Effect of coloring pigments on the properties of dental zirconia - A systematic review and meta-analysis

Abstract

Objectives

The effect of pigmentation on color, aging behavior, and mechanical properties of zirconia is debated. The purpose of the paper is to conduct a systematic review and meta-analysis of the studies that evaluated the effect of coloring pigments on the properties of zirconia.

Data Sources

A systematic electronic search was conducted using MEDLINE, Scopus, Cochrane Central, and EBSCO Host. A platform-specific search strategy was employed, using MeSH keywords and text words.

Study Selection

The inclusion criteria were in-vitro experimental studies that synthesized coloring pigments and evaluated their effect on zirconia. Case reports, narrative reviews, and systematic reviews were excluded. Commercially available pre-shaded zirconia, coloring liquids, and staining or glazing systems were not considered. Two investigators independently evaluated the risk of bias using Modified CONSORT. Meta-analysis employed a random-effects model and assessed heterogeneity with the I-square test and continuous data with the inverse variance test.

Conclusions

Transitional metal oxides are used in pre-shaded blocks and rare earth metals in coloring liquids. These pigments can alter grain size and lattice parameters, increase porosity, reduce translucency, improve aging resistance, and affect zirconia’s hardness, fracture toughness, and flexural strength. They demonstrated no effect on gingival fibroblast proliferation, indicating their cytocompatibility.

Introduction

In recent years, there has been a noticeable shift towards utilizing tooth-colored all-ceramic restorations, such as zirconia and glass-ceramics. Zirconia is known for its high fracture toughness and biocompatibility. Efforts were made to enhance its optical properties through three generations of development.^1–3 The initial generation, was the opaque 3 mol. % yttria-stabilized zirconia (YSZ). Subsequent second generation, employed 3 mol. % YSZ with decreased alumina content at grain boundaries, followed by the third generation that utilized 5 mol.% YSZ as a fully-stabilized cubic zirconia. The latest development, the fourth generation, involves 4 mol. % YSZ.^1–3

Despite these strides in refining the composition and microstructure of zirconia for high translucent YSZs, achieving a close resemblance to natural teeth remains a challenge.^1–4 Consequently, pigmentation techniques have been advocated to optimize aesthetic outcomes in zirconia restorations.^5–7 These involve incorporating metallic pigments into the zirconia powder during block formation, applying coloring liquids on pre-sintered zirconia, or staining and glazing of sintered zirconia.^5,8–10

The color in zirconia essentially comes from the electronic transition process of the coloring ions in the pigments that selectively absorb and reflect under visible light.¹¹ In addition to affecting optical properties, these coloring procedures alter the surface characteristics with reported high surface roughness, affect microstructure with changes in grain size with enhanced aging resistance, and further influence the mechanical characteristics of zirconia.^12–18 Few studies reported pigmenting zirconia does not impact the mechanical properties of YSZ.^{5,8–10,14,15} Conversely, other studies have indicated a potential negative impact on flexural strength and hardness due to an increase in porosity and grain size.^16–21 These coloring pigments can act as dopants within the zirconia grain boundaries or lattice during high-temperature sintering, potentially impacting its mechanical properties.²² Such reduced mechanical properties of colored zirconia can adversely affect its long-term performance. Given the conflicting literature, it is important to conduct a more comprehensive investigation into the relationship between optical, mechanical, and related properties of colored zirconia.

The purpose of the paper is to conduct a systematic review and meta-analysis of the studies that evaluated the effect of coloring pigments on the properties of dental zirconia. This will help in understanding how these pigments impact the physical, mechanical, optical, and biological properties of zirconia and can help in identifying which pigments can provide desired aesthetic outcomes while maintaining the properties of zirconia.

Methodology

Following the PRISMA 2020 checklist and PICO protocol, a research question was formulated: “What is the effect of coloring pigments on the properties of dental zirconia?” Here, dental zirconia was the population (P), the pigmentation technique served as the intervention (I), uncolored or unpigmented zirconia represented the control (C), and the outcomes (O) focused on the properties of dental zirconia.²³

Study characteristics for inclusion and exclusion criteria were defined, based on the PICO components, study design, type of dental material used, relevance to the topic, and conflict of interest with commercial products. Thereby, the inclusion criteria for study designs consisted of in vitro studies and experimental studies that synthesized and characterized the coloring pigments and evaluated their effect on the properties of zirconia. These studies must have evaluated properties ranging from microstructure, phase transformation, optical properties such as translucency, color and contrast ratio, surface characteristics such as surface roughness and wear, and mechanical properties such as hardness, fracture toughness, flexural strength and fatigue including biological properties such as cytocompatibility, cell viability, and cytotoxicity. Studies on all four generations of zirconia were included. The exclusion criteria were case reports, case series, letters to the editor, commentaries, narrative reviews, as well as systematic reviews. Studies that were not related to pigmentation, and those not related to dentistry, were excluded. Studies on dental materials other than the generations of dental zirconia were not considered. Studies on commercially available products of pre-shaded zirconia, coloring liquids, and staining or glazing systems were excluded, as the composition was not disclosed in most studies and to prevent conflict of interest.

A systematic electronic search was conducted across five databases, such as MEDLINE (PubMed), Scopus, Cochrane, EBSCO Host, and Google Scholar without date restrictions (last updated on January 21, 2025). The search was restricted to articles published in English. The strategy utilized MeSH terms and specific free-text terms from PubMed, which were tailored for compatibility with the other databases. Table 1 presents the MeSH terms and Boolean operators used for each database. Additionally, a manual review of grey literature sources, such as Shodhganga and Open Gray, along with reference lists from systematic and narrative reviews on pigmented or colored dental zirconia, was performed to identify studies not captured in the electronic search. The selection of databases was guided by the resources and accessibility available at our institution. To minimize search bias, a logic grid based on the PICO framework and Boolean operators was employed.

Table 1. Mesh terms and Boolean operators.

Database	Mesh Terms
Pubmed	#1	((((((((pigmentation) OR (prosthesis coloring)) OR (coloring agent)) OR (staining)) OR (shaded)) OR (pre-shaded)) OR (colored)) OR (shading technique)) OR (staining technique)
	#2	((dental zirconia) OR (zirconium oxide)) OR (yttria-stabilized tetragonal zirconia)) OR (zirconia)
	#3	((((translucency) OR (color)) OR (opacity)) OR (color parameters)) OR (optical properties)
	#4	((((((((((mechanical properties) OR (hardness)) OR (flexural strength)) OR (bond strength)) OR (fracture toughness)) OR (fracture load)) OR (fracture resistance)) OR (fatigue)) OR (cyclic fatigue)) OR (wear)) OR (wear resistance)
	#5	(((#1) AND (#2)) AND (#3)) AND (#4)
	#6	((#1) AND (#2)) AND (#3)
	#7	((#1) AND (#2)) AND (#4)
	#8	((#1) AND (#2)) AND (#5)
	#9	((#6) OR (#7)) OR (#8)
EBSCO HOST	S1	(pigmentation) OR (prosthesis AND coloring)) OR (coloring AND agent)) OR (staining)) OR (shaded)) OR (pre-shaded)) OR (colored)) OR (shading AND technique)) OR (staining AND technique)
	S2	dental zirconia OR zirconium oxide OR yttria stabilized tetragonal zirconia OR zirconia
	S3	Translucency OR color OR opacity OR color parameters OR optical properties
	S4	mechanical properties OR hardness OR flexural strength OR bond strength OR fracture resistance OR fatigue OR cyclic fatigue OR wear OR wear resistance OR fracture load OR fracture toughness
	S5	#S1AND #S2 AND #S3 AND #S4
	S6	S1 AND S2 AND S3
	S7	S1 AND S2 AND S4
	S8	S5 OR S6 OR S7
Scopus	#1	(pigmentation) OR (prosthesis coloring)) OR (coloring agent)) OR (staining)) OR (shaded)) OR (pre-shaded)) OR (colored)) OR (shading technique)) OR (staining technique
	#2	((dental AND zirconia) OR (zirconium AND oxide)) OR (yttria AND stabilized AND tetragonal AND zirconia)) OR (zirconia)
	#3	translucency OR color OR opacity OR color AND parameters OR optical AND properties
	#4	(mechanical AND properties) OR (hardness)) OR (flexural strength)) OR (bond AND strength)) OR (fracture AND toughness)) OR (fracture AND load)) OR (fracture AND resistance)) OR (fatigue)) OR (cyclic AND fatigue)) OR (wear)) OR (wear AND resistance)
	#5	#1AND #2AND #3AND #4
	#6	#1 AND #2 AND #3
	#7	#1 AND #2 AND #4
	#8	#5 OR #6 OR #7
Cochrane		dental zirconia OR zirconium oxide OR yttria stabilized tetragonal OR zirconia in Title Abstract Keyword AND fracture load OR fracture resistance OR fatigue OR cyclic fatigue OR wear resistance in Title Abstract Keyword AND mechanical properties OR hardness OR flexural strength OR bond strength OR fracture toughness in Title Abstract Keyword AND translucency OR color OR opacity OR color parameters OR optical properties in Title Abstract Keyword AND pre-shaded OR colored OR shading technique OR staining technique OR pigmentation OR prosthesis coloring OR coloring agent OR staining OR shaded in Title Abstract Keyword (Word variations have been searched)

All titles and abstracts were subject to the inclusion criteria of in vitro and experimental studies, studies published in English, and peer-reviewed indexed journals. Titles, abstracts, and keywords were initially screened based on inclusion and exclusion criteria, followed by a full-text evaluation by the authors (SG and ZP). Agreement between the authors was assessed using kappa statistics. The authors (SG and ZP) retrieved and individually reviewed the complete texts of relevant studies. Data from these chosen studies were subsequently extracted by the authors independently and were further agreed upon through discussions. In disagreements, a third reviewer was consulted for consensus (AK). Data extracted included authors, publication year, country, pigments used, zirconia type, results, and evaluated properties. The risk of bias in selected studies was assessed using a modified CONSORT checklist. Their quality was independently evaluated by the authors (SG and ZP).²⁴ Studies with a low to medium risk of bias and those that provided precise mean and standard deviation values along with the sample size were included in the meta-analysis. The mean difference was employed as the effect measure to evaluate the outcomes related to the properties of zirconia. Meta-analysis was conducted using JBI SUMARI (University of Adelaide, Australia), which employed a random-effects model for study variability and was used for assessing heterogeneity with the I-square test and continuous data with the inverse variance test.²⁵

Results

The study selection process is illustrated as a PRISMA flowchart in Figure 1. From the aggregated searches, a total of 1362 studies were identified. After removing duplicates and irrelevant studies, 174 studies were sought for retrieval. Ten studies were not retrievable due to a lack of access to the Chinese journal databases. Further, about 137 studies were excluded as 66 studies were systematic and narrative reviews, and 71 were deemed irrelevant to the topic, including studies that focused on commercial products, those that did not disclose the composition of coloring pigments, studies that were not related to pigmentation, for example, studies that used borosilicate glass for enhancing mechanical properties than pigmenting zirconia, and studies that did not report the outcome measures. Following the above exclusion criteria, the full texts of the remaining articles were thoroughly reviewed, resulting in 27 articles meeting the inclusion criteria.

Figure 1. PRISMA 2020 flow diagram of the search.

A significant agreement was observed between the two reviewers (SG and ZP) for articles screened through titles and abstracts and for those selected based on the full text (Kappa=0.9, p<0.0001). Data were classified into pigmentation techniques of pre-shaded zirconia and coloring liquids and are schematically illustrated in Figure 2. Most studies on pre-colored zirconia were from China, while studies on coloring liquids were primarily from Korea. Data extraction from each category are presented in Table 2 and Table 3. Of the 27 studies, 14 focused on pre-shaded zirconia, incorporating metal oxide powders into zirconia, and the other 13 studies utilized an infiltration technique, applying coloring liquids to pre-sintered zirconia.

Figure 2. Schematic illustration of the pigments used for coloring zirconia.

Table 2. Data extraction table of pre-shaded pigmented zirconia.

S.no	Author year	Country	Aim of the study	Pigments used	Pigmentation technique	Type of zirconia used	Results	Properties evaluated
1.	Wen 200838	China	To examine the mechanical properties and color of zirconia ceramics tinted with CeO2 and Er2O3.	CeO2 and Er2O3	CeO2 was added in concentrations of 1wt% to 4wt%, and Er2O3 at 0.2wt% and 0.6wt%	YSZ powders were isostatically pressed at a pressure of 200 MPa and sintered at a temperature of 1500°C for 2 h.	The color changed from yellow-green to yellow-red as the concentrations of CeO2 and Er2O3 rose. There was also a modest decrease in brightness (L value). Following coloring, the three-point bending strength decreased to 845 MPa, which is less than the 1301 MPa seen in uncolored zirconia. The fracture toughness also somewhat decreased. Both additives were found to have a more porous microstructure, according to SEM measurements.	Color CIE Lab coordinates Three-point bending strength Fracture toughness Microstructure
2.	Wen 201029	China	To study the effect of Fe2O3 and Bi2O3 on the color properties of dental zirconia	Fe2O3 and Bi2O3	Fe2O3 was added in 0.03 wt% intervals in the range of 0.03 to 0.15 wt,%, Bi2O3 was added in 0.05 wt,% intervals from 0.05 to 0.2 wt.%. The combination of two pigments, Fe2O3 was added in 0.03 wt.% intervals in the range of 0.06 to 0.15 wt.%, at each level, Bi2O3 was added in 0.05 wt.% intervals from 0.05 to 0.2 wt.%	YSZ powders were subject to cold isostatic pressing at 200 MPa and were subsequently sintered at 1500ºC for 2 h.	As the levels of Fe2O3 and Bi2O3 increased, the color shifted from yellow-green to yellow-red. Using both Fe2O3 and Bi2O3 together resulted in a further decrease in the L value, creating a color spectrum suitable for dental prosthetics.	Color CIE Lab coordinates Phase changes
3	Zhao 201326	Sweden China	To examine the feasibility of bicolored zirconia and assess its impact on mechanical properties.	Fe2O3	A white body and yellow as a bicolored block resulting from doping with a small amount of Fe2O3 (0.202 wt%) with a color gradient	Two commercial YSZ powders and YSZ-Yellow were subjected to layer-wise compaction using a cold isostatic press and were sintered at 900°C for milling and at 1450°C for 2 h.	The color gradient zone in the bicolored zirconia was relatively weak due to its heterogeneous microstructure, caused by granule packing defects and localized enrichment of Fe2O3.	Hardness Bending strength Density Microstructure Fractographic analysis
4	Chen 201430	China	To evaluate the color of zirconia ceramic incorporated with two types of rare earth oxides.	Fe2O3	Fe2O3 was added at 5 concentrations 0.03wt%, 0.06wt%, 0.09wt%, 0.12wt% 0.15wt% and CeO2 was added in 7 concentrations 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%.	YSZ powders were cold isostatically pressed at 200 MPa and sintered at 1500°C for 2 h.	The addition of cerium oxides resulted in a lightness of 85, with a slight decrease in the a value, shifting the color towards a more yellow-green hue. As the iron oxide content increased, the chroma intensified and the lightness decreased, causing the color to shift towards a red-yellow direction.	Color (CIE Lab coordinates) Phase changes Microstructure
5	Kaya 201331	China Sweden	To produce self-colored dental zirconia and characterize its microhardness, fracture toughness, color difference, microstructure, and phase formation.	Fe2O3	Fe2O3 content of 0.2 wt.% as coloring agent 0.029 and 0.143 wt,%	3 mol.% YSZ and colored zirconia with 0.2 wt.% of iron oxide were used. Proportions of 3 mol.% YSZ with colored zirconia (10-30-50-70%) were ball-milled and were uniaxially pressed at 300 MPa and sintered to 800º C for 1.5 h at a rate of 1ºC/min for debinding, and at 1100°C at a rate of 10ºC/min for 2 h for pre-sintering, and to 1500°C at a rate of 10ºC/min, for 2 h for sintering.	The presence of Fe2O3 did not impact the formation of the tetragonal zirconia phase. Although there was an increase in the density and microhardness of the pre-sintered zirconia, no significant changes were observed in the sintered zirconia. As the Fe2O3 content increased, the color of the samples shifted from white to yellowish-brown.	Micro-hardness, Fracture toughness, Color difference, Microstructure and Phase formation, bulk density and porosity
6.	Li 201439	China	To evaluate the influence of Pr6O11 and Er2O3 on the microstructure and mechanical properties of zirconia.	Pr6O11 and Er2O3	Concentrations of 0.001wt.%-0.01wt.% Pr6O11 and 0.04wt.%-0.8wt.% Er2O3 were used.	YSZ powders were ball milled for 24 h. After drying, the mixtures were uniaxially pressed at 200 MPa for 3 min and sintered at 1500°C for 2 h, with a heating and cooling rate of 5°C/min.	Phase composition and microstructure analysis indicated the presence of the tetragonal phase, with grain sizes ranging from 50 to 100 nm. The relative density was over 99.1%, and the linear shrinkage was nearly 20%. Vickers hardness exceeded 12.5 GPa. Although the three-point bending strength slightly decreased compared to the pure samples, it remained above 850 MPa, and the fracture toughness was measured to be over 4.9 MPa•m^1/2.	Phase changes Microstructure Vickers hardness Three point bending strength Linear shrinkage Fracture toughness
7.	Chen 201537	China	To assess the color of zirconia ceramic with metal oxides.	Fe2O3	Fe2O3 was added in five concentrations (0.03, 0.06,0.09, 0.12, and 0.15 wt%), CeO2 was added in seven concentrations (1, 1.5, 2, 2.5, 3, 3.5, and 4 wt%), and Bi2O3 was added in four concentrations (0.05, 0.1, 0.15, and 0.2 wt.%).	YSZ powders were cold isostatically pressed at 200 MPa and sintered at 1500°C for 2 h.	With CeO2 addition, zirconia’s lightness ranged from 84 to 87, with a slight decrease in a* value, shifting towards yellow-green. Increasing Fe2O3 intensified chroma, decreased lightness, shifting towards red-yellow. Higher Bi2O3 concentration shifted color to light saffron yellow. Tetragonal phase was identified. These findings suggest effective zirconia ceramic coloring with Fe2O3, CeO2, and Bi2O3 oxides.	Phase changes Microstructure Color (CIE Lab coordinates)
8.	Li Jiang 201542	China	To evaluate the effect of Fe2O3 on optical properties of zirconia	Fe2O3	Fe2O3 was used in concentrations of 0 wt.%, 0.02 wt.%, 0.04 wt.%, 0.06wt.%, 0.08 wt.% and 0.1 wt.%	YSZ powders and colored YSZ containing 0.102 wt.% Fe2O3, were used. Fe2O3 (0, 0.02, 0.04, 0.06, 0.08, and 0.1 wt.%), were pressed and sintered at 1450°C. There was no mention of the pressure applied.	Small additions of colorants impacted color and transmittance but not density and microstructure. With rising Fe2O3 content, L value decreased from 88.95 to 84.18, while b value increased from 3.43 to 12.02. Relative densities stayed at 99%, and SEM analysis revealed tetragonal retention in zirconia with Fe2O3 additions.	Sintering density Relative density Direct transmittance Color CIELab coordinates
9.	Kao 201733	Taiwan	To evaluate the effect of iron oxide on the sintering behavior of YSZ.	Fe2O3	Iron nitrate powder was dissolved into ethyl alcohol and further mixed with the YSZ powder by ball milling. The concentration of Fe2O3 was from 0 to 3.2 wt% and the amount of Fe ions in the YSZ was from 0 to 3 mol.%.	YSZ powders were ball-milled and dried at 100°C for 24 h and sieved.	Adding iron nitrate to YSZ powder enables a consistent color range spanning from different shades of yellow to red. This approach offers a broad spectrum of colors, catering to diverse needs in dental applications.	Phase changes Microstructure Color CIE Lab coordinates Hardness Toughness
10.	Takashi 201735	Japan	To evaluate fluorescence and properties of thulium-doped zirconia	Tm2O3	Thulium (Tm2O3) powder was added at different concentrations (0, 0.5, 0.8, 1.0, 1.2, and 1.5 wt%)	YSZ powders and two translucent dental zirconia (Zpex and Zpex-Smile) were wet ball-milled and dry-milled. The mixtures were cold isostatically pressed at 200 MPa for 5 min and sintered at 1500°C for 2 h. The heating and cooling rates were 10°C/min.	Zirconia doped with Tm2O3 exhibited blue fluorescence, peaking at 460 nm. Both Zpex and Zpex-Smile showed higher fluorescence intensity compared to YSZ, with Zpex displaying stronger fluorescence than Zpex-Smile. The maximum fluorescence intensity for Zpex was observed with 0.8 wt% Tm2O3 doping. Phase analysis indicated that YSZ and Zpex consisted mainly of tetragonal zirconia, while Zpex-Smile predominantly contained cubic phase zirconia. No changes were observed in the microstructure or physical properties of the zirconia specimens upon Tm2O3 doping.	Microstructure Flourescence Translucency Fracture toughness Hardness
11.	Willems 201934	Belgium	To assess how Fe2O3 affects the microstructure, mechanical and optical properties, and aging kinetics of zirconia ceramics.	Fe2O3	Various amounts of iron oxide in the 0.01–2 mol% range were added to the Zpex powder and ball-milled for 24 h in ethanol.	Zpex powder was doped with Fe2O3 The powders were debinded, dried, and granulated through sieving. Different amounts of iron oxide in the range of 0.01–2 mol.% were ball-milled and cold isostatically pressed at 300 MPa, followed by sintering at 1450°C for 2 h.	Elevated levels of Fe2O3 doping (0.5–2 mol.%) not only resulted in an inappropriate color for dental restorations but also induced a significant rise in the formation of larger-grained cubic phases and residual porosity. Consequently, this contributed to a decrease in density and mechanical properties.	Microstructure, mechanical and optical properties, and hydrothermal stability
12.	Alves 202027	Brazil	To assess how coloring pigments affect the translucency of dental zirconia with varying Fe2O3 content.	Fe2O3	Six sequential layers of ZrO2 with increasing Fe2O3 contents of 0 wt%, 0.027 wt%, 0.054 wt%, 0.082 wt%, 0.109 wt% and 0.136 wt% were processed as cylindrical blocks of 15 X 6 mm.	Commercial Zpex and Zpex Yellow powders were used. A Fe2O3 content of 0.136 wt% were added to the commercial powders, uniaxially pressed at 100 MPa for 60 seconds.	Varied amounts of Fe2O3 did not affect the densification, phase composition, or microstructure of zirconia. Fracture toughness and flexural strength remained stable with the Fe2O3 addition, of 7 MPa•m^1/2 and 1120–1150 MPa, across all layers. Increased Fe2O3 content led to higher hardness (1280–1330 HV) and contrast ratios (CR), resulting in decreased translucency. Additionally, color variation (ΔE) was influenced by thickness.	Relative density Phase assemblage Microstructure Hardness Translucency
13.	Takashi 202136	Japan	To assess the fluorescence and properties of thulium and erbium co-doped dental zirconia	Tm2O3 and Er2O3	Tm2O3 powder 0.8 wt% and Er2O3 powder at proportions from 0.1 to 0.8 wt% were used.	High-translucency yttria-stabilized dental zirconia powder was used. After ball-milling, the powders were cold isostatically pressed at 200 MPa for 5 min. They were then sintered at 300°C for 30 min and at 1500°C for 2 h with a heating rate of 10°C/min.	Zirconia co-doped with 0.8 wt% Tm2O3 powder and 0.3 to 0.5 wt% Er2O3 exhibited fluorescence akin to natural teeth. Phase analysis showed a tetragonal peak, with no significant changes in Vickers hardness, fracture toughness, or bending strength after adding Tm2O3 and Er2O3. Co-doping trace amounts of Tm2O3 and Er2O3 into high-translucency dental zirconia powder effectively enhances the aesthetic appeal of monolithic zirconia.	Fluorescence Translucency Vickers hardness Fracture toughness Bending strength
14.	Holz 201828	Portugal	To investigate the impact of Fe2O3, doping on color and mechanical characteristics.	Fe2O3	Wt.% of 0.1, 0.2 and 0.4 of α-Fe2O3 powder (Iron oxide III, Sigma-Aldrich, 99.99 %) was used.	Powders of Y-TZP were synthesized using the emulsion detonation method and mixed with Fe2O3 content. The mixture suspensions were milled, spray-dried, and uniaxially pressed at 94 MPa.	Iron oxide doping effectively imparted color to zirconia and demonstrated stability across varying temperature conditions. Phase analysis revealed an increase in unit cell volume with higher iron oxide content. While hardness and flexural strength were unaffected, a reduction in fracture toughness was observed.	Phase analysis Color evaluation Biaxial flexural strength Vickers Hardness Fracture toughness

Table 3. Data extraction table of pre-sintering pigmentation using coloring liquids.

S,no	Author year	Country	Aim of the study	Type of pigments used	Pigmentation technique used	Type of zirconia used	Results	Properties evaluated
1.	Shah 20089	USA	To assess the impact of cerium and bismuth coloring solutions on the microstructure, color, flexural strength, and aging resistance of dental zirconia.	Cerium chloride, cerium acetate and bismuth chloride were dissolved in distilled water at concentrations of 1, 5, and 10 wt %.	Samples were immersed for 30 min under consistent stirring in a closed container to prevent evaporation.	Commercial YSZ powders were subject to cold isostatic pressing to obtain green compacts.	The flexural strength decreased linearly as the concentration of cerium salts increased, ranging from 860.7 ± 172 MPa to 274.4 ± 67.3 MPa. The process of coloring did not influence the material’s resistance to degradation at low temperatures. At the lowest concentrations, cerium and bismuth salts caused noticeable color changes.	Flexural strength Aging resistance Microstructure
2.	Oh 201248	Korea	To assess the impact of transition metal dopants on the color and biaxial flexural strength of zirconia.	Aqueous metal chloride solutions containing chromium, molybdenum, or vanadium dopants were utilized.	Chromium trichloride hexahydrate, molybdenum trichloride, and vanadium trichloride were dissolved in distilled water at concentrations of 0.1, 0.3, 0.6, 1.1, 2.3, and 4.5 wt% They were immersed for 3 min (n=57). After drying, the specimens were sintered at 1450 °C in a furnace.	The zirconia samples were processed from a commercial YSZ.	Chromium and molybdenum oxides can be used for coloring zirconia.	Biaxial flexural strength and color
3.	Camposilvan 2012 53	Spain	To improve low temperature degradation resistance of zirconia through cerium infiltration without affecting its mechanical properties.	Cerium solutions were prepared. There was no mention of concentration.	Pre-sintered rods were soaked in a solution containing Cerium for 2 hours to ensure its infiltration in the open porosity and sintered at 1450 ºC for 2 h with a heating/cooling rate of 3 ºC/min.	YSZ powders were compacted by cold isostatic pressing (CIP) at 200 MPa for 10 minutes. The compacts were pre-sintered in the air for one hour in the furnace. The heating and cooling rates were 3 ºC/min.	Ceria was successfully incorporated into zirconia. The ceria content was higher in the superficial regions compared to the bulk, peaking at 4 mol.%. The microstructure and mechanical properties were not significantly impacted. Ceria ceramics effectively reduced low-temperature degradation (LTD) after artificial aging.	Aging sensitivity Density Porosity Fracture toughness Threelow-temperature point bending strength test
4.	Oh 201350	Korea	To assess the impact of transition metal dopants on the mechanical properties and biocompatibility of zirconia.	Metal chlorides (chromium, molybdenum and vanadium) were used to make aqueous metal chloride solutions. The solutions were prepared in concentrations of 0.3 and 4.5 wt.% for each dopant.	Pre-sintered zirconia specimens were immersed in aqueous metal chloride solutions and sintered at 1450 C for 2 h	Commercially available ZrO2 nanopowder was used.	Transition metal dopants slightly impaired the mechanical properties of zirconia. However, the addition of metal oxides did not affect the adhesion and proliferation of gingival fibroblasts.	Biaxial flexural strength
5.	Oh 2015 58	Korea	To assess the impact of metal chloride infiltration on the color and strength of zirconia.	Metal chloride solutions of 0.03-0.08 wt.% chromium and 0.03-0.07 wt.% terbium ions were prepared.	Pre-sintered YSZ were immersed in metal chloride solutions for 3 min.	YSZ was used.	The infiltration of chromium and terbium chloride solutions resulted in colors matching Vita shade guide A1, A2, and A3. The density of zirconia increased with the infiltration of these solutions. However, the bi-axial flexural strength of the sintered YSZ did not show statistically significant differences with this infiltration. Phase analysis indicated a tetragonal phase.	Density Color Flexural strength
6.	Camposilvan 2015 54	Spain	To investigate superficial defects, cracks, and pores after roughening and assess their ability to retain infiltrate..	A solution of cerium nitrate hexahydrate in distilled water was prepared.	After drying, the zirconia samples were placed inside a deformable mold. The mold was then closed and inserted into an isostatic press, applying a pressure of 50 MPa for 10 min.	YSZ spray-dried powders were isostatically pressed into cylindrical rods at 200 MPa for 10 minutes. The green body was then sintered in air at 1450°C in a tubular furnace, with a heating/cooling ramp of 31°C/min and a dwell time of 2 hours.	Aging resistance of cerium co-doped zirconia was enhanced.	Surface roughness Aging resistance
7.	Camposilvan 2015 55	Spain	To assess a technique for preventing hydrothermal degradation of zirconia.	Cerium nitrate hexahydrate solutions were prepared at concentrations of 50 wt.% and 75 wt.% in ethanol.	Pre-sintered zirconia blanks were soaked in the infiltrating solutions for 2 h. The solvent was evaporated in an oven at 60°C and sintered with heating/cooling rates of 3°C and a dwell time of 2 h at 1450°C.	YSZ spray-dried powders were isostatically pressed into cylindrical rods at 200 MPa for 10 minutes. The green body was then pre-sintered in air in a tubular furnace with heating/cooling ramps of 3°C/min and dwell times of one hour at 700°C (to remove processing additives) and at the final pre-sintering temperature.	An optimal compromise in properties was achieved for cerium-infiltrated zirconia. As the the sintering temperature increased, the fracture toughness improved with reduced strength and aging resistance.	Aging resistance Density Microstructure Hardness Fracture toughness Biaxial flexural strength
8.	Kim 201551	Korea	To assess and compare the shear bond strength (SBS) between resin cement and colored zirconia treated with metal chloride lining.	A 0.1 wt.% aqueous chromium chloride solution and 0.1 wt.% aqueous molybdenum chloride solution were used.	The samples were immersed in the prepared solutions for 3 min and dried.	5.4 wt.% YSZ was used. Resin cement and zirconia liner were used.	The coloring liquid improved the shear bond strength (SBS) between resin cement and zirconia treated with zirconia primer. Colored zirconia immersed in molybdenum chloride solution showed the highest SBS.	Shear bond strength between resin cement and zirconia.
9..	Kaplan 2017 49	Turkey Korea	To explore the coloring effects of molybdenum chloride and nickel chloride solutions on zirconia.	The solutions containing coloring ions were prepared by dissolving the required amount of analytically pure chloride powders (NiCl2 and MoCl3) in distilled water to achieve concentrations of 0.1, 0.25, and 0.5 wt%..	Pre-sintered samples were submerged in color solutions of varying concentrations for 5, 30, and 60 seconds.	YSZ powders with an average particle size of 0.04 μm were uniaxially pressed under a pressure of 25 MPa to form disc-shaped compacts. These compacts were then isostatically pressed at 100 MPa pressure in a cold isostatic press and pre-sintered at 950°C for 1 hour. The heating rate applied during pre-sintering was 3°C/min, and the cooling rate was 5°C/min.	The color produced depends on the type and concentration of the colorant solution, while the immersion time did not have any significant effect on coloring. Coloring solutions containing 0.1 and 0.25 wt% MoCl3 provided clinically adequate color, with the ΔE* value ranging from 5.16 to 6.42.	Phase changes Hardness Fracture toughness Surface roughness Wear
10.	Agingu 2018 13	China Japan	To examine the impact of coloring on the aging characteristics of dental zirconia.	The coloring liquid included Er(NO3)3 at a concentration of 0.30 mole/liter solvent, Pr(NO3)3 at 0.01 mole/liter solvent, Ce(NO3)3 at 0.10 mole/liter solvent, and Nd(NO3)3 at 0.10 mole/liter solvent.	Zirconia samples were dipped with plastic tweezers for 2 min at room temperature and dried with blotting paper.	Translucent YSZ was used.	The combined infiltration did not significantly affect the flexural strength of zirconia. Additionally, coloring reduced the impact of aging on the phase transformation of zirconia.	Aging resistance Flexural strength
11.	Jing 2019 59	China	To evaluate the effect of colouring on zirconia ceramics.	The dyeing solution, Er2O3, and Pr6O11 were calcined in a resistance furnace at 1150 K for 10,800 seconds to remove crystalline water from the raw materials. Following this, Er2O3 and Pr6O11 with various accurate weights were added into diluted nitric acid and heated to 360 K in a water bath.	The zirconia samples were soaked in the dye solution for 60 s, dried with filter paper and sintered for 10,800 s pressureless at 1700 K	YSZ was prepared by conventional ball-milling.	Praseodymium and erbium oxides showed yellow and red color respectively. The values of a and b of the mixed color are added up and the value of L increases.	Color CIE Lab Phase changes
12.	Periera 2020 56	Brazil Portugal France	To explore methods for producing thin Ce-TZP and Y-PSZ protective coatings aimed at enhancing low-temperature degradation resistance in zirconia.	12 mol.% cerium stabilized zirconia powder was used.	Double action pressing and dip-coating methods were used.	Commercial YSZ was used.	Suspensions containing a solid content of 23 wt.% and prepared with 12 mol.% ceria-stabilized zirconia exhibited coarsening porosity. These coatings improved low-temperature resistance in zirconia.	Color CIE Lab Aging resistance
13.	Kaplan 2021 57	Turkey	To investigate the effects of MoCl3 and NiCl2, on the cellular response of zirconia.	Molybdenum chloride and nickel chloride solutions were used for coloring. Predetermined amounts of MoCl2 and NiCl2-MoCl3 powders were dissolved in distilled water to prepare the coloring solutions	The zirconia samples were immersed in 0.1 wt % molybdenum chloride and 0.25 wt % combination of nickel chloride and molybdenum chloride solutions for 5 s. The infiltrated samples were at 100°C for drying and sintered at 1450°C for 2 h.	Tooth-colored zirconia were cold isostatically pressed at 100 MPa followed by pressureless sintering at 1450°C for 2 h.	Cell viability and proliferation studies revealed the absence of cytotoxicity of colored zirconia. Enhancement in cell attachment, adhesion and proliferation were observed.	Phase changes MTT assay

The risk of bias assessment for each of the selected studies is presented in Table 4. In pre-shaded dental zirconia, two studies were classified as a low risk of bias, while the rest were classified as medium risk. In coloring liquids, 5 studies were classified as low risk of bias and the rest of them as medium risk. Most studies were identified as medium risk, as they did not have a structured abstract, and had no explanation of the sample size determination, statistical analysis, and study limitations. Approximately 62% of pre-shading studies utilized iron oxide, while the remaining studies employed a combination of oxides. Additionally, 31% of the studies used cerium-based coloring liquids, whereas the others relied on transition metal oxides. Furthermore, 71% of the investigations primarily focused on 3 mol% yttria-stabilized zirconia (YSZ), with the rest investigating 5 mol% YSZ, Zpex, and Zpex Smile.

Table 4. Risk of bias assessment of selected studies of pre-shaded and infiltration techniques using coloring liquids using modified CONSORT checklist.

*Studies reporting only one or two items encoded as “No” were classified as having a low risk of bias, three to five items as a medium risk of bias, and more than five items as a high risk of bias.

Studies on pre-shaded technique
Author year	Abstract	Introduction		Methods				Results	Discussion	Funding	Overall risk of bias
	Structured	Back ground and rationale	Specific objectives and hypothesis	Intervention how and when	Outcome measures	Sample size	Statistical methods	Effect size and precision	Limitation/bias/imprecision/analysis
Wen 2008 38	No	Yes	Yes	Yes	Yes	No	No	Yes	No	No	Medium
Wen 201029	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Zhao 2013 26	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	No	Medium
Chen 2014 30	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Kaya 2013 31	No	Yes	Yes	Yes	Yes	No	No	Yes	No	No	Medium
Li 2014 39	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Chen 2015 37	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	Medium
Li Jiang 2015 42	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	No	No	Medium
Kao 2017 33	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Takashi 2017 35	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	Medium
Williems 2019 34	No	Yes	Yes	Yes	Yes	No	No	Yes	Yes	Yes	Medium
Alves 2020 27	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Takashi 2021 36	No	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Low
Holz 201828	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Low
Shah 2008 9	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	No	Medium
Oh 2012 48	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	Low
Camposilvan 2012 53	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Oh 2013 50	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	Medium
Oh 2015 58	No	Yes	Yes	Yes	Yes	No	Yes	Yes	No	No	Medium
Kim 2015 51	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	No	Low
Camposilvan 2015 54	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Camposilvan 2015 55	No	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Low
Kaplan 2017 57	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Agingu 2018 13	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Low
Jing 2019 59	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Periera 2020 56	No	Yes	Yes	Yes	Yes	No	No	Yes	No	Yes	Medium
Kaplan 2021 57	Yes	Yes	Yes	Yes	Yes	No	Yes	Yes	No	Yes	Low

There were a limited number of studies with similar outcomes and interventions in pre-shaded and coloring liquids. Meta-analysis could only be performed for pre-shaded zirconia, for properties of biaxial flexural strength and hardness. The meta-analysis included studies comparing the pre-shaded intervention group to the control group on the outcome of biaxial flexural strength and hardness.^26,27,28 The standardized mean difference (SMD) was 0.27 (95% CI: -0.24 to 0.79), with a p-value of 0.294. Heterogeneity among the studies was low (I² = 0). This result suggests a small, non-significant positive effect of the pre-shaded intervention on biaxial flexural strength compared to the control. The standardized mean difference was -0.55 (95% CI: -1.03 to -0.07), with a p-value of 0.024. Heterogeneity among the studies was low (I2 = 0). This result indicates a moderate, statistically significant reduction in hardness in the pre-shaded intervention group compared to the control group. The forest plot on biaxial strength of pre-shaded and control groups is presented in Figure 3 and that of hardness in Figure 4.

Figure 3. Forest plot of flexural strength of pre-shaded pigmented zirconia and control groups.

Figure 4. Forest plot of the hardness of pre-shaded pigmented zirconia and control groups.

Discussion

Commonly utilized pigments were transition metal ions like iron oxide, chromium, and bismuth oxides, as well as rare earth metals such as cerium, praseodymium, erbium, and thulium oxide. Transition metal ions, found in the d-block of the periodic table, exhibit different oxidation states due to their partially filled d orbitals, resulting in vibrant compounds. On the other hand, the rare earth series or lanthanides consists of 15 elements with atomic numbers 57 to 71, and are known for their unique properties.

Pre-shaded pigmented zirconia

Zirconia powders are doped with metal oxides and formed into green compacts through cold isostatic pressing. These green compacts are further sintered at recommended temperatures. Pre-shaded pigmented zirconia offers the benefits of excellent color stability, ease of use, and predictability, with no apparent impact on the mechanical characteristics of zirconia. Most studies investigated the addition of iron oxide to zirconia.^29–34 Other studies used thulium for fluorescence and combinations of cerium oxide with bismuth oxides, cerium oxide with erbium oxide, and praseodymium.^35–39

Iron oxide

The effect of iron oxide on color, hardness, flexural strength, fracture toughness, density, porosity, phase changes, and microstructure are discussed in the following sections. Most studies processed iron oxide-doped zirconia as a single sintered sample, another study approached a four-layered sample in varying concentrations of iron oxide to reproduce the color gradient in natural teeth and another produced a bi-layered bar with a color gradient between white zirconia and iron oxide colored zirconia.^{26, 27}

Increasing iron oxide concentration in zirconia reduces lightness (L) and increases a and b values, shifting color towards yellow-red.^{29–31,33,37,39} Iron oxide levels below 1 mol.% yields L values close to 60, a values close to 10, and b values close to 20, simulating natural teeth.^33,40 However, iron oxide increases porosity, and reduces translucency by 7-8% resulting in a darker color and a higher contrast ratio of zirconia.^{27,34,41–43} This is because substituting Fe+3 for Zr+4 generates oxygen vacancies, forming color centers that absorb light, which further reduces translucency and raises the contrast ratio.^34,43,44 Variations in color and transparency in layered iron oxide-doped zirconia can mimic shade transitions in natural teeth.²⁷

Adding iron oxide to zirconia doesn’t significantly affect its sintered hardness, but pre-sintered iron-doped zirconia showed higher hardness.^26,37 This increased hardness can accelerate tool wear during milling, thus reducing milling efficiency.³¹ In the bi-layered bar, the gradient zone between iron oxide and pure zirconia exhibited decreased hardness due to interior defects and porosities. Iron oxide-doped zirconia’s hardness decreases with its content above 0.5 mol.%.^34,44,45 In case of layered iron-oxide doped zirconia, the difference in hardness across the layers was attributed to iron oxide distribution in each layer.²⁶

Iron oxide-doped zirconia was observed to have decreased bending strength. This decreased bending strength was seen in iron oxide-doped admixed ceramics and was attributed to an increase in critical flaw size and large defects on the fractured surface.³⁷ The admixing procedure involved de-binding the commercial zirconia powders (for homogenous distribution) before adding iron oxide, followed by powder compaction without binder addition. This contrasts with the co-precipitated binder-containing commercial zirconia powders. Avoiding the de-binding step before compaction could have minimized the micro defects and improved the bending strength.³⁴ There was no noticeable difference in the bending strength values in layered iron oxide-doped zirconia.²⁷ The bending strength of the bi-layered zirconia bars was lower than that of the uncolored zirconia due to the defects at the color gradient.²⁶

The effect of iron oxide on the fracture toughness of zirconia is dependent on the content of iron oxide, the amount of tetragonal, monoclinic, and cubic phases, their phase transformability, and the grain size of zirconia. A typical 3 mol.% yttria-stabilized zirconia has 15 vol.% of cubic phase, with a grain size of 200 nm, and exhibits a fracture toughness value of 3.8 MPa.m^1/2. The effect of iron oxide on fracture toughness has not been significant till 0.143 wt.% as it did not affect its microstructure and grain size.³¹ Commercial zirconia such as undoped Zpex, Zpex-Yellow, and zirconia with 0.01-0.1 mol.% iron oxide, have a similar cubic phase content of 13-15 vol.%, with tetragonal phase and a grain size of 400 nm, exhibiting a higher fracture toughness of 4.5 MPa.m^1/2.³⁴ However, adding iron oxide beyond 1 mol. % increases monoclinic and cubic content with a reduced tetragonal phase and grain size, resulting in reduced fracture toughness.³³ The increased cubic phase depletes the yttria as a stabilizer and transforms to monoclinic, resulting in microcracks.³⁴ High fracture toughness of 7 MPa.m^1/2 was observed in layered iron oxide.²⁷

The addition of iron oxide till 0.143 wt. % increased the density of pre-sintered zirconia.³¹ Adding small amounts of iron oxide (0.02-0.1 wt. %) did not affect sintering and relative density.³⁹ In the layered iron oxide-doped zirconia, the density values were similar across the layers.²⁷ In the bi-layered zirconia, the density across the color gradient showed low values due to the defects across the junction.²⁶ Iron oxide-doped zirconia were completely sintered without any residual porosity or exaggerated grain growth.^31,46,47 In layered iron-doped zirconia, the residual porosity was 0.7% which can influence its optical properties.²⁷ The addition of iron oxide (up to 1 mol. %) promoted the sintering behavior of zirconia, with lowered temperatures required to initiate sintering and achieve maximum shrinkage rate and density.^26,33 The tetragonal phase was retained in almost all the studies. With an addition of 2 mol. %, there was reduced tetragonal, with an increase in cubic and monoclinic phases.^30,34

Combination oxides

The addition of bismuth oxide resulted in zirconia exhibiting a light saffron yellow color with a retained tetragonal phase, unaffected grain size, and absence of porosity.²⁹ Combinations of erbium oxide and cerium oxide contributed to yellow-green and yellow-red colors, respectively, with slight changes in color values.³⁷ Cerium oxide alone reduced bending strength and fracture toughness, while erbium oxide improved bending strength without affecting hardness.³⁵ Combined use of these colorants led to decreased lightness, increased a and b values, and darker color. Erbium oxide and praseodymium oxide maintained the tetragonal phase with grain sizes of about 0.5-1 µm and no porosity. Both additives did not affect density, shrinkage, or hardness, but reduced bending strength and fracture toughness, with higher deviations observed with erbium oxide.³⁹

Thulium and erbium oxide

Dental zirconia ceramics typically lack fluorescence, but thulium and erbium oxides are used to induce fluorescence.^35,36 Thulium oxide concentrations (0.8 wt.%) effectively produced blue fluorescence in studies on commercial zirconia such as Zpex, and Zpex-Smile, with a peak wavelength of 460 nm. However, fluorescence decreased at 1.5 wt.% due to concentration quenching, where increased thulium oxide led to energy transfer reduction. Zpex-Smile exhibited higher fluorescence intensity compared to Zpex due to its translucency and higher yttria content, facilitating UV energy transfer. While there was no influence on the tetragonal phase structure, Zpex-Smile displayed larger grain sizes and reduced fracture toughness with no change in hardness.^35,36 Co-doping with thulium oxide and erbium oxide showed blue-cyan fluorescence at 546 and 562 nm, unlike the only blue fluorescence at 460 nm of thulium oxide and green fluorescence of erbium oxide. Zirconia co-doped with thulium oxide and erbium oxide had fluorescence close to that of a natural tooth and was lower than those co-doped with each of those elements, due to the energy transfer between thulium and erbium. There was no effect on the translucency.^35,36

The tetragonal phase was retained with no change in grain size. However, crystal grain size increased in the samples containing higher amounts of thulium oxide and erbium oxide. Thulium and erbium oxide with a similar ionic radius to yttrium, form a solid solution and have a stabilizing effect on zirconia in the form of cubic crystals with a lower value of c/a lattice parameters. A reduced fracture toughness with no difference in the hardness and bending strength was observed.³⁶

Pre-sintering pigmentation using coloring liquids

Pre-shaded pigmented zirconia have problems in controlling the powder form, size, and quantity of pigments and their distribution in the zirconia powders.^48,49 The infiltration technique using coloring liquids is faster and easier and has a close resemblance to the tooth, with less impact on mechanical properties.^50–52 Furthermore, this technique affects the microstructural and crystallographic changes in zirconia, it has the potential to be explored for improving the aging resistance of zirconia.⁵

Coloring liquids

Infiltration techniques commonly used for coloring dental zirconia in the pre-sintered stage include rare earth elements such as cerium, and transitional elements such as molybdenum, vanadium, nickel, chromium, bismuth, erbium, and praseodymium. These metal ions are often in the form of chlorides, nitrates, or acetates, dissolved in distilled water or acetone at various concentrations. Chlorides, being highly soluble, decompose thermally in an atmospheric environment. The choice of solvent depends on pigment solubility. Immersion time (30 min to 2h), drying method, and temperature varied across studies.⁹

Cerium infiltration (rare earth element)

Cerium infiltration in zirconia led to increased grain size and uniform open porosity, three times higher than control zirconia.^9,53–55 Pressure-infiltrated cerium on zirconia displayed a 2-micron cerium-enriched layer at the superficial grain layer. This increase in grain size was attributed to reduced yttria drag, allowing cerium to diffuse into tetragonal grains.⁵⁵ Cerium dip-coated zirconia showed a larger cubic grain layer on the surface.⁵⁸ Tetragonal zirconia exhibited slight increases in lattice parameters, with cerium chloride showing the largest increase. Delta E values were significantly higher with cerium infiltration, yielding deep cream colors with increased concentration. Pale yellow colors were observed at low pre-sintering temperatures, with no color change at higher temperatures. Cerium dip-coating produced a white color with some translucency and high yellowness, varying with layer thickness.⁵⁶

Zirconia is prone to aging as oxygen vacancies fill with water in the oral cavity, leading to monoclinic phase peaks in control zirconia. Comparatively, cerium-infiltrated ceramics showed minimal monoclinic peaks (5%), indicating improved aging resistance. Cerium acts as an additional stabilizer, reducing oxygen vacancies through yttria and ceria diffusion into cubic and tetragonal grains. Infiltration on sintered zirconia, regardless of sandblasting or etching, enhances aging resistance. Cerium acetate infiltration exhibited higher flexural strength compared to cerium chloride, although an increase in cerium concentration led to decreased flexural strength with both salts.⁹ This decrease was attributed to observed increases in grain size and porosity in cerium-infiltrated zirconia. ⁹

Cerium infiltration did not affect fracture toughness values.⁵³ Hardness remained unchanged in aged thermally treated cerium-infiltrated sintered zirconia.⁵⁵ Density at the pre-sintered state showed 51.7% theoretical density with an apparent porosity of 47.2%. After sintering, the density reached 6.08 ± 0.02 g/cm³, close to the theoretical value of control zirconia.⁵³ Pre-sintering temperatures resulted in different densities and varying porosities, with porosity absent at 1300°C pre-sintering. Sandblasted infiltrated zirconia displayed reduced surface roughness compared to etched infiltrated zirconia.⁵⁴

Transitional metal infiltration

Bismuth infiltration deepens the orange color in zirconia by altering b values and slightly increasing lattice parameters of the tetragonal phase. Porosity increased significantly, due to bismuth’s low melting point. Higher bismuth chloride concentrations raised Delta E values and the density of zirconia due to bismuth’s higher atomic weight. Monoclinic content and flexural strength remain unchanged, indicating bismuth stabilizes the material and increases grain size without compromising strength.⁹

Chromium-infiltrated zirconia retains its tetragonal phase, with minor unit cell changes. While biaxial flexural strength decreases, it remains suitable for core ceramics, and higher chromium concentrations result in a redder hue.^48–51 Molybdenum infiltration produces a yellowish color and can replicate natural tooth color with short immersion times.⁵⁷ Despite lower strength at higher concentrations, properties remain acceptable for core ceramics without affecting gingival fibroblasts.^48–51 Vanadium chloride increases yellow color but reduces strength, with low fracture toughness.^48,50,58 Furthermore, these transitional dopants did not affect the proliferation of gingival fibroblasts, thereby indicating their cytocompatibility.⁵⁰ Nickel and molybdenum infiltration results in higher L values, specific b values, and noticeable color differences, with a rougher surface morphology that enhances cell attachment.⁵⁷ Increasing chromium decreases L and increases a and b values, while terbium has the opposite effect.⁵⁷ Combined erbium and praseodymium infiltration affects phase transformation and coloring, with erbium producing red hues and praseodymium yellow.^13,59 The meta-analysis of the studies on the pre-shaded technique reveals a non-significant impact on the flexural strength of pre-shaded zirconia, accompanied by a significant reduction in its hardness compared to the control.^26–28 Since pigmentation techniques have been found to reduce the mechanical properties of zirconia, future research could explore alternative methods to enhance its mechanical properties while incorporating pigmentation.

Conclusions

1. Iron oxide-doping in zirconia decreases lightness and increases chroma towards yellow-red, reducing translucency and increasing the contrast ratio. Hardness remains unchanged until 0.2 wt.%, but higher doping levels can wear out milling tools for pre-sintered zirconia. Iron oxide affects grain size and phase content, impacting fracture toughness. Higher iron oxide content increases density, reduces porosity, and aids sintering by requiring lower temperatures. Pre-shading with iron oxide doesn’t affect flexural strength but decreases hardness. Thulium oxide adds blue fluorescence to zirconia, is affected by the amount of yttria and translucency. Co-doping with thulium oxide and erbium oxide mimics natural tooth fluorescence.

2. Cerium infiltration improves the aging resistance of dental zirconia, but higher ceria concentrations increase color, porosity, and grain growth, and reduce its bending strength. Cerium acetates are more effective in enhancing mechanical properties than cerium chlorides. Pre-sintering temperatures of zirconia affect the extent of cerium infiltration and the color and porosity. Chromium infiltration mimics natural tooth shades without compromising flexural strength, but higher concentrations diminish strength and toughness. Molybdenum matches tooth color and higher concentrations boost resin cement bond strength. Vanadium weakens zirconia, while nickel affects color. Transitional metal dopants showed no impact on the proliferation of gingival fibroblasts, suggesting their cytocompatibility. Chromium and terbium yield VITA shade-like zirconia with better density. Erbium encourages abnormal grain growth, while praseodymium increases monoclinic content. Praseodymium and erbium oxides produce yellow and red zirconia, respectively.