1

Introduction

Glass ceramic systems have been advocated as the restoration of choice because they provide translucency similar to natural tooth [ ]. Despite the esthetic advantage of glass ceramics, the demands for stronger ceramic restorations have increased. As a result, high strength zirconia-based ceramics have broadened the range of their applications in dentistry [ ].

Zirconia ceramic has performed successfully as a framework material for dental restorations [ ]. Recently, monolithic zirconia restorations produced with computer-assisted design and computer-assisted manufacturing (CAD-CAM) technology have become widely used in dentistry because of their favorable mechanical properties, commonly used straightforward clinical preparation technique and the absence of veneering porcelain [ ]. The main disadvantage of anatomic contour zirconia restorations, compared to other ceramic systems, has been their lack of translucency [ ].

Recently, translucent zirconia has been marketed and it was reported that with improved translucency properties and various coloring technologies, these materials have vastly broadened the range of their use in dentistry [ ]. Translucent zirconia can be colored internally by using preshaded porous zirconia blanks [ ] or externally by immersion in [ ] or painting with the coloring solution [ ] before the sintering process. Kim et al. [ ] evaluated the effect of number of coloring liquid applications (6 groups: coloring liquid applied 1–5 times and a control group that had no coloring liquid application) on the optical properties of monolithic zirconia ceramics and reported that translucency of monolithic zirconia restorations was not affected by the number of coloring liquid applications. White and black backings that were not ideal were used to calculate the translucency parameter (TP) in their study [ ]. However, in the literature there is no information about the effect of shading technique (external or pre) on color stability and translucency of translucent zirconia.

Optical properties such as color stability and TP of ceramics are considered to affect the esthetic outcome of the ceramic restorations [ ]. It was reported that the translucency of dental porcelain is largely dependent on light scattering and thickness [ ]. Some studies [ ] investigated the effect of different thicknesses on the translucency of zirconia. They reported that translucency was significantly influenced by the type of ceramic and thickness and an inverse relationship was found between translucency and thickness [ ].

Different thicknesses may be needed for crown restorations depending on the intraoral conditions. Therefore, knowledge of the relationship between the translucency and thickness of restorative materials is essential to improve the esthetic outcome. However, information regarding the effect of different thicknesses on the optical properties of preshaded and externally shaded zirconia is lacking in the literature. The purpose of this study was to investigate the effect of shading technique (preshaded or externally shaded) and thickness on the color stability and relative translucency of translucent zirconia after coffee thermocycling. The first null hypothesis was that the type of shading technique and thickness would not affect the color of a new generation translucent zirconia after coffee thermocycling. The second null hypothesis was that the type of shading technique, thickness and coffee thermocycling would not affect the RTP of new generation translucent zirconia.

2

Materials and methods

Two types of CAD-CAM translucent zirconia materials, a preshaded zirconia (inCoris TZI C; Sirona Dental Systems GmbH, Bensheim, Germany) (Pre) and an externally shaded zirconia (inCoris TZI; Sirona Dental Systems GmbH, Bensheim, Germany) (Ext), were tested (n = 4 per group) for their color stability and relative translucency after thermocycling in coffee ( Table 1 ). The sintering shrinkage of translucent zirconia specimens was calculated per manufacturer recommendations to obtain rectangular plates of 3 different thicknesses (1, 1.5, 2 mm) after sintering process. Rectangular CAD-CAM blanks (20 × 19 × 15.5; L × W × D) were wet-sectioned in calculated thicknesses using a cutting machine (Vari/cut VC-50; Leco Corporation, St Josephs, MI, USA) and a slow-speed diamond-wafering blade (Buehler series 15 LC diamond; Illionis Tool Works Inc, Lake Bluff, Illinois, USA). Then, the specimens were cleaned under running water for 10 s.

Coloring process of Ext specimens was performed using dipping technique according to the manufacturer’s recommendations [ ]. They were predried in a drying cabinet for 10 min at 150 °C before coloring process [ ]. Submersible vessels were filled with the coloring liquid (A3 inCoris TZI coloring liquid; Sirona Dental Systems GmbH, Bensheim, Germany) and the specimens were placed using plastic tweezers. The specimens were immersed in the coloring liquid for 5 min according to manufacturer’s instructions [ ]. Pre and Ext specimens were predried in a drying cabinet for 10 min at 150 °C before sintering process [ ]. Pre and Ext specimens were then sintered in a sintering furnace (Programat S1 1600; Ivoclar Vivadent AG, Liechtenstein, Austria) using long-term sintering according to the manufacturer’s recommended heating rate, holding temperature and time parameters (heating at 25 °C/min to 800 °C, then at 15 °C/min to 1510 °C, dwelling for 120 min, followed by cooling at 30 °C/min down to 200 °C before removing from the furnace) [ ]. The total sintering time was 4 h. The sintered specimens were polished with silicon carbide abrasive papers (600-grit) under running water and the thickness of each specimen was measured with a digital caliper (Model number NB60; Mitutoyo American Corporation, Providence, RI, USA). Pre and Ext specimens were glazed using a thin layer of glaze material (Vita Akzent Plus Glaze powder; VITA Zahnfabrik, Bad Sackingen, Germany). Glazing was performed by the same operator and the glaze firings were performed at 900 °C for 1 min. After glazing, the specimens were stored in distilled water at 37 °C for 24 h.

The baseline spectral radiance (SR) of the zirconia specimens was measured using a non-contact spectral radiance measuring system consisting of SpectraScan PR705 spectroradiometer (Photo Research Inc.; Chatsworth, CA, USA) and a fiber optic light cable (Model 70050; Newport Stratford Inc., Stratford, CT) with a xenon arc lamp (300W; Oriel Instruments, Stratford, CT, USA). This system positioned on an optical table (Mecom Inc.; Rising Sun, OH) to achieve 0-degree observer and 45-degree illuminant angle [ ]. The distance between the zirconia specimens and the lens was fixed to 80 mm and the measuring diameter was 1.1 mm. SR (W/sr/m ² ) was obtained in the visible spectrum from 380 to 780 nm with a 2 nm interval on 3 different backings (black, gray and white) using SpectraWin software (v 2.0; Photo Research Inc., Chatsworth, CA, USA). The optical contacts between these backings and the specimens were obtained through a thin layer of saturated sucrose solution.

A certified white reflectance standard (S3796A; Labsphere Inc., North Sutton, NH) was measured before and after every zirconia specimen color measurement to optimize the accuracy of reflectance measurements. The reflectance calculations of each specimen were performed according to the following formula [ ]:

Spectral reflectance was converted to CIELAB values using Commission Internationale de l’Eclairage (CIE) illuminant A with 2 ° Standard Human Observation and D65 CIE Illumination.

After baseline measurements, the specimens were thermocycled for 10,000 cycles (Buchi 461 Water Bath; Buchi Corporation, New Castle, DE, USA) in the coffee solution (CTC) between 5 °C–55 °C (transfer time of 10 s and a dwell time of 30 s). To prepare the coffee solution (Maxwell House; Kraft, Canada), 1 round tablespoon of coffee was added to 177 mL of water according to the manufacturer’s instructions [ ]. The coffee in hot and cold baths was changed with freshly prepared coffee every 12 h. After 10,000 thermocycling, the specimens were washed under running water and brushed 10 times circumferentially with a toothpaste to remove the coffee extracts from the surface [ ]. Then, the specimens were dried with tissue paper. Reflectance and color measurements of each specimen were performed once again with the same protocol used in baseline measurements.

The color difference of translucent zirconia in different thicknesses between before and after coffee thermocycling was analyzed on the gray backing with the CIEDE2000 (ΔE ₀₀ ) color difference formula [ ]. In present study, the parametric factors of kL, kC, and kH were set to 1 [ ]. CIEDE2000 (ΔE ₀₀ ) values were evaluated in terms of perceptibility and acceptability. Perceptibility refers to detection of color difference between a tooth and adjacent colored restoration. Acceptability refers to the color difference that would be acceptable for that restoration [ ]. 50% perceptibility threshold was set as 0.8 ΔE* ₀₀ units, and 50% acceptability threshold was set as 1.8 ΔE* ₀₀ units according to Paravina et al.’s [ ] study.

An RTP for each translucent zirconia specimen was calculated from the color difference between when the specimen was on the opaque black (L* = 9.1, a* = −0.2, b* = 0.3) backing and when on the opaque white (L* = 93.7, a* = −0.3, b* = 2.9) backing using the following formula [ ]:

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