Abstract
Objectives
The study was conducted to compare the optical parameters of VM7 ® M-shade base dentin ceramics (VITA, Germany) for all ceramic restorations to the chemical composition across the 3D-MASTER ® shade system.
Methods
Three disc samples, 13 mm diameter and 1.4 mm thickness, were produced for each M-shade following the manufacturer’s instructions. Each disc was ground and polished to a thickness of 1.0 mm. Spectral light transmittance and reflectance data were recorded in the visible spectrum under the standard illuminant D65 and 2° observer at 10 nm intervals by using a computer-controlled spectrophotometer. Opacity, translucency and opalescence parameters were determined for each sample.
Results
(1) Spectral transmittance and reflectance in the short-wavelength range systematically decreased with increasing chroma number (M1, M2, M3) when compared within the same value (lightness) group.
(2) Spectral transmittance and reflectance decreased systematically across the whole visible spectrum with increasing value group number when compared within the same chroma group.
(3) Analysis of relationship between chemical composition and various optical parameters for all the samples showed the significant contribution of ZrO 2 and Y 2 O 3 substances to optical properties of the present material.
Significance
Systematic variations in optical properties of VM7 ® M-shade base dentin ceramics were observed throughout the 3D-MASTER ® shade system and were suggested to be caused by the fine structure of the sample which can interfere with shorter wavelengths in the visible spectrum.
1
Introduction
Veneering ceramics used in conjunction with high strength ceramic materials such as glass infiltrated spinell and alumina or fused alumina or zirconia materials are becoming increasingly advanced in their optical properties . Dental veneering ceramics for all-ceramic restorations should allow the operator to control the optical parameters of color (hue, chroma and value) as well as the translucency. Color and translucency may be determined from the transmitted light through a material, where the light source and detector are on opposite sides of the object, or from the reflected light where the light source and detector are on the same side of the object. Differences between the transmitted and reflected light is described as opalescence where a material is preferentially transmitting different wavelengths to those that are reflected. Enamel and dentin both exhibit opalescence resulting in the transmitted light being rich in longer wavelengths (red and orange) in comparison to the reflected light .
Modern dental ceramics also exhibits opalescence and this study sets out to quantify this using the ceramics in the 3D-MASTER ® shade system. These ceramics were chosen as they are arranged according to the three attributes of color and systematically change with value groups (lightness), notated 1–5 and chroma groups (color saturation) M1–M3 as described previously . Hue differences (color family) of L (yellow-shift) and R (red-shift) will not be considered in this study. The VM7 ® ceramic (VITA, Bad Sackingen, Germany) may be used as a veneering material for substructures with a coefficient of thermal expansion (CTE) of approximately 7 × 10 −6 /K, or independently when produced on a refractory of matching CTE. This ceramic material is composed of a fine two-phase all-glass structure and does not contain a crystal phase .
Since the knowledge of the optical properties of dental restorative materials is very important in achieving esthetic restorations, this study attempts to analyze optical properties of the fired VM7 ® base dentin ceramics. The authors suggest that the incident light entering a phase boundary between a matrix glass and the second phase glass particle is partly reflected, refracted or scattered at this boundary due to the differences in the refractive indices of both phases. To clarify this hypothesis, the study investigated the relationship between chemical compositions of the glass powders and the optical properties. The characterization of such properties will allow the materials to be selected appropriately.
2
Materials and methods
2.1
Sample preparation
Three disc shaped samples of VM7 ® base dentin ceramic were produced in each M-shade of the 3D-MASTER ® shade system. The ceramic powder was condensed in a silicone mold to a thickness of approximately 1.8 mm and a diameter of 15 mm. The condensed ceramic samples were removed from the mold and fired according to the manufacturer’s instructions. The samples were ground using a waterproof P240 silicon carbide abrasive paper until parallel sided and fired again resulting in a sample approximately 1.4 mm thick. The discs were ground to a final thickness of 1.0 mm with both faces finished with a waterproof P600 silicon carbide abrasive paper.
2.2
Recordings of spectral transmittance and reflectance and calculation of color coordinates
Spectral transmittance and reflectance data in the wavelength range of 360–740 nm under standard illuminant D65 and 2° observer were collected at 10 nm intervals using a computer-controlled spectrophotometer (CM-3600d, Konica Minolta Sensing, Inc., Osaka, Japan) with an integrating sphere accessory. Color coordinates, L * (lightness), a * (red-green chromaticity index), b * (yellow-blue chromaticity index), C * (chroma) and h (hue angle) were determined from the transmittance and reflectance data using a computer software (Spectra-Magic, Version 3.61, Konica Minolta Sensing, Inc., Osaka, Japan).
For each shade three measurements were obtained from three different samples. Average and standard deviation were calculated. Where reflectance measurements were carried out, the samples were placed on standard white and black backgrounds without optical coupling.
The transmittance and reflectance data at each wavelength was used to determine:
- (1)
Average transmittance ( T ) = sum of transmittance (%) at each wavelength divided by number of data points (39).
- (2)
Average reflectance ( R ) = sum of reflectance (%) at each wavelength divided by number of data points (39).
- (3)
Opacity of a sample was calculated according to the following equation:
where R b is the luminous reflectance of a sample with the black backing, and R w is the luminous reflectance of a sample with the white backing .
- (4)
Translucency parameter (TP) of a sample was calculated according to the following equation:
TP=[(LW−LB)2+(aW−aB)2+(bW−bB)2]1/2
TP = [ ( L W − L B ) 2 + ( a W − a B ) 2 + ( b W − b B ) 2 ] 1 / 2
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