The effect of ceramic thickness and number of firings on the color of a zirconium oxide based all ceramic system fabricated using CAD/CAM technology.

Abstract

Ceramics have a long history in fixed prosthodontics for achieving optimal esthetics and various materials have been used to improve ceramic core strength. However, there is a lack of information on how color is affected by fabrication procedure. The purpose of this study was to evaluate the effects of various dentin ceramic thicknesses and repeated firings on the color of zirconium oxide all-ceramic system (Lava™) fabricated using CAD/CAM technology.
Thirty disc-shaped cores, 12 mm in diameter with a 1 mm thickness were fabricated from zirconium oxide based all ceramic systems (Lava™, 3M ESPE, St Paul, MN, USA) and divided into three groups (n = 10) according to veneering with dentin ceramic thicknesses: as 0.5, 1, or 1.5 mm. Repeated firings (3, 5, 7, or 9) were performed, and the color of the specimens was compared with the color after the initial firing. Color differences among ceramic specimens were measured using a spectrophotometer (VITA Easyshade, VITA Zahnfabrik, Bad Säckingen, Germany) and data were expressed in CIELAB system coordinates. A repeated measures ANOVA and Bonferroni post hoc test were used to analyze the data (n = 10, α=.05).
L*a*b* values of the ceramic systems were affected by the number of firings (3, 5, 7, or 9 firings) (P<.001) and ceramic thickness (0.5, 1, or 1.5 mm) (P<.001). Significant interactions were present in L*a*b* values between the number of firings and ceramic thickness (P<.001). An increase in number of firings resulted in significant increase in L* values for both 0.5 mm and 1.5 mm thicknesses (P<.01, P=.013); however it decreased for 1 mm thickness (P<.01). The a* values increased for 1 mm and 1.5 mm thicknesses (P<.01), while it decreased for 0.5 mm specimens. The b* values increased significantly for all thicknesses (P<.01, P=.022). As the dentin ceramic thickness increased, significant reductions in L* values (P<.01) were recorded. There were significant increases in both a* and b* values (P<.01) as the dentin ceramic thickness increased.
The number of firings and dentin ceramic thickness have a definite effect on the final color of all ceramic system tested. The mean ΔE value increased as the dentin ceramic thicknesses increased for zirconium-oxide based all ceramic specimens tested. However, the mean ΔE values were less than 3.7ΔE units which is rated as a match in the oral environment.

Full text

Page 1

INTRODUCTION
Ceramics have a long history in fixed prosthodontics for achiev-
ing optimal esthetics. Yttria-stabilized tetragonal zirconia
(Y-TZP) is gaining use in dentistry due to its good mechani-
cal properties and superior biocompatibility.1It is currently used
as core material in all ceramic dental restorations,2,3implant
superstructures,4and orthodontic brackets.5Its superior
mechanical properties compared to other dental ceramics,
such as higher strength and fracture toughness,2,6,7which are
due to the transformation toughening mechanism, are similar
to that observed in quenched steel.
Color assessment is regarded as a complex psychophysiologic
process subject to numerous variables. Dentin is considered to
be the primary source of color for teeth, which is modified by
the thickness and translucency of overlying enamel. The
perceived color of natural teeth is a result of light reflected from
the enamel surface, in addition to the effect of light scattering
within enamel and dentin before it is ultimately reflected
back.8Clinically, it is important that ceramic restorations
reproduce the translucency and color of the natural teeth.
There are many factors affecting the match, such as translu-
cency, opalescence, fluorescence, surface texture, and shape.9,10
Most all ceramic systems require the combination of 2 lay-
ers of ceramic material, such as a strong ceramic core and a weak
veneering porcelain,11with different opacity, shade, and thick-
ness, to provide a natural appearance.12,13All ceramic restora-
tions without a metal substructure allow for greater light
DOI:10.4047/jap.2011.3.2.57
The effect of ceramic thickness and number of firings
on the color of a zirconium oxide based all ceramic
system fabricated using CAD/CAM technology
Vinay Chila Bachhav*, MDS, Meena Ajay Aras, MDS
Department of Prosthodontics, Goa Dental College and Hospital, Bambolim, Goa, India
PURPOSE. Ceramics have a long history in fixed prosthodontics for achieving optimal esthetics and various materials have been used to improve
ceramic core strength. However, there is a lack of information on how color is affected by fabrication procedure. The purpose of this study was
to evaluate the effects of various dentin ceramic thicknesses and repeated firings on the color of zirconium oxide all-ceramic system
(LavaTM) fabricated using CAD/CAM technology. MATERIALS AND METHODS. Thirty disc-shaped cores, 12 mm in diameter with a 1 mm
thickness were fabricated from zirconium oxide based all ceramic systems (LavaTM, 3M ESPE, St Paul, MN, USA) and divided into three groups
(n = 10) according to veneering with dentin ceramic thicknesses: as 0.5, 1, or 1.5 mm. Repeated firings (3, 5, 7, or 9) were performed, and the
color of the specimens was compared with the color after the initial firing. Color differences among ceramic specimens were measured using
a spectrophotometer (VITA Easyshade, VITA Zahnfabrik, Bad Sa ¨ckingen, Germany) and data were expressed in CIELAB system coordinates.
A repeated measures ANOVA and Bonferroni post hoc test were used to analyze the data (n = 10, α =.05). RESULTS. L*a*b* values of the
ceramic systems were affected by the number of firings (3, 5, 7, or 9 firings) (P<.001) and ceramic thickness (0.5, 1, or 1.5 mm) (P<.001). Significant
interactions were present in L*a*b* values between the number of firings and ceramic thickness (P<.001). An increase in number of firings result-
ed in significant increase in L* values for both 0.5 mm and 1.5 mm thicknesses (P<.01, P=.013); however it decreased for 1 mm thickness (P<.01).
The a* values increased for 1 mm and 1.5 mm thicknesses (P<.01), while it decreased for 0.5 mm specimens. The b* values increased sig-
nificantly for all thicknesses (P<.01, P=.022). As the dentin ceramic thickness increased, significant reductions in L* values (P<.01) were record-
ed. There were significant increases in both a* and b* values (P<.01) as the dentin ceramic thickness increased. CONCLUSION. The num-
ber of firings and dentin ceramic thickness have a definite effect on the final color of all ceramic system tested. The mean ΔE value increased
as the dentin ceramic thicknesses increased for zirconium-oxide based all ceramic specimens tested. However, the mean ΔE values were
less than 3.7ΔE units which is rated as a match in the oral environment. [J Adv Prosthodont 2011;3:57-62]
57
ORIGINAL ARTICLE
J Adv Prosthodont 2011;3:57-62
KEY WORDS. Dental porcelain, Color, CAD/CAM
Corresponding author: Vinay C. Bachhav.
Department of Prosthodontics, Goa Dental College and Hospital,
Rajiv Gandhi Medical Complex, Bambolim, Goa 403202, India
Tel. 91 9422783856: e-mail, vinaybachhav@gmail.com.
Received March 1, 2011 / Last Revison April 30, 2011 / Accepted May 9, 2011
ⓒ 2011 The Korean Academy of Prosthodontics
This is an Open Access article distributed under the terms of the Creative Commons
Attribution Non-Commercial License (http://creativecommons.org/licenses/by-
nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction
in any medium, provided the original work is properly cited.
*The zirconium oxide blocks used in this study were partly supported by 3M ESPE, India.

Page 2

transmission within the restoration, thereby improving the col-
or and translucency of the restoration; however, a perfect
esthetic tooth-colored restoration cannot be ensured.14If the major-
ity of light passing through a ceramic restoration is diffusely
transmitted and only part of it is scattered, the material will appear
translucent.15The 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.16Core translucency
was also identified as a primary factor in controlling esthetics
and a critical consideration in the selection of the materials.17
Some all ceramic core materials have high in vitro strength val-
ues.18,19However, an increase in crystalline content to achieve
greater strength generally results in greater opacity.17,20
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 CIELAB color parameters.21
The ΔE values are used to describe whether the changes in the
overall shade are perceivable to the human observer. This mag-
nitude of the color difference is based on the human per-
ception of color; color differences greater than 1 ΔE unit are
visually detectable by 50% of human observers.22However, under
uncontrolled clinical conditions, such small differences in
color would be unnoticeable because average color differences
below 3.7 have been rated as a match in the oral environment.23,24
Instrumental measurements can quantify color and allow com-
munication to be more uniform and precise. In addition to the
opacity and shade of the ceramic that determine the definitive
shade of an esthetic restoration, other factors, including
porcelain brand,8,25,26batches,25condensation techniques,25fir-
ing temperatures,26dentin thickness,27-29and number of porce-
lain firings,28,30can have an effect. Specific contributions of core
and veneer thickness to the appearance of layered ceramics were
determined,29and it was concluded that there was a significant
correlation between the thickness ratio of core and veneer ceram-
ics and the color of the restoration. Even when adequate
ceramic thickness exists, clinical shade matches are diffi-
cult to achieve31because there is a wide range of translucen-
cy among the core materials of all ceramic systems at clinically
relevant core thicknesses.16The thickness and the combination
of ceramic layers, such as the core, veneer, and other specialty
ceramic materials, have been shown to control the appearance
of all ceramic materials.15,23,32Antonson and Anusavice32found
that it was important to determine the translucency of a layered
core-veneering ceramic as a function of thickness. Moreover,
Heffernan et al.10,16investigated the influence of core and
core-veneering ceramic thickness on overall translucency.
Lee et al.33showed that the layered color of varied all ceram-
ic and veneer combinations was different depending on the type
of all ceramic core material, even though the thickness of the
layered specimen was set to 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.
The effect of multiple firings has been investigated in pre-
vious studies, and it has been reported that repeated firings did
not noticeably affect the color of dental ceramics.28,30,34,35
However, O’ Brien et al.25found perceivable differences (ΔE
= 1) between the color of ceramic specimens that were fired
3 and 6 times. Uludag et al.36and Ozturk et al.37reported per-
ceptual color changes in L*a*b* color parameters as the
number of firings increase. The purpose of this study was to
evaluate the effects of various dentin ceramic thicknesses
(0.5, 1, or 1.5 mm) and number of firings (3, 5, 7, or 9) on the
color of zirconium oxide all ceramic system fabricated using
CAD/CAM technology. The research hypothesis was that
the color difference would be determined relative to the firing
times and dentin ceramic thicknesses.
MATERIALS AND METHODS
The zirconia-based ceramic used in this study; the LavaTMAll-
ceramic system (3M ESPE Dental Products, St Paul MN,
USA) utilizes CAD/CAM technology to produce a densely sin-
tered and high strength zirconia framework with a 3% mol par-
tially yttria-stabilized zirconia polycrystal content.
Thirty disc-shaped cores, 12 mm in diameter and 1 mm thick
were fabricated from zirconia based all ceramic system using
CAD/CAM technology. These cores were divided into three
groups (n = 10) for veneering with dentin ceramic with thick-
nesses of 0.5, 1 and 1.5 mm. The cores were veneered with A1
(according to the VITA Shade Guide) dentin ceramic
(LavaCeram, 3M ESPE, St Paul, MN, USA) with the use of
the similar molds described by others25,28,32which allowed
standardization of the dentin thicknesses for each of group to
be studied. Specimens were then fired in a dental ceramic fur-
nace (Lava Furnace 200, 3M ESPE, St Paul, MN, USA).
The thickness of each group of specimens was then measured
with a digital caliper (Dial Caliper D, Aura Dental, Aura an der
Saale, Germany) with an accuracy of 0.02 mm, and correct-
ed with diamond rotary cutting instruments (ISO 173/016, Mani.
Inc, Utsunomiya, Tochigi, Japan) until the desired thickness
of dentin ceramic was achieved (0.5, 1, or 1.5 mm).
The color of each specimen was measured with a spec-
trophotometer (VITA Easyshade; VITA Zahnfabrik, Bad
Sa ¨ckingen, Germany). The spectrophotometer’ s CIE L*a*b*
output is based on D65 illuminant and a 2-degree standard
observer. Three measurements were made, and the average read-
ing was calculated for each specimen. Instrument was recal-
ibrated after measurement of each group (n = 10). The
CIELAB measurements make it possible to evaluate the
amount of perceptible color change in each specimen. The
CIELAB color space is a uniform 3-dimensional color order
58
The effect of ceramic thickness and number of firings on the color of a zirconium oxide based all-ceramic system fabricated using CAD/CAM technology
J Adv Prosthodont 2011;3:57-62
Bachhav VC et al.

Page 3

system. Equal changes in any of the 3 coordinates can be per-
ceived as visually similar. Total color differences were calculated
with use of the following equation.38
ΔE*= [(ΔL*)2+ (Δa*)2+ (Δb*)2]1/2
The L* coordinate is a measure of the lightness-darkness of
the specimen. The greater the L* is, the lighter the specimen.
The a* coordinate is a measure of the chroma along the red-
green axis. A positive a* relates to the amount of redness, and
a negative a* relates to 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. ΔL*,
Δa*, and Δb* are the differences in the CIE color-space
parameters of the 2 colors.38
The results of the testing were analyzed with statistical
software (SPSS, PC, Version 17.0; SPSS, Inc, Chicago, IL, USA).
Repeated measurements of the data (number of firings and ceram-
ic thicknesses) were analyzed with analysis of variance
(ANOVA) for significant differences. Bonferroni post hoc test
was used to perform pairwise comparisons (α =.05).
RESULTS
The analysis revealed that L*a*b* values of ceramic system
(Table 1) were affected by number of firings [3, 5, 7 or 9]
(P<.001) and dentin ceramic thickness [0.5 mm, 1 mm or 1.5
mm] (P<.001).
Significant interactions were present in L*, a* and b* values
within each group of dentin ceramic thickness i.e. 0.5 mm, 1
mm or 1.5 mm. An increase in number of firings resulted in
significant increase in L* values for both 0.5 mm and 1.5 mm
thicknesses (P<.01, P=.013); however it decreased for 1 mm
thickness (P<.01). The a* values increased for 1 mm and 1.5
mm thicknesses (P<.01), while it decreased for 0.5 mm spec-
imens. The b* values increased significantly for all thicknesses-
0.5 mm, 1 mm and 1.5 mm as the number of firings increased
(P<.01, P<.01, P=.022).
Significant interactions were also present in all color co-ordi-
nates i.e. L*, a* and b* values among all three groups of dentin
ceramic thicknesses and number of firings. As the dentin
ceramic thickness increased (0.5 mm - 1 mm - 1.5 mm),
significant reductions in L* values (P<.01) were recorded (Fig.
1). There were significant increases in both a* and b* values
(P<.01) as the dentin ceramic thickness increased (Fig. 2, 3).
Mean color difference (ΔE) values were calculated for
each group of thickness. The mean ΔE value increased as the
thicknesses increased. Mean ΔE values were less than 3 when
ceramic thickness increased from 0.5 mm to 1 mm (ΔE = 1.43)
and from 1 mm to 1.5 mm (ΔE = 2.38). It was higher than 3
(ΔE = 3.37) when ceramic thickness increased from 0.5 mm
to 1.5 mm.
The ΔE values were calculated for each specimen within each
group to determine mean color differences (ΔE), depending
on repeated firings (Table 2). The ΔE value was smaller
than 1 between firing 5 and 7, firing 5 and 9, and firing 7 and
9 for 0.5 mm thickness and between firing 3 and 9 for 1
mm thickness. The ΔE value was higher than 1 between all
firings for 1.5 mm specimens.
59
The effect of ceramic thickness and number of firings on the color of a zirconium oxide based all-ceramic system fabricated using CAD/CAM technology
J Adv Prosthodont 2011;3:57-62
Bachhav VC et al.
Table 1.Multivariate test results based on a repeated measures ANOVA for changes in color coordinates after repeated firings of ceramic
Effect Pillai's value
ΔL*Thickness.970
Number of firings.955
Number of firings×thickness.981
Δa*Thickness.993
Number of firings.955
Number of firings×thickness .999
Δb*Thickness.938
Number of firings.868
Number of firings×thickness .988
(P<.05 indicates significant difference; a: Exact statistics)
FHypothesis df
2
3
6
2
3
6
2
3
6
Error df
8
7
4
8
7
4
8
7
4
Sig.
< .001
< .001
< .001
< .001
< .001
< .001
< .001
< .001
< .001
131.241a
49.557a
33.537a
557.185a
49.988a
1247.562a
60.507a
15.289a
56.342a
Fig. 1.Means of L* for number of firings and dentin ceramic thicknesses.

Page 4

DISCUSSION
This in vitro study measured the color changes of ceramic spec-
imens prepared at different thicknesses and fired different num-
bers of times. The results of this study support the hypothesis
that the color difference varies with respect to the firing
times and dentin ceramic thicknesses. There were signifi-
cant differences in color change within groups.
Most all ceramic systems consist of a ceramic core with a thick-
ness of 0.5 to 1.0 mm and approximately 1.0 to 1.5 mm of space
available for veneering ceramic.16In the current study, the spec-
imens had ceramic thicknesses of 0.5, 1, or 1.5 mm, with a core
thickness of 1 mm. L* values, which reflect the brightness of
the specimens, decreased for both systems as the total thick-
ness of the specimens increased. Antonson and Anusavice32stud-
ied the effect of change in the thickness of ceramics on the con-
trast ratio of dental core and veneering ceramic, and concluded
that the contrast ratio was dependent on the type of the mate-
rial tested. Heffernan et al.10,16described the influence of
core material thickness on its translucency and the influence
of core plus ceramic veneer thickness on the overall translu-
cency of specimens. Shokry et al.29demonstrated that L*
values decreased for leucite reinforced and spinell ceramics as
the total thickness increased. The results of the present study
are in agreement with the previous studies12,29,31,36,37since the thick-
ness of the layered ceramic influenced the final shade, partially
due to the translucency, as the thicker ceramic disks were less
translucent.
As the dentin ceramic thickness increased, significant reduc-
tions in L* values were recorded. There were significant
increases in both a* and b* values as the dentin ceramic
thickness increased. These changes in L*, a* and b* values are
consistent with a study by Ozturk et al.37. An increase in the
number of firings resulted in an appreciable increase in L* val-
ues that resulted in darker specimens for 0.5 mm and 1.5 mm
thicknesses used in the present study. However it decreased for
1 mm thickness specimens. The a* color values of 1 mm and
1.5 mm thicknesses increased after repeated firings which result-
ed in redder specimens. However, the a* values decreased for
0.5 mm thickness specimens. The b* values increased appre-
ciably by the number of firings in all specimens of 0.5, 1 and
1.5 mm thicknesses. This resulted in yellower specimens.
The mean ΔE values increased as the dentin ceramic thick-
ness was increased. The ΔE value was smaller than 1 between
firing 5 and 7, firing 5 and 9 and firing 7 and 9 for 0.5 mm thick-
ness and between firing 3 and 9 for 1 mm thickness. The ΔE
value was higher than 1 between all firings for 1.5 mm spec-
imens. Color changes following repeated firings may also be
attributed to the color stability of metal oxides during firing which
can affect the resulting color of ceramic. Several studies
have suggested that certain metal oxides are not color stable
after they are subjected to firing temperatures, and color
changes of surface colorants after firing have demonstrated pig-
ment breakdown at firing temperatures.39-41
In the current study, mean color differences caused by var-
ious dentin thicknesses and repeated firings were below 3.7 ΔE
units, which is rated as a match in the oral environment.23,24
Ceramic systems in the present study exhibited visual color
changes during firing and demonstrated that changes in the thick-
60
The effect of ceramic thickness and number of firings on the color of a zirconium oxide based all-ceramic system fabricated using CAD/CAM technology
J Adv Prosthodont 2011;3:57-62
Bachhav VC et al.
Fig. 2.Means of a* for number of firings and dentin ceramic thicknesses. Fig. 3.Means of b* for number of firings and dentin ceramic thicknesses.
Table 2.Mean ΔE*ab values (SD) of LavaTMall ceramic specimens with
different thicknesses
Number of Mean ΔE (SD)
firings 0.5 mm
3 - 5 1.10 (0.57)
3 - 71.00 (0.42)
3 - 91.59 (0.44)
5 - 7 0.75 (0.26)
5 - 9 0.82 (0.42)
7 - 9 0.99 (0.44)
1 mm
1.44 (0.46)
2.19 (0.89)
0.78 (0.33)
1.19 (0.75)
1.11 (0.44)
1.64 (0.74)
1.5 mm
1.61 (1.47)
1.68 (1.40)
1.97 (1.06)
1.13 (0.35)
1.25 (0.47)
2.04 (0.16)

Page 5

ness and repeated firings of ceramic have an effect on the final
shade.
The results of this study suggest that dentin ceramic thick-
ness and the number of firings of the all ceramic system
tested significantly affect the final color of the all ceramic restora-
tions. These are important factors for the definitive color of the
restoration, and should be considered during shade selec-
tion and fabrication. The limitations of this study include
the in vitro use of a spectrophotometer to evaluate shade
differences of only 1 type of all ceramic material. Furthermore,
the specimens were disc shaped rather than shaped like
crowns. Further study of the clinical implications of the col-
or and translucency of consistent layers for all ceramic
restorations, such as core and veneer ceramics, luting cements,
and abutment, on the final shade of restorations after repeat-
ed firings, should be performed.
CONCLUSION
Within the limitations of this in vitro study, the following con-
clusions were drawn:
1. The number of firings and dentin ceramic thickness have
a definite effect on the final color of all ceramic system test-
ed and these factors should be considered during shade selec-
tion and fabrication of the restoration.
2. As the ceramic thickness increased, significant reductions
in L* values and significant increases in both a* and b* val-
ues were recorded. This resulted in darker, redder and yel-
lower specimens.
3. The mean ΔE value increased as the dentin ceramic
thicknesses increased for zirconium oxide based all-
ceramic specimens tested. However, the mean ΔE values
were less than 3.7ΔE units which is rated as a match in
the oral environment.
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