Inﬂuence of Different Treatments and Conditions on Optical Properties of Monolithic Zirconia: A Systematic Review

: The present systematic review aimed to evaluate the inﬂuence of different treatments and conditions on the optical properties of monolithic zirconia. An electronic search was performed using the following databases: PubMed (National Library of Medicine, Bethesda, MD, USA), Web of Science (Clarivate, London, UK), Scopus (Elsevier, Amsterdam, Netherlands), Google Scholar (Google, Mountain View, CA, USA), and Embase (Elsevier, Amsterdam, Netherlands), with no restrictions on publication year and language. Based on the PICO format, the primary research question of this review was: “What is the impact of different treatments and conditions on the optical properties of monolithic zirconia?” From 145 relevant articles, 12 studies were chosen for systematic review (qualitative synthesis). A modiﬁed version of the “Guidelines for Reporting Pre-Clinical In-Vitro Studies on Dental Materials” was used to assess the overall quality of the included studies and any bias within them. The included studies assessed the optical properties (such as color stability, translucency, and surface gloss) of monolithic zirconia and other relevant ceramic materials using different treatments and conditions including aging (i.e., artiﬁcial, simulated, chemical, and hydrothermal), grinding/occlusal adjustment, glazing/external staining, toothbrushing, bleaching, and artiﬁcial gastric acid exposure. All the included studies (100%) reported a signiﬁcant decrease ( p < 0.05) in the optical properties of monolithic zirconia samples as compared to those of other relevant ceramic materials. Overall, different treatments and conditions had a negative impact on the optical properties of monolithic zirconia. In conclusion, the optical features assessed for monolithic zirconia, such as color stability, translucency, and surface gloss, appeared to be signiﬁcantly compromised by different treatments and conditions including aging, grinding/occlusal adjustment, glazing/external staining, toothbrushing, bleaching, and artiﬁcial gastric acid exposure. However, the change was clinically not perceivable in 25% ( n = 3/12) of the included studies. Due to the heterogenic parameters in the included studies, it is recommended to perform future studies using standardized conditions with different stains of zirconia over an extended duration to obtain conclusive evidence


Introduction
Zirconia-based ceramics have been extensively used in restorative dentistry owing to their remarkable biocompatibility [1] as well as their tribological [2] and mechanical features [3][4][5]. Nonetheless, due to the opacity of zirconia, the design is required to be veneered with feldspathic ceramics to achieve acceptable outcomes. This bi-layered system of the core-ceramic design amalgamates the esthetics of the ceramic with the strength of zirconia [6,7].
However, veneered-zirconia restorations have shown increased clinical chipping rates of >30% [8][9][10], mandating a more invasive tooth preparation, and the fabrication procedure is more intricate. These clinical complications led to the development of monolithic zirconia with a modified microstructure possessing greater translucency and the incorporation of characterization pigments [11][12][13]. Compared to traditional metal-ceramic restorations and bi-layered zirconia crowns, monolithic zirconia possesses the benefit of reduced ceramic

Protocol and Registration
The current systematic review was conducted following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [37]. The protocol of this study has been registered in the Open Science Framework (OSF) Registries (https://doi.org/10.17605/OSF.IO/BRVT4).

Focused Question
For preparing and structuring the present systematic review, the focused question was formulated using the PICO format as follows [38]: Population: monolithic zirconia specimens (i.e., bars or plates); Intervention: monolithic aged zirconia; Comparison: nonaged zirconia ceramics; Outcome: effect of different treatments and conditions on optical properties of monolithic zirconia. Hence, the primary research question of this review is: "What is the impact of different treatments and conditions on the optical properties of monolithic zirconia?"

Information Sources and Search Strategy
With the help of a senior librarian specialized in health sciences database searches, an electronic search was undertaken using the following databases: PubMed (MEDLINE), Clarivate Analytics' Web of Science, Elsevier's Scopus, Google Scholar, and Embase, without no restrictions on publication year and language. The following search terms were used alone or in combination: "ceramic", "dental ceramic", "monolithic ceramic", "stained ceramic", "ceramic stain", "color", "color stability", "gloss", "surface hardness", "surface roughness", "ageing", "optical", "translucency", "translucent", "crystalline", "hydrothermal", "zirconia", "monolithic zirconia", "zirconium", "zirconium oxide", "Y-TZP", "yttria-partially stabilized zirconia", "yttria stabilized polycrystalline tetragonal zirconia", "yttria-stabilized tetragonal zirconia", "all-ceramic", "light scattering", and "light transmission", either as Medical Subject Heading (MeSH, National Library of Medicine, Bethesda, MD, USA) terms or keywords. These terms were utilized with Boolean operators "OR", "AND", and "NOT". Reference lists of the selected studies were subjected to thorough searches to identify potentially relevant studies. The cross-checking of the retrieved articles was performed to identify and remove duplicate articles, and the eligibility criteria (as determined by the PICO format) were applied to search for the studies.

Article Selection Procedure
The selection of articles was performed in the following three phases: An independent investigator (T.A.) screened titles and abstracts following the eligibility criteria. In case of ambiguity regarding any studies, a full-text assessment of the articles was performed.
Full-text assessment of all possible and included studies chosen from phase 1 was performed (T.A.).
The bibliographies of all studies chosen in phase 2 were assessed by independent investigator (T.A.) together, and full texts of potentially eligible articles were evaluated.

Data Extraction Process
Microsoft Excel was used for the data collection procedure and a standardized data extraction table was employed: (i) study details such as author, year, and country of publication; (ii) material composition; (iii) study groups; (iv) color assessment instruments; (v) study sample size; (vi) cleaning medium; (vii) specimen treatments; (viii) study outcomes; and (ix) general outcomes.

Quality Assessment
A modified version of the "Guidelines for Reporting Pre-Clinical In-Vitro Studies on Dental Materials" formulated by Faggion [39] was used to assess the overall quality of the included studies and any bias within them. In brief, the following items were evaluated in the individual included studies: (

Optical Properties Outcomes
The included studies assessed the influence of different treatments and conditions on the optical properties (such as color stability, translucency, and surface gloss) of monolithic zirconia and other relevant ceramic materials. Table 2 depicts the impact of different parameters on the optical properties of monolithic zirconia. These parameters include: (a) aging such as artificial [40], simulated [51], chemical [47], and hydrothermal [41]; (b) grinding/occlusal adjustment [42,48]; (c) glazing/external staining [43]; (d) tooth brushing [44,46,49,50]; (e) bleaching [45]; and (f) artificial gastric acid exposure [46].     Regarding the different aging procedures, varying outcomes were reported. One study found that the impact of aging on the translucency of the tested materials was nonsignificant [40]. Moreover, a color change after aging was greater in the zirconia samples treated with polishing paste as compared to other surface treatments. However, in the lithium disilicate material, no significant variation in color change was observed among the groups with varying surface treatments [40]. Another study revealed that hydrothermal aging decreased the translucency over the aging time for all tested materials [41]. Additionally, the shade of industrially pre-shaded 4Y-TZP was not affected by high-speed sintering [41]. Habib et al. [47], revealed that chemical aging significantly influenced the optical properties (i.e., color stability and translucency) of stained monolithic ceramics.
Regarding grinding/occlusal adjustments, one study reported that the pre-colored monolithic zirconia is preferable when grinding is required since it is less susceptible to the color alteration from grinding and eventually reduces the risk for restoration failure because of unacceptable discoloration resulting from material removal [42]. Herpel and colleagues [48] revealed that the color of monolithic zirconia stained by color infiltration alters linearly with the depth of material removal. Furthermore, staining that is resistant to occlusal adjustments is possible if the staining solution infiltrates adequately deep into the ceramic structure [48]. Farzin et al. [43], reported that the first phase of staining with both stains (i.e., value stain and yellow stain) caused more color change in all types of monolithic zirconia. Additionally, translucency increased after glazing and decreased after the first and second staining [43].
In terms of toothbrushing, Sehovic et al. [44], found that the color and gloss of stained monolithic ceramic materials changed significantly using toothbrush abrasion in vitro. However, the color changes were below the threshold value for detection by the human eye (∆E 1.8) [44]. Lee and co-workers [49] reported that toothbrushing had no effects on the color or translucency of the 5 mol% yttria-partially stabilized zirconia. Lee et al. [50], reported that brushing with several dentifrices markedly impacts the optical properties of monolithic zirconia finished with glazing or polishing procedures. Another study found that zirconia exposure to gastric acid with or without brushing will impact its color, irrespective of the application of coloring stains [46].
Regarding bleaching, bleaching with carbamide peroxide (i.e., home bleaching) 20% or hydrogen peroxide (i.e., office bleaching) 40% could perceptibly alter the color of externally stained monolithic zirconia, albeit within a clinically acceptable range [45]. Overall, all the included studies (100%) reported a significant decrease (p < 0.05) in the optical properties of monolithic zirconia samples as compared to those of other relevant ceramic materials, however, the change was clinically not perceivable in 25% (n = 3/12) of the included studies [44,45,48].

Quality Assessment Outcome
Eleven studies were graded as "moderate" quality [40][41][42][43][44][46][47][48][49][50][51], while only one study received an overall quality grade of "high" [45] (Table 3). All the included studies reported adequate background and objectives (introduction), intervention (methods), outcomes (methods), statistical analysis (methods), and outcomes and estimation (results) [40][41][42][43][44][45][46][47][48][49][50][51]. An adequate abstract was reported by all studies except one [50]. Randomization (methods) was conducted in five studies [44][45][46]50,51]; however, none of these studies reported the randomization procedure as well as the personnel involved in its implementation. A pre-determined sample size calculation (methods) was performed only by two studies [40,45]. All except two studies [41,51] reported the limitations in the discussion section. The funding status was mentioned by all but two studies [40,49], while three studies mentioned whether the full trial protocol was accessible [43,45,47] (Table 3). • Lithium disilicate ceramic was found to be more esthetic than monolithic zirconia ceramic in terms of color stability and translucency. • Color change after aging was higher in the zirconia specimens treated with polishing paste than other surface treatments. • However, in the lithium disilicate material, no significant difference in color change was found among the groups with different surface treatments.  • Zirconia exposure to gastric acid with or without brushing will affect its color, regardless the application of coloring stains.
(Habib et al., 2021) [47] • Chemical aging had significantly changed the color and decreased the translucency of all stained monolithic ceramics.

Discussion
This systematic review aimed to evaluate the influence of different treatments and conditions on the optical properties of monolithic zirconia as compared to other relevant ceramic materials. The optical properties (such as color stability, translucency, and surface gloss) of almost all the monolithic zirconia samples were significantly influenced by different treatments and conditions, including artificial, simulated, chemical, and hydrothermal aging, grinding/occlusal adjustment, glazing/external staining, toothbrushing, bleaching, and artificial gastric acid. For instance, according to Kurt et al. [40], a surface glaze could protect against aging impact for up to 1 year of clinical performance. Glazed monolithic zirconia demonstrated the greatest translucency parameters after and before artificial water spray aging as compared to those of only rubber polish with and without utilizing polishing paste [40].
The porosity generated on the surface after aging results in an increase in the incident light scattering that reduces translucency [52,53]. Moreover, after aging, the presence of the tetragonal and monoclinic stages in the structure decreases the translucency since the individual stage has varying refractive indices [52,54]. The aging method itself might be a variable influencing the optical properties of zirconia if varying methods are utilized. For example, constant immersion in a highly acidic solution simulating gastric acid with highly acidic pH (i.e., 1.2) at 37 • C for 4 days had a significant effect on the translucency of pre-sintered zirconia (i.e., 3-mol%Y-TZP; Prettau, Zirkonzhan, and Zenostar, Ivoclar Vivadent, Zirkonzahn, Atlanta, GA, USA) [46]. This protocol was repeated 208 times, representing four regurgitation sessions/week, for 12 months. This period under these specific conditions of time, temperature, and pH is regarded as equal to 10 years of clinical performance. Nevertheless, Kulkarni et al. [55] found that the utilization of acidic solution (i.e., pH = 2.0) with intermittent exposure (i.e., 2 min immersion, 2 min immersion in distilled water, and eventually 2 h storage in water at 37 • C) did not significantly influence the optical properties (i.e., translucency parameter) of zirconia ceramic. This procedure was repeated for a duration equal to 108 h of exposure [55].
Volpato and colleagues [56] found that the colorimetric features of zirconia were maintained after an accelerated aging procedure. Nonetheless, these investigators employed an aging method in an autoclave without exposure to ultraviolet light [47]. According to Dikicier and colleagues [57], the mean color difference in the zirconia samples was 1.29. This value is much lower than the mean color stability values reported by most of the studies included in this systematic review (i.e., <2.0). This outcome might be due to the utilization of the veneered samples fabricated from the colored pre-sintered block, and hence, a separate coloring method before sintering was not used by the authors of one of the included studies [40]. Some reports mentioned that color change in ceramics owing to aging might be related to metal oxides. Metallic pigments are incorporated for the color shading of ceramic, and these oxides are easily dissolved under ultraviolet light [57][58][59]. Consequently, in the present review, the significant color change in the zirconia might also be because of the dissolution of metallic oxides resulting from the ultraviolet radiation used during the aging protocol.
The association of aging with surface treatments also influences the color stability of ceramics. According to Atay and colleagues [60], the color change was greater in polished materials followed by the glazed materials after aging. In the present review, the color change score of the polishing paste-treated zirconia samples was revealed to be greater as compared to that of other relevant ceramic materials. Nevertheless, the color change score was not influenced by the surface treatment in the lithium disilicate material. Hence, it is indicated that the material in the polishing paste underwent interaction with the aging conditions and resulted in more differences in the color of zirconia samples.
The findings of this review revealed that lithium disilicate samples demonstrated significantly greater translucency as compared to monolithic zirconia samples, which is in agreement with the outcomes of several other studies [61][62][63]. Although the transparency of monolithic zirconia was increased as compared to traditional zirconia owing to the modifications performed while the production stage, it is still not comparable with even traditional lithium disilicate at the same material thickness.
The methodology-associated limitations of the included studies are important to recognize. First, most of the studies used zirconia from a single manufacturer. An amalgamation of varying shades and types of monolithic zirconia from multiple manufacturers is recommended since materials from different manufacturers may behave differently. Hence, it is recommended to use zirconia from multiple manufacturers in future experiments. Second, some studies performed aging for a limited time to simulate the clinical life of the prosthesis. Additionally, the aging process was performed only in water vapor and at standard temperatures. Further research should be performed over an extended period in the presence of different conditions, such as colored drinks, cigarettes, saliva, and different enzymes for a better reflection of clinical conditions. Third, 5 out of the 12 included studies did not assess the effect of translucency on the color of zirconia. It is recommended to incorporate translucency into the analysis and to investigate the color variations between different ceramic materials. Fourth, the in vitro studies permit standardization via the provision of a more precise estimate of the impact of material reduction alone on color stability in the absence of other potentially confounding variables, clinical studies are recommended for better reflection of the clinical setting of occlusal adjustments and restoration success based on the perception of patients. Moreover, most of the studies used zirconia with a similar yttria content. Zirconia with different yttria contents should be used in future investigations. Additionally, a few studies used one type of toothpaste with limited toothbrushing strokes. Since toothpaste with a higher amount of radioactive dentin abrasion will result in more intense wear of the characterized stains, it is recommended to perform a greater number of toothbrush strokes in future studies to assess the impacts of longer periods of simulated toothbrushing on the optical features of monolithic zirconia. Finally, enhancements in shade manufacturing and the optical characteristics of zirconia material are recommended to minimize the consequences of restoration failure and to ensure the long-lasting satisfaction of patients.

Conclusions
The optical features assessed for monolithic zirconia, such as color stability, translucency, and surface gloss, appeared to be compromised by various treatments and conditions, including aging, grinding/occlusal adjustment, glazing/external staining, toothbrushing, bleaching, and artificial gastric acid exposure. Due to the heterogeneity of the methodology and study conditions, it is recommended to perform future studies using standardized aging conditions (i.e., saliva, coloring drinks, cigarettes, and various enzymes), which reflect the clinical setting, using different stains of zirconia over an extended duration of aging to obtain conclusive evidence.