Search

Purpose: The addition of silicon carbide whiskers has been known to reinforce ceramics. This study compared the flexural strength of a glass-infiltrated alumina dental ceramic with various amounts of silicon carbide whiskers incorporated into the alumina matrix., Materials and Methods: Three groups with a total of 30 specimens were included: glass-infiltrated alumina dental ceramic with 0, 0.125, and 0.25 wt% silicon carbide whiskers. The flexural strength was measured using a three-point bending test, and the data were analyzed with one-way analysis of variance. Scanning electron microscopy was used to examine the fracture surfaces of all three materials., Results: There were no significant differences in the flexural strength among the three groups. Glass-infiltrated alumina dental ceramic failed predominantly by transgranular fracture and to a lesser degree by intergranular fracture., Conclusion: The addition of silicon carbide whiskers did not change the flexural strength of glass-infiltrated alumina dental ceramic.

Origin of renal arteries was investigated in 76 cadavers. The right renal artery most of cases originated under the angle of 75 degrees. The left renal artery originated usually under the angle of 85 degrees. In investigated 152 kidneys the accessory renal vessels were observed in 17 specimens.

Purpose: The biaxial flexural strength and fracture toughness for 3 representative types of ceramic core materials were studied to (1) ascertain the ranking of the 3 ceramic types for strength and toughness, and (2) provide clinicians with more information to predict clinical outcomes. The former aim was deemed important in view of the importance of flaw size in the relationship between failure stress and fracture toughness of brittle materials., Materials and Methods: The 3 representative ceramic types included a leucite-reinforced glass, a glass-infiltrated sintered alumina, and a high-purity, high-density alumina. The biaxial flexural strength was measured with the piston-on-3-ball method. The plane-strain fracture toughness was measured with the short-bar chevron-notch method., Results: The biaxial flexural strengths of the high-purity, high-density alumina and glass-infiltrated sintered alumina ceramic core types were significantly higher than that of the leucite-reinforced glass ceramic type. The glass-infiltrated sintered alumina was significantly higher in fracture toughness than the high-purity, high-density alumina type, which was significantly higher than the leucite-reinforced glass., Conclusion: All materials surpassed the minimum strength requirement outlined by the International Standards Organization, and they also had similar strength variability according to their Weibull moduli. The glass-infiltrated sintered alumina and the high-purity, high-density alumina types were significantly stronger and tougher than currently used core materials. However, while the glass-infiltrated sintered alumina had a higher fracture toughness than the high-purity, high-density alumina, it had a lower flexural strength.

This article reviews the current status of standardization in orthodontic products. The role of the American National Standards Institute and the American Dental Association is described. A review of laboratory testing for the evaluation of frictional forces and bond strength in orthodontics is discussed. The need for standardization in test protocols is emphasized to obtain valuable interpretation and use of data generated by researchers. Standardization of test protocols will lead to improved standards for orthodontic products and ultimately higher-quality products for orthodontists and their patients.

The in vitro marginal fit of three all-ceramic crown systems (In-Ceram, Procera, and IPS Empress) was compared. All crown systems were significantly different from each other at P = 0.05. In-Ceram exhibited the greatest marginal discrepancy (161 microns), followed by Procera (83 microns), and IPS Empress (63 microns). There were no significant differences among the various stages of the crown fabrication: core fabrication, porcelain veneering, and glazing. The facial and lingual margins exhibited significantly larger marginal discrepancies than the mesial and distal margins.

This study compared the firing shrinkages, flexural strengths, and chemical solubilities of two low-fusing porcelains formulated for use with titanium (Procera, Nobel Biocare, Göteborg, Sweden; and Duceratin, Degussa, Plainfield, NJ) to those of a conventional low-fusing feldspathic porcelain (Vita VMK 68, Vident, Baldwin Park, CA). Procera demonstrated the highest mean firing shrinkage and was significantly different from the other two porcelains. There were no significant differences among the porcelains in flexural strength and chemical solubility.

The tensile mechanical properties of welded titanium joints were studied, and intact titanium was used as controls. Welded joints were fabricated with either a stereographic laser-welding technique or a gas tungsten arc welding technique. The effect of heat treatment following a simulated porcelain application was also investigated. Heat-treated laser welds had significantly lower ultimate tensile strengths. Heat treatment had no effect on the modulus of elasticity or elongation, but generally significantly decreased the yield strength of the titanium specimens. The gas tungsten are welding specimens had significantly higher yield strengths and elastic moduli than the other two groups. The elongation of the control specimens was significantly greater than the elongation of the gas tungsten arc welding specimens, which was in turn significantly higher than that of the laser-welded specimens.

The working times of five poly(vinyl siloxane) impression materials were estimated by evaluating the dimensional accuracy of stone dies of impressions of a standard model made at successive time intervals. The stainless steel standard model was represented by two abutments having known distances between landmarks in three dimensions. Three dimensions in the x-, y-, and z-axes of the stone dies were measured with a traveling microscope. A time interval was rejected as being within the working time if the percentage change of the resultant dies, in any dimension, was statistically different from those measured from stone dies from previous time intervals. The absolute dimensions of those dies from the rejected time interval also must have exceeded all those from previous time intervals. Results showed that the working times estimated with this method generally were about 30 seconds longer than those recommended by the manufacturers.

The resistance to staining of two low-fusing porcelains (Procera and Duceratin) for use with titanium and one conventional feldspathic porcelain (Vita VMK 68) were examined visually according to a protocol similar to the ADA Specification No. 69. The color deviation was also quantified using a colorimeter. Forty samples of each product were prepared using manufacturer's instructions and the samples divided into two groups of 20. The samples of one group were immersed in methylene blue, and the control group in distilled water. Of the twenty samples from each group, ten were ground to remove the glaze. The color values for each sample were recorded prior to and after immersion in the staining medium. The color deviation value, Delta E, for each sample was calculated. Visually discernible stain was present on the unglazed samples of all three porcelain products immersed in methylene blue. No stain was visually detectable on the glazed samples or the unglazed samples immersed in distilled water for all three porcelain products. The objective evaluation confirmed that the unglazed porcelain products had statistically higher color deviation after immersion in methylene blue.

For esthetic reasons, pigments and opacifiers are added to porcelains used in restorative dentistry. The purpose of this study was to use Kubelka-Munk theory (Kubelka and Munk, 1931) to predict and analyse the colors of porcelains modified by the addition of two pigments and an opacifying agent. The base porcelain was composed of 88% potassium feldspar, 6% quartz and 6% kaolin. The porcelain was modified by the addition of a yellow (Pr-Zr-Si) or a brown (Fe-Cr-Zn) stain and an opacifier (10% SnO in base porcelain). After firing at 1200 degrees C for 30 min, reflectance spectra of the various combinations were obtained with a spectrophotometer. Reflectance spectra, except at low wavelengths, were influenced by increased scattering due to the addition of the opacifier. Calculated values from Kubelka-Munk theory for absorption coefficients and scattering coefficients with appropriate correction factors were compared with the values from the reflectance spectra of the combinations. In general, good agreement was obtained if the scattering coefficient of the opacifier is set equal to 1.0. Using the L*a*b* transform of the CIE color space, it was found that this transform provided uniform color intervals for equal changes in pigment concentration.

Porcelains and resin composites exposed to acidulated phosphate fluorides (APF) have been reported to result in increased roughness, loss of weight, and loss of specular reflectance (gloss). Six samples of five commercial porcelains were subjected to five four-minute treatments with APF gels. Samples were then subjected to a nine-day cyclic staining procedure that utilized a tea, coffee, and mucin mixture. Changes in reflectance were then measured by means of a Minolta Chromameter (CR121) and converted to CIE L* a* b* values at illuminant D65 against a white background. delta L*, delta a*, delta b*, and delta E values were calculated. There was a substantial decrease in the L* value (lightness) for all porcelains. The average L* value for APF-treated and then stained porcelains was 43.6, for the stained-untreated samples, 48.2, and for untreated-unstained porcelain, 53.5. For three of the five porcelains, the differences in L* between treated and untreated stained porcelains were statistically significant. Changes in a* and b* values were also found to be consistent with but not as large as the changes in L*.

The luminescence spectra were obtained for a series of porcelain teeth and powders currently used in this country. Comparison was made between the spectra of these materials and the spectra of plastic teeth and natural tooth structures. It was found that many of the porcelain teeth and powders have strikingly different luminescence characteristics than those of natural teeth.

Color changes resulting from immersion of four composite resins in hot distilled water, coffee and tea were measured by a tristimulus colorimeter. Changes are shown as CIE deltaE values and converted to Munsell deltaH, deltaV and deltaC values.

Luminescence spectra of several non-radioactive luminophors that may be suitable for dental porcelain have been obtained. It was found that porcelain composition and maturation temperature have large effects on excitation and emission spectra. Most of the luminophors had excitation maxima lower in wavelength than have been observed for natural teeth.

The translucency and colour of a composite resin are characterized by two wavelength-dependent parameters: the absorption coefficient, K and the scattering coefficient, S. These parameters were calculated according to the Kubelka-Munk (1931) equations for two commercial resins. The calculations were based on reflectance spectra of samples of different thicknesses which were placed on a black and a white background. Employing the values of these parameters, calculated spectra were obtained of samples on the same as well as different backgrounds. The difference between the experimental and the calculated spectra was evaluated in terms of Friele-MacAdam-Chickering (Hemmendinger, 1970) colour differences. The Munsell colour values, corresponding to the spectra were evaluated. It was found that the colour of a resin sample of a specified thickness which is placed on a particular background may be predicted to a fair degree of accuracy from calculations employing the K and S values of the resin and the reflectance spectrum of the background. It has been demonstrated that illuminating the sample at 45 degrees instead of using diffuse illumination, affected the results only to a small extent. The effect of internal reflections at the resin surface on the relfectivity on the sample is discussed. An expression has been derived for computing the reflectivity of a sample on any background from its transmittance and reflectivity on a completely absorbing background.

The irradiance in the wavelength of 450 to 500 nm of nine visible light-curing units was measured at 120 V input voltage. The highest irradiance was more than 20 times that of the lowest. Irradiance was also measured at 90 to 130 V input. Irradiance and depth of cure at lower input voltages were also examined.

The ultraviolet light color stability of seven commercial composite resins was evaluated after 1, 8, and 15 days of exposure. Color differences between exposed and unexposed specimens stored for identical time periods were determined. Samples exposed to ultraviolet light showed large changes in Munsell Hue and Chroma, with smaller but significant changes in Value. Unexposed samples showed small changes in the Munsell components in some cases opposite those observed for the exposed samples. Statistical analysis showed that although significant color changes were observed, brands of composite resins could not be distinguished by length of storage in the dark. Time of exposure was a significant variable at 24 hours and 8 days. At 15 days a number of composite resins did not undergo additional significant color change. Scanning electron microscope showed a significant roughening of the surface of exposed composites with resin breakdown and exposure of the composite filler.

Eyeglasses with protective filters are recommended for users of visible light-curing units to decrease the intensity of blue light reaching the eyes. The transmission curves of 20 commercially available filter glasses are obtained. The irradiance from eight curing units through these glasses is measured. The reduction in irradiance through the filter glasses is related to the transmission curves.