Effect of transparency and abutment tooth color on the final color of shade-gradient zirconia crowns

Abstract

Purpose: This study aimed to compare the effects of two different color shades (A2, A3) and thicknesses of translucent zirconia materials on translucency and to compare the masking ability of these materials on different colored abutment teeth (dentin, resin, and metal).

Methods: Highly transparent zirconia (ZR Lucent Supra, Shofu) was used to fabricate plate specimens with five different thicknesses (0.5, 1.0, 1.5, 2.0, and 2.5 mm), and monolithic zirconia crowns of the maxillary left central incisor. Three different colored abutment teeth (dentin, resin, and gold-colored) were fabricated. The translucency parameter (TP) and color difference (ΔE₀₀) were calculated from the L*, a*, and b* obtained by measurement. The color difference (ΔE₀₀) between crowns of different abutment tooth colors and reference color specimens was calculated.

Results: The TP values decreased significantly as the thickness of the specimens increased. The ΔE₀₀ was significantly different for gold-colored teeth compared to dentin- and resin-colored teeth (P < 0.05).

Conclusion: The color difference between the reference values was large for highly translucent shade-gradient monolithic zirconia crowns, regardless of the shade, when the abutment-tooth color was gold. The study findings indicate that to achieve a highly esthetic crown, the color of the abutment tooth and the thickness of the crown should be considered.

Introduction

Dental zirconia, a tetragonal zirconia polycrystal (TZP) material that contains 3 mol% (3Y) of yttria, was introduced to clinical dentistry nearly 20 years ago [1]. Due to its high strength, toughness, biocompatibility, and low translucency, dental zirconia is widely used as a framework and for the fabrication of porcelain-layered zirconia crowns. The clinical course of porcelain-layered zirconia crowns is almost the same as that of metal-ceramic crowns over a long-term clinical course of approximately 10 years [2]. However, porcelain-layered zirconia crowns have been clinically reported to undergo fracture and chipping of the layering material [2,3], with a recent increase in need for single-layered monolithic zirconia (MZ) crowns that do not require material layering [4,5,6].

Recently, by adjusting the amount of alumina and yttria added to dental zirconia, using zirconia with appropriate mechanical properties and translucency has become possible. Materials such as 3Y-TZP with improved translucency and partially stabilized zirconia (PSZ) containing 4 mol% (4Y), 5 mol% (5Y), and 6 mol% (6Y) of yttria have been developed to improve esthetics [1]. Furthermore, shade-gradient zirconia crowns with gradient layers of translucent zirconia disks have been developed. Multi-layer types with several layers of pre-shade and mixed-composition types with TZP and PSZ of different compositions are currently commercially available. Because these crowns have a stacked structure from the cervical to the incisal area, the fabricated zirconia crowns have a color closer to that of natural teeth compared to non-shade gradient zirconia materials. However, harmonizing the color with natural teeth can be difficult because of the fixed color-gradient structure. The final finishing method for MZ crowns is polishing or glazing, or glazing after color matching by staining. For using tooth-colored materials with gradient structures, color matching by staining is necessary if it is difficult to match the unique color of the patient's natural teeth.

The color of MZ crowns can be influenced by the manufacturing process, technical procedures, abutment-tooth material, luting agent, and restorative characteristics (translucency, thickness, shade, surface texture, gloss, and contour) [7,8,9,10,11]. Although several studies have examined the color and associated factors of MZ restorations [10,11,12,13,14], establishing clinical practice guidelines for the esthetics of MZ restorations remains difficult due to multiple effectors, diverse research methods, and numerous zirconia products with different optical properties. Dentists can clinically control some factors such as abutment-tooth material, luting agent, and thickness of the restoration. The color of the abutment tooth is related to the material of the abutment tooth, while the thickness of the restoration depends on the preparation of the abutment tooth. Since translucent MZ with high yttria content is a relatively new material, the influence of ceramic thickness and background type on restoration color-matching is insufficient. The color of the background and the thickness of the restoration affect the color of the final crown, and hence, these should be considered. The tabs in the VITA Classical shade guide are not systematically arranged within the color space, and colors are not uniform across the tabs [15]. Although the VITA Classical shade guide represents chroma and hue, the lack of criteria for lightness makes accurate shade selection more difficult than the VITA 3D-Master shade guide. A2 and A3 shades indicate that the A3 shade has more chroma (deeper in color) and less lightness than the A2 shade [15]. However, ascertaining all the colors in the same A3 shade is difficult since different manufacturers produce slightly different shades of colors. For many esthetic restorative materials, although the material colors conform to the VITA Classical shade guide scale, this scale is only a rough approximation to the clinical reality of tooth color [16]. Furthermore, commercially available glass-ceramic shades for computer-aided design/computer-aided manufacturing (CAD-CAM) machining showed color differences exceeding the acceptable threshold between the corresponding nominal VITA Classical shade guide [17].

In clinical practice, zirconia disks of parts that are no longer needed after fabrication of a fixed dental prosthesis are sometimes sintered and used as color samples. Therefore, evaluating the color of zirconia disks and zirconia crowns is considered to be a useful clinical indicator. Furthermore, measuring the final color of crowns considering translucency parameter (TP), L*, a*, and b* values and abutment tooth colors of A2 and A3 shades for specific high translucent zirconia materials, will help dentists to make decisions when selecting abutment-tooth materials and zirconia materials and their colors in their daily clinical practice. In addition, it is a challenge for clinicians and technicians to achieve a color that matches that of the dentition by covering non-vital teeth or metal abutments with restorations made from highly translucent materials [18]. Therefore, because the final color of the restoration is easily influenced by the color of the abutment teeth, the dentist, technician, or patient may not be able to achieve the desired color [18].

The objectives of this study were, first, to compare the effects of two different color shades (A2 and A3) and thicknesses of translucent zirconia materials on translucency, and second, to compare the masking ability of these materials on different colored abutment teeth (dentin, resin, and metal). The null hypotheses of this study were: first, there is no difference in translucency between the materials tested, and second, the thickness of the crown and abutment tooth color does not affect the color of highly translucent shade-gradient MZ crowns.

Table 1 Materials used

Materials	Materials	Product name	Manufacturer	Lot number
Abutment tooth	dentin color	Die Color Wax, dentin	Shofu, Kyoto, Japan	072001
	resin core color	Die Color Wax, resin core	Shofu	072001
	gold core color	Die Color Wax, dentin	Shofu	072001
		Nice Fit, gold	Shofu	022001
Plate and crown	5 mol% yittria-partialy stabilized zircoia	ZR Lucent Supra, A2	Shofu	DBU140F02D
Plate and crown	5 mol% yittria-partialy stabilized zircoia	ZR Lucent Supra, A3	Shofu	DBU140F29C

Fig. 1 Specimens for measuring the TP values

a: A2 shade, b: A3 shade

Fig. 2 Specimens for measuring the L*, a*, b*, and ΔE₀₀ values

a: three types of abutment teeth, b: two types of MZ crowns, c: three measurement regions common to the TP and ΔE₀₀ values

Materials and Methods

Table 1 lists the materials used in the experiments.

Translucency parameter (TP)

A highly translucent shade-gradient zirconia (ZR Lucent Supra, Shofu, Kyoto, Japan) in shades A2 and A3 was used to fabricate 11 mm (H) × 11 mm (W) plate specimens with five different plate thicknesses (0.5, 1.0, 1.5, 2.0, and 2.5 ± 0.01 mm) (Fig. 1). Zirconia was used to fabricate the specimen using a dental CAD-CAM system (Shofu S-Wave CAD-CAM System, Shofu). Specimens were sintered using a sintering furnace (Austromat 674i, Dekema, Freilassing, Germany) according to the manufacturer's instructions with conventional sintering schedules. The plate specimens after sintering were flattened using a surface grinder (PFG500DXAL II, Okamoto, Annaka, Japan) and polishing disk (#325, #800). Subsequently, the specimens were polished with a dental rubber abrasive (Zircoshine HP, Shofu, number of revolutions 20,000 min^-1), followed by a final polishing compound (ZirGloss, Shofu) and polishing brush (Pivot Brush HP, Shofu, number of revolutions 10,000 min^-1). A total of 25 plate-shaped specimens were fabricated, with five specimens for each thickness.

Color of the MZ crowns

A model tooth (A55A-211, Nissin, Kameoka, Japan) used was a preparation of the maxillary left central incisor. The model tooth was prepared using round-end diamond points (102R, 106RD, SF106RD, Shofu) to attain deep chamfer finish lines. Impressions of the modified model tooth were made using a vinyl polysiloxane impression material (Examix fine regular, GC, Tokyo, Japan). Dentin-colored and resin-colored wax (Die Color Wax, Shofu) were injected into the impression to recreate the dentin- and resin-core abutment teeth. Furthermore, as a cast metal core, a gold-colored spacer (Nice Fit, Shofu) was applied to a – dentin-colored abutment tooth to reproduce three types of abutment teeth made of different materials (Fig. 2a).

A dental CAD-CAM system (Shofu S-Wave CAD/CAM system, Shofu) was used to fabricate MZ crowns (ZR Lucent Supra, Shofu). The abutment tooth was attached to the dental model, and three-dimensional shape data of the dental model were obtained using a desktop scanner (Shofu S-Wave scanner D900, Shofu). Crowns were designed using CAD software (Dental Manager, 3 Shape, Copenhagen, Denmark) based on the three-dimensional shape data of the abutment tooth. The standard tessellation language (STL) data for the crown was the same as that used in previous research [10]. The labial thickness of the crown was 0.8 mm in the cervical region, 0.9 mm at the center, and 2.0 mm at the incisal margin, with a cement spacer of 50 µm, according to a previous study [10]. Zirconia disk was milled using a CAM machine (DWX-52DCi, Shofu) and sintered using a conventional sintering schedule (Austromat 674i, Dekema). The specimens were polished with a dental rubber abrasive (Zircoshine HP, Shofu), followed by a final polishing compound (ZirGloss, Shofu) and polishing brush (Pivot Brush HP, Shofu) (Fig. 2b). Eight crown specimens of each shade were used, and a total of 48 crowns were fabricated.

To obtain reference colors for crown measurements, a dental CAD-CAM system was used to create shade-gradient zirconia disks in shades A2 and A3, similar to the zirconia disks used to prepare the plate specimens and crowns. To avoid the color of the background, it was formed into a plate shape with a thickness of 10 mm. These plate-shaped zirconia were used as reference samples for MZ crown measurements.

Measurement of TP values

The TP values were measured using a spectrophotometer (CM-600d, Konica Minolta, Tokyo, Japan) under the following conditions: wavelength, 400-700 nm; bandwidth, 10.0 nm; data interval, 10.0 nm; and a pulsed xenon light source. The specimens were placed on standard white plates (W) (CM-A247, Konica Minolta, L* 98.1, a* −0.5, and b* 2.8) and standard black plates (B) (CM-A260, Konica Minolta, L* 4.7, a* −0.1, and b* 0.0) to acquire the measurements at three locations (cervical, middle, and incisal regions), and the average value was used as the measurement value (Fig. 1a). The translucency parameter (TP) values were calculated from the L*, a*, and b* values of the CIE L*a*b* color system derived using the following equation:

TP = [(L_W*−L_B*)² + (a_W*−a_B*⁾² + (b_W*−b_B*)²]^1/2

where W and B refer to a specimen placed on a black and white background, respectively.

Calculation of color difference

The fabricated MZ crown was placed on the maxillary model with the prepared abutment tooth (Fig. 2a). The model was placed in a dedicated check box, and color measurements were acquired using a non-contact dental spectrophotometer (Crystaleye, Olympus, Tokyo, Japan). Measurements were repeated three times from three separate locations on the labial surface (cervical, middle, and incisal regions), and the average of the three measurements was used as the measurement value on each site. The color difference (ΔE₀₀) between crowns of different abutment tooth colors and reference color specimens was calculated using the following formula.

ΔE₀₀ = {[(L_i−L_j ∕K_LS_L)]² + [(C_i−C_j ∕K_CS_C)]² + [(H_i−H_j ∕K_HS_H)]²

+ R_T [(C_i−C_j ∕ K_CS_C)] × [(H_i−H_j ∕ K_HS_H)]}^1∕2

where L refers to the lightness; C refers to the chroma; H refers to the hue; the subscripts i and j denote values obtained from different periods; S_L, S_C, and S_H represent weighting functions; K_L, K_C, and K_H are parametric factors (set to 1 in the present study); and R_T is the rotation function [19].

Statistical analysis

The Kolmogorov-Smirnov test was primarily used. Bartlett test was used to evaluate the homogeneity of variance. When homoscedasticity was shown in the obtained measured values, Tukey-Kramer honestly significant difference test as a post-hoc test after one-way analysis of variance was used to evaluate the TP, L*, a*, b*, and ΔE₀₀ values obtained, and multiple comparison tests were performed (α = 0.05). If homoscedasticity was not observed, the Kruskal-Wallis test was performed (JMP Pro 16.0.0, SAS, Cary, NC, USA).

Table 2 Reference values of L*, a*, b* of background and zirconia block

	White background	Black background	A2			A3
			cervical	middle	incisal	cervical	middle	incisal
L*	97.01	2.69	72.84	74.61	70.69	69.23	72.96	70.80
a*	−0.80	−0.06	2.66	1.27	−0.41	4.48	2.93	0.97
b*	2.46	−0.51	20.85	19.20	11.46	25.10	22.68	15.34

Fig. 3 Mean values (error bars = standard deviations) of TP for the measurement regions observed in the A2 shade and A3 shade specimens (n = 5)

No significant differences are observed between the same letters. a: A2 shade specimens, b: A3 shade specimens

Fig. 4 Mean values (error bars = standard deviations) of L*, a*, and b* observed in the A2 shade and A3 shade MZ crowns (n = 8)

a, b, c: A2 shade MZ crowns, d, e, f: A3 shade MZ crowns, a, d: L* values, b, e: a* values, c, f: b* values

Results

Table 2 presents the reference values for the black and white backgrounds, L*, a*, and b* of the A2 and A3 blocks.

TP values

Figure 3 presents the TP values for the plate specimens in A2 and A3 shades, which decreased for both shades as the thickness of the specimen increased. A comparison of the TP values for different shades revealed that A2 had slightly larger TP values than A3 for the same thickness and measurement region. Statistical analysis revealed no significant difference in the TP values between the crowns of both shades for the same thickness.

L*, a*, b*, and color difference values

Figure 4 presents a graph of the L*, a*, and b* values in the three measurement regions. The L* values for both shades of crown did not change for the dentin- or resin-core abutment tooth; however, the L* values tended to decrease when the abutment tooth color was gold. The measurements of the gold-abutment tooth in all regions for both shades of crowns were significantly different (P < 0.05) from those of the dentin- and resin-core abutment teeth. The a* values of the A3-shade crowns in all measurement regions were greater than those of the A2-shade crowns. The a* values for both shades of crowns tended to decrease toward the cervical, middle, and incisal regions. The abutment tooth color of the gold-abutment tooth was significantly different (P < 0.05) from those of the dentin- and resin-core abutment teeth, as well as the L* values, in all measurement regions for both shades of crowns. The b* values of the A3-shade crowns were higher than those of the A2-shade crowns in all measurement regions. The b* and a* values of both shades of crowns were lower in the cervical, middle, and incisal regions in that order. The tooth color of the gold-abutment tooth was significantly different (P < 0.05) from that of the dentin- and resin-core abutment teeth, as well as the L* and a* values, in all measurement regions for both shades of crowns.

Figure 5 presents the ΔE₀₀ graphs according to the measurement region. Gold had the highest ΔE₀₀ values for all measurement regions, and the highest values were observed in the middle of the crowns for both shades of crown in the gold-abutment tooth. The abutment tooth color of the gold-abutment teeth was statistically significantly different (P < 0.05) from that of the dentin- and resin-core abutment teeth in all measurement regions for both shades of crowns.

Fig. 5 Mean values (error bars = standard deviations) of ΔE00 between the reference value and final color of MZ crowns due to different abutment tooth colors (n = 8)

a: A2 shade MZ crowns, b: A3 shade MZ crowns

Discussion

The present study evaluated the TP value and final color of shade-gradient MZ crowns using different abutment tooth colors. The hypothesis that the TP, abutment tooth color, and measurement area have no effect on the final color of MZ crowns was rejected.

Flat-plate specimens were used in this study as standardized white and black backgrounds were required for the measurement of TP values. A non-contact dental spectrophotometer, commonly used in clinical practice, was used to measure the final color of the MZ crowns. The abutment tooth and crown seated in the artificial model were measured in a dedicated checkbox to prevent light from entering the device. The non-contact spectrophotometer used in this study uses a 7-band light emitting diode (LED) light source, diffuse reflection type with a 45/0° geometry, and spectral estimation method, and has high reproducibility and standardization of measurement results [20]. The TP values measured by this device were evaluated using the CIE L*a*b* uniform perceptual color space based on the international standard CIE1976 [21] to compare with other previous literature. However, the CIEDE2000 color difference (ΔE₀₀) formula was used to evaluate the color difference. A recent study reported that this new formula provides a better fit than the previously used CIELAB (ΔE_ab) formula, with an improved correlation between visual and calculated color differences [19,20,21,22].

The TP values of A2 and A3 shades were almost equivalent for the same thickness and measurement area in this study. Thicker ceramic specimens resulted in lower TP values, which is consistent with the results of previous studies [14]. The translucency of shade-gradient zirconia crowns decreases as light is physically blocked due to the increase in the thickness of the crown [13]. Therefore, a small change in thickness of the MZ ceramic can result in a large change in translucency. The relationship between the thickness of zirconia and translucency varies with each product; however, this may be difficult to visualize [12]. The TP values for 3Y high translucency zirconia (Lava Plus High Translucency) decrease from 16.5 to 14.0 as the thickness increased from 0.4 mm to 1.0 mm [23]. Furthermore, a reduction in the thickness of conventional 3Y-TZP (BruxZir) as a framework from 2.0 to 1.0 mm results in an increased in TP from 2.27 to 5.34 [13]. The translucency increases significantly when the thickness of 4Y and 5Y translucent zirconia (Katana STML, UTML) decreases from 1.5 mm to 0.8 mm. These specimens showed lower translucency (11.68-15.36) than human enamel, and TP values increased significantly with increasing yttria content [24]. The TP values for human enamel and dentin with a thickness of 1.0 mm are 18.1 and 16.4, respectively [25], suggesting that the 1.0-mm thick zirconia used in this experiment was as transparent as enamel and dentin.

The TP values are closely related to the particle size of zirconia, yttria content, and percentage of chemical impurities [13]. The interaction of light with zirconia particles results in some light being reflected when light hits the surface of zirconia some of the light is reflected; however, most of the light is scattered by grain boundaries and internal defects, whereas the remaining light passes through the pores [26]. The transparency of zirconia increases as the yttria content and particle size increase [24,27,28]. Therefore, the TP value of 1.5 mm 5Y-PSZ used in this study was 11.2-12.3. Although the products were different, they were considered to show the same transparency as the TP value of 12.6 reported for 1.5-mm 5Y-PSZ by Cho et al. [24].

Considering reports that TP values are influenced by a crystal structure [12,24], it seems necessary to include TP values as a factor affecting the final reproduced color of the zirconia crowns [18]. Furthermore, the linear relationship between ceramic thickness and TP values indicates that TP values are a promising guide for material selection [29]. The thickness of the crown depends on the final crown shape and the amount of abutment tooth preparation, both of which are dependent on the condition of the abutment and the adjacent teeth, as well as the dentist's skill and experience. Ideal masking results can be achieved by controlling crown thickness, but the color and material of the abutment tooth will vary with the patient and clinical situation [30]. Because it is difficult to reproduce all crown thicknesses in vitro and achieve the desired crown thickness in vivo, TP may predict clinically esthetic results and serve as a reference value to guide ceramic selection [18].

MZ (5Y-PSZ) has been reported to show higher color variation on a silver-metal background than on copper-colored metal or dark-colored abutment teeth [31]. However, the reports on biocompatible gold-colored cores and 5Y-PSZ color among metal-colored cores are limited. In this experiment, the dentin color was selected assuming a healthy dentin, resin color assuming a resin composite core, and gold alloy color assuming a cast metal core that would avoid metal tattoos and could be applied in the anterior region. The L*, a*, and b* values for the gold abutment tooth deviated from those for the other conditions. Substituting the values for gold abutment tooth into the formula for ΔE₀₀ resulted in an ΔE₀₀ that was significantly larger than that of the other abutment tooth colors in all measurement regions for both shades of crowns. However, no significant difference was observed in the L*, a*, and b* values of the dentin and resin core abutment tooth colors; therefore, the ΔE₀₀ did not differ significantly. A ΔE₀₀ value of <0.8 (50:50% perception threshold) indicates no clinical color difference detectable by the human eye when determining the color difference between all-ceramic crowns and natural teeth [31]. The ΔE₀₀ was calculated for shade-gradient zirconia crowns in the present study, and the value of shade-gradient zirconia blocks of the same material was used as reference; however, a ΔE₀₀ value of <0.8 was not observed for MZ crowns. A ΔE₀₀ value of <1.8 is considered an acceptable color difference (50:50% acceptable threshold) [31], and all dentin and resin core specimens had a ΔE₀₀ of <1.8, except for the middle and incisal regions of the A3 shade resin core crown.

The thickness of the ceramic and background color influence the color compatibility of MZ crowns with medium translucency (CopraSmile). The combination of A2 and A3 shade abutment tooth backgrounds and 1.1-mm thick zirconia ceramics reportedly resulted in acceptable color compatibility [11]. Tabatabaian noted that the abutment tooth became smaller as the thickness of the restoration increased, which may compromise the structure of the abutment tooth. Therefore, similar shades must be selected for abutment teeth and restorations [32]. Other studies reported that the thickness of MZ crowns affects the final color [12,26,33] and that the minimum thickness of zirconia with medium translucency (CopraSmile) to achieve acceptable masking was at least 1.0 mm [9]. Tabatabaian et al. reported that greater thicknesses were necessary to mask darker shades [9] and concluded that for optimal masking, the minimum thickness of conventional 3Y-TZP as a framework should be 0.4 mm for composite resins of A1 and A3.5 shades, 0.6 mm for amalgams, and 0.8 mm for nickel-chromium alloys [9]. For 3Y-TZP (Katana HTML), the masking ability of 1.5-mm thick specimens is better than that of the 0.8-mm thick specimens on a darker shade background (ND9). Optimal masking ability is shown by 0.8-mm thick specimens of 3Y-TZP, 4Y-PSZ (Katana STML), and 5Y-PSZ (Katana UTML) on natural dentin shade background (ND3). In the case of lithium disilicate glass ceramics, a thickness of 1.5 mm has been shown to be insufficient for masking metallic colors (gold-, silver-, copper-, and titanium-colored background) [34,35]. These studies reported that using a more opaque/white luting agent or increasing the thickness of the cement layer could mask the abutment tooth color and modify the final color of the crown. However, the use of opaque luting agents can result in undesirable color results in the final appearance of the ceramic restoration. Instead, Park et al. stated that 0.8-mm thick specimens of 5.5 mol% of yttria (5.5Y)-PSZ and lithium disilicate glass ceramics failed to mask titanium-colored background, while 1.5-mm thick specimens were sufficient to mask titanium-colored background [36]. Therefore, since the zirconia used in this study is 5Y-PSZ, a 1.5-mm thickness may be able to mask the gold-colored background. By choosing the appropriate thickness and appropriate ceramic material, it is considered to be the most appropriate method to mask the background color. A limitation of this in vitro study is that it does not consider the effect of the luting agents on the final color of shade gradient zirconia crowns. Careful selection of luting agents plays an important role in achieving optimal esthetics when using all-ceramic crowns [37]. Opaque luting agents produce lighter colors, whereas transparent luting agents produce darker colors owing to the changes in the L* value [38]. The background effect becomes important as the luting agent becomes more transparent [37]. Malkondu et al. reported that color changes occurred after luting MZ ceramics regardless of the luting agent type [14]. However, the transparency and masking ability of the crown after seating may change depending on the color and transparency of the adhesive luting agent used to seat it. Previous studies have not shown the effect of adhesive resin luting agents on the final color of MZ crowns, so the effect of resin luting agents was not considered in this study [10]. However, it is said that a liquid coupling medium such as glycerin is necessary to avoid the influence of air on optical properties and to minimize light scattering due to the difference in refractive index between air and ceramics [39,40]. Therefore, as light passes through the set 50 µm cement space, any increase in scattered light should be considered to increase the opacity of the resulting color and decrease its translucency. Thus, further studies should be conducted to determine the effect of luting agent type and cement space on the final color of ceramic restorations. In addition, only two shades of zirconia from one brand were evaluated. Hence, different resin luting agents and zirconia brands/shades need to be tested in future research. Objective data obtained using a non-contact dental spectrophotometer or a spectrophotometer could help determine the amount of abutment tooth preparation, crown thickness, and abutment material selection for the color of the abutment tooth in clinical practice.

The TP values for transparency decreased significantly as the thickness of the specimens increased. The color difference between the reference values was large for highly translucent shade-gradient MZ crowns, regardless of the shade, when the abutment-tooth color was gold. Thus, the color of the abutment tooth and the thickness of the crown should be considered when fabricating highly translucent shade-gradient MZ crowns to achieve a highly esthetic final color of the crown.

Abbreviations

CAD-CAM: computer-aided design-computer-aided manufacturing; ΔE₀₀: color difference; LED: light emitting diode; MZ: monolithic zirconia; PSZ: partially stabilized zirconia; STL: standard tessellation language; TP: transparency parameter; TZP: tetragonal zirconia polycrystal; 3Y: 3 mol% of yttria; 4Y: 4 mol% of yttria; 5Y: 5 mol% of yttria; 5.5Y: 5.5 mol% of yttria

Ethical Statements

Not applicable

Conflicts of Interest

Not applicable

Funding

This study was supported by Grants-in-Aid for Scientific Research (C: 21K10026 and C: 22K10107) from the Japan Society for the Promotion of Science and Miyata Research Grant E.

Author Contributions

ST: data curation, methodology, resources, visualization, writing – original draft, writing – review and editing; SM: conceptualization, data curation, formal analysis, investigation, methodology, resources, software, validation, visualization, writing – original draft, writing – review and editing; TF: conceptualization, data curation, methodology, resources, visualization, writing – review and editing; KSM: conceptualization, methodology, resources, visualization, writing – review and editing; YI: conceptualization, formal analysis, methodology, writing – review and editing; KA: conceptualization, methodology, writing – review and editing; MF: conceptualization, formal analysis, funding acquisition, methodology, project administration, supervision, validation, writing – review and editing. All authors read and approved the final version of the manuscript.

ORCID iD

• ST: [e-mail], NA

  SM*: [e-mail], https://orcid.org/0000-0002-5564-9670

  TF: [e-mail], NA

  KSM: [e-mail], https://orcid.org/0000-0003-2959-6840

  YI: [e-mail], https://orcid.org/0000-0003-1545-4120

  KA: [e-mail], https://orcid.org/0009-0007-9918-3829

  MF: [e-mail], https://orcid.org/0000-0001-9507-769X

Acknowledgments

The authors thank Shofu for supporting this study by providing the zirconia ceramics.

Data Availability Statements

All data generated or analyzed during the current study are available from the corresponding author on reasonable request.

References

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