W.J. O'Brien

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, ΔE 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, 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 ΔE. An estimate of the accuracy of Eq. 1 was 0.41 ΔE units when ΔE 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 ΔE units when ΔE CIE was less than 4.0. The new color-difference equation provides a means for estimation of AE CIE L*a*b* color difference between dental shades with Munsell notation. This equation will be useful for estimation of small AE CIE L*a*b* values for shade-guide teeth that are designated in terms of Munsell notation.

Optical Properties of Direct Restorative Materials

The contrast ratio, light reflectivity, scattering coefficient, and absorption coefficient of four composites and an unfilled resin were calculated algebraically from reflection spectrophotometric data using Kubelkas equations. The correlation coefficient between calculated and experimental values of contrast ratio was 0.9996. Values of infinite optical thickness ranged from 4.19 to 6.70 mm.

Translucency of Human Dental Enamel

Translucency of human dental enamel was determined by total transmittance of wavelengths from 400 to 700 nm. The transmission coefficient at 525 nm was 0.481 mm-1. Total transmission of light through human dental enamel increased with increasing wavelength. Human tooth enamel is more translucent at higher wavelengths. The translucency of wet human enamel and enamel after dehydration was also measured by total transmittance. The transmission coefficient at 525 nm decreased from 0.482 to 0.313 mm-1 after dehydration and was reversed on rehydration. The decrease in translucency occurred as a result of the replacement of water around the enamel prisms by air during dehydration.

The Surface Roughness and Gloss of Composites

The contrast gloss and the average roughness were measured for four commercial composite filling materials. Using a factorial design, each material was subjected to four available finishing methods. A significant difference was found in the contrast gloss among finishing methods, and a significant linear regression is given which relates the inverse of the contrast gloss to the average roughness. Surface gloss is proposed as playing a major role in the esthetic appearance of composite restorations.

Double-layer Color Effects in Porcelain Systems

The color of an unshaded body porcelain was determined at three thicknesses on white, gray, and three chromatic backings. Spectral absorption and scattering coefficients of the porcelain were determined from the diffuse reflectance at one thickness on the white and gray backings. These optical coefficients, when utilized with the Kubelka-Munk reflectance theory, accurately predicted the color of the other sample configurations studied. The scattering of the body porcelain was found to decrease with increasing wavelength within the visible spectrum, in accordance with scattering theory for particles not substantially less than the wavelength of the scattered light. For the filtering effects of a translucent material in optical contact with a backing, the Kubelka-Munk reflectance theory described the interaction between the optical absorption and scattering within the translucent material and the reflectance of the backing.

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.

Transverse Strength of Aluminous and Feldspathic Porcelain

The transverse strength of aluminous porcelain was compared with that of feldspathic porcelain and the effect of surface roughness and environmental moisture on these strengths was determined. Aluminous porcelain was superior to feldspathic porcelain in transverse strength and the transverse strength of both types of porcelain was affected adversely by environmental moisture.

Light Scattering and Gloss of an Experimental Quartz-filled Composite

For samples of polymethylmethacrylate with and without quartz filler, the inverse of the contrast-gloss ratio is shown to be related to surface roughness and to the optical scattering coefficient. This finding adds to the importance of optical scattering, which has been widely studied because of its relation to color and translucency of materials. Furthermore, optical scattering by composite fillers is shown to be linearly related to the concentration of the filler material within the range of concentrations studied. Quartz fillers were incorporated at concentrations from 5 to 20 weight percent and were short fibers or granular powder, with the granular particles ranging in median equivalent spherical diameter from 15 to 3.3 μm. The efficiency of optical scattering for the granular quartz filler increased as the size of the filler decreased.

Stress induced phase transformation of a cesium stabilized leucite porcelain and associated properties.

OBJECTIVES The addition of CS2O to dental porcelains might provide a means for controlling the thermal expansion and toughness of feldspathic porcelains. The purpose of this investigation was to determine for a leucite porcelain the effect of CS2O content upon its coefficient of thermal expansion (alpha), toughness, hardness, and content of low (tetragonal) leucite and high (cubic) leucite. METHODS In order to determine the amount of low leucite in the specimens, an x-ray calibration curve for low leucite and an internal standard of copper was obtained using quantitative x-ray diffraction techniques. Utilizing a stress induced phase transformation between low and high leucite, an x-ray intensity conversion ratio (r) was determined for high leucite so that the calibration curve for low leucite could be used to determine the amount of high leucite present in the experimental porcelains. Specimens were prepared with various amounts of CS2O (0.0, 0.5, 1.0, 1.5, 2.0 mol%) so that, except for the as-received porcelain (A), all had the same thermal history. Specimens were tested for content of low and high leucite, hardness (Vickers), toughness (indentation-strength method), and coefficient of thermal expansion (alpha) over two temperature ranges (30-500 degrees C and 30-640 degrees C). Fractured surfaces were examined with a scanning electron microscope (SEM). For each property, specimen groups were compared for statistical differences. These comparisons were statistically analyzed using analysis of variance (ANOVA) and Fishers protected least significant differences (PLSD). RESULTS Quantitative x-ray examination of abraded and heat-treated specimens demonstrated that a stress induced phase transformation occurred which could be reversed by heat treatment. The conversion ratio was determined as r = 1.93 +/- 0.29. The addition of CS2O lowered the wt% of low leucite from 63 +/- 6% to 0% and increased the amount of high leucite from 0% to 62 +/- 5%. ANOVA showed that the addition of CS2O had a significant effect on alpha (p < 0.0001), hardness (p < 0.005), and toughness (p < 0.0001). CS2O significantly (PLSD) lowered the alpha (p < 0.0001), hardness (p < 0.01), and toughness (p < 0.0001) of a high-content leucite porcelain. SIGNIFICANCE A stress induced phase transformation of high leucite to low leucite was demonstrated and, as a consequence, suggested the potential for phase transformation toughening. The alpha of a leucite porcelain could be controlled by stabilizing high (cubic) leucite to room temperature with the fraction of high leucite dependent upon the amount of CS2O added. A method was developed to determine the amount of high leucite present in a porcelain.

Thermal Stress Failure of Porcelain Bonded to a Palladium-Silver Alloy

Cracking or rupturing, due to thermal stress, of porcelain bonded to a palladium-silver alloy indicated that porcelain-metal thermal compatibility was dependent on: the difference in thermal expansion coefficients of the porcelain and metal; the geometry and dimensions of the samples; and the porcelain-metal thickness ratio. A spherical configuration was more sensitive to thermal expansion coefficient differences than was a disc configuration. A higher incidence of cracking resulted from an increase in both specimen size and porcelain-metal thickness ratio.