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Optical properties and light irradiance of monolithic zirconia at variable thicknesses
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Optical properties and light irradiance of monolithic zirconia at variable thicknesses Taiseer A. Sulaiman a,b,c,∗,1 , Aous A. Abdulmajeed a,b , Terrence E. Donovan c , André V. Ritter c , Pekka K. Vallittu b,d , Timo O. Närhi a,b,e , Lippo V. Lassila b,d a
Department of Prosthetic Dentistry, Institute of Dentistry, University of Turku, Turku, Finland Turku Clinical Biomaterials Centre (TCBC), University of Turku, Turku, Finland c Department of Operative Dentistry, School of Dentistry, University of North Carolina, Chapel Hill, NC, USA d Department of Biomaterials Science, Institute of Dentistry, University of Turku, Turku, Finland e Clinic of Oral Diseases, Turku University Central Hospital, Turku, Finland b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The aims of this study were to: (1) estimate the effect of polishing on the surface
Received 28 September 2014
gloss of monolithic zirconia, (2) measure and compare the translucency of monolithic zirco-
Received in revised form
nia at variable thicknesses, and (3) determine the effect of zirconia thickness on irradiance
28 April 2015
and total irradiant energy.
Accepted 25 June 2015
Methods. Four monolithic partially stabilized zirconia (PSZ) brands; Prettau® (PRT, Zirkon-
Available online xxx
zahn), Bruxzir® (BRX, Glidewell), Zenostar® (ZEN, Wieland), Katana® (KAT, Noritake), and one fully stabilized zirconia (FSZ); Prettau Anterior® (PRTA, Zirkonzahn) were used to fabri-
Keywords:
cate specimens (n = 5/subgroup) with different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm).
Monolithic zirconia
Zirconia core material ICE® Zircon (ICE, Zirkonzahn) was used as a control. Surface gloss
Surface gloss
and translucency were evaluated using a reflection spectrophotometer. Irradiance and total
Translucency parameter
irradiant energy transmitted through each specimen was quantified using MARC® Resin
Contrast ratio
Calibrator. All specimens were then subjected to a standardized polishing method and
Light cure irradiance
the surface gloss, translucency, irradiance, and total irradiant energy measurements were repeated. Statistical analysis was performed using two-way ANOVA and post-hoc Tukey’s tests (p < 0.05). Results. Surface gloss was significantly affected by polishing (p < 0.05), regardless of brand and thickness. Translucency values ranged from 5.65 to 20.40 before polishing and 5.10 to 19.95 after polishing. The ranking from least to highest translucent (after polish) was: BRX = ICE = PRT < ZEN < KAT < PRTA (p < 0.05). The ranking from least to highest total irradiant energy was: BRX < PRT < ICE = ZEN < KAT = PRTA (p < 0.05). There was an inverse relationship between translucency, irradiant energy, and thickness of zirconia and the amount was brand dependent (p < 0.05).
∗ Corresponding author at: Department of Prosthetic Dentistry, Institute of Dentistry, University of Turku, Lemminkäisenkatu 2, FI-20520 Turku, Finland. Tel.: +358 2 333 8379; fax: +358 2 333 8390. E-mail addresses: taabab@utu.fi, [email protected] (T.A. Sulaiman). 1 Present address: Department of Operative Dentistry, School of Dentistry, University of North Carolina, 5405F Koury Oral Health Science Building, CB 7450, Chapel Hill, NC 27599, USA. Tel.: +1 919 537 3161.
http://dx.doi.org/10.1016/j.dental.2015.06.016 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Sulaiman TA, et al. Optical properties and light irradiance of monolithic zirconia at variable thicknesses. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.06.016
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Significance. Brand selection, thickness, and polishing of monolithic zirconia can affect the ultimate clinical outcome of the optical properties of zirconia restorations. FSZ is relatively more polishable and translucent than PSZ. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Monolithic zirconia restorations have been increasingly used in restorative dentistry given their favorable properties, reasonable esthetics, simple clinical technique, and relative low cost compared to cast gold restorations. One notable disadvantage of full contour zirconia restorations, compared to other all-ceramic systems, is their lack of translucency owing to zirconia’s high refractive index mismatch between its particles and the matrix and due to the dispersed particles (slightly greater in size to the wavelength of the incident light) [1]. While improvements in zirconia’s translucency have been claimed by many manufacturers, a better understanding of zirconia’s optical properties, including surface gloss and translucency, can help inform material development and selection for improved esthetic outcomes. Surface gloss may be defined as the degree a surface approaches that of a mirror, and is used primarily as a measure of surface shine [2]. Specular reflection is defined as light reflected from a smooth surface at a defined angle; whereas diffuse reflection is produced by rough surfaces that tend to reflect light in all directions. The rougher the surface the lower the gloss, meaning the specular component is weak and the diffused light is stronger. A spectrophotometer measures the spectral reflectance of the specimen. It can operate two different measuring geometries of specular component excluded (SCE) where it excludes the specular reflectance of light, and specular component included (SCI) where it includes the specular reflectance of light [3]. Surface gloss can be estimated by a spectrophotometer. By lighting two xenon lamps in quick succession, the system can provide virtually simultaneous SCE/SCI measurements and also enable the calculation of 8◦ gloss. The difference between the SCE and SCI reflectance (E*SCE − SCI) gives estimation about the surface gloss [4,5]. Another method has been reported in the literature for measuring the surface gloss performed by using a small-area glossmeter, and the gloss measurements were expressed in Gloss units (GU) [6]. The amount of light that is absorbed, reflected, and transmitted depends on the amount of crystals within the core matrix, their chemical nature, and their size compared to the incident light wavelength [7]. The translucency of dental porcelain is largely dependent on light scattering [8] and thickness [9–11]. If the majority of light passing through a ceramic is intensely scattered and diffusely reflected, the material will appear opaque. If only part of the light is scattered and most is diffusely transmitted, the material will appear translucent [12]. Porcelain translucency is usually expressed by contrast ratio (CR) and/or translucency parameter (TP) [13]. The CR is defined as the ratio of illuminance (Y) of the test material when placed over a black background (Yb ) to the illuminance
of the same material when placed over a white background (Yw ), where 0 is most translucent and 1 is most opaque [14]. The TP is defined as the color difference (E) between a uniform thickness of a material over a white and a black backing [13]. The higher the TP value the more opaque the material. In most studies, the translucency of dental ceramics was mainly studied at a certain thickness, generally, the thinnest recommended by the manufacturers [15]. In clinical situations, ceramic restorations with various thicknesses are required, depending on the different conditions of the tooth to be restored. Therefore, an accurate knowledge of the relationship between the translucency and thickness of restorative materials is fundamental to improving the esthetic outcome of restorations [15]. Zirconia and its opacity at variable thicknesses can have an immense effect on light cure irradiation. Light transmission and curing efficiency of light activated resin luting agents is influenced by the shade and the thickness of the restorative material. Generally, the thicker the restoration or the darker its shade, the more critical the irradiance of the incident light is to achieve optimal photopolymerization of the material [16,17]. Other factors can also affect the degree of cure such as ceramic translucency, resin cement composition, and polymerization type as well as the curing light’s output power, curing duration, and distance [18]. The International Organization for Standardization (ISO) recommends irradiance for polymerization lights of 300 mW/cm2 , and the standard depth-of-polymerization requirement is 1.5 mm [19]. Optimal cure is always critical because inadequately polymerized resin cements are prone to have altered mechanical properties and dimensional stability with decreased bonding to tooth structures resulting in microleakage, decreased biocompatibility, discoloration, and postoperative sensitivity [20]. The aims of this study were to (1) estimate and compare the effect of polishing on the surface gloss of monolithic zirconia, (2) measure and compare the translucency of zirconia at variable thicknesses, and (3) determine the effect of thickness on irradiance and total irradiant energy. The null hypotheses tested for this study were: (1) there is no difference in surface gloss before and after polishing; (2) translucency of the zirconia material is not influenced by the thickness, and (3) light transmittance though zirconia is not affected by its thickness.
2.
Materials and methods
2.1.
Sample preparation
Four brands of monolithic partially stabilized zirconia (PSZ) and a fully stabilized zirconia (FSZ) were studied in this study, and a zirconia core (ICE Zircon) was used as a control
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Table 1 – Monolithic zirconia brands used in the study. Brand name
Code
Partially stabilized zirconia (PSZ) Prettau Zirconia
Manufacturer
PRT
Zirkonzahn, Taufers, Italy
Bruxzir Zirconia
BRX
Wieland Zenostar Translucent
ZEN
Katana High Translucent
KAT
Glidewell Laboratories, Irvine, USA Ivoclar Vivadent, Principality of Liechtenstein Kurary Noritake INC, Noritake, Japan
Fully stabilized zirconia (FSZ) Prettau Anterior
PRTA
Zirkonzahn, Taufers, Italy
<12% Y2 O3 , <1% Al2 O3 , max. 0.02% SiO2 , max. 0.01% Fe2 O3 , max. 0.04% Na2 O
Control (PSZ) ICE Zircon
ICE
Zirkonzahn, Taufers, Italy
4%–6% Y2 O3 , <1% Al2 O3 , max. 0.02% SiO2 , max. 0.01% Fe2 O3 , max. 0.04% Na2 O
(Table 1). Square shaped specimens (10 mm × 10 mm, n = 5/per subgroup) were cut into different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm) using a cutting device (Struers Secotom50, Copenhagen, Denmark). In green stage, each specimen was sequentially ground to the specific thickness using silicon carbide grinding paper (FEPA #1200, 2400, 4000) (Struers LaboPol 21, Struers A/S, Rodovre, Denmark). The final thickness (±0.1 mm) was measured using a digital caliper (Mitutoyo Corporation, Kanagawa, Japan). The specimens were sintered according to the manufacturers’ instructions using the manufacturers’ furnace for each brand. The specimens were cleaned ultrasonically in distilled water for 10 min before testing (Quantrex 90, L&R Ultrasonics Manufacturing Co., Kearny, NJ, USA) then air-dried individually for 20 s. Optical, light cure irradiance and total irradiant energy measurements described following were conducted (baseline measurements). Then one side of each specimen was polished by a single experienced operator (TS) using a straight lab handpiece (K5plus, Kavo, Germany) connected to an electrical control unit (K-control 4960, Kavo, Germany) with diamond polishers (Zircpol Plus and Zircoshine Plus, Diatech, Switzerland) followed by a polishing paste (Zircon-Brite; Dental Ventures of America Inc., Corona, CA, USA) at a constant speed of 10,000 rpm under constant pressure and standard time in a single directed motion, following manufacturers’ instructions. The specimens were not glazed in this experiment. The specimens were ultrasonically cleaned and dried as previously explained. Then the optical, light cure irradiance and total irradiant energy measurements were conducted after polishing.
2.2.
Optical measurements
To estimate the surface gloss (E*SCE − SCI), TP, and CR values of each specimen, a reflection spectrophotometer (CM-700d, Konica Minolta Sensing Inc., Tokyo, Japan) was used according to the CIE 1976 L*a*b* color scale relative to the CIE standard illuminant D65 (as defined by the International Commission on Illumination) which corresponds to “average” daylight (including ultraviolet wavelength region with a correlated color temperature of 6504 K). The SCE and SCI geometries
Composition 4–6% Y2 O3 , <1% Al2 O3 , max. 0.02% SiO2 , max. 0.01% Fe2 O3 , max. 0.04% Na2 O Unknown Unknown (ZrO2 + HfO2 + Y2 O3 ) > 99.0%, yttrium oxide (Y2 O3 ) > 4.5–≤6.0%, hafnium oxide (HfO2 ) ≤5.0%, other oxides ≤1.0%
were determined according to the CIE L*a*b* color scale using standard illuminant D65 over white (CIE L* = 98.1, a* = −0.5 and b* = 2.8) and black (CIE L* = 4.7, a* = −0.1 and b* = 0.0) background. Differences in surface gloss (E*SCE − SCI) values were calculated by the following equation: E ∗ SCE − SCI 2
2 1/2
2
= [(L ∗ SCE − SCI) +(a ∗ SCE − SCI) +(b ∗ SCE − SCI) ]
Calibration of the spectrophotometer was executed before measurement of each specimen. Then the TP of each specimen was obtained by calculating the color difference between the specimen against the white background and against the black background using the following equation: 2
2
2 1/2
TP = [(Lb ∗ −Lw∗) + (ab ∗ −aw∗) + (bb ∗ −bw∗) ]
where L* refers to the lightness, a* to redness to greenness, and b* to yellowness to blueness. The CR values were calculated from the spectral reflectance of light of the specimen (Y) over a black background (Yb ) and over a white background (Yw ), using the following equation: CR =
2.3.
Yb Yw
Light irradiation and transmittance
The specimens were evaluated to determine the amount of light irradiance (i.e. amount of light received by the specimen), and total irradiant energy through each thickness. A LED light-curing unit (Elipar S10, 3 M ESPE, St. Paul, MN, USA) with light irradiance of 1200 mW/cm2 , wavelength range 430–480 nm and curing time 10 s was used. Light energy transmitted through each specimen, was quantified by MARC® Resin Calibrator (BlueLight analytics Inc., Halifax, Canada) whereby each specimen was placed on the “bottom surface” of the resin calibrator. A specially made jig was used to ensure the stability and proper placement of the light-curing unit. To
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determine the actual irradiance of the light-curing unit (without a specimen), plastic rings with a 10 mm diameter opening of the same thicknesses were made to compare the irradiance of the light before and after specimen placement.
2.4.
Results for TP are presented in (Table 3). There was a significant differences in TP (p < 0.05) at different thicknesses, regardless of zirconia brand and polishing process. As the thickness of the specimens increased, the translucency decreased. There was no statistically significant difference among any of the specimens before and after polishing, nor was there any statistically significant difference among the specimens on a black background versus specimens on the white background. Post-hoc Tukey’s test indicated that PRTA was the most translucent. However, there was no significant difference between BRX, ICE, and PRT. Ranking from least translucent to most translucent was: BRX = PRT = ICE < ZEN < KAT < PRTA. Results for CR are present in (Table 4). Likewise, CR of the specimens investigated was brand and thickness dependent. Ranking from least translucent to most was: BRX < ICE = PRT = ZEN < KAT < PRTA (p < 0.05). Results for light cure irradiance and total irradiant energy are shown in Fig. 1a and b. There was a significant difference for the amount of light irradiance and total irradiant energy transmitted through the specimens. The amount of light irradiance and energy decreased as the thickness of the specimen increased (p < 0.05) regardless of the brand. Post-hoc Tukey’s test revealed that BRX showed the lowest values in terms of light irradiance and total irradiant energy, while PRTA and KAT showed the highest values. There was no statistical significance between ICE and ZEN. The ranking for the amount of light irradiance through specimens from least to highest was: BRX < PRT < ICE = ZEN < KAT = PRTA (p < 0.05). As the thickness increases, there was a strong positive Pearson correlation between TP and irradiance (0.973 value), and a strong negative correlation between CR and irradiance (0.884 value) (p < 0.01).
Statistical analysis
The independent variables were zirconia brand and thickness, and dependent variables were TP, CR, and irradiance. Statistical analysis was performed with Statistical Package for the Social Sciences (Version 21.0; SPSS. Inc, Chicago, IL). The distribution of data was normal, which was determined by the Kolmogorov–Smirnov test. Thus, the data were analyzed using two-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test. Differences were considered significant at 95% confidence level. Measurement of correlation between TP, CR, and irradiance was performed using the Pearson Correlation test.
3.
Results
Results of surface gloss (E*SCE − SCI) measurements are presented in Table 2. The E*SCE − SCI revealed that all specimens (placed over a white or black background) regardless of the brand had significantly higher surface gloss after polishing (p < 0.05). Thickness had no influence on surface gloss (p > 0.05). Post-hoc Tukey’s test for (E*SCE − SCI) revealed that PRTA and BRX had the highest surface gloss value, while ZEN had the lowest surface gloss value. Ranking from the lowest to the highest surface gloss value were: ZEN < PRT < KAT < PRTA = BRX (p < 0.05).
Table 2 – Mean surface gloss (E*SCE − SCI) measurements before and after polishing with a white and black background. Thickness (mm)
*ICE
White background PRT
0.5 0.7 1.0 1.2 1.5 2.0
1.50 1.51 1.53 1.51 1.52 1.51
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
1.51 1.53 1.53 1.55 1.53 1.54
6.53 6.61 6.75 6.60 6.64 6.58
1.44 1.49 1.47 1.47 1.49 1.52
7.78 7.79 7.81 7.90 7.92 7.93
1.19 1.14 1.16 1.17 1.16 1.14
9.64 9.64 9.60 9.65 9.66 9.62
1.80 1.78 1.79 1.80 1.79 1.81
6.18 6.15 6.17 6.16 6.16 6.17
1.92 1.94 1.95 1.92 1.95 1.93
7.58 7.59 7.57 7.60 7.61 7.62
Black background PRT
0.5 0.7 1.0 1.2 1.5 2.0
1.60 1.60 1.61 1.60 1.62 1.61
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
1.63 1.66 1.62 1.62 1.63 1.64
8.21 8.58 8.44 8.67 8.43 8.58
1.84 1.83 1.88 1.82 1.89 1.89
10.52 10.45 10.54 10.50 10.51 10.57
1.22 1.26 1.25 1.21 1.25 1.21
10.32 10.32 10.39 10.38 10.38 10.37
1.94 1.91 1.94 1.92 1.95 1.91
7.83 7.80 7.82 7.79 7.80 7.79
2.08 2.10 2.11 2.08 2.11 2.10
9.58 9.59 9.56 9.52 9.55 9.54
*ICE: intended for core use only, therefore no polishing was performed. Regardless of the brand, there was a significantly higher surface gloss after polishing (p < 0.05). Thickness had no influence on surface gloss (p > 0.05).
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Table 3 – Mean translucency parameter (TP) values before and after polishing placed over a white background. Thickness (mm)
*ICE
0.5 0.7 1.0 1.2 1.5 2.0
16.59 14.41 11.47 9.92 8.52 6.66
PRT
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
17.13 15.50 12.46 10.62 8.73 6.38
16.97 14.50 11.16 9.82 7.76 5.61
20.40 17.60 15.82 14.82 12.04 9.74
19.95 17.23 15.03 13.48 11.52 9.17
17.76 15.03 12.32 10.53 8.00 5.65
17.03 14.22 11.58 9.66 7.33 5.10
18.90 15.91 13.95 11.84 9.47 7.46
17.99 15.27 12.98 10.92 8.75 6.83
18.23 16.71 14.51 12.83 10.76 8.50
17.57 15.11 13.42 11.69 9.78 7.78
*ICE: intended for core use only, therefore no polishing was performed.
Table 4 – Mean contrast ratio (CR) values before and after polishing placed over a white background. Thickness (mm)
*ICE
0.5 0.7 1.0 1.2 1.5 2.0
0.86 0.88 0.90 0.92 0.93 0.95
PRT
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
0.85 0.87 0.90 0.92 0.93 0.95
0.86 0.88 0.91 0.92 0.94 0.96
0.82 0.84 0.85 0.86 0.89 0.91
0.82 0.84 0.86 0.87 0.89 0.91
0.86 0.89 0.92 0.93 0.95 0.97
0.87 0.90 0.92 0.94 0.96 0.97
0.84 0.86 0.88 0.90 0.92 0.94
0.84 0.87 0.89 0.91 0.93 0.94
0.84 0.85 0.87 0.89 0.91 0.93
0.84 0.86 0.88 0.90 0.91 0.93
*ICE: intended for core use only, therefore no polishing was performed.
a
ICE
900
PRT
ZEN
BRX
KAT
b
PRTA
PRT
ZEN
BRX
KAT
PRTA
9
800
8 ENERGY J/cm2
700 MEAN IRRADIANCE mW/cm2
ICE
10
600 500 400 300
7 6 5 4 3
200
2
100
1 0
0 0.0
0.5
1.0 1.5 AVERAGE THICKNESS (mm)
2.0
2.5
0.0
0.5
1.0 1.5 AVERAGE THICKNESS (mm)
2.0
2.5
Fig. 1 – (a) Mean light irradiance for the investigated monolithic zirconia brands at various thicknesses. Red line indicates minimum amount of light irradiance required to initiate polymerization [38]. The irradiance level of the light curing unit through the hallowed discsplastic rings at 0.5, 0.7, 1, 1.2, 1.5 and 2 mm thickness was 1759, 1725, 1710, 1686, 1633 and 1294 mW/cm2 , respectively. (b) Total irradiant energy for the investigated monolithic zirconia brands at various thicknesses and 10 s curing time. Total irradiant energy can be calculated from the result of light irradiance for each specimen multiplied by 10 s by multiplying the irradiance by the curing time. Based on the manufacturer’s curing time for resin-based cements, the energy necessary for polymerization can be determined. (For interpretation of the color information in this figure legend, the reader is referred to the web version of the article.)
4.
Discussion
Based on the results of the current study, it would be reasonable to classify monolithic zirconia as a semi-translucent ceramic. The relative opacity of monolithic zirconia still overcomes the manufacturer’s will and the clinicians demand in delivering a more translucent restoration to match zirconia’s success in physical properties.
There was a significant difference between the brands of zirconia in relation to their optical properties, yet it may be difficult to perceive by the eye. It is worth noting that all of the experimented brands are considered to be partially stabilized zirconia (yttria content between 4 and 6%) with exception to PRTA which is considered a fully stabilized zirconia (yttria content < 12%) containing more cubic form zirconia and hence the better translucency values. Differences in gloss (polishability) and translucency values may closely relate to
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the given zirconia’s grain size, yttria content, and percentage of chemical impurities. A reflection spectrophotometer was used to measure the effect of polishing on the surface gloss of monolithic zirconia. This compact device has a numerical gloss control system, eliminating the need for a mechanical gloss trap. There has been no consensus on the angle at which the light beam should strike the specimen surface [21]. A 60◦ angle has been recommended to measure a medium glossy material and a 20◦ angle was recommended to measure highly glossy surface (ISO 2813 standard) [22]. Barucci-Pfister et al. used a 45◦ angle for specular gloss measurements of polished resin composite specimens which was recommended previously for ceramic materials [21]. The spectrophotometer has integrated spheres that can measure reflected color in both geometries SCE and SCI. These spheres are also designed to keep all the reflected light within the system and usually have a port to control the handling of the surface gloss [24]. If the light trap covers the specular port, surface gloss is absorbed and therefore removed from the measurement. If a diffuse white material covers the specular port, specular gloss is reflected back into the sphere and remains in the measurement [23]. As the surface becomes glossier, more specular light is present at the specular port. Therefore the difference between SCE and SCI reflectance may give an opinion about the surface gloss of the specimen. In the present study, there was a strong correlation between the polishing process and the surface gloss. By polishing the zirconia surfaces, regardless of the brand and thickness, the glossier the specimen became. Therefore, the null hypothesis that the E*SCE − SCI was not influenced by polishing was rejected. It was interesting to note that the thickness of the specimens had no effect on the surface gloss, as gloss represents a surface phenomenon and due to the opaque nature of zirconia material allowing very minimum amount of light to pass through. Although some brands showed more gloss than others, meaning you can get a more polished surface depending on the brand of zirconia used. Polishing is a technique sensitive step relying on many factors such as pressure used, time, grit size, and direction of motion [24]. The more a surface reflects light, the less selective absorption is observed. If surface conditions enhance light reflection, transmission is proportionally reduced [25]. When light reflection is increased, the color of the object tends to be more luminous and of a higher value. In objects with a certain degree of light transmission, reflection of the surface reduces the amount of light that crosses the object [26]. In the present study, the L* value increased for most of the specimens after polishing, supporting the idea that polishing increased the reflected light, thus making it more luminous and lighter value. Also, the translucency of zirconia decreased after polishing, which can be accepted due to the fact that the smoother the surface, the more light is reflected, thus decreasing the amount of light passing through the material. However, other studies support the fact that polishing zirconia surfaces decreases its lightness [27], and the reason for this conflict might be owed to the brand of zirconia used, polishing protocol, and materials used. Zirconia’s opacity can be related to two factors; the particle size of the zirconia (±0.4 m) is larger than the incident wavelength of light (0.1 m) causing the light to scatter thus
decreasing its transmittance, and due to the fact that there is a refractive index mismatch between its particles and the matrix causing more light to be refracted than transmitted [1]. Thus, the less crystalline content and a refractive index close to that of the matrix cause less scattering of light, and the more translucent the material can appear [28]. The crystalline content was not a variable in this study, as all the tested material was zirconia. The thickness of the specimen was considered as a variable to see how it can influence its translucency, and it was also interesting to compare and examine how the translucencies of the different brands relate to one another and most importantly how they relate to the translucency of enamel and dentin. The TP value of human dentin with a thickness of 1.0 mm has been determined to be 16.4 and that of human enamel 18.7 [29]. The TP of specimens tested at 1.0 mm thickness ranged between 11.16 and 15.3, and is still less than the TP of enamel and dentin, giving the conclusion that zirconia needs further improvement to provide a better optical match to natural teeth [30], similarly to glass ceramics (TP values between 14.9 and 19.6 at 1.0 mm thickness). However, as a general finding, the translucency of all the tested specimens decreased as the thickness increased, which agrees with studies on other restorative material as composite resin and glass ceramics [31–33] that the relative translucency is in an exponential relationship to the thickness of the material. So the second null hypothesis for this study, that increasing the zirconia thickness has no effect on its translucency can be rejected. CR is another parameter to measure the relative translucency of ceramics. CR is a ratio of reflectance values, versus TP uses color differences. Both parameters were included in this study (CR and TP) to compare both values to other similar findings in the literature. In a previous study [34], the contrast ratio of the specimens was obtained at thicknesses of 0.7, 1.1, 1.25, and 1.5 mm. They suggested that the contrast ratio of dental ceramics was linearly related to the thickness. It was also interesting to determine the CR values of the specimens, as the perception threshold for translucency has been determined in a study by Liu et al. [14]. They determined that the overall mean translucency perception threshold of all subjects was 0.07, and 50% of the study population perceived a 0.06 CR difference in translucency. Thus, it was possible in the present study to compare, through CR values, what is perceptible to the eye in terms of translucency between different brands and between the thicknesses of the same brand. Accordingly, it can be said that despite the opacity of monolithic zirconia, it does possess some translucency and does allow for some light to pass through. Baldissara et al. [35] compared the translucency of several zirconia copings and demonstrated that they all allowed light to pass through the material to some degree. Even the least translucent had the 42.1% translucency of a controlled glass ceramic. A TP below 2.0 was considered to be opaque enough to block out a black background [15]. Therefore, all brands of zirconia studied in the present study should be considered as having a certain degree of translucency even at the thickness of 2.0 mm. Although there was a statistically significant difference when comparing translucency between the brands, it is most
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probably not perceptible to the eye at the same thickness. It was also important to note, based on Liu et al. study, comparing the translucency of the same brand at two thicknesses, there will have to be at least a 0.6 mm increase in thickness between the two, to achieve the 0.06 difference in translucency. In the present study, zirconia brand, thickness, and their interaction with curing light were evaluated. Considering the use of a single light curing unit with known irradiance, light irradiance, and total irradiant energy varied with different brands and thicknesses. This is an important matter as many dentists use resin based cements for cementing monolithic zirconia crowns. Light transmission through any ceramic can be influenced by a number of factors, including ceramic thickness, shade, and translucency. It can also be influenced by the polymerization type, resin cement composition, curing light’s output, curing duration, and distance [36,37]. As mentioned before, “polymerization lights with an intensity of 300 mW/cm2 appear to effectively polymerize most resin-based composite materials when appropriate polymerization times are used.” [19]. Also, a study by Rueggeberg and Caughman [38], suggested that the adequate irradiance for a light-curing unit is 400 mW/cm2 in order to initiate polymerization of resin-based material, the duration of curing was for 60 s. Light irradiance expresses the amount of light taken up by the material, which is considered of minimal importance to resin polymerization versus the total irradiant energy (defined as the mathematical product of the curing light irradiance (mW/cm2 ) multiplied by the exposure duration in seconds). By knowing the curing time recommended by the manufacturers and the total energy output from the light-curing unit, the amount of irradiant energy needed to polymerize the resin cement can be determined, and any material thickness that does not allow such energy, will affect the degree of polymerization. Pazin et al. [39] showed that the ceramic thickness is a critical factor in developing the hardness in indirectly activated dual-cured resin luting agents. In a study by Peixoto et al. [40], where they evaluated the effect of shade and thickness of porcelain in light transmission, they found that for most shades there was a significant decrease in light transmission as the sample porcelain thickness increased. In the present study, light transmittance through the zirconia specimens proved to be brand and thickness dependent. Most of the specimens, regardless of the brand fell under the 400 mW/cm2 at thicknesses more than 1.65 mm, which raises a doubt for the sufficiency of light irradiance needed to initiate polymerization under the mentioned thickness. BRX showed questionable irradiance levels at thicknesses over 0.7 mm, which can be considered as a serious issue in resin polymerization. It has been reported that the degree of conversion of dual-polymerizing cements was influenced under different brands of monolithic zirconia at variable thicknesses, and increasing the curing time may be beneficial for achieving a more optimal degree of conversion [41]. Based on the present investigation, it is clear that increasing the thickness of the zirconia specimens reduces light penetration. Therefore, the third null hypothesis for this study, where the light transmittance though zirconia is not affected by its thickness can be rejected.
5.
7
Conclusion
Based on the findings of this study, it can be concluded that: 1. Polishing of monolithic zirconia increases its surface gloss. The degree of surface gloss is brand dependent, and thickness shows no effect on the gloss. 2. Different brands of zirconia have different translucencies, which is highly influenced by the thickness of the material. FSZ is relatively more translucent than PSZ. 3. Total irradiant energy and irradiance through zirconia is brand and thickness dependent. Certain thickness of zirconia may have an effect on the polymerization of resinbased cements. 4. The translucency of monolithic zirconia is still far from being considered as an alternative to enamel or even dentin in the esthetic zones.
Acknowledgments We would like to thank Drake Dental Laboratory (Charlotte, NC), Mr. Lee Culp (Microdental Dental Laboratories) and Kuraray Noritake Dental Inc., for their help in providing the zirconia used in this study.
references
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ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 5 ) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Optical properties and light irradiance of monolithic zirconia at variable thicknesses Taiseer A. Sulaiman a,b,c,∗,1 , Aous A. Abdulmajeed a,b , Terrence E. Donovan c , André V. Ritter c , Pekka K. Vallittu b,d , Timo O. Närhi a,b,e , Lippo V. Lassila b,d a
Department of Prosthetic Dentistry, Institute of Dentistry, University of Turku, Turku, Finland Turku Clinical Biomaterials Centre (TCBC), University of Turku, Turku, Finland c Department of Operative Dentistry, School of Dentistry, University of North Carolina, Chapel Hill, NC, USA d Department of Biomaterials Science, Institute of Dentistry, University of Turku, Turku, Finland e Clinic of Oral Diseases, Turku University Central Hospital, Turku, Finland b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The aims of this study were to: (1) estimate the effect of polishing on the surface
Received 28 September 2014
gloss of monolithic zirconia, (2) measure and compare the translucency of monolithic zirco-
Received in revised form
nia at variable thicknesses, and (3) determine the effect of zirconia thickness on irradiance
28 April 2015
and total irradiant energy.
Accepted 25 June 2015
Methods. Four monolithic partially stabilized zirconia (PSZ) brands; Prettau® (PRT, Zirkon-
Available online xxx
zahn), Bruxzir® (BRX, Glidewell), Zenostar® (ZEN, Wieland), Katana® (KAT, Noritake), and one fully stabilized zirconia (FSZ); Prettau Anterior® (PRTA, Zirkonzahn) were used to fabri-
Keywords:
cate specimens (n = 5/subgroup) with different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm).
Monolithic zirconia
Zirconia core material ICE® Zircon (ICE, Zirkonzahn) was used as a control. Surface gloss
Surface gloss
and translucency were evaluated using a reflection spectrophotometer. Irradiance and total
Translucency parameter
irradiant energy transmitted through each specimen was quantified using MARC® Resin
Contrast ratio
Calibrator. All specimens were then subjected to a standardized polishing method and
Light cure irradiance
the surface gloss, translucency, irradiance, and total irradiant energy measurements were repeated. Statistical analysis was performed using two-way ANOVA and post-hoc Tukey’s tests (p < 0.05). Results. Surface gloss was significantly affected by polishing (p < 0.05), regardless of brand and thickness. Translucency values ranged from 5.65 to 20.40 before polishing and 5.10 to 19.95 after polishing. The ranking from least to highest translucent (after polish) was: BRX = ICE = PRT < ZEN < KAT < PRTA (p < 0.05). The ranking from least to highest total irradiant energy was: BRX < PRT < ICE = ZEN < KAT = PRTA (p < 0.05). There was an inverse relationship between translucency, irradiant energy, and thickness of zirconia and the amount was brand dependent (p < 0.05).
∗ Corresponding author at: Department of Prosthetic Dentistry, Institute of Dentistry, University of Turku, Lemminkäisenkatu 2, FI-20520 Turku, Finland. Tel.: +358 2 333 8379; fax: +358 2 333 8390. E-mail addresses: taabab@utu.fi, [email protected] (T.A. Sulaiman). 1 Present address: Department of Operative Dentistry, School of Dentistry, University of North Carolina, 5405F Koury Oral Health Science Building, CB 7450, Chapel Hill, NC 27599, USA. Tel.: +1 919 537 3161.
http://dx.doi.org/10.1016/j.dental.2015.06.016 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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Significance. Brand selection, thickness, and polishing of monolithic zirconia can affect the ultimate clinical outcome of the optical properties of zirconia restorations. FSZ is relatively more polishable and translucent than PSZ. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Monolithic zirconia restorations have been increasingly used in restorative dentistry given their favorable properties, reasonable esthetics, simple clinical technique, and relative low cost compared to cast gold restorations. One notable disadvantage of full contour zirconia restorations, compared to other all-ceramic systems, is their lack of translucency owing to zirconia’s high refractive index mismatch between its particles and the matrix and due to the dispersed particles (slightly greater in size to the wavelength of the incident light) [1]. While improvements in zirconia’s translucency have been claimed by many manufacturers, a better understanding of zirconia’s optical properties, including surface gloss and translucency, can help inform material development and selection for improved esthetic outcomes. Surface gloss may be defined as the degree a surface approaches that of a mirror, and is used primarily as a measure of surface shine [2]. Specular reflection is defined as light reflected from a smooth surface at a defined angle; whereas diffuse reflection is produced by rough surfaces that tend to reflect light in all directions. The rougher the surface the lower the gloss, meaning the specular component is weak and the diffused light is stronger. A spectrophotometer measures the spectral reflectance of the specimen. It can operate two different measuring geometries of specular component excluded (SCE) where it excludes the specular reflectance of light, and specular component included (SCI) where it includes the specular reflectance of light [3]. Surface gloss can be estimated by a spectrophotometer. By lighting two xenon lamps in quick succession, the system can provide virtually simultaneous SCE/SCI measurements and also enable the calculation of 8◦ gloss. The difference between the SCE and SCI reflectance (E*SCE − SCI) gives estimation about the surface gloss [4,5]. Another method has been reported in the literature for measuring the surface gloss performed by using a small-area glossmeter, and the gloss measurements were expressed in Gloss units (GU) [6]. The amount of light that is absorbed, reflected, and transmitted depends on the amount of crystals within the core matrix, their chemical nature, and their size compared to the incident light wavelength [7]. The translucency of dental porcelain is largely dependent on light scattering [8] and thickness [9–11]. If the majority of light passing through a ceramic is intensely scattered and diffusely reflected, the material will appear opaque. If only part of the light is scattered and most is diffusely transmitted, the material will appear translucent [12]. Porcelain translucency is usually expressed by contrast ratio (CR) and/or translucency parameter (TP) [13]. The CR is defined as the ratio of illuminance (Y) of the test material when placed over a black background (Yb ) to the illuminance
of the same material when placed over a white background (Yw ), where 0 is most translucent and 1 is most opaque [14]. The TP is defined as the color difference (E) between a uniform thickness of a material over a white and a black backing [13]. The higher the TP value the more opaque the material. In most studies, the translucency of dental ceramics was mainly studied at a certain thickness, generally, the thinnest recommended by the manufacturers [15]. In clinical situations, ceramic restorations with various thicknesses are required, depending on the different conditions of the tooth to be restored. Therefore, an accurate knowledge of the relationship between the translucency and thickness of restorative materials is fundamental to improving the esthetic outcome of restorations [15]. Zirconia and its opacity at variable thicknesses can have an immense effect on light cure irradiation. Light transmission and curing efficiency of light activated resin luting agents is influenced by the shade and the thickness of the restorative material. Generally, the thicker the restoration or the darker its shade, the more critical the irradiance of the incident light is to achieve optimal photopolymerization of the material [16,17]. Other factors can also affect the degree of cure such as ceramic translucency, resin cement composition, and polymerization type as well as the curing light’s output power, curing duration, and distance [18]. The International Organization for Standardization (ISO) recommends irradiance for polymerization lights of 300 mW/cm2 , and the standard depth-of-polymerization requirement is 1.5 mm [19]. Optimal cure is always critical because inadequately polymerized resin cements are prone to have altered mechanical properties and dimensional stability with decreased bonding to tooth structures resulting in microleakage, decreased biocompatibility, discoloration, and postoperative sensitivity [20]. The aims of this study were to (1) estimate and compare the effect of polishing on the surface gloss of monolithic zirconia, (2) measure and compare the translucency of zirconia at variable thicknesses, and (3) determine the effect of thickness on irradiance and total irradiant energy. The null hypotheses tested for this study were: (1) there is no difference in surface gloss before and after polishing; (2) translucency of the zirconia material is not influenced by the thickness, and (3) light transmittance though zirconia is not affected by its thickness.
2.
Materials and methods
2.1.
Sample preparation
Four brands of monolithic partially stabilized zirconia (PSZ) and a fully stabilized zirconia (FSZ) were studied in this study, and a zirconia core (ICE Zircon) was used as a control
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Table 1 – Monolithic zirconia brands used in the study. Brand name
Code
Partially stabilized zirconia (PSZ) Prettau Zirconia
Manufacturer
PRT
Zirkonzahn, Taufers, Italy
Bruxzir Zirconia
BRX
Wieland Zenostar Translucent
ZEN
Katana High Translucent
KAT
Glidewell Laboratories, Irvine, USA Ivoclar Vivadent, Principality of Liechtenstein Kurary Noritake INC, Noritake, Japan
Fully stabilized zirconia (FSZ) Prettau Anterior
PRTA
Zirkonzahn, Taufers, Italy
<12% Y2 O3 , <1% Al2 O3 , max. 0.02% SiO2 , max. 0.01% Fe2 O3 , max. 0.04% Na2 O
Control (PSZ) ICE Zircon
ICE
Zirkonzahn, Taufers, Italy
4%–6% Y2 O3 , <1% Al2 O3 , max. 0.02% SiO2 , max. 0.01% Fe2 O3 , max. 0.04% Na2 O
(Table 1). Square shaped specimens (10 mm × 10 mm, n = 5/per subgroup) were cut into different thicknesses (0.5, 0.7, 1.0, 1.2, 1.5, and 2.0 mm) using a cutting device (Struers Secotom50, Copenhagen, Denmark). In green stage, each specimen was sequentially ground to the specific thickness using silicon carbide grinding paper (FEPA #1200, 2400, 4000) (Struers LaboPol 21, Struers A/S, Rodovre, Denmark). The final thickness (±0.1 mm) was measured using a digital caliper (Mitutoyo Corporation, Kanagawa, Japan). The specimens were sintered according to the manufacturers’ instructions using the manufacturers’ furnace for each brand. The specimens were cleaned ultrasonically in distilled water for 10 min before testing (Quantrex 90, L&R Ultrasonics Manufacturing Co., Kearny, NJ, USA) then air-dried individually for 20 s. Optical, light cure irradiance and total irradiant energy measurements described following were conducted (baseline measurements). Then one side of each specimen was polished by a single experienced operator (TS) using a straight lab handpiece (K5plus, Kavo, Germany) connected to an electrical control unit (K-control 4960, Kavo, Germany) with diamond polishers (Zircpol Plus and Zircoshine Plus, Diatech, Switzerland) followed by a polishing paste (Zircon-Brite; Dental Ventures of America Inc., Corona, CA, USA) at a constant speed of 10,000 rpm under constant pressure and standard time in a single directed motion, following manufacturers’ instructions. The specimens were not glazed in this experiment. The specimens were ultrasonically cleaned and dried as previously explained. Then the optical, light cure irradiance and total irradiant energy measurements were conducted after polishing.
2.2.
Optical measurements
To estimate the surface gloss (E*SCE − SCI), TP, and CR values of each specimen, a reflection spectrophotometer (CM-700d, Konica Minolta Sensing Inc., Tokyo, Japan) was used according to the CIE 1976 L*a*b* color scale relative to the CIE standard illuminant D65 (as defined by the International Commission on Illumination) which corresponds to “average” daylight (including ultraviolet wavelength region with a correlated color temperature of 6504 K). The SCE and SCI geometries
Composition 4–6% Y2 O3 , <1% Al2 O3 , max. 0.02% SiO2 , max. 0.01% Fe2 O3 , max. 0.04% Na2 O Unknown Unknown (ZrO2 + HfO2 + Y2 O3 ) > 99.0%, yttrium oxide (Y2 O3 ) > 4.5–≤6.0%, hafnium oxide (HfO2 ) ≤5.0%, other oxides ≤1.0%
were determined according to the CIE L*a*b* color scale using standard illuminant D65 over white (CIE L* = 98.1, a* = −0.5 and b* = 2.8) and black (CIE L* = 4.7, a* = −0.1 and b* = 0.0) background. Differences in surface gloss (E*SCE − SCI) values were calculated by the following equation: E ∗ SCE − SCI 2
2 1/2
2
= [(L ∗ SCE − SCI) +(a ∗ SCE − SCI) +(b ∗ SCE − SCI) ]
Calibration of the spectrophotometer was executed before measurement of each specimen. Then the TP of each specimen was obtained by calculating the color difference between the specimen against the white background and against the black background using the following equation: 2
2
2 1/2
TP = [(Lb ∗ −Lw∗) + (ab ∗ −aw∗) + (bb ∗ −bw∗) ]
where L* refers to the lightness, a* to redness to greenness, and b* to yellowness to blueness. The CR values were calculated from the spectral reflectance of light of the specimen (Y) over a black background (Yb ) and over a white background (Yw ), using the following equation: CR =
2.3.
Yb Yw
Light irradiation and transmittance
The specimens were evaluated to determine the amount of light irradiance (i.e. amount of light received by the specimen), and total irradiant energy through each thickness. A LED light-curing unit (Elipar S10, 3 M ESPE, St. Paul, MN, USA) with light irradiance of 1200 mW/cm2 , wavelength range 430–480 nm and curing time 10 s was used. Light energy transmitted through each specimen, was quantified by MARC® Resin Calibrator (BlueLight analytics Inc., Halifax, Canada) whereby each specimen was placed on the “bottom surface” of the resin calibrator. A specially made jig was used to ensure the stability and proper placement of the light-curing unit. To
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determine the actual irradiance of the light-curing unit (without a specimen), plastic rings with a 10 mm diameter opening of the same thicknesses were made to compare the irradiance of the light before and after specimen placement.
2.4.
Results for TP are presented in (Table 3). There was a significant differences in TP (p < 0.05) at different thicknesses, regardless of zirconia brand and polishing process. As the thickness of the specimens increased, the translucency decreased. There was no statistically significant difference among any of the specimens before and after polishing, nor was there any statistically significant difference among the specimens on a black background versus specimens on the white background. Post-hoc Tukey’s test indicated that PRTA was the most translucent. However, there was no significant difference between BRX, ICE, and PRT. Ranking from least translucent to most translucent was: BRX = PRT = ICE < ZEN < KAT < PRTA. Results for CR are present in (Table 4). Likewise, CR of the specimens investigated was brand and thickness dependent. Ranking from least translucent to most was: BRX < ICE = PRT = ZEN < KAT < PRTA (p < 0.05). Results for light cure irradiance and total irradiant energy are shown in Fig. 1a and b. There was a significant difference for the amount of light irradiance and total irradiant energy transmitted through the specimens. The amount of light irradiance and energy decreased as the thickness of the specimen increased (p < 0.05) regardless of the brand. Post-hoc Tukey’s test revealed that BRX showed the lowest values in terms of light irradiance and total irradiant energy, while PRTA and KAT showed the highest values. There was no statistical significance between ICE and ZEN. The ranking for the amount of light irradiance through specimens from least to highest was: BRX < PRT < ICE = ZEN < KAT = PRTA (p < 0.05). As the thickness increases, there was a strong positive Pearson correlation between TP and irradiance (0.973 value), and a strong negative correlation between CR and irradiance (0.884 value) (p < 0.01).
Statistical analysis
The independent variables were zirconia brand and thickness, and dependent variables were TP, CR, and irradiance. Statistical analysis was performed with Statistical Package for the Social Sciences (Version 21.0; SPSS. Inc, Chicago, IL). The distribution of data was normal, which was determined by the Kolmogorov–Smirnov test. Thus, the data were analyzed using two-way analysis of variance (ANOVA) followed by Tukey’s post-hoc test. Differences were considered significant at 95% confidence level. Measurement of correlation between TP, CR, and irradiance was performed using the Pearson Correlation test.
3.
Results
Results of surface gloss (E*SCE − SCI) measurements are presented in Table 2. The E*SCE − SCI revealed that all specimens (placed over a white or black background) regardless of the brand had significantly higher surface gloss after polishing (p < 0.05). Thickness had no influence on surface gloss (p > 0.05). Post-hoc Tukey’s test for (E*SCE − SCI) revealed that PRTA and BRX had the highest surface gloss value, while ZEN had the lowest surface gloss value. Ranking from the lowest to the highest surface gloss value were: ZEN < PRT < KAT < PRTA = BRX (p < 0.05).
Table 2 – Mean surface gloss (E*SCE − SCI) measurements before and after polishing with a white and black background. Thickness (mm)
*ICE
White background PRT
0.5 0.7 1.0 1.2 1.5 2.0
1.50 1.51 1.53 1.51 1.52 1.51
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
1.51 1.53 1.53 1.55 1.53 1.54
6.53 6.61 6.75 6.60 6.64 6.58
1.44 1.49 1.47 1.47 1.49 1.52
7.78 7.79 7.81 7.90 7.92 7.93
1.19 1.14 1.16 1.17 1.16 1.14
9.64 9.64 9.60 9.65 9.66 9.62
1.80 1.78 1.79 1.80 1.79 1.81
6.18 6.15 6.17 6.16 6.16 6.17
1.92 1.94 1.95 1.92 1.95 1.93
7.58 7.59 7.57 7.60 7.61 7.62
Black background PRT
0.5 0.7 1.0 1.2 1.5 2.0
1.60 1.60 1.61 1.60 1.62 1.61
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
1.63 1.66 1.62 1.62 1.63 1.64
8.21 8.58 8.44 8.67 8.43 8.58
1.84 1.83 1.88 1.82 1.89 1.89
10.52 10.45 10.54 10.50 10.51 10.57
1.22 1.26 1.25 1.21 1.25 1.21
10.32 10.32 10.39 10.38 10.38 10.37
1.94 1.91 1.94 1.92 1.95 1.91
7.83 7.80 7.82 7.79 7.80 7.79
2.08 2.10 2.11 2.08 2.11 2.10
9.58 9.59 9.56 9.52 9.55 9.54
*ICE: intended for core use only, therefore no polishing was performed. Regardless of the brand, there was a significantly higher surface gloss after polishing (p < 0.05). Thickness had no influence on surface gloss (p > 0.05).
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Table 3 – Mean translucency parameter (TP) values before and after polishing placed over a white background. Thickness (mm)
*ICE
0.5 0.7 1.0 1.2 1.5 2.0
16.59 14.41 11.47 9.92 8.52 6.66
PRT
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
17.13 15.50 12.46 10.62 8.73 6.38
16.97 14.50 11.16 9.82 7.76 5.61
20.40 17.60 15.82 14.82 12.04 9.74
19.95 17.23 15.03 13.48 11.52 9.17
17.76 15.03 12.32 10.53 8.00 5.65
17.03 14.22 11.58 9.66 7.33 5.10
18.90 15.91 13.95 11.84 9.47 7.46
17.99 15.27 12.98 10.92 8.75 6.83
18.23 16.71 14.51 12.83 10.76 8.50
17.57 15.11 13.42 11.69 9.78 7.78
*ICE: intended for core use only, therefore no polishing was performed.
Table 4 – Mean contrast ratio (CR) values before and after polishing placed over a white background. Thickness (mm)
*ICE
0.5 0.7 1.0 1.2 1.5 2.0
0.86 0.88 0.90 0.92 0.93 0.95
PRT
PRTA
BRX
ZEN
KAT
Before
After
Before
After
Before
After
Before
After
Before
After
0.85 0.87 0.90 0.92 0.93 0.95
0.86 0.88 0.91 0.92 0.94 0.96
0.82 0.84 0.85 0.86 0.89 0.91
0.82 0.84 0.86 0.87 0.89 0.91
0.86 0.89 0.92 0.93 0.95 0.97
0.87 0.90 0.92 0.94 0.96 0.97
0.84 0.86 0.88 0.90 0.92 0.94
0.84 0.87 0.89 0.91 0.93 0.94
0.84 0.85 0.87 0.89 0.91 0.93
0.84 0.86 0.88 0.90 0.91 0.93
*ICE: intended for core use only, therefore no polishing was performed.
a
ICE
900
PRT
ZEN
BRX
KAT
b
PRTA
PRT
ZEN
BRX
KAT
PRTA
9
800
8 ENERGY J/cm2
700 MEAN IRRADIANCE mW/cm2
ICE
10
600 500 400 300
7 6 5 4 3
200
2
100
1 0
0 0.0
0.5
1.0 1.5 AVERAGE THICKNESS (mm)
2.0
2.5
0.0
0.5
1.0 1.5 AVERAGE THICKNESS (mm)
2.0
2.5
Fig. 1 – (a) Mean light irradiance for the investigated monolithic zirconia brands at various thicknesses. Red line indicates minimum amount of light irradiance required to initiate polymerization [38]. The irradiance level of the light curing unit through the hallowed discsplastic rings at 0.5, 0.7, 1, 1.2, 1.5 and 2 mm thickness was 1759, 1725, 1710, 1686, 1633 and 1294 mW/cm2 , respectively. (b) Total irradiant energy for the investigated monolithic zirconia brands at various thicknesses and 10 s curing time. Total irradiant energy can be calculated from the result of light irradiance for each specimen multiplied by 10 s by multiplying the irradiance by the curing time. Based on the manufacturer’s curing time for resin-based cements, the energy necessary for polymerization can be determined. (For interpretation of the color information in this figure legend, the reader is referred to the web version of the article.)
4.
Discussion
Based on the results of the current study, it would be reasonable to classify monolithic zirconia as a semi-translucent ceramic. The relative opacity of monolithic zirconia still overcomes the manufacturer’s will and the clinicians demand in delivering a more translucent restoration to match zirconia’s success in physical properties.
There was a significant difference between the brands of zirconia in relation to their optical properties, yet it may be difficult to perceive by the eye. It is worth noting that all of the experimented brands are considered to be partially stabilized zirconia (yttria content between 4 and 6%) with exception to PRTA which is considered a fully stabilized zirconia (yttria content < 12%) containing more cubic form zirconia and hence the better translucency values. Differences in gloss (polishability) and translucency values may closely relate to
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the given zirconia’s grain size, yttria content, and percentage of chemical impurities. A reflection spectrophotometer was used to measure the effect of polishing on the surface gloss of monolithic zirconia. This compact device has a numerical gloss control system, eliminating the need for a mechanical gloss trap. There has been no consensus on the angle at which the light beam should strike the specimen surface [21]. A 60◦ angle has been recommended to measure a medium glossy material and a 20◦ angle was recommended to measure highly glossy surface (ISO 2813 standard) [22]. Barucci-Pfister et al. used a 45◦ angle for specular gloss measurements of polished resin composite specimens which was recommended previously for ceramic materials [21]. The spectrophotometer has integrated spheres that can measure reflected color in both geometries SCE and SCI. These spheres are also designed to keep all the reflected light within the system and usually have a port to control the handling of the surface gloss [24]. If the light trap covers the specular port, surface gloss is absorbed and therefore removed from the measurement. If a diffuse white material covers the specular port, specular gloss is reflected back into the sphere and remains in the measurement [23]. As the surface becomes glossier, more specular light is present at the specular port. Therefore the difference between SCE and SCI reflectance may give an opinion about the surface gloss of the specimen. In the present study, there was a strong correlation between the polishing process and the surface gloss. By polishing the zirconia surfaces, regardless of the brand and thickness, the glossier the specimen became. Therefore, the null hypothesis that the E*SCE − SCI was not influenced by polishing was rejected. It was interesting to note that the thickness of the specimens had no effect on the surface gloss, as gloss represents a surface phenomenon and due to the opaque nature of zirconia material allowing very minimum amount of light to pass through. Although some brands showed more gloss than others, meaning you can get a more polished surface depending on the brand of zirconia used. Polishing is a technique sensitive step relying on many factors such as pressure used, time, grit size, and direction of motion [24]. The more a surface reflects light, the less selective absorption is observed. If surface conditions enhance light reflection, transmission is proportionally reduced [25]. When light reflection is increased, the color of the object tends to be more luminous and of a higher value. In objects with a certain degree of light transmission, reflection of the surface reduces the amount of light that crosses the object [26]. In the present study, the L* value increased for most of the specimens after polishing, supporting the idea that polishing increased the reflected light, thus making it more luminous and lighter value. Also, the translucency of zirconia decreased after polishing, which can be accepted due to the fact that the smoother the surface, the more light is reflected, thus decreasing the amount of light passing through the material. However, other studies support the fact that polishing zirconia surfaces decreases its lightness [27], and the reason for this conflict might be owed to the brand of zirconia used, polishing protocol, and materials used. Zirconia’s opacity can be related to two factors; the particle size of the zirconia (±0.4 m) is larger than the incident wavelength of light (0.1 m) causing the light to scatter thus
decreasing its transmittance, and due to the fact that there is a refractive index mismatch between its particles and the matrix causing more light to be refracted than transmitted [1]. Thus, the less crystalline content and a refractive index close to that of the matrix cause less scattering of light, and the more translucent the material can appear [28]. The crystalline content was not a variable in this study, as all the tested material was zirconia. The thickness of the specimen was considered as a variable to see how it can influence its translucency, and it was also interesting to compare and examine how the translucencies of the different brands relate to one another and most importantly how they relate to the translucency of enamel and dentin. The TP value of human dentin with a thickness of 1.0 mm has been determined to be 16.4 and that of human enamel 18.7 [29]. The TP of specimens tested at 1.0 mm thickness ranged between 11.16 and 15.3, and is still less than the TP of enamel and dentin, giving the conclusion that zirconia needs further improvement to provide a better optical match to natural teeth [30], similarly to glass ceramics (TP values between 14.9 and 19.6 at 1.0 mm thickness). However, as a general finding, the translucency of all the tested specimens decreased as the thickness increased, which agrees with studies on other restorative material as composite resin and glass ceramics [31–33] that the relative translucency is in an exponential relationship to the thickness of the material. So the second null hypothesis for this study, that increasing the zirconia thickness has no effect on its translucency can be rejected. CR is another parameter to measure the relative translucency of ceramics. CR is a ratio of reflectance values, versus TP uses color differences. Both parameters were included in this study (CR and TP) to compare both values to other similar findings in the literature. In a previous study [34], the contrast ratio of the specimens was obtained at thicknesses of 0.7, 1.1, 1.25, and 1.5 mm. They suggested that the contrast ratio of dental ceramics was linearly related to the thickness. It was also interesting to determine the CR values of the specimens, as the perception threshold for translucency has been determined in a study by Liu et al. [14]. They determined that the overall mean translucency perception threshold of all subjects was 0.07, and 50% of the study population perceived a 0.06 CR difference in translucency. Thus, it was possible in the present study to compare, through CR values, what is perceptible to the eye in terms of translucency between different brands and between the thicknesses of the same brand. Accordingly, it can be said that despite the opacity of monolithic zirconia, it does possess some translucency and does allow for some light to pass through. Baldissara et al. [35] compared the translucency of several zirconia copings and demonstrated that they all allowed light to pass through the material to some degree. Even the least translucent had the 42.1% translucency of a controlled glass ceramic. A TP below 2.0 was considered to be opaque enough to block out a black background [15]. Therefore, all brands of zirconia studied in the present study should be considered as having a certain degree of translucency even at the thickness of 2.0 mm. Although there was a statistically significant difference when comparing translucency between the brands, it is most
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probably not perceptible to the eye at the same thickness. It was also important to note, based on Liu et al. study, comparing the translucency of the same brand at two thicknesses, there will have to be at least a 0.6 mm increase in thickness between the two, to achieve the 0.06 difference in translucency. In the present study, zirconia brand, thickness, and their interaction with curing light were evaluated. Considering the use of a single light curing unit with known irradiance, light irradiance, and total irradiant energy varied with different brands and thicknesses. This is an important matter as many dentists use resin based cements for cementing monolithic zirconia crowns. Light transmission through any ceramic can be influenced by a number of factors, including ceramic thickness, shade, and translucency. It can also be influenced by the polymerization type, resin cement composition, curing light’s output, curing duration, and distance [36,37]. As mentioned before, “polymerization lights with an intensity of 300 mW/cm2 appear to effectively polymerize most resin-based composite materials when appropriate polymerization times are used.” [19]. Also, a study by Rueggeberg and Caughman [38], suggested that the adequate irradiance for a light-curing unit is 400 mW/cm2 in order to initiate polymerization of resin-based material, the duration of curing was for 60 s. Light irradiance expresses the amount of light taken up by the material, which is considered of minimal importance to resin polymerization versus the total irradiant energy (defined as the mathematical product of the curing light irradiance (mW/cm2 ) multiplied by the exposure duration in seconds). By knowing the curing time recommended by the manufacturers and the total energy output from the light-curing unit, the amount of irradiant energy needed to polymerize the resin cement can be determined, and any material thickness that does not allow such energy, will affect the degree of polymerization. Pazin et al. [39] showed that the ceramic thickness is a critical factor in developing the hardness in indirectly activated dual-cured resin luting agents. In a study by Peixoto et al. [40], where they evaluated the effect of shade and thickness of porcelain in light transmission, they found that for most shades there was a significant decrease in light transmission as the sample porcelain thickness increased. In the present study, light transmittance through the zirconia specimens proved to be brand and thickness dependent. Most of the specimens, regardless of the brand fell under the 400 mW/cm2 at thicknesses more than 1.65 mm, which raises a doubt for the sufficiency of light irradiance needed to initiate polymerization under the mentioned thickness. BRX showed questionable irradiance levels at thicknesses over 0.7 mm, which can be considered as a serious issue in resin polymerization. It has been reported that the degree of conversion of dual-polymerizing cements was influenced under different brands of monolithic zirconia at variable thicknesses, and increasing the curing time may be beneficial for achieving a more optimal degree of conversion [41]. Based on the present investigation, it is clear that increasing the thickness of the zirconia specimens reduces light penetration. Therefore, the third null hypothesis for this study, where the light transmittance though zirconia is not affected by its thickness can be rejected.
5.
7
Conclusion
Based on the findings of this study, it can be concluded that: 1. Polishing of monolithic zirconia increases its surface gloss. The degree of surface gloss is brand dependent, and thickness shows no effect on the gloss. 2. Different brands of zirconia have different translucencies, which is highly influenced by the thickness of the material. FSZ is relatively more translucent than PSZ. 3. Total irradiant energy and irradiance through zirconia is brand and thickness dependent. Certain thickness of zirconia may have an effect on the polymerization of resinbased cements. 4. The translucency of monolithic zirconia is still far from being considered as an alternative to enamel or even dentin in the esthetic zones.
Acknowledgments We would like to thank Drake Dental Laboratory (Charlotte, NC), Mr. Lee Culp (Microdental Dental Laboratories) and Kuraray Noritake Dental Inc., for their help in providing the zirconia used in this study.
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Please cite this article in press as: Sulaiman TA, et al. Optical properties and light irradiance of monolithic zirconia at variable thicknesses. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.06.016