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The effect of repeated firings on the color of an alumina ceramic system with two different veneering porcelain shades
The effect of repeated firings on the color of an alumina ceramic system with two different veneering porcelain shades V. Sahin, DDS, PhD,a B. Uludag, DDS, PhD,b A. Usumez, DDS, PhD,c and S. E. Ozkir, DDS, PhDd Kirikkale University, Kirikkale, Turkey; Ankara University, Ankara, Turkey; Gaziantep University, Gaziantep, Turkey Statement of problem. Possible sources of processing variables in porcelain firing include thickness and color of the opaque; thickness, color, and translucency of the body and enamel layers; firing temperature; and number of firings. Purpose. The purpose of this in vitro study was to investigate the color changes of an alumina ceramic system veneered with different veneering porcelain shades and fired different numbers of times. Material and methods. Twenty disc-shaped ceramic specimens (10 mm in diameter, with a core thickness of 1 mm), with 2 different veneering porcelain shades (A1, A3), were fabricated from an alumina ceramic system (Turkom-Cera) (n=10). Repeated firings (3, 5, 7, or 9 firings) were performed, and color differences (ΔE) were determined using a spectrophotometer. Repeated-measures ANOVA was used to analyze the data (number of firings, veneering porcelain color). The Duncan test and paired 2-tailed tests were used for multiple comparisons (α=.05). Results. The L*a*b* values of the ceramic system were affected by the number of firings (3, 5, 7, or 9) (P<.005) and veneering porcelain shade (P<.001). Significant interactions were present between the number of firings and the veneering porcelain shade for L* (P=.002), a* (P=.001), and b* (P=.001) values. A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade specimens became lighter after an increased number of firings. For both A1 and A3 veneering porcelain shades, the a* value decreased after repeated firings, which resulted in less reddish specimens, and the b* value decreased after repeated firings, which resulted in less yellowish specimens. Conclusions. Imperceptible (ΔE<1.6) and clinically acceptable color changes (ΔE<3.7) were demonstrated by the alumina ceramic system tested. (J Prosthet Dent 2010;104:372-378)
Clinical Implications
Veneering porcelain shade and number of firings should be carefully considered to obtain an acceptable color match of the definitive restorations. The use of a ceramic rather than a metal core permits greater light transmission within a crown, thereby improving the color and translucency of a restoration.1 Combining the strength of ceramic cores and the esthetics of veneering porcelains and using layering techniques allow dental laboratory technicians to fabricate an individual-
ized restoration with exceptional esthetics.2 At present, there are 5 methods for fabricating ceramic crowns: condensation and sintering, cast and ceramming, pressing, slip casting, and computer-aided design/computer-aided manufacturing (CAD/CAM) milling of ceramic blocks or presintered blocks.3,4
Most ceramic systems require the combination of 2 layers of ceramic material, such as a high-strength ceramic core and a weaker veneering porcelain,5 with different opacity, shade, and thickness, to provide a natural appearance.6,7 A successful color match is an important aspect of any esthetic restoration.8 However, a perfectly matched,
Assistant Professor, Department of Prosthodontics, Faculty of Dentistry, Kirikkale University. Professor, Department of Prosthodontics, Faculty of Dentistry, Ankara University. c Professor, Department of Prosthodontics, Faculty of Dentistry, Gaziantep University. d Clinician, Ministry of Health, Bozuyuk Oral Health Center, Bilecik, Turkey. a
b
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December 2010 esthetic, tooth-colored restoration cannot be ensured, despite the improved color and translucency of layered ceramic restorations. Layered ceramic restorations are influenced by the core thickness, veneer thickness, and by their interaction.9 If the majority of light passing through a ceramic restoration is diffusely transmitted and only part of it is scattered, the material will appear translucent.10 The amount of light that is absorbed, reflected, and transmitted depends on the quantity of crystals within the core matrix, their chemical nature, and the size of the particles compared to the incident light wavelength.11 Kelly et al12 identified core translucency as a primary factor in controlling esthetics and a critical consideration in the selection of materials. Some ceramic core materials have high in vitro strength values.13,14 However, an increase in crystalline content to achieve greater strength generally results in greater opacity.11,12,15 Clinical perceptibility of color differences has been the subject of numerous investigations. The Commission Internationale de l’Eclairage (CIE) recommended calculating color difference (ΔE) based on CIE L*a*b* color parameters.16 Delta E values are used to describe whether the changes in the overall shade are perceivable to the human observer. This magnitude of color difference is based on the human perception of color; color differences greater than 1 ΔE unit are visually detectable by 50% of human observers.17 However, under uncontrolled clinical conditions, such small differences in color would be unnoticeable because average color differences below 3.7 ΔE have been rated as a match in the oral environment.18,19 Instrumental measurements can quantify color and allow communication to be more uniform and precise. The development of more advanced computerized colorimeters and spectrophotometers has increased their use in dental research.18 Spectrophotometers measure the reflectance or transmittance factors of an object
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one wavelength at a time. They are useful in the measurement of surface color20 and have been used to measure the spectral curves of porcelains and extracted teeth.21,22 Spectrophotometers have been shown to be superior in color matching as compared to conventional methods using shade guides.23 There are several factors that affect the ability of a ceramic system to produce an acceptable match with corresponding shade guides.24 In addition to the opacity and shade of the ceramic, other factors, including porcelain brand,21,25,26 batch,25 condensation technique,25 firing temperature,26 dentin thickness,9,27,28 and number of porcelain firings,28,29 can affect the definitive shade of an esthetic restoration. Studies examining color changes of surface colorants after firing have demonstrated pigment breakdown at firing temperatures.30-32 Specific contributions of core and veneer thickness to the appearance of layered ceramics were determined,9 and it was concluded that there was a significant correlation between the thickness ratio of core and veneer ceramics and the color of the restoration. Even when adequate ceramic thickness exists, clinical shade matches are difficult to achieve24 because there is a wide range of translucency among the core materials of ceramic systems at clinically relevant core thicknesses.11 The thickness and combination of ceramic layers, such as the core, veneer, and other specialty ceramic materials, have been shown to impact the appearance of ceramic materials.10,18,33 Antonson and Anusavice33 found that it was important to determine the translucency of a layered core-veneering ceramic as a function of thickness. Heffernan et al11,34 investigated the influence of core and core-veneering ceramic thickness on overall translucency. Lee et al35 showed that the layered color of various ceramic and veneer combinations was different depending on the type of ceramic core material, even though the thickness of the layered specimen was set at 1.5 mm. These studies, however, did not
address the particular contribution of each variable: the difference in core substructure, the changes in veneer thickness, or the number of firings. Several studies investigated the effect of repeated firings on the color of ceramic systems.25,29,36-41 The effect of repeated firings on the ceramic color in metal ceramic systems has been shown to be minimal,29,36,37 although one study reported perceptible color changes.25 Color stability of ceramiccore systems subjected to a repeated number of firings was shown to be clinically acceptable39,40,41; however, perceptible color changes were reported for only one ceramic-core system investigated.39 Ceramic-core specimens subjected to a repeated number of firings exhibited better color stability compared to metal ceramic specimens, despite the imperceptible color changes.38 In addition to the development of advanced dental material technologies, the increased demand for esthetic restorations has resulted in the use of ceramic restorations in several applications.39 Recently, new dental materials and techniques have been introduced to fabricate esthetic ceramic restorations with improved strength and marginal adaptation. This becomes more important for posterior areas, where forces reach as high as 522 N in an average person, and are much higher than in the anterior region.42,43 To provide satisfactory posterior ceramic restorations, strong alumina cores have been produced. Turkom-Cera ceramic material (Turkom-Ceramic (M), Kuala Lumpur, Malaysia), Procera AllCeram (Nobel Biocare AB, Göteborg, Sweden), and VITA In-Ceram (VITA Zahnfabrik, Bad Säckingen, Germany) are 3 ceramic systems that incorporate a high alumina core. These cores differ in their manufacturing process and are also intrinsically different in that Procera AllCeram contains a densely sintered alumina core, whereas Turkom-Cera and VITA In-Ceram are made of a high alumina core, which is subsequently crystal hardened or glass infiltrated.44
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Volume 104 Issue 6 However, alumina cores tend to be opaque and require the use of veneering porcelain to mask the core and provide the desired contours.45,46 The purpose of this study was to evaluate the effects of 2 different veneering porcelain shades (A1, A3) and number of firings (3, 5, 7, or 9) on the color of an alumina-based ceramic system. The research hypothesis was that color differences would occur relative to the firing times and veneering porcelain shades.
MATERIAL AND METHODS A custom-made plastic mold with a diameter of 10 mm and thickness of 1 mm was prepared. The mold was filled with alumina gel (Turkom-Cera Alumina Gel, batch no. TC02CGP-06; Turkom-Ceramic (M)). Disc-shaped Turkom-Cera specimens were removed from the mold after the gel was completely dry, and the specimens were fired in a vacuum furnace (Programat P300; Ivoclar Vivadent AG, Schaan, Liechtenstein) (temperature increase rate, 50°C/min; holding temperature, 1150°C; holding time, 5 minutes), cleaned using the built-in cleaning program to avoid any impurities resulting from previous operations. The sintered specimens were hardened using crystal powder (Turkom-Cera Crystal Powder, batch no. TC09P1P-06; Turkom-Ceramic (M)). The crystal powder was mixed with water and applied on the sintered specimens in the same furnace for 30 minutes at1150°C. The excess crystals were removed using a laboratory micromotor (Ultimate 500; NSK Nakanishi, Inc, Kanuma, Japan) with diamond rotary cutting instruments (863.204.016; Gebr Brasseler GmbH, Lemgo, Germany) at slow speed. Twenty specimens were obtained. Conventional veneering ceramic (VITA VM 7; VITA Zahnfabrik) was applied on the cores to a thickness of 1 mm. The specimens were divided into 2 groups. The first group consisted of 10 specimens with an A1 veneering porcelain shade (VITA VM 7, batch
no. 23810; VITA Zahnfabrik), and the second group consisted of 10 specimens with an A3 veneering porcelain shade (VITA VM 7, batch no. 21000; VITA Zahnfabrik). The sample sizes for this study were derived from the sample sizes used in previous color assessment studies.38-40 No power analysis was performed to determine adequate sample size. Two shades from the A group of a shade guide (VITA Classical Shade Guide; VITA Zahnfabrik) were selected, as this group accounts for at least 65% of clinical shade selections.1 A custom-made plastic mold was used to standardize the veneering porcelain thickness. An apparatus that was used in previous studies27,29,39-41 was modified for the current study. The mold was prepared in 2 pieces. The first piece (I) had a 10-mm-diameter cylindrical cavity in the middle. The second piece (II) had a piston that was attached to the first piece with a screw system so that it could rise and descend in the cavity. The upper surface of the apparatus was divided into 20 equal units. When it was calibrated, the piston moved downward with a sensitivity of 0.05 mm with a 1-unit turn of the lower piece. The piston descended 1.0 mm with 20 turns of the attached lower piece. The required ceramic thickness was determined by adjusting the depth of the cavity above the piston by turning the attached lower piece the necessary amount. Veneering porcelain slurry was condensed and hand vibrated; excess moisture was removed with absorbent paper tissue to minimize porosity. The condensed specimens were fired in a vacuum furnace (Programat P300; Ivoclar Vivadent AG) (temperature increase rate, 55°C/min; holding temperature, 900°C; holding time, 1 minute). After glazing the specimens, the thickness of each disc was verified with a micrometer (Praecimeter S; Renfert GmbH, Hilzingen, Germany). The color of each specimen was measured with a spectrophotometer (VITA Easyshade; VITA
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Zahnfabrik). This system captures the color coordinates using a D65 illuminant (color temperature 6227°C, a mathematical construct equivalent to average daylight in the Northern Hemisphere) and a viewing angle of 2 degrees. The color of the specimen was measured with the glazed surface facing up against a neutral grey background.9 The CIE L*a*b* values were determined from 3 measurements of each specimen, and transferred to a personal computer (Toshiba America, Inc, New York, NY) for analysis. The instrument calibration was evaluated after measurement of each group (n=10), and the instrument was recalibrated. The CIE L*a*b* measurements make it possible to evaluate the amount of perceptible color change in each specimen. The CIE L*a*b* color space is a uniform 3-dimensional (3-D) color order system. Equal changes in any of the 3 coordinates can be perceived as visually similar. Total color differences were calculated with use of the following formula47: ∆E= [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2 The L* coordinate is a measure of the lightness-darkness of the specimen. The greater the L* value is, the lighter the specimen. The a* coordinate is a measure of the chroma along the redgreen axis. A positive a* relates to the amount of redness, and a negative a* relates to the greenness of a specimen. The b* coordinate is a measure of the chroma along the yellow-blue axis; that is, a positive b* relates to the amount of yellowness, while a negative b* relates to the blueness of the specimen. Delta L*, ∆a*, and ∆b* are the differences in the CIE colorspace parameters of the 2 colors.47 The color difference value (∆E) represents the numerical distance between L*a*b* coordinates of 2 colors. Under ideal observation conditions, when the ∆E value of 2 colors is less than 1 unit (∆E<1), the 2 colors can be judged to match.17 When measured color differences are within the 1 to 2 ∆E range, correct judgments are made frequently by observers. A ΔE value of 1.6 was determined to be
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December 2010 a color difference that could not be detected by the human eye.48 When ∆E values are greater than 2 ∆E units, all observers can apparently detect color difference between 2 colors.17 The clinically acceptable limit of the color difference value is considered 3.7 ∆E units.17 Data were analyzed with statistical software (SPSS v. 10.0; SPSS, Inc, Chicago, Ill). Repeated-measures analysis of variance (ANOVA) was used to analyze the data (number of firings, veneering porcelain color) for
significant differences. The Duncan test was used to perform multiple comparisons (α=.05).
both A1 and A3 veneering porcelain shades, the a* value decreased after repeated firings, which resulted in less reddish specimens, and the b* value decreased after repeated firings, which resulted in less yellowish specimens. A1 and A3 veneering porcelain shades had statistically significant differences for all of the parameters tested for the individual number of firings. The results of the ANOVA of L*a*b* color parameters (number of firings and different veneering porcelain
RESULTS The mean values and multiple comparison test results for each individual parameter tested are presented in Table I. A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade specimens became lighter after an increased number of firings. For
Table I. Mean values and multiple comparison test results for individual parameters tested Number of Firings Property
Shade
3
5
7
9
A1
92.37Aa
92.26Aa
92.25Aa
92.24Aa
A3
86.23Cb
86.66ABb
86.59Bb
86.83Ab
A1
1.72De
1.44Ee
1.28Fe
1.27Fe
A3
4.49Ed
4.73Dd
4.36EFd
4.30Fd
A1
25.48Gh
25.28Gh
24.45Hh
24.48Hh
A3
37.79Hg
38.25Gg
37.63Hg
37.46Hg
L*
a*
b*
Uppercase letters indicate significant differences between number of firings; lowercase letters indicate differences between shades (P<.05).
Table II. Multivariate test results based on 2-factor repeated-measures ANOVA for changes in color coordinates after repeated firings of ceramic Pillai’s Value
Numerator df
Denominator df
F
P
Number of firings
0.307
3
22
3.25a
.041
Number of firings x color
0.490
3
22
7.06a
.002
Number of firings
0.723
3
22
19.14a
<.001 .001
Parameter
Effect
L*
a*
b*
Number of firings x color
0.521
3
22
7.99a
Number of firings
0.765
3
22
23.82a
<.001
22
7.57a
.001
Number of firings x color aExact statistic
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0.508
3
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Table III. Mean ∆E values of 2 different colors of ceramic specimens after repeated firings
Number of Firings
ΔE (A1)
ΔE (A3)
3-5
0.61
0.93
3-7
1.28
0.65
3-9
1.43
0.88
5-7
0.95
0.86
5-9
1.19
1.05
7-9
0.68
0.56
shades) for the ceramic system are listed in Table II. The L*a*b* values of the ceramic system were affected by the number of firings (3, 5, 7, or 9 firings) (P<.005) and veneering porcelain shade (P<.001). Statistically significant interactions were present between the number of firings and the veneering porcelain shade for L* (P=.002), a* (P=.001), and b* (P=.001) values. The ∆E values were calculated for each specimen within each group (Table III) to determine mean color differences (∆E), depending on repeated firings. The highest ∆E value was 1.43 for specimens with the A1 veneering porcelain shade, between the third and ninth firings; however, for specimens with the A3 veneering porcelain shade, the highest ∆E value was 1.05, between the fifth and ninth firings.
DISCUSSION This in vitro study investigated the color changes of alumina ceramic specimens veneered with different veneering porcelain shades and fired different numbers of times. The results of this study support the hypothesis that color differences would occur relative to the number of firings and veneering porcelain shades. Possible sources of processing variables in porcelain firing include thickness and color of the opaque; thickness, color, and translucency of the body
and enamel layers; firing temperature; and number of firings.25 Results of the current study indicated imperceptible color changes for the tested aluminabased ceramic system subjected to a repeated number of firings. A previous study found ceramic-core systems to be more color stable compared to metal ceramic systems as the number of firings increased.38 The aluminabased ceramic system investigated in the current study can be expected to exhibit more color stability compared to metal ceramic systems, as the results of these studies are in agreement. It was previously reported that an increase in the number of firings resulted in a decrease in L* and an increase in a* and b* color values of In-Ceram and IPS Empress specimens with different veneering ceramic thicknesses.41 In the current study, repeated firings resulted in less reddish and yellowish specimens for the A1 veneering porcelain shade, and lighter, less reddish, and less yellowish specimens for the A3 veneering porcelain shade. Results of another study investigating the color changes of a zirconia ceramic system with 2 different veneering porcelain shades after repeated firings indicated an increase in the L* value and a decrease in the a* value for both A1 and A3 veneering porcelain shades, resulting in lighter and greener specimens; however, the b* value was not influenced by the number of firings for the A1 veneer-
The Journal of Prosthetic Dentistry
ing porcelain shade and increased for the A3 veneering porcelain shade, resulting in more yellowish specimens.40 The differences between these 2 previous studies and the current study may be attributed to the optical properties of different core materials, as a zirconia ceramic system was found to be the least translucent ceramic system and more opaque than a glass-infiltrated alumina ceramic system or the IPS Empress system.11 In addition, the VITA instrument used in the current study, which was found to have both reliability and accuracy values greater than 90%,20 may be sensitive to translucency changes, and some change may be related to the vitrification of the veneering porcelain. As the porcelain is fired, its translucency may change and the measurements may be slightly altered. Heffernan et al34 stated that an additional firing resulted in a significant difference in translucency rankings of ceramic systems and a decrease in the opacity of all veneered materials, except the completely opaque zirconia and metal ceramic specimens. Color change after repeated firings may also be attributed to the color instability of metal oxides during firing, which can affect the resulting color of ceramic. Several studies30-32 have suggested that certain metal oxides are not color stable after they are subjected to firing temperatures. It was reported that yellow- and orangehued stains were the least color stable at the manufacturers’ recommended firing temperatures.30,32 However, another study indicated that blue was the most unstable stain, while orange demonstrated the greatest color stability at higher firing temperatures.31 In the present study, 2 veneering porcelain shades were selected, A1 and A3. The results showed that A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade specimens became lighter after an increased number of firings, and lower ∆E values were observed in A3 than A1 porcelain veneering material. This result
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December 2010 may be explained by the fact that the A3 veneering porcelain may mask the opaque core more than the A1 veneering porcelain. Clinical success and color stability of ceramic restorations depend on laboratory and clinical variables. Ceramic systems in this study exhibited color differences that could not be detected by the human eye under firing conditions following the manufacturers’ instructions. The influence of the background substrate on the definitive appearance of ceramic specimens is well established. Neutral colors such as white, grey, and black are, by definition, colors that have no hue. Neutral grey was selected as a background to minimize the influence of background hue on the color measurement of the specimens.9 Finally, ceramic restorations should be luted to the tooth substrate using a luting agent17 with a shade and thickness that contribute to the esthetic appearance of the restorations. Therefore, further studies on the interaction of ceramic materials with luting agents and other substrates are needed. The limitations of this study include the in vitro use of a spectrophotometer to evaluate shade differences of only a single type of ceramic material. No power analysis was performed to determine adequate sample size. Furthermore, the specimens were disc shaped rather than crown shaped. Additional studies are needed to investigate the interaction of different core colors with repeated firings compared to conventional metal ceramic systems.
CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. L*a*b* values of the ceramic systems were affected by the number of firings (3, 5, 7, or 9 firings) and veneering porcelain shade (A1 or A3). 2. A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade
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specimens became lighter after an increased number of firings. For both A1 and A3 veneering porcelain shades, the a* value decreased after repeated firings, which resulted in less reddish specimens (P=.001), and the b* value decreased after repeated firings, which resulted in less yellowish specimens (P=.001). 3. The mean color differences caused by repeated firings were imperceptible (ΔE<1.6) and represent clinically acceptable color changes (ΔE<3.7).
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Volume 104 Issue 6 34.Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA. Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 2002;88:10-5. 35.Lee YK, Cha HS, Ahn JS. Layered color of all-ceramic core and veneer ceramics. J Prosthet Dent 2007;97:279-86. 36.Barghi N, Goldberg J. Porcelain shade stability after repeated firing. J Prosthet Dent 1977;37:173-5. 37.Yilmaz B, Ozçelik TB, Wee AG. Effect of repeated firings on the color of opaque porcelain applied on different dental alloys. J Prosthet Dent 2009;101:395-404. 38.Pires-de-Souza Fde C, Casemiro LA, Garcia Lda F, Cruvinel DR. Color stability of dental ceramics submitted to artificial accelerated aging after repeated firings. J Prosthet Dent 2009;101:13-8. 39.Ozturk O, Uludag B, Usumez A, Sahin V, Celik G. The effect of ceramic thickness and number of firings on the color of two all-ceramic systems. J Prosthet Dent 2008;100:99-106. 40.Celik G, Uludag B, Usumez A, Sahin V, Ozturk O, Goktug G. The effect of repeated firings on the color of an all-ceramic system with two different veneering porcelain shades. J Prosthet Dent 2008;99:203-8.
41.Uludag B, Usumez A, Sahin V, Eser K, Ercoban E. The effect of ceramic thickness and number of firings on the color of ceramic systems: an in vitro study. J Prosthet Dent 2007;97:25-31. 42.Bakke M, Holm B, Jensen BL, .Michler L, Möller E. Unilateral, isometric bite force in 8-68-year-old women and men related to occlusal factors. Scand J Dent Res 1990;98:149-58. 43.Pallis K, Griggs JA, Woody RD, Guillen GE, Miller AW. Fracture resistance of three allceramic restorative systems for posterior applications. J Prosthet Dent 2004;91:561-9. 44.AL-Makramani BM, Razak AAA, Abu-Hassan MI. Comparison of the load at fracture of Turkom-Cera to Procera AllCeram and In-Ceram all-ceramic restorations. J Prosthodont 2009;18:484-8. 45.Andersson M, Odén A. A new all-ceramic crown. A dense-sintered high-purity alumina coping with porcelain. Acta Odontol Scand 1993;51:59-64. 46.Webber B, McDonald A, Knowles J. An in vitro study of the compressive load at fracture of Procera AllCeram crowns with varying thickness of veneer porcelain. J Prosthet Dent 2003;89:154-60.
47.Knispel G. Factors affecting the process of color matching restorative materials to natural teeth. Quintessence Int 1991;22:525-31. 48.Ishikawa-Nagai S, Yoshida A, Sakai M, Kristiansen J, Da Silva JD. Clinical evaluation of perceptibility of color differences between natural teeth and all-ceramic crowns. J Dent 2009;37 Suppl 1:e57-63. Corresponding author: Dr Volkan Sahin University of Kirikkale, Faculty of Dentistry Department of Prosthodontics 71200 Kirikkale TURKEY Fax: +90 318 225 06 85 E-mail: [email protected] Copyright © 2010 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications
Veneering porcelain shade and number of firings should be carefully considered to obtain an acceptable color match of the definitive restorations. The use of a ceramic rather than a metal core permits greater light transmission within a crown, thereby improving the color and translucency of a restoration.1 Combining the strength of ceramic cores and the esthetics of veneering porcelains and using layering techniques allow dental laboratory technicians to fabricate an individual-
ized restoration with exceptional esthetics.2 At present, there are 5 methods for fabricating ceramic crowns: condensation and sintering, cast and ceramming, pressing, slip casting, and computer-aided design/computer-aided manufacturing (CAD/CAM) milling of ceramic blocks or presintered blocks.3,4
Most ceramic systems require the combination of 2 layers of ceramic material, such as a high-strength ceramic core and a weaker veneering porcelain,5 with different opacity, shade, and thickness, to provide a natural appearance.6,7 A successful color match is an important aspect of any esthetic restoration.8 However, a perfectly matched,
Assistant Professor, Department of Prosthodontics, Faculty of Dentistry, Kirikkale University. Professor, Department of Prosthodontics, Faculty of Dentistry, Ankara University. c Professor, Department of Prosthodontics, Faculty of Dentistry, Gaziantep University. d Clinician, Ministry of Health, Bozuyuk Oral Health Center, Bilecik, Turkey. a
b
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December 2010 esthetic, tooth-colored restoration cannot be ensured, despite the improved color and translucency of layered ceramic restorations. Layered ceramic restorations are influenced by the core thickness, veneer thickness, and by their interaction.9 If the majority of light passing through a ceramic restoration is diffusely transmitted and only part of it is scattered, the material will appear translucent.10 The amount of light that is absorbed, reflected, and transmitted depends on the quantity of crystals within the core matrix, their chemical nature, and the size of the particles compared to the incident light wavelength.11 Kelly et al12 identified core translucency as a primary factor in controlling esthetics and a critical consideration in the selection of materials. Some ceramic core materials have high in vitro strength values.13,14 However, an increase in crystalline content to achieve greater strength generally results in greater opacity.11,12,15 Clinical perceptibility of color differences has been the subject of numerous investigations. The Commission Internationale de l’Eclairage (CIE) recommended calculating color difference (ΔE) based on CIE L*a*b* color parameters.16 Delta E values are used to describe whether the changes in the overall shade are perceivable to the human observer. This magnitude of color difference is based on the human perception of color; color differences greater than 1 ΔE unit are visually detectable by 50% of human observers.17 However, under uncontrolled clinical conditions, such small differences in color would be unnoticeable because average color differences below 3.7 ΔE have been rated as a match in the oral environment.18,19 Instrumental measurements can quantify color and allow communication to be more uniform and precise. The development of more advanced computerized colorimeters and spectrophotometers has increased their use in dental research.18 Spectrophotometers measure the reflectance or transmittance factors of an object
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one wavelength at a time. They are useful in the measurement of surface color20 and have been used to measure the spectral curves of porcelains and extracted teeth.21,22 Spectrophotometers have been shown to be superior in color matching as compared to conventional methods using shade guides.23 There are several factors that affect the ability of a ceramic system to produce an acceptable match with corresponding shade guides.24 In addition to the opacity and shade of the ceramic, other factors, including porcelain brand,21,25,26 batch,25 condensation technique,25 firing temperature,26 dentin thickness,9,27,28 and number of porcelain firings,28,29 can affect the definitive shade of an esthetic restoration. Studies examining color changes of surface colorants after firing have demonstrated pigment breakdown at firing temperatures.30-32 Specific contributions of core and veneer thickness to the appearance of layered ceramics were determined,9 and it was concluded that there was a significant correlation between the thickness ratio of core and veneer ceramics and the color of the restoration. Even when adequate ceramic thickness exists, clinical shade matches are difficult to achieve24 because there is a wide range of translucency among the core materials of ceramic systems at clinically relevant core thicknesses.11 The thickness and combination of ceramic layers, such as the core, veneer, and other specialty ceramic materials, have been shown to impact the appearance of ceramic materials.10,18,33 Antonson and Anusavice33 found that it was important to determine the translucency of a layered core-veneering ceramic as a function of thickness. Heffernan et al11,34 investigated the influence of core and core-veneering ceramic thickness on overall translucency. Lee et al35 showed that the layered color of various ceramic and veneer combinations was different depending on the type of ceramic core material, even though the thickness of the layered specimen was set at 1.5 mm. These studies, however, did not
address the particular contribution of each variable: the difference in core substructure, the changes in veneer thickness, or the number of firings. Several studies investigated the effect of repeated firings on the color of ceramic systems.25,29,36-41 The effect of repeated firings on the ceramic color in metal ceramic systems has been shown to be minimal,29,36,37 although one study reported perceptible color changes.25 Color stability of ceramiccore systems subjected to a repeated number of firings was shown to be clinically acceptable39,40,41; however, perceptible color changes were reported for only one ceramic-core system investigated.39 Ceramic-core specimens subjected to a repeated number of firings exhibited better color stability compared to metal ceramic specimens, despite the imperceptible color changes.38 In addition to the development of advanced dental material technologies, the increased demand for esthetic restorations has resulted in the use of ceramic restorations in several applications.39 Recently, new dental materials and techniques have been introduced to fabricate esthetic ceramic restorations with improved strength and marginal adaptation. This becomes more important for posterior areas, where forces reach as high as 522 N in an average person, and are much higher than in the anterior region.42,43 To provide satisfactory posterior ceramic restorations, strong alumina cores have been produced. Turkom-Cera ceramic material (Turkom-Ceramic (M), Kuala Lumpur, Malaysia), Procera AllCeram (Nobel Biocare AB, Göteborg, Sweden), and VITA In-Ceram (VITA Zahnfabrik, Bad Säckingen, Germany) are 3 ceramic systems that incorporate a high alumina core. These cores differ in their manufacturing process and are also intrinsically different in that Procera AllCeram contains a densely sintered alumina core, whereas Turkom-Cera and VITA In-Ceram are made of a high alumina core, which is subsequently crystal hardened or glass infiltrated.44
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Volume 104 Issue 6 However, alumina cores tend to be opaque and require the use of veneering porcelain to mask the core and provide the desired contours.45,46 The purpose of this study was to evaluate the effects of 2 different veneering porcelain shades (A1, A3) and number of firings (3, 5, 7, or 9) on the color of an alumina-based ceramic system. The research hypothesis was that color differences would occur relative to the firing times and veneering porcelain shades.
MATERIAL AND METHODS A custom-made plastic mold with a diameter of 10 mm and thickness of 1 mm was prepared. The mold was filled with alumina gel (Turkom-Cera Alumina Gel, batch no. TC02CGP-06; Turkom-Ceramic (M)). Disc-shaped Turkom-Cera specimens were removed from the mold after the gel was completely dry, and the specimens were fired in a vacuum furnace (Programat P300; Ivoclar Vivadent AG, Schaan, Liechtenstein) (temperature increase rate, 50°C/min; holding temperature, 1150°C; holding time, 5 minutes), cleaned using the built-in cleaning program to avoid any impurities resulting from previous operations. The sintered specimens were hardened using crystal powder (Turkom-Cera Crystal Powder, batch no. TC09P1P-06; Turkom-Ceramic (M)). The crystal powder was mixed with water and applied on the sintered specimens in the same furnace for 30 minutes at1150°C. The excess crystals were removed using a laboratory micromotor (Ultimate 500; NSK Nakanishi, Inc, Kanuma, Japan) with diamond rotary cutting instruments (863.204.016; Gebr Brasseler GmbH, Lemgo, Germany) at slow speed. Twenty specimens were obtained. Conventional veneering ceramic (VITA VM 7; VITA Zahnfabrik) was applied on the cores to a thickness of 1 mm. The specimens were divided into 2 groups. The first group consisted of 10 specimens with an A1 veneering porcelain shade (VITA VM 7, batch
no. 23810; VITA Zahnfabrik), and the second group consisted of 10 specimens with an A3 veneering porcelain shade (VITA VM 7, batch no. 21000; VITA Zahnfabrik). The sample sizes for this study were derived from the sample sizes used in previous color assessment studies.38-40 No power analysis was performed to determine adequate sample size. Two shades from the A group of a shade guide (VITA Classical Shade Guide; VITA Zahnfabrik) were selected, as this group accounts for at least 65% of clinical shade selections.1 A custom-made plastic mold was used to standardize the veneering porcelain thickness. An apparatus that was used in previous studies27,29,39-41 was modified for the current study. The mold was prepared in 2 pieces. The first piece (I) had a 10-mm-diameter cylindrical cavity in the middle. The second piece (II) had a piston that was attached to the first piece with a screw system so that it could rise and descend in the cavity. The upper surface of the apparatus was divided into 20 equal units. When it was calibrated, the piston moved downward with a sensitivity of 0.05 mm with a 1-unit turn of the lower piece. The piston descended 1.0 mm with 20 turns of the attached lower piece. The required ceramic thickness was determined by adjusting the depth of the cavity above the piston by turning the attached lower piece the necessary amount. Veneering porcelain slurry was condensed and hand vibrated; excess moisture was removed with absorbent paper tissue to minimize porosity. The condensed specimens were fired in a vacuum furnace (Programat P300; Ivoclar Vivadent AG) (temperature increase rate, 55°C/min; holding temperature, 900°C; holding time, 1 minute). After glazing the specimens, the thickness of each disc was verified with a micrometer (Praecimeter S; Renfert GmbH, Hilzingen, Germany). The color of each specimen was measured with a spectrophotometer (VITA Easyshade; VITA
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Zahnfabrik). This system captures the color coordinates using a D65 illuminant (color temperature 6227°C, a mathematical construct equivalent to average daylight in the Northern Hemisphere) and a viewing angle of 2 degrees. The color of the specimen was measured with the glazed surface facing up against a neutral grey background.9 The CIE L*a*b* values were determined from 3 measurements of each specimen, and transferred to a personal computer (Toshiba America, Inc, New York, NY) for analysis. The instrument calibration was evaluated after measurement of each group (n=10), and the instrument was recalibrated. The CIE L*a*b* measurements make it possible to evaluate the amount of perceptible color change in each specimen. The CIE L*a*b* color space is a uniform 3-dimensional (3-D) color order system. Equal changes in any of the 3 coordinates can be perceived as visually similar. Total color differences were calculated with use of the following formula47: ∆E= [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2 The L* coordinate is a measure of the lightness-darkness of the specimen. The greater the L* value is, the lighter the specimen. The a* coordinate is a measure of the chroma along the redgreen axis. A positive a* relates to the amount of redness, and a negative a* relates to the greenness of a specimen. The b* coordinate is a measure of the chroma along the yellow-blue axis; that is, a positive b* relates to the amount of yellowness, while a negative b* relates to the blueness of the specimen. Delta L*, ∆a*, and ∆b* are the differences in the CIE colorspace parameters of the 2 colors.47 The color difference value (∆E) represents the numerical distance between L*a*b* coordinates of 2 colors. Under ideal observation conditions, when the ∆E value of 2 colors is less than 1 unit (∆E<1), the 2 colors can be judged to match.17 When measured color differences are within the 1 to 2 ∆E range, correct judgments are made frequently by observers. A ΔE value of 1.6 was determined to be
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December 2010 a color difference that could not be detected by the human eye.48 When ∆E values are greater than 2 ∆E units, all observers can apparently detect color difference between 2 colors.17 The clinically acceptable limit of the color difference value is considered 3.7 ∆E units.17 Data were analyzed with statistical software (SPSS v. 10.0; SPSS, Inc, Chicago, Ill). Repeated-measures analysis of variance (ANOVA) was used to analyze the data (number of firings, veneering porcelain color) for
significant differences. The Duncan test was used to perform multiple comparisons (α=.05).
both A1 and A3 veneering porcelain shades, the a* value decreased after repeated firings, which resulted in less reddish specimens, and the b* value decreased after repeated firings, which resulted in less yellowish specimens. A1 and A3 veneering porcelain shades had statistically significant differences for all of the parameters tested for the individual number of firings. The results of the ANOVA of L*a*b* color parameters (number of firings and different veneering porcelain
RESULTS The mean values and multiple comparison test results for each individual parameter tested are presented in Table I. A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade specimens became lighter after an increased number of firings. For
Table I. Mean values and multiple comparison test results for individual parameters tested Number of Firings Property
Shade
3
5
7
9
A1
92.37Aa
92.26Aa
92.25Aa
92.24Aa
A3
86.23Cb
86.66ABb
86.59Bb
86.83Ab
A1
1.72De
1.44Ee
1.28Fe
1.27Fe
A3
4.49Ed
4.73Dd
4.36EFd
4.30Fd
A1
25.48Gh
25.28Gh
24.45Hh
24.48Hh
A3
37.79Hg
38.25Gg
37.63Hg
37.46Hg
L*
a*
b*
Uppercase letters indicate significant differences between number of firings; lowercase letters indicate differences between shades (P<.05).
Table II. Multivariate test results based on 2-factor repeated-measures ANOVA for changes in color coordinates after repeated firings of ceramic Pillai’s Value
Numerator df
Denominator df
F
P
Number of firings
0.307
3
22
3.25a
.041
Number of firings x color
0.490
3
22
7.06a
.002
Number of firings
0.723
3
22
19.14a
<.001 .001
Parameter
Effect
L*
a*
b*
Number of firings x color
0.521
3
22
7.99a
Number of firings
0.765
3
22
23.82a
<.001
22
7.57a
.001
Number of firings x color aExact statistic
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0.508
3
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Table III. Mean ∆E values of 2 different colors of ceramic specimens after repeated firings
Number of Firings
ΔE (A1)
ΔE (A3)
3-5
0.61
0.93
3-7
1.28
0.65
3-9
1.43
0.88
5-7
0.95
0.86
5-9
1.19
1.05
7-9
0.68
0.56
shades) for the ceramic system are listed in Table II. The L*a*b* values of the ceramic system were affected by the number of firings (3, 5, 7, or 9 firings) (P<.005) and veneering porcelain shade (P<.001). Statistically significant interactions were present between the number of firings and the veneering porcelain shade for L* (P=.002), a* (P=.001), and b* (P=.001) values. The ∆E values were calculated for each specimen within each group (Table III) to determine mean color differences (∆E), depending on repeated firings. The highest ∆E value was 1.43 for specimens with the A1 veneering porcelain shade, between the third and ninth firings; however, for specimens with the A3 veneering porcelain shade, the highest ∆E value was 1.05, between the fifth and ninth firings.
DISCUSSION This in vitro study investigated the color changes of alumina ceramic specimens veneered with different veneering porcelain shades and fired different numbers of times. The results of this study support the hypothesis that color differences would occur relative to the number of firings and veneering porcelain shades. Possible sources of processing variables in porcelain firing include thickness and color of the opaque; thickness, color, and translucency of the body
and enamel layers; firing temperature; and number of firings.25 Results of the current study indicated imperceptible color changes for the tested aluminabased ceramic system subjected to a repeated number of firings. A previous study found ceramic-core systems to be more color stable compared to metal ceramic systems as the number of firings increased.38 The aluminabased ceramic system investigated in the current study can be expected to exhibit more color stability compared to metal ceramic systems, as the results of these studies are in agreement. It was previously reported that an increase in the number of firings resulted in a decrease in L* and an increase in a* and b* color values of In-Ceram and IPS Empress specimens with different veneering ceramic thicknesses.41 In the current study, repeated firings resulted in less reddish and yellowish specimens for the A1 veneering porcelain shade, and lighter, less reddish, and less yellowish specimens for the A3 veneering porcelain shade. Results of another study investigating the color changes of a zirconia ceramic system with 2 different veneering porcelain shades after repeated firings indicated an increase in the L* value and a decrease in the a* value for both A1 and A3 veneering porcelain shades, resulting in lighter and greener specimens; however, the b* value was not influenced by the number of firings for the A1 veneer-
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ing porcelain shade and increased for the A3 veneering porcelain shade, resulting in more yellowish specimens.40 The differences between these 2 previous studies and the current study may be attributed to the optical properties of different core materials, as a zirconia ceramic system was found to be the least translucent ceramic system and more opaque than a glass-infiltrated alumina ceramic system or the IPS Empress system.11 In addition, the VITA instrument used in the current study, which was found to have both reliability and accuracy values greater than 90%,20 may be sensitive to translucency changes, and some change may be related to the vitrification of the veneering porcelain. As the porcelain is fired, its translucency may change and the measurements may be slightly altered. Heffernan et al34 stated that an additional firing resulted in a significant difference in translucency rankings of ceramic systems and a decrease in the opacity of all veneered materials, except the completely opaque zirconia and metal ceramic specimens. Color change after repeated firings may also be attributed to the color instability of metal oxides during firing, which can affect the resulting color of ceramic. Several studies30-32 have suggested that certain metal oxides are not color stable after they are subjected to firing temperatures. It was reported that yellow- and orangehued stains were the least color stable at the manufacturers’ recommended firing temperatures.30,32 However, another study indicated that blue was the most unstable stain, while orange demonstrated the greatest color stability at higher firing temperatures.31 In the present study, 2 veneering porcelain shades were selected, A1 and A3. The results showed that A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade specimens became lighter after an increased number of firings, and lower ∆E values were observed in A3 than A1 porcelain veneering material. This result
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December 2010 may be explained by the fact that the A3 veneering porcelain may mask the opaque core more than the A1 veneering porcelain. Clinical success and color stability of ceramic restorations depend on laboratory and clinical variables. Ceramic systems in this study exhibited color differences that could not be detected by the human eye under firing conditions following the manufacturers’ instructions. The influence of the background substrate on the definitive appearance of ceramic specimens is well established. Neutral colors such as white, grey, and black are, by definition, colors that have no hue. Neutral grey was selected as a background to minimize the influence of background hue on the color measurement of the specimens.9 Finally, ceramic restorations should be luted to the tooth substrate using a luting agent17 with a shade and thickness that contribute to the esthetic appearance of the restorations. Therefore, further studies on the interaction of ceramic materials with luting agents and other substrates are needed. The limitations of this study include the in vitro use of a spectrophotometer to evaluate shade differences of only a single type of ceramic material. No power analysis was performed to determine adequate sample size. Furthermore, the specimens were disc shaped rather than crown shaped. Additional studies are needed to investigate the interaction of different core colors with repeated firings compared to conventional metal ceramic systems.
CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. L*a*b* values of the ceramic systems were affected by the number of firings (3, 5, 7, or 9 firings) and veneering porcelain shade (A1 or A3). 2. A1 shade specimens maintained their L* value independent of the number of firings, whereas A3 shade
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specimens became lighter after an increased number of firings. For both A1 and A3 veneering porcelain shades, the a* value decreased after repeated firings, which resulted in less reddish specimens (P=.001), and the b* value decreased after repeated firings, which resulted in less yellowish specimens (P=.001). 3. The mean color differences caused by repeated firings were imperceptible (ΔE<1.6) and represent clinically acceptable color changes (ΔE<3.7).
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Volume 104 Issue 6 34.Heffernan MJ, Aquilino SA, Diaz-Arnold AM, Haselton DR, Stanford CM, Vargas MA. Relative translucency of six all-ceramic systems. Part II: core and veneer materials. J Prosthet Dent 2002;88:10-5. 35.Lee YK, Cha HS, Ahn JS. Layered color of all-ceramic core and veneer ceramics. J Prosthet Dent 2007;97:279-86. 36.Barghi N, Goldberg J. Porcelain shade stability after repeated firing. J Prosthet Dent 1977;37:173-5. 37.Yilmaz B, Ozçelik TB, Wee AG. Effect of repeated firings on the color of opaque porcelain applied on different dental alloys. J Prosthet Dent 2009;101:395-404. 38.Pires-de-Souza Fde C, Casemiro LA, Garcia Lda F, Cruvinel DR. Color stability of dental ceramics submitted to artificial accelerated aging after repeated firings. J Prosthet Dent 2009;101:13-8. 39.Ozturk O, Uludag B, Usumez A, Sahin V, Celik G. The effect of ceramic thickness and number of firings on the color of two all-ceramic systems. J Prosthet Dent 2008;100:99-106. 40.Celik G, Uludag B, Usumez A, Sahin V, Ozturk O, Goktug G. The effect of repeated firings on the color of an all-ceramic system with two different veneering porcelain shades. J Prosthet Dent 2008;99:203-8.
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47.Knispel G. Factors affecting the process of color matching restorative materials to natural teeth. Quintessence Int 1991;22:525-31. 48.Ishikawa-Nagai S, Yoshida A, Sakai M, Kristiansen J, Da Silva JD. Clinical evaluation of perceptibility of color differences between natural teeth and all-ceramic crowns. J Dent 2009;37 Suppl 1:e57-63. Corresponding author: Dr Volkan Sahin University of Kirikkale, Faculty of Dentistry Department of Prosthodontics 71200 Kirikkale TURKEY Fax: +90 318 225 06 85 E-mail: [email protected] Copyright © 2010 by the Editorial Council for The Journal of Prosthetic Dentistry.
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