Antagonist tooth wear is considered one of the significant problems when using dental ceramics. It is complex and multifactorial process which has some functional and esthetic complications.¹,² The complexity of tooth wear process makes it difficult to conduct adequate clinical studies on this issue; therefore, in vitro studies using wear-simulator machines which simulate oral conditions in controlled-experimental way were introduced in 1996.³,⁴,⁵ These machines can reduce confounding variables such as pH, review viscosity, and flow rate of the saliva that affects enamel wear.⁶

Determining a dental ceramic that simultaneously has enough strength without the disadvantage of increased enamel wear, has important clinical role in prosthodontics and the science of dental materials.⁷,⁸ Some studies showed that the amount of antagonist enamel wear opposing dental ceramics is strongly material dependent.⁹,¹⁰ For example, Stawarczyk et al. showed that conventional zirconia resulted in significant higher antagonist wear than monolithic zirconia⁹ and Rosentritt et al. concluded that feldspathic porcelains provided higher wear than zirconia specimens.¹⁰

Because of increasing patient's demands for having more dental esthetic,¹¹ veneered zirconia porcelains have been widely used because of their excellent esthetic and mechanical properties,¹²,¹³ however, high wear rate of their antagonist teeth have been reported by many in vitro studies.¹⁴,¹⁵,¹⁶ In order to reduce enamel wear of veneered zirconia, translucent, and no veneered zirconia porcelains have been established.¹⁷,¹⁸

The wear behavior of zirconia has not been understood completely.¹⁹,²⁰ Many studies have evaluated the effects of different polishing and glazing techniques on the enamel wear opposing dental porcelains.²,²¹,²²

Since there is a controversy about the effect of different ceramic systems on the wear of enamel because of different environmental and testing conditions, this systematic review was undertaken to evaluate and report the wear behavior of polished and glazed feldspathic and zirconia crowns in published articles up to January 2020.

This systematic review was Performed Using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement.²³

Inclusion criteria

Participants

Studies assessing tooth wear process in the experimental condition.

Intervention

Studies using Y-TZP zirconia.

Comparison

Studies comparing feldspathic porcelain with Y-TZP zirconia

Outcomes

Studies which evaluated the antagonist's enamel or steatite.

Types of studies

All in vitro studies were included in this review.

Studies published in English and published the starting date of databases to January 2020 were considered for inclusion in this review.

Exclusion criteria

(1) Incomplete data reporting, (2) systematic review articles, (3) absence of enamel wear evaluation or steatite wear evaluation, (4) use of veneered zirconia, (5) absence of clear method/materials, (6) in vivo studies, (7) high bias articles, (8) case reports, and (9) conference papers were excluded from the study.

Search strategy

A systematic search of electronic databases was conducted on Web of Science, MEDLINE (via PubMed), Cochrane Library, EMBASE, and Scopus. Additional hand searching was conducted through the references of included studies. In the primary search, the terms used were “dental ceramic,” “enamel,” “wear,” and “feldspathic,” “zirconia,” “Y-TZP,” “polished,” and “glazed.” No publication year limits were applied during the electronic searches. English articles were selected in this paper. The search strategies in PubMed, Embase, and Scopus are presented in Appendix I.

Study selection

Related titles and abstracts were screened by two independent reviewers and articles without an available abstract were excluded. Discrepancies between two reviewers were solved by discussion. The Cohen's Kappa value was used for inter-observer reliability and it should be greater than 80%. In case of unsolved disagreements, the third reviewer decided to include the article or not. After appraising of abstracts, potentially eligible articles were regained in full text.

Assessment of methodological quality

Methodological quality regarding the risk of bias in selected articles was assessed by one of the authors according to the criteria as set by the Cochrane collaboration's tool.²⁴ Allocation concealment was not applicable for this systematic review and therefore its column was removed Table 1. A meta-analysis could not be performed, as the test parameters differed from one study to the other. Therefore, the results of included studies were descriptively reported and discussed.{Table 1}

The information extracted from the articles is summarized in Table 2.{Table 2}

One hundred and thirty-two articles were identified through electronic database searching and one article was found through hand-searching by checking the references of included articles. Among the initially 133 articles, 59 duplicated articles were removed, and finally, 52 articles were screened and among them, only 16 articles remained for full-text regaining and 11 studies were excluded for inappropriate study design, intervention, and outcome Figure 1.Figure 1

Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram of the selection process.

Among studies which were critically appraised in this systematic review, 16 were in low level of bias risks, remains were high risks. Near 78% of these studies were free of reporting and other biases and almost 60% had adequate sequence generation, but in 60% of studies, there were incomplete outcome data. In addition, blinding was not applicable in these study designs. The first and second null hypotheses of this study stating that there is no difference in the wear number of antagonists of zirconia and feldspathic specimens, and glazing and polishing techniques do not affect antagonist's wear of zirconia and feldspathic groups. The findings of this study rejected both hypotheses with the following reasons:

One of the reasons that can explain lower antagonist wear of zirconia comparing with feldspathic porcelains is zirconia's smaller particle (fine grain) which can result in smoother and monotonic surface. Another reason is higher fracture toughness of zirconia (9 MPa) comparing with feldspathic porcelain (0.73 MPa). Low fracture toughness of feldspathic porcelain can lead to microfractures on its surface which results in prominences and roughness. This increasing roughness makes stress on enamel antagonist and leads to abrasion.³³

High fracture toughness in Y-TZP zirconia may be explained by phase transformation from the tetragonal to monoclinic phase due to external pressure, which leads to a volume expansion of about 3%–5%. It creates compressive strength and prevents cracks spreading and therefore makes it less surface roughness.³⁴

Glazed zirconia showed greater wear compared with polished zirconia, although the surface of glazed zirconia results in smooth, esthetic, and hygienic surface but the glaze layer can be easily removed during function or occlusal adjustment, and the underlying rough ceramic surface exposed and can cause aggressive damage of enamel antagonists.¹⁵,²⁶,²⁷,²⁸,²⁹,³⁰,³¹,³⁵ In some in vivo studies the wear behavior of glazed and polished zirconia prostheses are comparable.³⁶,³⁷,³⁸ SEM images of polished zirconia showed a surface with a more fine-grained and homogeneous texture, therefore polishing techniques decrease the surface roughness of zirconia and subsequently the wear of natural antagonists.²⁷

The low wear rate of enamel antagonist to monolithic zirconia comparing with conventional ones can be explained by lower mechanical properties of this material.⁹

Clinical studies demonstrated disputed wear manners of monolithic zirconia crowns, that some resulted similarity³⁶,³⁷,³⁸ or greater³⁹,⁴⁰ wear of natural teeth antagonist in compare to enamel-enamel tooth contact.

Although it is a common perception that hardness of a material is directly proportional to its abrasiveness, several studies have shown that hardness alone cannot explain the abrasive properties of a substance.¹¹ It seems that the wear of a material is more related to the level of its surface roughness than the hardness, and therefore various studies suggest that surface roughness of porcelain is important factor to predict antagonist wear of porcelains.⁴¹,⁴² Physical grain or crystal size plays an important role in the surface topography and roughness of porcelains, too.

Lohbauer and Reich evaluated the amount of wear on the antagonist occlusal surfaces of clinically placed monolithic zirconia premolar and molar crowns (LAVA Plus, 3M ESPE). They showed mean wear of 200 μm as admissible. The monolithic zirconia crowns (LAVA Plus) showed acceptable antagonist wear rates after 2 years in situ, regardless of natural enamel or ceramics as antagonist materials;⁴³ and of course, monolithic zirconia crowns are well known with low amount of antagonist wear when opposed to monolithic zirconia itself.⁴⁴

The result of Habib et al. investigation showed monolithic zirconia has more wear effect to natural teeth in comparison to lithium disilicate and composite resin.⁴⁵ Pereira et al. demonstrated that the wear of composite resin opposed to monolithic zirconia is greater than opposing bovine enamel.⁴⁶ Gundugollu et al. conducted that glazed polished monolithic zirconia produce more wear of the opposed enamel in comparison to unglazed polished monolithic zirconia.⁴⁷

The results of Esquivel-Upshaw et al. investigation showed that polished monolithic zirconia (Lava Plus) has a similar amount of wearing of opposing enamel to metal-ceramic and enamel antagonists.³⁸

Low-fusing feldspathic porcelains showed lower antagonist wear in comparing with traditional feldspathic porcelains but there was no significant difference between the enamel wear of two low-fusing porcelains.³² These results can be described by large grain particle size of traditional feldspathic porcelain which contributes to a much more irregular surface and therefore an increase in abrasive qualities. The similarity between surface roughness of two low-fusing porcelains can explain the same wear amount of their antagonists.¹⁴,²⁵,³²

Human enamel has been considered the best choice for wear studies, however, there are some complications to using them such as natural variations in shape and physical properties, and therefore natural teeth require extensive preparation and standardization which can be difficult due to their variations. Some studies used industrial hydroxyapatite called steatite as a substrate for human natural enamel and showed that it can be a reliable substrate in wear studies instead of human natural teeth.²⁶,²⁹ Wear measurement of steatite opposed to zirconia and feldspathic groups revealed the same results as enamel specimens.¹⁰

We suggest to researchers to use steatite in further wear studies because it allows the standardization of antagonistic conditions, however, it has some differing mechanical properties such as higher hardness (steatite: 680 HV; enamel: 330 HV) or initial roughness (steatite: 1.7 μm, enamel: 0.9 μm).³⁰

The wear behavior of ceramics in clinical practice is a combination of 2-Body and 3-Body wear, but evaluation of methods of included studies showed that they use either 2-Body or 3-Body wear simulator, and none of them use combination of them to simulate oral conditions near to reality.²⁷,²⁸,⁴⁸ It should be noted that 3-Body wear may reduce the effect of surface roughness due to the presence of a third body, and therefore selecting a proper liquid mediator such as artificial saliva should be considered.²⁶

There are two common methods for decreasing roughness, glazing, and polishing. The amount of roughness is related to use polishing system it would be between 0.08 and 0.9 mm. The roughness of polished zirconia is approximate to glazed one.⁴⁹

Different finishing and polishing procedures for zirconia specimens affected antagonistic enamel wear, vertically and volumetrically;⁵⁰ although there was no significant difference in antagonist wear of feldspathic porcelains by different polishing techniques.¹⁴ This observation can be explained by their different microstructure and surface properties. It should be noted that not all materials can be made smoothed. Large grain porcelains such as feldspathic porcelains are the good samples of these materials.¹⁴,¹⁵,³² Different glazing techniques can cause significantly different antagonists wear of zirconia groups. For example, using of glaze spray (GS) made significantly less enamel's wear than using glaze ceramic (GC). Both GC and GS can provide a thin smooth layer in comparison with unglazed zirconia, but their glaze layer will be worn in a short period, approximately after 6 months under clinical conditions. SEM analysis showed that after removing the glaze layer, the surface under GS was smoother than GC, therefore results in low enamel wear.⁹,³⁰,⁵¹

The zirconia materials show similar composition, but differ in sintering temperature and resulting grain sizes. Especially sintering conditions may influence microstructure, longevity and hydrolytic performance, and finally, wear behavior of material.⁹,¹⁴,¹⁶

The zirconia or feldspathic specimens should be polished before any surface treatment because when the staining becomes worn, a rough surface may accelerate wear.¹⁵,¹⁶,³¹ Finally, this systematic review recommends using polished monolithic zirconia to reduce antagonist wear in clinical practice.

Under the limitations of the present study, there was a significant difference between antagonist wear of zirconia and feldspathic specimens among included studies, which zirconia specimens showed wear-friendly behavior than feldspathic ones, and also among different surface treatments, polishing leads to less enamel wear than glazing.

Financial support and sponsorship

This article is funded Tabriz University of Medical Sciences, Tabriz, Iran.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.

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