1

Introduction

Zirconia ceramic has performed successfully as a framework material for dental restorations over a decade exhibiting good biocompatibility and high mechanical properties . Due to its whitish-opaque appearance, zirconia ceramic would look unnatural for esthetic dental restorations. Recently, improved translucency and various coloring technologies enable the matching of natural tooth color and thereby, zirconia ceramic as a monolithic design has vastly broadened the range of its applications in dentistry.

Zirconia ceramic can be colored by means of applying either a layer of stain or liner over the sintered zirconia surface ; immersion in or painting with the coloring solution in the partially sintered state; or fabricating pre-shaded porous zirconia blanks . Application of a liner material could result in a weak link between the zirconia substrate and the veneering layer . With regard to an infiltration technique, an additional step of dipping or painting is required . Moreover, the resulting color would not be homogenous , and coloring ions only penetrate to a certain depth . Thus, fabrication of pre-shaded zirconia blanks which have more uniform color has been investigated. Several techniques, such as co-precipitation of coloring ions and subsequent calcination , a heterogenous nucleation method , and mixing metal oxides with the zirconia starting powder have been introduced to obtain pre-shaded zirconia ceramics. Furthermore, the homogenous and even multi-colored zirconia blanks could be made through a specific approach in which zirconia powders are coated with coloring substrates . It has been speculated that coloring pigments decreased the flexural strength and fracture toughness of zirconia ceramics. Unlike those studies, Pittayachawan et al. and Sedda et al. reported that coloring procedures did not have any adverse effects on the flexural strength of zirconia ceramics.

Several studies investigated the optical properties of natural dentitions and therefore, the reproduction of color and translucency of natural teeth would be an ultimate goal for esthetic dental restorations. Recent study evaluated the effect of coloring on the optical properties of monolithic zirconia ceramics. Translucency was not affected by the number of coloring. However, the study used a specific coloring technique, such as a brush-infiltration method. Since there have been very few studies regarding the optical properties of pre-shaded monolithic zirconia ceramics, the purposes of this in vitro study were to evaluate the color parameters and translucency of recently marketed pre-colored monolithic zirconia ceramics and to compare with those of veneered zirconia and lithium disilicate glass ceramics. In this study, the evaluation of the optical properties was made with a diffuse-reflected spectrophotometer which has been previously used for color assessments in the dental field . The null hypotheses were that there would be no differences in the color and translucency among pre-colored monolithic zirconia ceramics of different brands and shades, that there would be no differences in the color and translucency among different shades of the same ceramic brand, and that there would be no differences in the color and translucency among different ceramic brands of the same nominal shade.

2

Materials and methods

Four different brands of various shades of pre-colored monolithic zirconia, 5 different shades of veneered zirconia, and 3 different shades with different levels of translucency of lithium disilicate glass ceramics were tested in this study, and un-colored monolithic zirconia served as a control ( Table 1 ). For the tested specimens, nominal Vita A shades from each manufacturer were selected as this group represented common clinical shade selections . The shade B2 veneered zirconia which displayed a yellow shade range was also compared. Square-shaped specimens were prepared (17.0 × 17.0 × 1.5 mm, n = 5) by using a horizontal grinding machine (HRG-150, AM Technology, Asan, Korea). For multi-colored monolithic zirconia blanks, the specimens were cut from both the uppermost and the lowermost layers. For veneered zirconia specimens, 0.5-mm thick un-colored zirconia cores were veneered with 5 different shades of feldspathic porcelain. All specimens were then sequentially polished (coarse, medium-coarse, and super-fine grit, Edenta AG, Hauptstrasse, Switzerland) to diamond paste (LegabrilDiamond, Metalor Dental AG, Biel/Bienne, Switzerland). The final thicknesses of all specimens were set to 1.5 mm and the thicknesses were measured at each of 4 locations by a single operator with a digital caliper (Digimatic micrometer, Mitutoyo, Tokyo, Japan) with a resolution of 0.01 mm. The specimens were cleaned in an ultrasonic bath of isopropyl alcohol for 5 min before the measurements.

Relative spectral reflectance against a white polytetrafluoroethylene (PTFE) background (GM29021020, X-Rite, Melbourne, Australia; CIE L ^* = 93.95, a ^* = − 0.64, and b ^* = 2.04), a black ceramic tile (CM-A101B, Konica Minolta, Tokyo, Japan; CIE L ^* = 0.01, a ^* = − 0.02, and b ^* = − 0.01), and a neutral grey ceramic tile (CM-A101DFG, Konica Minolta, Tokyo, Japan; CIE L ^* = 56.79, a ^* = − 2.25, and b ^* = 3.02) were measured with a spectrophotometer (Color i5, X-Rite, Melbourne, Australia) from 360 to 750 nm at 10-nm intervals. The optical configuration of the instrument was tri-beam diffuse/8-degree and the specular component was excluded. A 10-mm diameter aperture was used, and a 10-mm diameter measurement area was employed. CIELab values relative to standard illuminant D65, with the CIE 1931 2-degree standard colorimeter observer were calculated by the use of a software program (Color iQC, X-Rite, Melbourne, Australia) . Before the color measurements, the calibration of the spectrophotometer was performed. The optical fluid whose refractive index was 1.98 (Series H, Cargille Labs, Cedar Grove, NJ, USA) was placed between each specimen and a background as a coupling medium . For the lithium disilicate glass ceramic specimens, the optical fluid of n = 1.55 (Series A, Cargille Labs, Cedar Grove, NJ, USA) was used. Five repeated measurements per each specimen were carried out and thus, 25 data were collected for each group. For the specimens from multi-colored monolithic zirconia blanks (KML and KMD), top surfaces of the uppermost layers (KMLU and KMDU) and bottom surfaces of the lowermost layers (KMLL and KMDL) were measured.

Average values of L ^* , a ^* , and b ^* against a neutral grey background were used to calculate the CIEDE2000 color difference ( ΔE00ΔE00
Δ E 00
) between monolithic zirconia and the corresponding nominal shade of veneered zirconia or lithium disilicate glass ceramic specimens: ΔE00=ΔE00=
Δ E 00 =
[(ΔL′KLSL)2+(ΔC′KCSC)2+(ΔH′KHSH)2+RT(ΔC′KCSC)(ΔH′KHSH)]1/2
[ ( Δ L ′ K L S L ) 2 + ( Δ C ′ K C S C ) 2 + ( Δ H ′ K H S H ) 2 + R T ( Δ C ′ K C S C ) ( Δ H ′ K H S H ) ] 1 / 2
, where ΔC‘
Δ C ‘
and ΔH‘
Δ H ‘
are the differences in chroma and hue for a pair of specimens. SL
S L
, SC
S C
, and SH
S H
are the weighting functions for the lightness, chroma, and hue and the parametric factors, KL
K L
, KC
K C
, and KH
K H
are the correction terms for variations in experimental conditions. RT
R T
, a rotation function, is applied to account for the interaction between chroma and hue differences in the blue region . The CIEDE2000 color difference ( ΔE00
Δ E 00
) was calculated by using an Excel spreadsheet implementation by Sharma . The parametric factors were set to 1 . The ΔE00
Δ E 00
values were compared with 50:50% perceptibility threshold (PT) and 50:50% acceptability threshold (AT) values reported in the previous studies . The translucency parameter (TP) value of each specimen was obtained by calculating the CIEDE2000 color difference ( ΔE00
Δ E 00
) of the specimen against a white and a black background . The color coordinates and TP values of pre-colored monolithic zirconia ceramics of different brands and shades, of different shades of the same ceramic brand, and of different ceramic brands of shade A2 including KML group (A 1.5–2 intended by the manufacturer) were compared.

To determine the crystalline phases of each monolithic zirconia group, 1 randomly selected specimen from each group was subjected to X-ray diffraction (XRD) analysis by using Cu-Kα radiation at 1.5406 Å (D8 ADVANCE, Bruker, Karlsruhe, Germany) . Scans were performed in the 2-theta range between 20 and 60-degrees at a scan speed of 2-degrees per minute with a step size of 0.02-degrees.

The Shapiro-Wilk test was performed to verify the normal distributions of CIE L ^* , a ^* , b ^* and TP values of each group. To identify the significant differences in the color coordinates and TP values among ceramic specimens of different brands and shades, among different shades of the same ceramic brand, and among different ceramic brands of the corresponding A2-shade, 1-way ANOVA, Scheffé post hoc, and Pearson correlation testing were carried out (α = 0.05). For all of the analyses, the statistical software (IBM SPSS Statistics for Windows, v23.0, IBM Corp., Chicago, IL, USA) was used. Statistical power analyses were applied to assess the results of 1-way ANOVA, Scheffé post hoc, and Pearson testing (G ^* Power version 3.1.9.2, Duesseldorf University, Dusseldorf, Germany) with an α = 0.05.