Effect of surface treatments on color stability of lithium disilicate: a systematic review and meta-analysis

Introduction

The use of ceramics in dentistry has evolved significantly since its inception in the 19th century, driven by advancements in material properties and bonding techniques. Modern dental ceramics are broadly classified into metal-based and metal-free systems, including layering and press ceramics as well as analog and digitally fabricated restorations. Among these, lithium disi­licate (Li2Si₂O₅) ceramics have gained prominence due to their excellent esthetic and mechanical properties. The IPS e.max® system, introduced in 2005, represents a significant milestone as the first fully integrated all-ceramic system. It is known for its superior optical charac­teristics, variable translucency, and enhanced flexural strength, making it applicable in both press and CAD/CAM technologies. Lithium disilicate ceramics, characterized by an average flexural strength of approximately 400 MPa and excellent translucency, are widely used for anterior and posterior restorations [1]. Esthetic qualities of dental ceramics, including translucency and color matching, are critical factors influencing their clinical success, allowing restorations to blend seamlessly with natural dentition. However, mechanical integrity of lithium disilicate is equally vital in ensuring the long-term durability of restorations [2].
Surface treatments, such as polishing and glazing, play a crucial role in determining the final characteristics of lithium disilicate ceramics, particularly their optical properties, surface roughness, and overall clinical performance [3]. Polishing, typically performed with fine abrasives, aims to smooth the ceramic surface after adjustments made with diamond burs. This process enhances translucency and light transmittance while reducing surface roughness, thereby improving esthetic outcomes [4]. It is particularly recommended for cases requiring occlusal adjustments, as it helps restore the ceramic’s optical properties post-grinding [3], and contributes to the long-term survival of restorations by minimizing the risk of catastrophic failures [5].
Conversely, glazing involves the application of a thin, glassy layer that is fired to create a smooth, glossy finish [4]. Glazed surfaces generally exhibit lower surface roughness and higher flexural strength compared with polished surfaces, making them preferable for restorations, without further occlusal adjustments [4]. The glazing process, performed before final cementation, is critical for maintaining chromatic stability and resistance to staining in ceramic restorations. However, occlusal adjustments made after cementation result in the removal of the glazed layer, increasing surface roughness and altering the restoration’s optical properties by reducing light reflection, thereby impacting the final color perception [6].
Several external factors influence color stability of lithium disilicate ceramics over time, including exposure to thermocycling, staining beverages [6], acce­lerated artificial aging [7], toothbrushing [8], and acidic environments [9]. These simulated oral conditions have been extensively studied to evaluate their impact on the long-term color stability of lithium disilicate restorations.
Given that color alterations can significantly affect the longevity and esthetic quality of restorations [10], understanding the comparative effects of polishing and glazing on color stability is crucial. Although both polishing and glazing have been shown to influence color stability, previous studies have reported inconsistent findings due to differences in experimental designs, aging protocols, and evaluation criteria. This variability underscores the need for a systematic review and meta-analysis to clarify the comparative effectiveness of each surface treatment method.
The aim of this review was to evaluate which surface treatment, i.e., polishing or glazing, offers superior color stability for lithium disilicate ceramics. By synthesizing the current evidence, this review sought to provide a comprehensive understanding of the clinical implications of surface treatment choices in ceramic restorations.
The null hypothesis was that there is no significant difference in the color stability of lithium disilicate ceramic restorations between polished and glazed surface treatments.

Material and methods

This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses for Scoping Reviews (PRISMA-S) guidelines [11]. A comprehensive literature search was performed using three electronic databases: PubMed, Web of Science, and Scopus. The search strategy incorporated various key words, including “dental ceramics,” “color change,” “surface treatment,” and “lithium disili­cate.”
Inclusion criteria for this systematic review and meta-analysis were designed to ensure the selection of relevant and high-quality studies. Only in vitro studies published in English between 2015 and January 1, 2025, were considered. Studies had to focus exclusively on lithium disilicate ceramics, and directly compare polish­ed and glazed surfaces. Additionally, studies were required to report color change (ΔE values) with extractable data, including mean, standard deviation (SD), and sample size (N). Studies evaluating the effects of polish­ing and glazing on color stability as well as those incor­porating aging protocols, such as thermocycling, staining beverages, or accelerated artificial aging, were included.
Studies on zirconia-reinforced lithium disilicate or other ceramic materials not exclusively focused on lithium disilicate, were excluded. Also, research that did not directly compare polished and glazed surfaces, without extractable ΔE data sufficient for meta-analysis, or did not report color stability as a primary or secondary outcome, were excluded. In addition, non-English-written publications, articles published outside the 2015 to January 1, 2025 timeframe, and all reviews, case reports, clinical studies, or studies not conducted in vitro, were omitted. These criteria were established to ensure the inclusion of studies, which directly addressed the research question regarding the effect of surface treatments on the color stability of lithium disilicate ceramics. Figure 1 presents the flowchart of the study selection process.
Statistical analysis was performed using RevMan Web, applying a random-effects model due to expected heterogeneity. Standardized mean difference (SMD) with 95% confidence intervals (CI) was employed for ΔE comparisons, and heterogeneity was evaluated with I² statistics. Confidence intervals were calculated using the Hartung-Knapp-Sidik-Jonkman method, and hetero­geneity (t²) was estimated using the restricted maximum-likelihood method.

Results

A total of 8 studies were included in the meta-analysis (Table 1), comparing the color stability of polished vs. glazed surfaces after aging. The pooled analysis showed that polished lithium disilicate exhibited significantly higher ΔE values compared with glazed lithium disilicate, indicating greater color change over time. The standardized mean difference (SMD) was 1.81 (95% CI: 0.67-2.95, p = 0.007), favoring the glazed group for better color stability (Figure 2).
The heterogeneity among studies was high (I² = 93%), suggesting substantial variability in aging methods, measurement techniques, and polishing/glazing protocols. Despite this high heterogeneity, the findings remained statistically significant, indicating a consistent trend favoring glazed lithium disilicate. The quality of the in vitro studies was assessed using the QUIN tool (Table 1) [12].
Overall, the results suggested that glazing enhances the long-term color stability of lithium disilicate restorations compared with polishing, as polished surfaces tend to undergo more discoloration after aging.

Discussion

This review investigated the effect of surface treatments on the color stability of lithium disilicate ceramics, comparing polished and glazed restorations. According to the results obtained from the studies, polished lithium disilicates exhibited significantly higher ΔE values than glazed lithium disilicates (p = 0.007), indicating a greater degree of color change over time. These results reject the null hypothesis and suggest that glazing is more effective in preserving the long-term color stability.
The results of this meta-analysis further support this trend by categorizing studies based on a clinically perceptible ΔE threshold of 3.7. Studies reporting ΔE ≥ 3.7, including Kanat-Erturk et al. [13] (SMD = 3.34), Gül et al. [14] (SMD = 2.60), Saleh et al. [15] (SMD = 2.72), and Al-Qousy et al. [9] (SMD = 2.19), demonstrated significant differences, suggesting that polishing leads to noticeable color changes. In contrast, studies with ΔE < 3.7, such as Alencar-Silva et al. [8] (SMD = –0.72), Kurt et al. [7] (SMD = 0.43, not significant), Ghazaee et al. (SMD = 1.77), and Sarıyer et al. [16] (SMD = 2.33), reported varied outcomes, with some research still indicting significant differences in color stability.
Most studies included in the meta-analysis favored glazing over polishing for maintaining color stability [7, 9, 13-17]. However, Alencar-Silva et al. [8] reported contrasting outcomes, where polished lithium disilicate exhibited greater color stability than glazed restorations. This discrepancy may be due to variations in surface roughness, as the polished groups in their study exhibited significantly lower roughness values than the glazed groups. In contrast, findings from several other studies suggested that glazing generally results in a smoother surface than polishing, with consistently lower roughness values [4, 18].
The effectiveness of surface treatments in reducing roughness varies depending on the method used. Manual finishing and polishing for 60 seconds as well as the application of a glazing paste, were found to be the most effective techniques for reducing surface roughness in CAD/CAM silica-based glass-ceramics. Additionally, polishing for 60 seconds yielded the highest gloss levels, though the final result varied depending on the specific ceramic material used [19].
Mechanical polishing has been shown to enhance surface smoothness and improve color stability. Systems, such as Eve Diapro and Diatech ShapeGuard, significantly decreased surface roughness and color change compared with untreated surfaces, with no significant difference between the two polishing systems [20]. A study has suggested that both glazing and polishing significantly reduced roughness and root mean square roughness values than untreated surfaces, with no statistically significant differences between the two in some cases [21]. Kulvarangkun et al. [22] found that polishing resulted in smoother surfaces than glazing in lithium disilicate and zirconia, contradicting the general assumption that glazing provides the smoothest finish.
Thermocycling played a significant role in color stability by altering surface texture. Lithium disilicate restorations exhibited increased light reflection after thermocycling, likely due to partial removal or degradation of the staining layer. These changes affected surface texture, leading to reduced color stability and a less uniform esthetic appearance in dental restorations [23]. All studies included in the meta-analysis simulated oral conditions, which were expected to influence the results in both groups.
Moreover, surface treatments affected the optical properties of lithium disilicate ceramics, particularly lightness (L* value). Grinding increased surface roughness, reducing translucency and light transmittance, leading to a decrease in L* values. Conversely, polishing reduced roughness, improved translucency, and potentially increased the L* value [3]. The interaction between surface finishing procedures, staining solutions, and material thickness, substantially influenced L* changes. For example, mechanical polishing combined with immersion in tea solutions showed the highest change in lightness, highlighting how surface treatments can cause noticeable variations in the L* over time [24].
Palla et al. [6] reported that nonglazed pressed lithium disilicate restorations immersed in tea exhibited clinically unacceptable color change (ΔE = 4.99 > 3.7). This was associated with a rougher surface, reduced reflectance, and altered surface crystallinity. Conversely, Alp et al. [25] reported that the color change of polished lithium disilicate was perceptible but clinically acceptable.
The number of glaze firings influenced color stability. A study comparing one versus two glaze firings found that ceramics subjected to two glaze firings exhibited less color change over time than those with only one firing [26]. Furthermore, the composition and translucency of lithium disilicate ceramics also affected their color stability. High translucency materials tended to show more color change compared with low translucency ones, especially when subjected to surface treatments and staining [27].
This review has several limitations, including high heterogeneity among the included studies due to variations in aging protocols and measurement techniques. Additionally, all data were derived from in vitro studies, which may not fully replicate clinical conditions. The limited sample size and variability in surface treatment methods further restrict the generalizability of the findings.

Conclusions

Glazed lithium disilicate ceramics exhibit better long-term color stability than polished surfaces. Their resistance to discoloration supports their preferred use in esthetically demanding restorations.

Disclosures

1. The approval of the Bioethics Committee for the research: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References