Highlights
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We investigated thickness dependence of optical properties of a Ce-TZP/Al 2 O 3 nanocomposite.
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A spectrophotometer was used to collect spectral transmittance data.
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The Ce-TZP/Al 2 O 3 nanocomposite showed substantially low light transmittance.
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Translucency parameter decreased exponentially with sample thickness.
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High opalescence was observed when the sample thickness was nearly 0.3 mm.
Abstract
Objectives
This study was conducted to investigate thickness dependence of light transmittance, translucency and opalescence of a commercially available fully-sintered ceria-stabilized zirconia/alumina nanocomposite for dental all-ceramic restorations.
Methods
Three disk samples of 16 mm in diameter and thickness ranging from 0.2 to 0.6 mm with 0.1 mm increment each were cut from a fully-sintered rod-shaped Ce-TZP/alumina nanocomposite (NANOZR, Panasonic Healthcare, Japan) and polished flat by using diamond slurry. Spectral light transmittance data under the CIE standard illuminant D65 were recorded at 10 nm intervals from 360 to 740 nm using a computer-controlled spectrophotometer. Average transmittance, translucency and opalescence parameters were determined as a function of sample thickness. Optical properties of a fully-sintered yttria-stabilized tetragonal zirconia polycrystals (Cercon ® base, DeguDent GmbH, Germany) were also investigated as a reference. Two-way ANOVA was performed to determine the significant differences in various optical parameters among types of ceramic and thicknesses at α = 0.05.
Results
Results of the two-way ANOVA showed that the average transmittance, translucency and opalescence parameters of both ceramic materials were significantly influenced by the type of ceramic and thickness ( p < 0.001). Light transmittance of the NANOZR was significantly lower than that of the Cercon ® base. For both ceramic materials, average transmittance of light and translucency parameter decreased with sample thickness following exponential functions. The NANOZR showed substantially higher opalescence parameters exceeding 20 CIE units when the sample thickness was nearly 0.3 mm. The prominent characteristics of high opalescence and low transmittance of light in the NANOZR was considered to be caused by its specific very fine interpenetrated intragranular microstructure and by a large difference of refractive indices of Ce-TZP and alumina components.
Significance
High opalescence and low transmittance of light of the ceria-stabilized zirconia/alumina nanocomposite (NANOZR) are attractive properties for use as a substructure in fabricating porcelain-veneering-type esthetic all-ceramic restorations.
1
Introduction
Recent developments of various types of high-strength ceramic materials have led to their increasing applications to dental prostheses such as inlays, crowns, and load-bearing frameworks of long-span bridges. Among them zirconia-based ceramics are well known to possess excellent mechanical properties due to its transformation toughening mechanisms . Typically, Y-TZP (tetragonal zirconia polycrystals stabilized with 3 mol% yttria) is one of the major zirconia ceramics used in dentistry . The Y-TZP has been reported to show a flexural strength in the 800–1000 MPa range and a fracture toughness in the 6–8 MPa m 1/2 range evaluated by the indentation-fracture method .
Nawa et al. developed a high strength and high toughness CeO 2 -stabilized tetragonal zirconia polycrystals (Ce-TZP)/alumina nanocomposite. This ceramic nanocomposite possesses a novel interpenetrated intragranular microstructure, in which small alumina and Ce-TZP particles are trapped within Ce-TZP and alumina grains, respectively . For an optimum composition with 0.05 mol% TiO 2 doped 10Ce-TZP/30 vol% Al 2 O 3 composite, both high strength (950 MPa) and high toughness (18.3 MPa m 1/2 for the indentation-fracture method) were achieved .
The development of this tough and strong Ce-TZP/alumina nanocomposite triggered the increased number of research articles, in which mechanical properties , low-temperature aging degradation , biocompatibility , and wear resistance were examined. Results of these various investigations guaranteed the safe application of this Ce-TZP/Al 2 O 3 nanocomposite to substructures of dental all-ceramic restorations and bearing materials in orthopedic total joint replacements. Since this material is the toughest ceramic currently available for fixed dental prostheses, the thickness of Ce-TZP/alumina nanocomposite frameworks can be reduced to 0.3 mm . Moreover, manufacturers and clinical researchers recommend the thickness of zirconia core should be in the range 0.3–0.5 mm .
Regarding the optical properties of zirconia-based core materials, Baldissara et al. evaluated the translucency of eight commercially available different ceramics using the direct transmission method and demonstrated a relatively higher translucency of Lava™ Frame (3 M ESPE, St. Paul, USA) and a least translucent character of Cercon ® base (DeguDent GmbH, Hanau, Germany) made of Y-TZP although a small amount of light passed through this material. Recently, Kanchanavasita et al. compared contrast ratio of six commercially available zirconia-based dental ceramics and reported a relatively higher translucency of inCoris TZI (Sirona Dental Systems GmbH, Bensheim, Germany) and an opaque character of Cercon ® base. Wang et al. investigated relationship between translucency and thickness of various dental glass ceramics and zirconia ceramics. They reported that all of the zirconia ceramics showed some degree of translucency and their translucency parameters decreased exponentially with thickness . However, little information is available concerning optical properties of the Ce-TZP/alumina nanocomposite.
Since veneering enamel and dentin porcelains are translucent, optical properties such as light transmittance, translucency parameter and opalescence parameter of the substructure ceramics are considered to affect esthetic outcome of the final all-ceramic restoration. Translucency is defined as the property of a material by which a major portion of the transmitted light undergoes scattering . If the majority of light passing through a ceramic is intensely scattered and diffusely reflected, the material will appear more opaque . If only part of the light is scattered and most is diffusely transmitted, the material will appear more translucent . Opalescence is the process by which a material appears yellowish-red in transmitted light and blue in the scattered light perpendicular to the transmitted light . The phenomenon is named after the appearance of opal stone .
Knowledge of optical properties of the Ce-TZP/Al 2 O 3 nanocomposite is important when this material is used as a substructure (framework) of dental all-ceramic restorations. The objectives of the present study were to investigate the effects of sample thickness on light transmittance, translucency and opalescence of a fully-sintered Ce-TZP/alumina nanocomposite material. Since typical thickness of the frameworks made of zirconia ceramics ranges 0.3–0.5 mm in clinical practice , samples with thickness ranging from 0.2 to 0.6 mm, covering the above-mentioned thickness range in clinical practice, were examined in the present study. To compare optical properties of the Ce-TZP/alumina nanocomposite with those of the conventional zirconia material, a Y-TZP ceramic was used as a reference. The null hypotheses tested were that the average transmittance of light, translucency and opalescence parameters would not be affected by the type of ceramic or thickness.
2
Materials and methods
2.1
Sample preparation
Three disks of 16 mm in diameter and thickness of 0.22 to 0.62 mm with 0.1 mm increment each were cut from a fully-sintered rod-shaped Ce-TZP/alumina nanocomposite (NANOZR, Panasonic Healthcare, Ehime, Japan), where full sintering was performed at 1450 °C for 2 h. Both circular faces of the cut disks were polished consecutively using a series of diamond slurry starting from the average grain size of 12 μm down to 6 μm. The final thicknesses of the polished disks were 0.21, 0.31, 0.41, 0.51, and 0.61 mm. Since these series of mechanical cutting and polishing processes can induce partial transformation from the tetragonal to monoclinic crystal structure of Ce-TZP at and near the cut surface of the samples , the polished samples were heat-treated at 1000 °C for 5 min to recover the original tetragonal crystal structure of the Ce-TZP phase.
Similarly, three disks of 16 mm in diameter and thickness of 0.23 to 0.63 mm with 0.1 mm increment each were cut from a fully-sintered rod-shaped yttria-stabilized tetragonal zirconia polycrystals (Cercon ® base, DeguDent GmbH, Hanau-Wolfgang, Germany), where full sintering was performed at 1350 °C for 6 h. The disks were polished following the above-mentioned methods and heat-treated at 1000 °C for 5 min to recover the original tetragonal crystal structure. The final thicknesses of the samples for optical property measurements were 0.22, 0.32, 0.42, 0.51, and 0.62 mm.
2.2
Recordings of spectral transmittance and reflectance and calculation of color coordinates
The disk samples prepared above were mounted on a computer-controlled spectrophotometer (CM-3600d, Konica Minolta Sensing, Inc., Osaka, Japan) with an integrating sphere accessory and their spectral transmittance data were collected at 10 nm intervals from 360 to 740 nm under the CIE (Commission Internationale de l’Eclairage) standard illuminant D65 and 2° observer. Where reflectance measurements were carried out, the samples were placed on standard white and black backgrounds.
Various optical parameters were obtained as follows.
- (1)
Average transmittance (AT) = sum of transmittance (%) at each wavelength divided by number of data points (39).
- (2)
Translucency parameter (TP) of a sample was calculated according to the following equation:
<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='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]1/2TP=[(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 / 2where, the subscript W refers to color coordinates with the white backing and the subscript B refers to those with the black backing .
- (3)
Opalescence parameter (OP) of a sample was determined according to the following equation:
where, the subscript T refers to the transmitted light and the subscript R refers to the reflected light with the black backing .
The relationship between the thickness and AT and TP values for each ceramic was analyzed with a regression analysis of exponential function :
y = a ⋅ exp ( b x )
where, y is the observed AT or TP value, x is thickness of a sample, and a and b are constants.
2.3
Statistical analysis
The effects of type of ceramic and material thickness on the AT, TP, and OP values of the two ceramic materials were analyzed with a two-way analysis of variance (ANOVA) at α = 0.05 using the statistical analysis software (JMP Pro, version 11.2.0, SAS Institute Inc., Cary, NC, USA).
2
Materials and methods
2.1
Sample preparation
Three disks of 16 mm in diameter and thickness of 0.22 to 0.62 mm with 0.1 mm increment each were cut from a fully-sintered rod-shaped Ce-TZP/alumina nanocomposite (NANOZR, Panasonic Healthcare, Ehime, Japan), where full sintering was performed at 1450 °C for 2 h. Both circular faces of the cut disks were polished consecutively using a series of diamond slurry starting from the average grain size of 12 μm down to 6 μm. The final thicknesses of the polished disks were 0.21, 0.31, 0.41, 0.51, and 0.61 mm. Since these series of mechanical cutting and polishing processes can induce partial transformation from the tetragonal to monoclinic crystal structure of Ce-TZP at and near the cut surface of the samples , the polished samples were heat-treated at 1000 °C for 5 min to recover the original tetragonal crystal structure of the Ce-TZP phase.
Similarly, three disks of 16 mm in diameter and thickness of 0.23 to 0.63 mm with 0.1 mm increment each were cut from a fully-sintered rod-shaped yttria-stabilized tetragonal zirconia polycrystals (Cercon ® base, DeguDent GmbH, Hanau-Wolfgang, Germany), where full sintering was performed at 1350 °C for 6 h. The disks were polished following the above-mentioned methods and heat-treated at 1000 °C for 5 min to recover the original tetragonal crystal structure. The final thicknesses of the samples for optical property measurements were 0.22, 0.32, 0.42, 0.51, and 0.62 mm.
2.2
Recordings of spectral transmittance and reflectance and calculation of color coordinates
The disk samples prepared above were mounted on a computer-controlled spectrophotometer (CM-3600d, Konica Minolta Sensing, Inc., Osaka, Japan) with an integrating sphere accessory and their spectral transmittance data were collected at 10 nm intervals from 360 to 740 nm under the CIE (Commission Internationale de l’Eclairage) standard illuminant D65 and 2° observer. Where reflectance measurements were carried out, the samples were placed on standard white and black backgrounds.
Various optical parameters were obtained as follows.
- (1)
Average transmittance (AT) = sum of transmittance (%) at each wavelength divided by number of data points (39).
- (2)
Translucency parameter (TP) of a sample was calculated according to the following equation:
TP=[(L*W−L*B)2+(a*W−a*B)2+(b*W−b*B)2]1/2TP=[(L∗W−L∗B)2+(a∗W−a∗B)2+(b∗W−b∗B)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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