Exploring the Effect of In Vitro Aging Protocols on the Optical Properties and Crystalline Structure of High-Translucency (HT) Zirconia Used in Dentistry: A Systematic Review

Abstract

Zirconia crowns are capping materials used in dentistry for tooth capping and are very popular due to their optical properties and natural-looking visuals. In vitro aging protocols measure zirconia’s optical properties, which are vital in order for it to look natural. This study aims to conduct a systematic review to explore the effect of in vitro aging protocols on the optical properties and crystalline structure of high-translucency (HT) zirconia. A correlation matrix was obtained using Microsoft Excel, which was later transferred into SPSS for confirmatory factor analysis (CFA) and hierarchal clustering and to obtain a dendrogram in order to display the distribution of clusters for each key term relevant to the study. Further, for qualitative analysis, 17 studies were screened and reviewed. The result demonstrates that high translucency has been observed in the crystalline structure of zirconia capping. However, quantitative and qualitative results did not demonstrate the in vitro protocol; instead, neglect of the in vitro protocol’s vitality in zirconia crown capping was alarming. Overall, zirconia has good optical properties when combined with catalysts such as aluminum and lithium to obtain a more sustainable crystalline structure.

1. Introduction

Zirconia is a widely used dental material due to its high strength, biocompatibility, and esthetic properties. However, zirconia can undergo aging or degradation when exposed to oral environments over time, which can affect its mechanical, chemical, and optical properties [1,2,3,4]. High-translucency (HT) zirconia is a relatively new type of zirconia that has been developed to improve its esthetic properties. HT zirconia has a high level of translucency, which makes it more similar in appearance to natural teeth. However, the effect of aging on the optical properties and crystalline structure of HT zirconia is still not fully understood [4]. In vitro aging protocols are used to simulate the effects of long-term exposure to oral environments on zirconia. These protocols typically involve exposing zirconia samples to various solutions, such as artificial saliva or acidic environments, for a specific period of time. The aging process can lead to changes in the surface roughness, phase transformation, and optical properties of zirconia [5,6,7]. In vitro aging protocols have become an important tool for simulating the effects of long-term exposure to oral environments on dental materials. The aim is to optimize optical properties and high translucency in zirconia crowns using an in vitro aging protocol.

In summary, the use of in vitro aging protocols to simulate the effects of oral environments on dental materials is an important tool for predicting the long-term behavior of these materials in clinical settings. HT zirconia is chosen since it resembles natural teeth. Changes in optical properties over time can result in changes in the appearance of these restorations. Furthermore, changes in the physical structure of zirconia upon aging can alter its durability and stability. Therefore, the use of aging protocols to better understand the changes in the structural and optical properties of HT zirconia can help guide the conditions in which HT zirconia is preferable to other materials for tooth capping. Several studies have investigated the effect of aging on the properties of HT zirconia, including its surface roughness, mechanical properties, and optical properties. However, there is a need to provide a collective conclusion to all these recent studies incorporating new and improved methodologies. Systematic reviews can help to synthesize the available literature and provide a quantitative estimate of the effects of different aging protocols on HT zirconia properties, which can be useful for clinicians and researchers in selecting appropriate aging protocols and predicting the long-term behavior of HT zirconia restorations.

2. Materials and Methods

2.1. Inclusion and Exclusion Criteria

The focus of this study was on the optical and crystalline structure of HT zirconia in the process of zirconia capping for dental care using an in vitro aging protocol. The following inclusion criteria were used: studies incorporating age protocols to study their effects on either the crystalline structure or the optical properties or both of HT zirconia, in vitro studies, studies published between 1 January 2019 and 30 December 2023, and papers published in English. The exclusion criteria were as follows: studies using zirconia for purposes other than dentistry, studies not published in English, studies published before 1 January 2019 or after 30 December 2023, and studies not using in vitro methodology.

2.2. Search Strategy

A thorough search of the literature published between 1 January 2019 and 30 December 2023 was conducted in the online database “Scopus”. This time period was selected to synthesize the findings of only recent papers so that research methodologies are up-to-date on the current trends and advancements in techniques. The following key search terms were used: “zirconia”, “optical properties”, “high translucency”, and “in vitro aging protocols”.

2.3. Sampling

A total of 354 studies were extracted, from which 170 studies were found relevant to the topic after duplication was removed. Further, an analysis was performed based on the 170 studies that remained. For the qualitative analysis, 170 studies were screened to extract only the most relevant studies to be discussed in this paper in detail.

2.4. Tools and Techniques

A systematic literature review approach was adopted. The data were imported into a visualization software named VOSviewer version 1.6.20. First, the data were analyzed using the keywords that were found most relevant to the topic, which gave a clustered network of the data. The software automatically assigned each study and the keywords a unique ID, providing the frequency distribution of a massive data set. For extracting a correlation matrix, the data obtained from VOSviewer were imported into Microsoft Excel version 2016. The data were sorted to obtain a correlation matrix in Excel. This matrix was finally imported into the Statistical Package for Social Sciences (SPSS) version 27 for data analysis.

2.5. Data Analysis (Quantitative Results)

Component factor analysis (CFA) and hierarchal clustering with charts for visualization of the results were obtained for quantitative results. These analyses are the standard techniques for performing a systematic literature review.

2.6. Eligibility Criteria (Qualitative Results)

After the initial screening, a total of 354 articles were retrieved. Later scanning through VOSviewer software and removing duplicates, 170 articles were retrieved. The detailed screening of the remaining articles by considering only the titles of each study led to the exclusion of 87 articles. The remaining 97 articles were then observed based on their abstracts, which led to the removal of 34 articles, and then their titles, topics, relevance, and language were checked, which ended up with the removal of 17 more articles. The keywords were brought into focus, and only studies that exclusively discussed optical properties and measured these properties with in vitro aging protocols were chosen, which excluded 36 more studies. A total of 10 studies remained for qualitative systematic analysis. The search for this study was performed following the standards of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (Figure 1).

2.7. Registration

The protocol for this systematic review, titled “Exploring the Effect of In Vitro Aging Protocols on the Optical Properties and Crystalline Structure of High-Translucency (HT) Zirconia Used in Dentistry”, was registered in the Prospective International Register of Systematic Reviews (PROSPERO) with ID CRD42024579359. The review aims to systematically assess and synthesize existing studies on how different in vitro aging protocols impact the optical properties and crystalline structure of HT zirconia, a widely used material in restorative dentistry. By documenting these effects, the review seeks to provide a comprehensive understanding of how aging processes may influence the long-term performance and aesthetic outcomes of HT zirconia restorations, contributing valuable insights to both clinical practice and materials science. PROSPERO, a global repository, ensures transparency and reduces duplication in systematic reviews by registering detailed protocols in various fields, including healthcare, public welfare, education, and more.

3. Results

3.1. Quantitative Analysis

Table 1. Correlation analysis.

Key Terms	Term 1	Term 2	Term 3	Term 4	Term 5	Term 6	Term 7	Term 8	Term 9	Term 10	Term 11	Term 12	Term 13	Term 14
Term 1	1	−0.03964	−0.049	0.120801	−0.0668	−0.04605	−0.04294	0.120801	−0.0799	−0.04294	−0.06208	−0.1408	−0.05711	0.051768
Term 2	−0.03964	1	−0.04522	−0.03659	−0.06165	0.108034	−0.03964	−0.03659	−0.07375	−0.03964	−0.0573	−0.12084	−0.05271	0.064919
Term 3	−0.049	−0.04522	1	−0.04522	−0.07621	−0.05254	0.083216	−0.04522	−0.01289	0.083216	0.024734	−0.07051	−0.06516	−0.06803
Term 4	0.120801	−0.03659	−0.04522	1	−0.06165	−0.04251	−0.03964	0.136179	−0.07375	−0.03964	−0.0573	−0.12084	−0.05271	−0.05504
Term 5	−0.0668	−0.06165	−0.07621	−0.06165	1	−0.07163	0.034591	−0.06165	−0.06426	0.034591	−0.02328	−0.1902	−0.01018	−0.09275
Term 6	−0.04605	0.108034	−0.05254	−0.04251	−0.07163	1	−0.04605	−0.04251	−0.00292	−0.04605	0.034475	−0.15892	−0.06124	−0.06395
Term 7	−0.04294	−0.03964	0.083216	−0.03964	0.034591	−0.04605	1	−0.03964	0.008301	−0.04294	−0.06208	−0.20004	−0.05711	−0.05963
Term 8	0.120801	−0.03659	−0.04522	0.136179	−0.06165	−0.04251	−0.03964	1	−0.07375	−0.03964	−0.0573	0.13435	0.071748	−0.05504
Term 9	−0.0799	−0.07375	−0.01289	−0.07375	−0.06426	−0.00292	0.008301	−0.07375	1	−0.0799	−0.05175	−0.23189	0.030588	−0.045
Term 10	−0.04294	−0.03964	0.083216	−0.03964	0.034591	−0.04605	−0.04294	−0.03964	−0.0799	1	0.04561	−0.0223	−0.05711	−0.05963
Term 11	−0.06208	−0.0573	0.024734	−0.0573	−0.02328	0.034475	−0.06208	−0.0573	−0.05175	0.04561	1	0.01058	0.000983	−0.0862
Term 12	−0.1408	−0.12084	−0.07051	−0.12084	−0.1902	−0.15892	−0.20004	0.13435	−0.23189	−0.0223	0.01058	1	−0.26603	−0.27777
Term 13	−0.05711	−0.05271	−0.06516	−0.05271	−0.01018	−0.06124	−0.05711	0.071748	0.030588	−0.05711	0.000983	−0.26603	1	−0.0793
Term 14	0.051768	0.064919	−0.06803	−0.05504	−0.09275	−0.06395	−0.05963	−0.05504	−0.045	−0.05963	−0.0862	−0.27777	−0.0793	1

demonstrates the correlation between the terms, which gives a significantly good association between items (Table 1). A value of 1 demonstrates a good correlation between items; the study results gave a value of 1 for each item correlation with a significant p-value of 0.05 or lower (Table 1).

Figure 2 is a visualization of the overlay network that demonstrates the most relevant literature clustering based on keywords.

Table 2. Communalities.

Key Terms	Initial	Extraction
TERM 1	1.000	0.551
TERM 2	1.000	0.615
TERM 3	1.000	0.645
TERM 4	1.000	0.406
TERM 5	1.000	0.839
TERM 6	1.000	0.547
TERM 7	1.000	0.707
TERM 8	1.000	0.644
TERM 9	1.000	0.612
TERM 10	1.000	0.550
TERM 11	1.000	0.486
TERM 12	1.000	0.847
TERM 13	1.000	0.668
TERM 14	1.000	0.701

demonstrates the commonalities obtained from CFA, which is the initial extraction for the data. The majority of the values obtained were above the standard value of 0.5, which shows an appropriate extraction of the 14 terms.

The 14 terms were subdivided into six measure factors, which explains 62.998% of the variation in the data. The variation gradually dropped from 13.010% to 8.595% for factors 1 to 6, respectively. The eigenvalues gave a value above 1 for each of these factors, resulting in the formulation of six factors under the 14 terms presented (

Table 3. Total variance explained.

Key Terms	Initial Eigenvalues			Extraction Sums of Squared Loadings			Rotation Sums of Squared Loadings
	Total	% of Variance	Cumulative %	Total	% of Variance	Cumulative %	Total	% of Variance	Cumulative %
TERM 1	1.998	14.273	14.273	1.998	14.273	14.273	1.821	13.010	13.010
TERM 2	1.840	13.140	27.413	1.840	13.140	27.413	1.529	10.922	23.932
TERM 3	1.469	10.496	37.908	1.469	10.496	37.908	1.491	10.648	34.580
TERM 4	1.300	9.284	47.192	1.300	9.284	47.192	1.426	10.187	44.767
TERM 5	1.159	8.278	55.471	1.159	8.278	55.471	1.349	9.635	54.403
TERM 6	1.054	7.527	62.998	1.054	7.527	62.998	1.203	8.595	62.998
TERM 7	0.984	7.031	70.029
TERM 8	0.886	6.329	76.358
TERM 9	0.850	6.071	82.430
TERM 10	0.723	5.163	87.593
TERM 11	0.648	4.628	92.221
TERM 12	0.609	4.348	96.568
TERM 13	0.480	3.432	100.000
TERM 14	5.551 × 10−17	3.965 × 10−16	100.000

).

The scree plots demonstrate a gradual fall throughout the 14 terms. However, only six reached above the level of 1 on the scale of eigenvalues. All the terms below 1 were neglected (Figure 3).

The evaluation of the component matrix gave a variable result for the six factors obtained in the results (

Table 4. Component matrix.

Key Terms	Component
	1	2	3	4	5	6
TERM 1		0.616
TERM 2			−0.665
TERM 3		−0.419		0.458	−0.435
TERM 4		0.517
TERM 5					0.677
TERM 6			−0.579
TERM 7						−0.462
TERM 8	−0.614
TERM 9	0.503				−0.454
TERM 10		−0.470
TERM 11		−0.485
TERM 12	−0.815
TERM 13				−0.621
TERM 14		0.434				0.418

). For this reason, varimax rotation was considered to evaluate the exact value on which each factor defines each term (

Table 5. Rotated component matrix.

Key Terms	Component
	1	2	3	4	5	6
TERM 1	0.604
TERM 2				−0.703
TERM 3					0.413	−0.447
TERM 4	0.627
TERM 5						0.878
TERM 6				−0.703
TERM 7					0.828
TERM 8	0.667
TERM 9			0.687
TERM 10				0.431
TERM 11	−0.486				−0.446
TERM 12		−0.677	−0.489
TERM 13			0.734
TERM 14		0.816

). The specific values were obtained on the rotation of the eighth iteration of the data. Terms 1, 4, and 8 compose component 1, which is our first factor. Term 14 is defined by the second factor, term 13 by the third factor, term 7 by the fifth factor, and term 5 by the sixth factor. The terms that obtained factor loading that were negative or below 0.7 were not considered (Table 5).

In the output,

Table 6. Agglomeration schedule.

Stage	Cluster Combined		Coefficients	Stage Cluster First Appears		Next Stage
	Cluster 1	Cluster 2		Cluster 1	Cluster 2
1	1	4	1.558	0	0	2
2	1	8	1.612	1	0	11
3	2	6	1.623	0	0	8
4	3	10	1.717	0	0	5
5	3	11	1.896	4	0	9
6	5	7	1.908	0	0	9
7	9	13	1.921	0	0	10
8	2	14	2.062	3	0	11
9	3	5	2.123	5	6	10
10	3	9	2.236	9	7	12
11	1	2	2.268	2	8	12
12	1	3	2.315	11	10	13
13	1	12	2.852	12	0	0

follows the proximity matrix. The agglomeration schedule demonstrates the clustering of the observations; in this case, it is terms that denote keywords. Each stage consists of combined results from two cases to form a cluster. The analysis uses the algorithm obtained by the distance and linkage assigned to each stage. Due to the combination of two terms in one cluster, it forms 13 stages instead of 14. Terms 2 and 3 are combined in the first stage, and the Euclidean distance between them is the smallest among all pairs. With the increase in values, the coefficients that the clusters combined in a given stage are more heterogeneous than those in the previous stage (Table 6).

Figure 4 is a visualization of the occurrence of each term in each cluster. Terms 12 and 13 have the highest number of clusters while occurring together in the same document, whereas terms 4 to 8 have the lowest number of occurrences together in the documents obtained (Figure 4).

In term 14, there existed 5 clusters; however, in term 11, there were 6 clusters. In term 8, 8 clusters were found; however, in term 5, 10 clusters were grouped. Term 3 contained 13 clusters, and term 1 contained 14 clusters. These clusters are groups of studies that contain the literature relayed to the terms. If the dendroid is studied horizontally, clusters that contain terms 14 and 12 contain the highest amount of clusters formed, which shows a high occurrence of the terms that fall between the 14th and 13th terms (Figure 5).

3.2. Qualitative Analysis

Table 7. A tabular form of the literature extracted.

Study No. (Scopus Database)	Authors	Title	Year	Source Title	Abstract	Author Keywords
1	Avcıoğlu et al. [8]	Radiation shielding and mechanical properties of mullite–zirconia composites fabricated from investment-casting shell waste	2023	Journal of Materials Research and Technology	Good mechanical strength, with flexural and compressive strength.	Ceramic recycling, gamma radiation shielding, investment casting shell waste, mechanical properties, mullite–zirconia ceramic composite
2	Moon et al. [9]	Influence of additive firing on the surface characteristics, Streptococcus mutans viability and optical properties of zirconia	2021	Materials	Decreased surface roughness, contact angle, S. mutans viability, and translucency. Second stage: no critical difference in surface characteristics, S. mutans viability, and optical property.	Contact angle, firing, roughness, S. mutans, translucency, zirconia
3	Almohammed et al. [10]	Optical properties of five esthetic ceramic materials used for monolithic restorations: A comparative in vitro study	2022	Ceramics	Monolithic multilayer zirconia ceramics: lower optical properties. Monolithic multilayer polymer-infiltrated hybrid ceramic (significantly affected by aging). Lithium disilicate ceramic (better optical properties).	Aging, color, lithium disilicate, monolithic, multilayer, opalescence, optical properties, polymer-infiltrated ceramic, translucency, zirconia
10	Mešić et al. [11]	Effect of aging on the microstructure and optical properties of translucent ZrO2 ceramics	2021	Acta Stomatologica Croatica	All polished specimens displayed significant color change. No translucency of specimens due to the aging protocols.	Cubic zirconia, monolithic zirconia, translucent zirconia
13	De Souza et al. [12]	Flexural strength and crystalline stability of monolithic translucent zirconia subjected to grinding, polishing and thermal challenges	2020	Ceramics International	Surface treatment: The Grind (higher roughness, greater flexural strength). Grind + Polish (intermediate roughness, flexural strength). Aging (minor roughness, reduced flexural strength).	Finishing, mechanical properties, surfaces, ZrO2
14	Kim [13]	Optical and mechanical properties of highly translucent dental zirconia	2020	Materials	Lithium disilicate and alumina affect the translucency, opalescence, or fluorescence qualities of zirconia.	Fluorescence, lithium disilicate, opalescence, translucency, zirconium oxide
15	Owen et al. [14]	Diffusion in doped and undoped amorphous zirconia	2021	Journal of Nuclear Materials	Zirconia exhibits higher oxygen diffusivity compared with equivalent undoped crystalline systems. Similar diffusivities are observed between amorphous and crystalline doped systems.	Amorphous, diffusion, grain boundaries, lanthanides, ZrO2
22	Ruiz-Aguilar et al. [15]	Fabrication, characterization, and in vitro evaluation of β-TCP/ZrO2-phosphate-based bioactive glass scaffolds for bone repair	2022	Boletin de la Sociedad Espanola de Ceramica y Vidrio	Zirconia had a significantly positive effect on cell proliferation as well as in the cell adhesion.	Bioactive glass, calcium phosphate, hydroxyapatite, scaffold, zirconia
27	Lim et al. [16]	Composition, processing, and properties of biphasic zirconia bioceramics: Relationship to competing strength and optical properties	2022	Ceramics International	Consistent with literature reporting, the materials rank in decreasing order for strength but increasing order for translucency. However, for a given composition, the data suggest that the strengths of densely sintered increases with translucency.	Crystal structure, density, microstructure, strength, tetragonality, translucency
28	Tayal et al. [17]	Comparative evaluation of the effect of One Coat 7 Universal and Tetric N-Bond Universal adhesives on shear bond strength at resin–zirconia interface: An in vitro study	2021	Journal of Conservative Dentistry	Both universal adhesives increased the bond strength at the resin–zirconia junction. Tetric N-Bond Universal exhibited the highest strength.	10-Methacryloyloxydecyl dihydrogen phosphate, adhesive, universal, zirconia
54	Kannaiyan et al. [18]	Impact of different surface treatments on flexural strength and surface roughness of zirconia implant material: An in vitro study	2021	World Journal of Dentistry	The highest surface roughness was recorded in the SLA-treated zirconia flexural strength.	Dental implant, flexural strength, surface roughness, zirconia
83	Tanaka et al. [19]	Mechanical properties of partially stabilized zirconia for dental applications	2019	Journal of Asian Ceramic Societies	Ceria-based nano-zirconia: double the fracture toughness and bending strength Yttria-based zirconia: excellent dental prosthesis strength.	CAD/CAM, dental, mechanical properties, zirconia
123	Ha et al. [20]	Comparison of shear strength of metal and ceramic orthodontic brackets cemented to zirconia depending on surface treatment: An in vitro Study	2019	European Journal of Dentistry	In terms of surface treatment, no statistical differences were found between the different groups. Metal brackets have a greater bond strength than ceramic brackets when cemented to zirconia.	Brackets, cementation, surface treatment, zirconia
176	Shetty et al. [21]	An in vitro study to evaluate the effect of artificial aging on translucency, contrast ratio, and color of zirconia dental ceramic at different sintering levels	2022	Coatings	An increase in sintering temperature increased the translucency.	Aging, ceramic, color stability, contrast ratio, esthetics, sintering, spectrophotometry, translucency, zirconia
273	El-Mahdy et al. [22]	Strain gauge analysis and fracture resistance of implant-supported PEKK hybrid abutments restored with two crown materials: An in vitro study	2023	Dental and Medical Problems	The zirconia group recorded significantly higher strain and fracture resistance values than the composite group (p < 0.001).	Composite resins, dental implants, implant abutments, polyetherketoneketone, zirconia
277	Guan et al. [23]	Thickness dependence of the crystallization and phase transition in ZrO2 thin films	2023	Journal of Advanced Ceramics	--	Crystallization, monoclinic (m), phase transition, tetragonal (t), zirconia
328	Gad et al. [24]	Translucency of nanoparticle-reinforced pmma denture base material: An in vitro comparative study	2021	Dental Materials Journal	Highest translucency in pmma with an increase in concentration. Denture base resin decreased the translucency.	Esthetics, nanoparticle, optical properties, polymethyl methacrylate, removable dental prostheses

demonstrates the most relevant studies out of the 170 studies that were obtained with the help of VOSviewer. The keywords that were most relevant in the selection of the 17 studies were zirconia, dentistry, in vitro aging protocol, optical properties, and translucency. The table presents a diverse array of studies investigating various aspects of zirconia and related materials, encompassing topics such as mechanical properties, optical characteristics, surface treatments, and biomedical applications [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24].

4. Discussion

The results demonstrate the density of the literature in each study concerning the keywords. Quantitative analysis showed much relevance in 170 studies. The clusters of the literature concerning the key terms that were most relevant to the study aim were huge. Zirconia was found most effective on the optical properties and translucency of the capping. Surface treatment was also found to significantly improve due to the use of zirconium oxide (Figure 1). With regard to zirconia, coating, composites, flexural strength, hardness, mechanical properties, and microstructure are the most repeatedly discussed properties in dentistry (Table 3 and Figure 3). With the help of an agglomeration schedule, we determined that composite zirconium ceramics are useful in optimizing flexure strength (Table 6). Since the measuring protocol was in vitro, especially in the case of aging of the zirconia material, we determined that in vitro is a robust technique for measuring zirconia in dentistry. However, in vitro protocols have not been given much attention as the clusters do have in vitro significantly occurring in the data (Figure 1).

Qualitative analysis showed that although zirconia has been considered as an important capping material, its appropriate analysis by measuring the situation of the crown capping with in vitro aging protocols has not been specified in the literature in abundance. The advancement in measuring protocols is important to suggest the improvement and possible lack of any important factor. With regard to zirconia, the literature emphasized using catalysts to strengthen its productivity and improve its sustainability [13,14,19,24]. The optical properties and translucency of zirconia capping in the mentioned studies showed significantly positive results [11,13,16,21]. Zirconia capping has been observed to lose its translucency and optical properties with age [10,11]. After aging, surface porosity is generated, elevating incident light scattering, which in turn reduces translucency [25,26]. Additionally, the presence of monoclinic and tetragonal stages after aging reduces translucency as a result of the individual stage having different refractive indices [25,27]. An aging protocol might influence the optical properties of different methods of aging used. For instance, one aging method immersed pre-sintered zirconia in a highly acidic solution (pH 1.2) for 4 days, which significantly affected its translucency [28], whereas another protocol utilizing an acidic solution of pH 2 with intermittent exposure found that it did not alter the optical properties significantly [29]. Another property that has been detected is flexural compressive strength, which is a vital property of a tooth cap material [8,12]. However, the absence of literature on in vitro aging protocols is alarming.

Several studies have investigated the effect of different aging protocols on the Several studies have investigated the effect of different aging protocols on the properties of HT zirconia. For example, one study evaluated the effect of aging on the surface roughness and surface chemistry of HT zirconia using atomic force microscopy and X-ray photoelectron spectroscopy. The results showed that aging caused an increase in surface roughness and a decrease in the number of oxygen-containing functional groups on the surface of HT zirconia [30,31]. Another study investigated the effect of aging on the mechanical properties of HT zirconia using four-point bending tests. The results showed that aging caused a decrease in the flexural strength of HT zirconia, indicating that the aging process can hurt the mechanical properties of this material [32,33,34]. Other studies have focused on the effect of aging on the optical properties of HT zirconia. For example, one study evaluated the effect of aging on the color stability of HT zirconia using a spectrophotometer. The results showed that aging caused a significant increase in color change in HT zirconia, which can hurt its esthetic properties [7,35]. In contrast, a study showed that the colorimetric properties of zirconia were retained after it was subjected to an accelerated aging procedure [36]. Another study investigated the effect of aging on the translucency of HT zirconia using a spectrophotometer and found that aging caused a decrease in the translucency of this material. These findings suggest that the aging process can have a significant impact on the optical properties of HT zirconia, which can affect its esthetic performance [37,38,39].

This study is based on an extremely thorough research methodology with strict eligibility criteria, ensuring that only studies that fit the inclusion and exclusion criteria were included. Pre-defined key terms were used to search the literature to find relevant and specific studies from the database. This systematic review, including only recent studies, helps in synthesizing the latest knowledge regarding the effects of different aging protocols on the optical and structural properties of HT zirconia. As such, it helps in improving the understanding of the updated methodologies on the subject. Furthermore, this study holds significance for various professionals in the field of dentistry, including practitioners, dental surgeons, and dental hygienists. The findings shed light on the absence of literature on in vitro aging protocols, which may require further attention and development.

This systematic review also has certain weaknesses. This study used only one database, i.e., Scopus, to search for relevant literature. Therefore, future research can incorporate various databases in the reviews, such as PubMed and Embase, so that more studies fitting the criteria of the review can be assessed. Owing to the rigorous inclusion and exclusion criteria, only 13 studies were compared, which is a small number and limits the robustness of the comparison in the systematic review. Furthermore, heterogeneity in individual studies in terms of different sample sizes, protocols, and durations of exposure also influences the comparison. The literature on the subject itself is also quite limited, restricting the comprehensiveness of the review.

5. Conclusions

This study aimed to gather published evidence regarding the zirconia capping, optical properties, and translucency of the material measured with an in vitro aging protocol. The results obtained showed that the use of zirconia in dentistry has been discussed several times in the literature in terms of optical properties, translucency, surface material, coating, aging, etc. Qualitative analysis showed that zirconia, by adopting a catalyst such as aluminum, lithium, etc., to obtain a more sustainable crystalline structure, gives significantly improved optical properties and high translucency. In vitro was not preferred as a measuring protocol in the majority of the studies (n = 170); however, after screening for qualitative analysis, it was found that the in vitro aging protocol has indeed been considered important in a few cases and has been adopted. This study holds significant implications for dentists in determining the conditions in which HT zirconia is the most suitable for the patient. Even though this review is focused on providing a detailed analysis of the impact of different protocols on the properties of zirconia, more research is needed on this subject since the in vitro aging protocol has not been witnessed in the literature in abundance, which leaves a gap in the literature, making the study of zirconia along with the properties of in vitro aging protocols important. Overall, this review contributes to the existing body of literature on in vitro aging protocols to assess the optical and structural properties of HC zirconia.

6. Future Recommendation

Based on the results of this study, future study of zirconia with a catalyst to improve its properties with a unique crystalline structure is recommended. Further, a more sustainable and long-lasting capping material must be investigated concerning zirconia measuring with an in vitro aging protocol.