This study was aimed to investigate the effect of the number of coloring liquid applications on the optical properties of monolithic zirconia.
Eighteen monolithic zirconia specimens (27.6 mm × 27.6 mm × 2.0 mm) were fabricated and divided into 6 groups ( n = 3). Each group was designated by the number of A2-coloring liquid applications (Group I to Group V) and Group O as a control. Color and spectral distribution of the specimens were measured with a double-beam spectrophotometer. CIE L *, a * and b * relative to the standard illuminants D65 were measured in reflectance and transmittance modes. Color difference <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='(ΔEab*)’>(ΔE∗ab)(ΔEab*)
( Δ E a b * )
, translucency parameter (TP) and opalescence parameter (OP) were calculated. All measurements were performed on five different areas of each specimen. All data were analyzed by ANOVA and multiple comparison Scheffé test, Pearson correlations and linear regression analysis ( α = 0.05).
With the increase of the number of coloring liquid applications, CIE L * ( R 2 = 0.878) and OP values ( R 2 = 0.701) were decreased, but CIE b * ( R 2 = 0.938) was increased. However, TP values were not significantly changed. The color differences among groups ranged from 1.3 to 15.7 <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*’>ΔE∗abΔEab*
Δ E a b *
units. Strong correlation was found between OP and Δ b * ( R 2 = 0.982, P < .01).
Within the limitations of this study, it can be concluded that the number of coloring liquid applications with a single shade affects the lightness, yellow chromaticity and opalescence of monolithic zirconia, although its translucency cannot be controlled by the coloring procedure.
Zirconia has been introduced and widely used as an esthetic material in dentistry. Due to its opaque white color, it has been used as a framework material and has been veneered with feldspathic porcelain. One of the major shortcomings of zirconia-based restorations is a cohesive failure of the veneering porcelain . Thereby, CAD/CAM-generated monolithic restoration systems which consist of a single material without any veneering, have been developed . Nowadays the advantages of monolithic zirconia restorations with an increased mechanical stability make them possible to expand their clinical indications . In a recent clinical report , elimination of veneered porcelain on posterior zirconia crowns and fixed dental prostheses was performed for a clinical trial and presented an acceptable esthetic result. This clinical trial included several technical procedures, such as the application of coloring liquids on the pre-sintered zirconia block, surface characterizations, glazing and polishing of zirconia restorations, in order to mimic natural tooth color.
Determination of the color of teeth and porcelain crowns can be described by the “double layer effect”. This means that the resulting color appears from a diffuse reflectance of the inner dentin or opaque porcelain layer filtered by the scattering of outer translucent layer . Therefore, interactions of the optical scattering and absorption coefficients, thickness of the outer translucent material, and reflectance of the background substructure, can influence the changes of overall color parameters . In contrast, the creation of esthetic monolithic zirconia restorations could be challenging since they are full-contour mono-layered restorations.
Although monolithic zirconia restorations have been introduced in dentistry, there have been few studies reporting coloring effect on their optical properties at a clinically relevant thickness. Furthermore, there is no standardization of coloring in terms of color, translucency and opalescence parameters of the monolithic zirconia restorations.
The purpose of this study was to investigate the effect of the number of coloring liquid applications on the color, translucency and opalescence of monolithic zirconia. The null hypothesis to be tested was that there was no significant difference in the optical properties between monolithic zirconia ceramics with the different number of coloring liquid applications.
Materials and methods
Monolithic zirconia-based ceramic specimens were investigated in this study; BruxZir which is yttria-stabilized tetragonal zirconia polycrystal ( Table 1 ). Eighteen square-shaped, pre-sintered zirconia block (34.0 mm × 34.0 mm × 2.7 mm) were fabricated using a cutting machine (618 slicer, Harig, Niles, IL, USA).
|Type||Brand name||Characteristic||Batch No.||Manufacturer|
|Monolithic zirconia||BruxZir||Yttria-stabilized tetragonal zirconia polycrystal||B 186338||Glidewell Laboratories, Newport Beach, CA, USA|
|Coloring liquid||Tanaka ZirColor (A2 shade)||(R)-p-mentha-1,8-diene, 50–75%
Stoddard solvent, 10–25%
|401350297||Tanaka Dental, Skokie, IL, USA|
|Glass ceramic||IPS e.max Press||HT (A2 shade)||Lithium disilicate glass ceramic||626320||Ivoclar Vivadent AG, Schaan, Liechtenstein|
|LT (A2 shade)||605273|
|MO (MO1 shade)||596756|
|HO (HO1 shade)||596753|
Tanaka ZirColor of A2 shade was used as a coloring liquid ( Table 1 ). It is designed to be brushed on and dried quickly with no drying time between each application and, thus no preheating is necessary before sintering. The coloring liquid was applied according to the manufacturer’s recommendations with a synthetic nylon fiber brush (No. 156, Hwahong, Hwasung-si, Kyunggi-Do, Korea). These specimens were divided into six groups ( n = 3) according to the number of coloring liquid applications. The specimen with no application was used as a control.
Group O (control group): no application
Group I: one time of application
Group II: two times of application
Group III: three times of application
Group IV: four times of application
Group V: five times of application
All specimens were then fired in a zirconia sintering furnace (LHT 0217, Nabertherm GmbH, Bahnhofstr, Germany). The sintering cycle was controlled as followings: The temperature was raised to 950 °C for 1.5 h and maintained for 2 h, and then raised up to 1550 °C for 1.5 h and maintained for 3 h. After sintering process, the shrinkage of specimens was circa 20%. The mean size of sintered specimens was 27.6 mm × 27.6 mm, verified with a Vernier caliper (Mitutoyo, Tokyo, Japan).
The grinding procedure was performed on the opposite side of colored surface of each specimen to adjust the final thickness to 2.0 mm by the horizontal grinding machine (HRG-150, AM Technology, Kyunggi-Do, Korea). Final thickness was checked with a digital height gauge (Digimicro ME-50HA, Nikon Corp., Tokyo, Japan) with the accuracy of 1 μm on five different sites (center and each corner of specimen) of each specimen. The thickness of specimens ranged from 1.79 mm to 2.03 mm. Colored surface of the specimen was neither grinding nor polishing after completion of sintering. All specimens were ultrasonically cleaned in distilled water for 5 min before testing.
Color and spectral distribution were taken with a double-beam spectrophotometer (Cary 5000 UV–vis–NIR Spectrophotometer, Agilent Technologies Inc., Santa Clara, CA, USA) using an integrating sphere attachment. The specular reflectance component was excluded (SCE mode) by gloss trap inserted. Relative reflectance data was recorded in the visible range from 380 to 780 nm at 5 nm intervals. Measurements were recorded in Commission Internationale de l’Eclairage (CIE) 1976 L * a * b * color space (CIELAB) relative to the standard illuminant D65 and CIE 1964 10° supplementary standard observer in the reflectance mode over a white background (CIE L * = 99.9701, a * = −0.0711 and b * = 0.0499) and a black background (CIE L * = 4.7487, a * = −1.6749 and b * = −1.5844), and in the transmittance mode. The white standard was polytetrafluoroethylene (PTFE) plate (SRS-99-020, Spectralon ® Reflectance Standards, Labsphere Inc., North Sutton, NH, USA) and the black background was a black tile (CM-A101B, Konica Minolta Optics Inc., Tokyo, Japan). The spectrophotometer for this study was equipped with an integrating sphere of 150 mm diameter made with sintered PTFE. The geometry for the reflection measurements was 8 °: de (eight degree: diffuse geometry, specular component excluded). The aperture size was 19 mm in diameter for the reflectance measurement. For the transmittance measurement, opaque black polyvinyl chloride (PVC) plate supported measuring aperture to make the aperture size 10 mm × 15 mm, because the original aperture size of the instrument was 10 mm × 35 mm for the transmittance measurement. The specimens of 27.6 mm × 27.6 mm for this study provided adequate area for color measurement. The white PTFE standard was used for zero/base correction before reflectance color measurement.
Color coordinates, CIE L *, a * and b *, were determined from the transmittance and reflectance data using a computer software (Cary WinUV Software, Agilent Technologies Inc., Santa Clara, CA, USA). Each value was measured on five different areas of each specimen including the center of specimen by moving it to each quadrant direction slightly. Since the beam size was 1 mm × 5 mm, an effort was made not to overlap the beam on each measurement. A drop of distilled water whose refractive index is 1.7, was placed between specimen and the background for better optical contact . Average L *, a * and b * values were used to calculate CIE 1976 a , b (CIELAB) color difference, <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='ΔEab*’>ΔE∗abΔEab*
Δ E a b *
of each groupset using the following equation .
Δ E a b * = [ ( Δ L * ) 2 + ( Δ a * ) 2 + ( Δ b * ) 2 ] 1 / 2
where the L * coordinate represents the brightness of an object, the a * value represents the red or green chroma, and the b * value represents the yellow or blue chroma and Δ L *, Δ a * and Δ b * indicate the differences between the CIE L *, a * and b * color parameters of two specimen groups.
The translucency parameter (TP) was obtained by calculating the color difference of the specimen over the black and white backgrounds with the following equation .
TP = [ ( L B * − L W * ) 2 + ( a B * − a W * ) 2 + ( b B * − b W * ) 2 ] 1 / 2
where subscript B refers to the color coordinates over a black background and the subscript W refers to those over a white background. TP value of zero corresponds to a completely opaque material. The greater the TP value, the higher the actual translucency of the material.
For the translucency measurement, IPS e.max Press (Ivoclar Vivadent AG, Schaan, Liechtenstein) lithium disilicate glass ceramic specimens with four different levels of translucency, which were 25.0 mm × 25.0 mm × 2.0 mm, were also prepared ( n = 3); HT (high translucency), LT (low translucency), MO (medium opacity) and HO (high opacity) ( Table 1 ). Five different areas of each specimen were measured over the black and white background in the reflectance mode.
The opalescence parameter (OP) was calculated as the difference in yellow-blue and red-green coordinates between the transmitted and reflected colors using the following equation .
OP = [ ( CIE a T * − CIE a R * ) 2 + ( CIE b T * − CIE b R * ) 2 ] 1 / 2