused-to-metal body porcelains. The purpose of this study was to investigate the strengthening mechanism for the magnesia core material. Six batches of the magnesia core material were made by reacting magnesia with a silica glass with holding times ranging from 17 to 120 min. The flexural strength was measured using three-point loading according to the ISO specification for dental ceramics. The forsterite content was measured using quantitative x-ray diffraction. A statistically significant correlation was found between the forsterite content and flexural strength. The proposed mechanism for strengthening is the precipitation of fine forsterite crystals in the glass matrix surrounding unreacted magnesia. Longer reaction times produced more dissolution of magnesia and subsequent precipitation of forsterite. This method results in a new strengthening mechanism for dental ceramics which have previously relied on the incorporation of alumina, leucite or ceramic whiskers.

Differences in color between fired porcelain and shade guides.

The inaccuracy of premixed porcelain shades may cause errors when color matching porcelain crowns. Most brands of porcelain are labeled to match shades of the Vita shade guide, but produce slightly different colors from this guide upon firing. The purpose of this study was to quantify in CIE delta E* units the color differences between the Vita shade guide colors and four commercial porcelains for metal ceramic crowns. Two operators prepared shade-guide teeth from six shades of four brands of porcelain. Opaque, body, and incisal layers were fired in the form of shade-guide teeth on Vita ceramic carriers used for making custom shade-guide teeth. The colors of these teeth were measured with a Beckman spectrophotometer with an integrating sphere. The average delta E* values for the differences between the colors of the Vita shade guide and the fired porcelains for each of the brands were 3.1, 2.9, 4.1, and 2.0, respectively, for the first operator and 4.1, 2.6, 2.8, and 1.6, respectively, for the second operator. The color difference between the custom shade-guide teeth and the Vita master shade guide were significantly affected by both brands and shades. The overall average error resulting from the differences in colors between the Vita shade guide and fired porcelains was 3.0 for the first operator and 2.8 for the second operator. The mean delta E* between the teeth prepared by the two operators was 3.6. The color difference between the teeth made by the two operators was not significantly affected by brands or shades.

Sources of color variation on firing porcelain.

The final color matching of porcelain crowns depends upon the accuracy of the original shade matching by the dentist and variables introduced during processing. Possible sources of processing variables include thickness and color of the opaque, thickness, color, and translucency of the body and enamel layers, firing temperature, and number of firings (Miller, 1987). These processing variables can lead to an error in shade match. The purpose of this study was to quantify, in CIE delta E units: (1) the shade variations when the same batches are fired, (2) the shade variations between different batches, and (3) the differences in color produced by the multiple firing. Three lots of six shades of four commercial brands were included in this study. The color variation of the opaque samples (mean delta E was 0.46) was generally lower than that of the body/opaque samples (mean delta E was 0.86). The average color variation for three different batches of the body/opaque samples was 1.44. The average color difference produced as a result of multiple firings was 1.00 after six firings, compared with the color after three firings.

Coverage errors of two shade guides.

Dental shade guides contain a limited selection of colors compared to those found in human teeth. An error in shade selection is thus introduced, since many tooth colors must be defined by approximation to the nearest shade of the guide. The Bioform and Vita Lumin guides, and a combination of the two, were spectrophotometrically compared to the published colors of 335 human teeth. The minimum CIE L*a*b* color difference was calculated for each tooth using each shade guide. The average of these color differences was defined as the coverage error. The Bioform and Vita shade guide coverage errors were not significantly different, but the coverage error was significantly lower when the combination was used. The use of both guides in shade selection is recommended to reduce the coverage error.

A new, small-color-difference equation for dental shades.

Traditionally, dental-shade-guide standards are designated in terms of Munsell hue (H), value (V), and chroma (C). However, delta E color differences proposed as ADA tolerances for shade guides are in the CIE L*a*b* system. The purpose of this study was to evaluate a new color-difference equation, delta EM = C delta H/5 + 7 delta V + 4 delta C for estimation of small color differences by Munsell parameters. The published values of the Bioform shade-guide tooth colors determined with a Beckman spectrophotometer were used. Color differences among 276 combinations of the 24 Bioform shade-guide colors were calculated with Eq. 1, with use of the Munsell notation, and also with the CIE L*a*b* equation for delta E. An estimate of the accuracy of Eq. 1 was 0.41 delta E units when delta E CIE was below 4.0. The Vita shade-guide colors were determined with a Beckman spectrophotometer. This data set contained 16 samples, and 120 combinations were used for calculation of color difference. An estimate of the accuracy for this set of data was 0.35 delta E units when delta E CIE was less than 4.0. The new color-difference equation provides a means for estimation of delta E CIE L*a*b* color difference between dental shades with Munsell notation. This equation will be useful for estimation of small delta E CIE L*a*b* values for shade-guide teeth that are designated in terms of Munsell notation.

One-dimensional color order system for dental shade guides.

The purpose of this study was to re-arrange the master Bioform shade guide into a long-range one-dimensional color system based upon color difference. Although most shade guides may show local order when arranged according to hue, long-range order has not been established. However, shade guide arrangement according to a logical color order would be an advantage to the user. The first step in determining the color order was to measure the color of the shade guide teeth. A methodology was developed for measuring the color by use of a reflectance spectrophotometer. The precision of measurement was determined to be equal to CIE L*a*b* delta E of 0.5. Spectra were obtained and converted into CIE L*a*b* and Munsell notation. The measured colors of the Bioform shades ranged from a Munsell hue of 0.9 Y to 3.5 Y; a value of 6.6 to 7.8; and a chroma of 1.9 to 4.1. The teeth were then arranged visually from light to dark. The correlation coefficient between the visual ranking and color difference was 0.95. There was an inverse correlation between visual ranking and Munsell value, with a correlation coefficient of 0.90. Therefore, the sequence according to color difference provided the better agreement with visual perception.