Does Repolishing Affect the Gloss and Roughness of Lithium Disilicate and Monolithic Zirconia Ceramics?

Abstract

Purpose Maintaining the surface quality of ceramic restorations after clinical adjustments is critical for both aesthetic outcomes and long-term oral health, yet the optimal approach to restoring gloss and smoothness remains unclear. The purpose of this study is to investigate the effect of different surface finishing and grinding procedures on the surface gloss and roughness of three different monolithic lithium disilicate ceramics and one monolithic ultra-translucent zirconia ceramic. Materials and Methods A total of 104 specimens (1.5 × 12 × 14 mm) were prepared from four ceramic materials: LiSi CEREC Tessera (CT), GC Initial LiSi (LS), IPS e.max CAD (EC), and zirconia disc (KATANA UTML (KAT)). Each was divided into two subgroups based on surface finishing (mechanical polishing or glazing; n = 10). Gloss and surface roughness were measured using a glossmeter and a profilometer, respectively. One specimen per subgroup was analyzed under SEM at ×1000 magnification. Results Gloss and roughness values were analyzed with the two-way robust ANOVA test and multiple comparisons were made with Bonferroni correction. The significance level was set at p < 0.05. Mechanical polishing, glazing, and repolishing increased the gloss values of the materials, with the KAT group achieving the highest gloss in the repolishing groups. The lowest gloss values were observed in the grinding groups. Additionally, these surface treatments reduced the roughness of the surface of all the materials. Conclusions Surface finishing procedures significantly influenced the gloss and roughness of monolithic lithium disilicate and zirconia ceramics. Mechanical polishing systems performed similarly or better than glazing. However, selecting an appropriate polishing system for each material is essential.

1. Introduction

Computer-aided design–computer-aided manufacturing (CAD-CAM) ceramic materials facilitate a fully digital workflow, from impression acquisition to final restoration, offering both clinical reliability and high patient satisfaction [1,2]. Lithium disilicate ceramics are widely favored as monolithic ceramic systems for anterior and posterior single crowns [3]. Although glass ceramics offer aesthetic benefits, there has been a growing demand for stronger ceramic restorations [4]. The utilization of anatomically contoured zirconia restorations, known as monolithic zirconia, without the need for veneer porcelain, is on the rise in dentistry due to its superior optical and mechanical properties [5,6,7]. To overcome the issue of limited translucency in conventional zirconia, a new high-translucency monolithic zirconia has been developed for practical application in clinical use [7].

The final-stage adjustments made on these ceramic restorations using diamond burs are essential for removing occlusal interferences, optimizing the contours of the restorations, and enhancing their aesthetic appearance [8]. These adjustments remove the glaze layer, increasing surface roughness, reducing reflected light, and affecting the restoration’s color and gloss while also raising the risk of external staining [9,10]. Ceramic polishing kits are widely used for smooth-polishing the surfaces of ceramic restorations after adjustment. Studies have explored whether ceramic polishing kits can effectively restore ceramic surfaces to a state similar to the original glazed ceramic surfaces prior to any adjustments [11,12,13]. It has been strongly emphasized that maintaining a smooth ceramic surface during clinical use is crucial for preventing the initiation and progression of microcracks and minimizing the wear of opposing teeth [14]. Therefore, applying a surface finishing procedure after grinding is crucial to restore and enhance the surface properties of ceramic restorations [15,16].

Most recent studies agree that the glazing procedure can produce a sufficiently smooth surface on ceramic restorations [17,18,19,20]. Conversely, the polishing process can yield a surface that closely resembles the properties of a natural tooth [21]. Following adjustments of dental ceramics, the laboratory process for reglazing is a time-consuming process. Moreover, once the restoration has been cemented in the mouth, reglazing is no longer feasible. Consequently, mechanical polishing of the adjusted restoration offers a viable solution to achieve a smooth surface, and it can be conducted both intraorally and extraorally [22]. The search for effective clinical polishing methods after adjusting porcelain restorations is of great importance as it reduces the treatment time and enables cementation to be completed in the same session. Reglazing of the restoration requires multiple visits for the patient, as it is performed by glaze firing in the dental laboratory [23]. Patients’ desire for a natural tooth appearance can be attained by achieving the right contour, smoothness, and glossiness [24]. Maintenance of a smooth restoration is essential not only for the health of the tooth and the surrounding periodontal tissues but also for the aesthetic aspect of the restoration [25]. Conversely, rough surfaces create an environment conducive to plaque accumulation and staining, potentially resulting in plaque-induced gingivitis, secondary cavities, and adverse effects on gloss [26,27]. Surface profilometry is convenient for quantitative assessment of surface roughness [28]. Surface roughness of the restorations is an important factor for bacterial adhesion. It was reported that a further reduction in Ra below a threshold level of 0.2 μm had no effect on supra- and subgingival microbiological adhesion or colonization [29].

Gloss is defined as a specific light intensity reflectance on a surface, with the incident angle equal and opposite to the reflectance angle [30]. A value of 0 GU indicates a completely non-reflective surface, while 100 GU corresponds to a polished opal glass with the highest refractive surface [31]. In addition, gloss measurements at a 60° angle are considered to be more clinically reliable [32]. The existing literature lacks clarity on the most effective surface finishing method to smooth rough surfaces that arise during the clinical adjustment of monolithic all-ceramic restorations. Additionally, there is inadequate information regarding the impact of surface finishing protocols on surface roughness and gloss. The main objective of this study was to examine how various surface finishing and grinding procedures influence the surface gloss and surface roughness of different CAD-CAM restorative materials. The null hypotheses were that for each restorative material (1) there was no significant difference in surface roughness and gloss values between specimens that underwent only polishing and those that underwent only glazing, and (2) mechanically polishing glazed samples after grinding with a diamond bur did not affect surface roughness or gloss values.

2. Materials and Methods

Based on the results of the power analysis (G*Power software v3.1.10, Heinrich Heine University, Düsseldorf, Germany), it was determined that at least 5 specimens were required in each group to achieve 95% confidence (1-α), 95% test power (1-β), and an effect size of f = 0.57797. To ensure reliability, 10 specimens were prepared for each subgroup (n = 10). Each ceramic group was divided into two subgroups based on the surface treatment applied—mechanical polishing and glazing. In total, 104 specimens (n = 104) were prepared, comprising 20 samples for each CAD-CAM material for surface treatment protocols and 6 additional samples per material for SEM analysis. The materials used in the study are listed in

Table 1. Materials used.

Material	Product	Manufacturer	Lot Number
Lithium disilicate ceramic	IPS e.max CAD (EC)	Ivoclar Vivadent, Schaan, Liechtenstein	YB54FL
Lithium disilicate ceramic	CEREC Tessera (CT)	Dentsply Sirona, York, PA, USA	16012436
Lithium disilicate ceramic	GC Initial LiSi (LS)	GC, Tokyo, Japan	2107161
Monolithic zirconia	KATANA UTML (KAT)	Kuraray Noritake, Tainai City, Japan	EFJTM
Mechanical polishing set used for lithium disilicate ceramics	G&Z Instrumente/DPLT 14 RA Set	EVE Ernst Vetter GmbH, Birkenfeld, Germany	473021
Mechanical polishing set used for monolithic zirconia	G&Z Instrumente/DCAT 14 RA Set	EVE Ernst Vetter GmbH, Birkenfeld, Germany	473724
Glaze paste and liquid used for IPS e.max CAD	IPS Ivocolor Glaze Paste	Ivoclar Vivadent, Schaan, Liechtenstein	Z038YF
	IPS Ivocolor Mixing Liquid allround		Z03PP9
Glaze paste and liquid used for CEREC Tessera	Universal Overglaze High Flu	Dentsply Sirona, York, PA, USA	21002882
	Universal Stain and Glaze Liquid		21002448
Glaze paste and liquid used for GC Initial LiSi Block	GC Initial IQ Lustre Pastes ONE/NF	GC, Tokyo, Japan	2205021
	GC Initial IQ Lustre Pastes ONE Diluting Liquid		2204111
Glaze paste used for KATANA UTML	Cerabien ZR FC Paste Stain Glaze	Kuraray Noritake, Tainai City, Japan	EJABU
Diamond bur with fine grain (red band) (27–76 µm grain size) used for grinding	Meisinger 881Z3-017-FG	Hager & Meisinger GmbH, Neuss, Germany	R14777

.

Three different lithium disilicate ceramics were used in the present study: CEREC Tessera blocks (CT) (Dentsply Sirona, York, PA, USA); GC Initial LiSi (LS) Block (GC, Tokyo, Japan); IPS e.max CAD (EC) (Ivoclar Vivadent, Schaan, Liechtenstein); and zirconia disc (KATANA (KAT)). All blocks were selected in A2 color, high translucency (HT), and C14 size. The blocks were cut with a thickness of 1.5 ± 0.05 mm using a diamond cutting disc (Buehler Diamond Wafering Blade, Series 15 LC, Buehler, IL, USA) with a low-speed sectioning device (Isomet 1000, Buehler, Lake Bluff, IL, USA) and samples were prepared in accordance with the block dimensions (1.5 mm × 12 mm × 14 mm). Crystallization for EC was performed using the Programat P310 ceramic furnace (Ivoclar Vivadent, Schaan, Liechtenstein) following the manufacturer’s instructions. The lithium disilicate blocks (LS and CT) were also crystallized. KATANA 5Y-PSZ monolithic zirconia (Kuraray Noritake, Tainai City, Japan) was used in this study, with a disc of 18 mm thickness and A2 color selected based on the sample dimensions. The zirconia specimens were designed using the Exocad DentalCAD 3.0 Galway (Darmstadt, Germany) software and milled on a Yenadent D15 milling machine (Yena Makina, Istanbul, Turkey). After milling, the samples were sintered in a Protherm PLF 110/6 dental furnace (Protherm Furnaces, Ankara, Turkey) according to the manufacturer’s sintering parameters. The thickness of all lithium disilicate and zirconia samples was checked with an electronic caliper (Absolute Digimatic Caliper, Mitutoyo, Kawasaki, Japan). To standardize the surface quality, the specimens were polished using 600-, 800-, 1000-, and 1200-grit silicon carbide papers (3M ESPE, St. Paul, MN, USA) under running water for 30 s. The samples were cleaned in 100% distilled water for 10 min in an ultrasonic cleaner (Ultrasonic Cleaner Cd 4820, Codyson, Shenzhen, China) before finishing.

Mechanical polishing of the lithium disilicate samples was carried out with the G&Z Instrumente DPLT-14 RA (EVE Ernst Vetter GmbH, Keltern, Germany) set, specially produced for mechanical polishing of feldspathic ceramics and lithium disilicate ceramics; it was used in order according to the grain size and color code. Mechanical polishing of the monolithic zirconia specimens was performed with the G&Z Instrumente DCAT-14 RA set (EVE Ernst Vetter GmbH, Germany), which is specially designed for mechanical polishing of zirconia ceramics. The polishing process was applied with a contra-angle rotary instrument (Contra angle 500, Kavo, Germany) under water cooling at a speed of 12,000 rpm according to the manufacturer’s recommendation, and carried out by a single investigator. The specimens were polished for 30 s in one direction and for another 30 s at 90 degrees to the first direction [33,34,35,36].

Glazing applications were performed with the glazing pastes and liquids recommended by the manufacturer for each different ceramic group. All samples were fired in the glaze programs recommended by the manufacturers. The glazed specimens in each ceramic group were ground with a red-striped fine-grit (27–76 µm) diamond bur (Meisinger 881Z3-017-FG, Hager & Meisinger GmbH, Neuss, Germany) especially recommended for grinding ceramic restorations [37,38,39].

Surface roughness measurements of the samples were made using a profilometer device (SJ-210, Mitutoyo, Japan) before the surface finishing procedures (control) and after the mechanical polishing, glazing, and grinding processes were completed. Three parallel readings were performed per sample and the Ra parameter (µm) was evaluated. The surface roughness was further observed by scanning electron microscope analysis (Zeiss, Evo LS10, Jena, Germany) at a ×1000 magnification to investigate the effects of the surface finishing procedures. A glossmeter (Novo Curve Benchtop Glossmeter 60°, Rhopoint, UK) with a 60° angle was used for the gloss evaluation [40,41,42]. Gloss measurements were made of the samples before the surface finishing (control) and after the completion of mechanical polishing, glazing, and grinding. During the measurement, the samples were covered with a black film container to block the external light. Three gloss unit (GU) values were calculated from each specimen and the average value was calculated.

Statistical Analysis

The datasets were analyzed with statistical software (SPSS V23; IBM Corp, Armonk, NY, USA). The conformity to the normal distribution was examined using the Shapiro–Wilk and Kolmogorov–Smirnov tests. The Pearson correlation coefficient was used to analyze the relationship between normally distributed gloss and roughness values, and the Spearman’s rho correlation coefficient was used to analyze the relationship between non-normally distributed gloss and roughness values. The analysis results are presented as mean ± standard deviation (SD) and median (minimum–maximum). The significance level was set at p < 0.05. The descriptive statistics and multiple comparison results of the gloss and roughness values were evaluated according to the interaction of the material and surface treatment in the glazing, grinding, and repolishing groups. Compliance with the normal distribution was analyzed by the Shapiro–Wilk test. Gloss and roughness values that did not conform to the normal distribution according to the ceramic and surface treatment were analyzed with the two-way robust ANOVA test and multiple comparisons were made with Bonferroni correction. The analysis results are presented as the median (minimum–maximum). The significance level was set at p < 0.05.

3. Results

3.1. Surface Roughness (Ra)

The mean ± SD and median (minimum–maximum) values of the surface roughness values obtained after mechanical polishing of the monolithic CAD-CAM ceramic materials used in this study are presented in

Table 2. Comparison of the roughness values after mechanical polishing for each ceramic.

	Control		Mechanical Polishing		Test Statistic	p
	Mean ± SD	Median (Min–Max)	Mean ± SD	Median (Min–Max)
CT	0.19 ± 0.04	0.18 (0.15–0.26) b	0.09 ± 0.01	0.09 (0.07–0.11) ab	Z = −2.803	0.005
LS Block	0.2 ± 0.02	0.19 (0.17–0.26) ab	0.09 ± 0.02	0.08 (0.06–0.13) b	Z = −2.805	0.005
EC	0.2 ± 0.01	0.2 (0.18–0.21) ab	0.09 ± 0.02	0.08 (0.07–0.13) b	Z = −2.805	0.005
KAT	0.25 ± 0.05	0.26 (0.18–0.31) a	0.11 ± 0.02	0.12 (0.09–0.13) a	Z = −2.805	0.005
Test Statistic	${\chi }^2$= 9.594		${\chi }^2$= 11.744
p	0.022		0.008

Z: Wilcoxon test;${\chi }^2$: Kruskal–Wallis test; mean ± SD (standard deviation); median (minimum–maximum); ^a,b: there is no difference between ceramics with the same letter in each column.

. After the mechanical polishing process for all ceramic groups, the median roughness values decreased and a statistically significant difference was found compared to the control readings (p = 0.005). The median roughness value decreased for CT from 0.18 µm to 0.09 µm; for LS from 0.19 µm to 0.08 µm; for EC from 0.2 µm to 0.08 µm; and for KAT from 0.26 µm to 0.12 µm.

The mean ± SD and median (minimum–maximum) values of the surface roughness values obtained after glazing, grinding, and mechanical polishing (repolishing) of the monolithic CAD-CAM ceramic materials used in this study are presented in

Table 3. Comparison of the roughness values according to the ceramic and the process applied to each ceramic (glazing, grinding, and repolishing).

	Control	Glazing	Grinding	Repolishing	Test Statistic	p
CT	0.19 ± 0.01	0.13 ± 0.02	0.81 ± 0.08	0.16 ± 0.01	${\chi }^{2\ast }$= 29.727	<0.001
	0.18 (0.17–0.21) BC	0.13 (0.11–0.16) A	0.82 (0.67–0.93) B	0.17 (0.15–0.18) AC(a)
LS Block	0.19 ± 0.03	0.12 ± 0.01	0.84 ± 0.08	0.18 ± 0.01	${\chi }^{2\ast }$= 27.12	<0.001
	0.19 (0.16–0.24) BC	0.13 (0.11–0.14) A	0.84 (0.71–0.96) B	0.18 (0.17–0.2) AC(b)
EC	0.19 ± 0.02	0.12 ± 0.03	0.77 ± 0.12	0.17 ± 0.02	${\chi }^{2\ast }$= 26.04	<0.001
	0.19 (0.15–0.23) BC	0.13 (0.09–0.18) A	0.76 (0.61–0.98) B	0.17 (0.15–0.2) AC(ab)
KAT	0.24 ± 0.05	0.11 ± 0.03	0.72 ± 0.13	0.16 ± 0.02	${\chi }^{2\ast }$ = 30	<0.001
	0.24 (0.17–0.35) BC	0.11 (0.07–0.14) A	0.74 (0.47–0.9) B	0.17 (0.11–0.18) AC(a)
Test Statistic	${\chi }^2$= 8.693	${\chi }^2$= 5.587	${\chi }^2$= 6.461	${\chi }^2$= 12.459
p	0.051	0.134	0.091	0.006

${\chi }^{2\ast }$: Friedman test; ${\chi }^2$: Kruskal–Wallis test; mean ± standard deviation; median (minimum–maximum); ^a,b: no difference between ceramics with the same lower case letter within each column; ^A–C: no difference between roughness values with the same upper case letter within each row.

. No significant differences (p < 0.05) were observed among the ceramic materials in the control, glazing, and grinding protocols. A statistically significant difference was found between the median roughness values in the repolishing groups according to the ceramics (p = 0.006). The value obtained for LS Block was higher and differed from the roughness value obtained from CT and KAT. There was no statistically significant difference (p < 0.05) between EC and LS Block. A statistically significant difference was observed in the median surface roughness values across all ceramic materials based on the applied surface treatment protocols (p < 0.001). While the highest roughness values were obtained in the grinding process, they differed from the values of the repolishing and glazing groups. No statistically significant difference (p < 0.05) was found between the repolishing and glazing groups. The roughness values of the glazing groups were lower than the control readings and showed significant differences. The mean ± SD and median (minimum–maximum) values of the surface roughness values obtained after glazing and mechanical polishing of the materials and the differences between the groups are presented in

Table 4. Comparison of roughness values according to the ceramic and surface finishing process.

	Glazing		Mechanical Polishing		Test Statistic	p
	Mean ± SD	Median (Min–Max)	Mean ± SD	Mean (Min–Max)
CT	0.13 ± 0.02	0.13 (0.11–0.16)	0.09 ± 0.01	0.09 (0.07–0.11) ab	U = 0.000	<0.001
LS Block	0.12 ± 0.01	0.13 (0.11–0.14)	0.09 ± 0.02	0.08 (0.06–0.13) b	U = 5.000	0.001
EC	0.12 ± 0.03	0.13 (0.09–0.18)	0.09 ± 0.02	0.08 (0.07–0.13) b	U = 14.000	0.006
KAT	0.11 ± 0.03	0.11 (0.07–0.14)	0.11 ± 0.02	0.12 (0.09–0.13) a	U = 44.500	0.677
Test Statistic	${\chi }^2$= 5.587		${\chi }^2$= 11.744
p	0.134		0.008

U: Mann–Whitney U test; ${\chi }^2$: Kruskal–Wallis test; mean ± SD (standard deviation); median (minimum–maximum); ^a,b: there is no difference between ceramics with the same letter within each column.

. A statistically significant difference was observed in the surface roughness values of all lithium disilicate ceramic groups, depending on whether glazing or polishing was applied (p < 0.05). The median surface roughness values in the mechanical polishing groups were lower than those in the glazing groups. No statistically significant difference was found in the surface roughness values of the KAT ceramic following the different surface finishing procedures (p = 0.677). Two-way analysis of variance (ANOVA) revealed that neither the ceramic type (p = 0.766) nor the interaction between ceramic type and surface treatment (p = 0.217) had a statistically significant effect on the surface roughness value. However, the surface treatment protocol itself showed a statistically significant effect on the Ra value (p < 0.001).

3.2. Surface Gloss (GU)

The mean ± SD and median (minimum–maximum) values of the gloss values obtained after mechanical polishing of the monolithic CAD-CAM ceramic materials used in this study are presented in

Table 5. Comparison of the gloss values after mechanical polishing for each ceramic.

	Control		Mechanical Polishing		Test Statistic	p
	Mean ± SD	Median (Min–Max)	Mean ± SD	Median (Min–Max)
CT	64.37 ± 6.81	64.82 (54.2–75.33)	104.33 ± 3.03 c	103.77 (99.97–111.6)	t = −18.555	<0.001
LS Block	62.79 ± 6.94	60.15 (55.43–76.47)	102.34 ± 4.27 bc	102.6 (94.1–108.17)	t = −12.003	<0.001
EC	62.37 ± 5.9	61.03 (54.37–75.9)	95.76 ± 5.66 b	95.37 (85.4–103.47)	t = −13.666	<0.001
KAT	69.29 ± 8.37	68.25 (56.93–84.73)	161.7 ± 15.72 a	167.25 (129.37–179.93)	t = −16.035	<0.001
Test Statistic	F = 2.023		F = 48.902
p	0.128		<0.001

t: Paired two-sample t test; F: one-way analysis of variance; mean ± SD (standard deviation); median (minimum–maximum); ^a–c: there is no difference between ceramics with the same letter in each column.

. No statistically significant difference was found in the mean gloss values of the control readings across the different ceramic materials (p = 0.128). A statistically significant difference was observed in the mean gloss values after mechanical polishing, depending on the type of ceramic material (p < 0.001). Here, the highest average value was obtained in the KAT ceramic and it differs from other ceramics (161.7 GU) (KAT > CT > LS∼EC). A statistically significant difference was observed between the average gloss values after mechanical polishing and those of the control readings in all the ceramic groups (p < 0.001). Mechanical polishing increased the gloss values of all the ceramic materials.

The mean ± SD and median (minimum–maximum) values of the gloss values obtained after glazing, grinding, and mechanical polishing (repolishing) of the monolithic CAD-CAM ceramic materials used in this study are presented in

Table 6. Comparison of the gloss values according to the ceramic and the process applied to each ceramic (glazing, grinding, and repolishing).

	Control	Glazing	Grinding	Repolishing	Test Statistic	p
CT	64.2 ± 5.54	101.98 ± 2.71	14.13 ± 0.35	86.22 ± 3.68	${\chi }^{2\ast }$ = 30	<0.001
	62.55 (58.83- 75.77) BC(ab)	102.95 (96.77–105.07) A	14.03 (13.8–15) B(ac)	84.77 (83.2–94.57) AC(b)
LS Block	67.95 ± 6.7	101.29 ± 2.43	13.46 ± 0.17	84.34 ± 2.15	${\chi }^{2\ast }$ = 30	<0.001
	68.12 (58.07–76.5) BC(ab)	100.37 (97.3–105.87) A	13.42 (13.23–13.73) B(b)	83.7 (81.63–89.07) AC(b)
EC	62.05 ± 5.9	100 ± 1.8	13.92 ± 0.25	87.55 ± 7.56	${\chi }^{2\ast }$= 28.92	<0.001
	59.85 (54.23–71.93) BC(b)	99.6 (98.07–102.53) A	13.88 (13.63–14.37) B(bc)	85.57 (79.03–102.73) AC(b)
KAT UTML	72.34 ± 3.71	101.84 ± 2.62	15.9 ± 1.75	104.92 ± 9.95	${\chi }^{2\ast }$= 27.12	<0.001
	72.38 (66.87–76.9) BC(a)	102 (98.2–107.4) AC	15.3 (13.83–18.53) B(a)	104.6 (89.8–122.13) A(a)
Test Statistic	${\chi }^2$= 13.537	${\chi }^{2 }$= 5.702	${\chi }^2$= 28.303	${\chi }^{2 }$= 20.75
p	0.004	0.127	<0.001	<0.001

${\chi }^{2\ast }$: Friedman test; ${\chi }^2$: Kruskal–Wallis test; mean ± standard deviation; median (minimum–maximum); ^a–c: no difference between ceramics with the same lower case letter within each column; ^A–C: no difference between gloss values with the same upper case letter within each row.

. A statistically significant difference was observed in the median gloss values of the control readings among the different ceramic materials (p = 0.004). The median value obtained in the EC ceramic and the value obtained in the KAT ceramic showed a significant difference. No statistically significant difference was found in the median gloss values of the glaze groups among the different ceramic materials (p = 0.127). A statistically significant difference was observed in the median gloss values of the grinding groups across the different ceramic materials (p < 0.001). While the highest gloss value was obtained in KAT, it did not differ from the value obtained in EC and LS Block, and there was no difference between KAT and CT. A statistically significant difference was observed in the median gloss values of the repolishing groups among the different ceramic materials (p < 0.001). Here, the value obtained in the KAT ceramic was higher than the gloss values obtained from all the other ceramics (104.6 GU). A statistically significant difference was found between the median gloss values obtained from different surface treatments in all ceramic groups (p < 0.001). The values of the control readings and glazing groups differed from each other for all lithium disilicate ceramic materials used in this study; gloss median values increased with glaze application. The gloss values in the grinding groups were significantly lower than those in the glazing and repolishing groups. The gloss values, which declined following the grinding process, significantly increased after repolishing. No significant difference was found between the glazing and repolishing groups for any of the ceramic materials.

The mean ± SD and median (minimum–maximum) values of the gloss values obtained after glazing and mechanical polishing of the materials and the differences between the groups are presented in

Table 7. Comparison of gloss values according to the ceramic and surface finishing process.

	Glazing		Mechanical Polishing		Test Statistic	p
	Mean ± SD	Median (Min–Max)	Mean ± SD	Median (Min–Max)
CT	101.98 ± 2.71	102.95 (96.77–105.07)	104.33 ± 3.03	103.77 (99.97–111.6) b	U = 27.50	0.088
LS Block	101.29 ± 2.43	100.37 (97.3–105.87)	102.34 ± 4.27	102.6 (94.1–108.17) b	U = 38.00	0.364
EC	100 ± 1.8	99.6 (98.07–102.53)	95.76 ± 5.66	95.37 (85.4–103.47) b	U = 27.00	0.082
KAT UTML	101.84 ± 2.62	102 (98.2–107.4)	161.7 ± 15.72	167.25 (129.37–179.93) a	U = 0.00	<0.001
Test Statistic	${\chi }^2$= 5.702		${\chi }^2$= 28.879
p	0.127		<0.001

U: Mann–Whitney U test; ${\chi }^2$: Kruskal–Wallis test; mean ± SD (standard deviation); median (minimum–maximum); ^a,b: there is no difference between ceramics with the same letter within each column.

. A statistically significant difference was found between the gloss values obtained in the KAT ceramics after glazing (102 GU) and mechanical polishing (167.25 GU) (p < 0.001). For CT, LS Block, and EC, no significant difference was found between the groups (p > 0.050).

Two-way ANOVA demonstrated that the effect of ceramic type, surface treatment, and ceramic- surface treatment interaction was statistically significant on the gloss variable (p < 0.001).

3.3. Correlation Between Gloss and Surface Roughness

No statistically significant correlation was found between the gloss and roughness values of the control readings (p = 0.683). There was no statistically significant correlation between the gloss and roughness values after glazing (p = 0.539). There was no statistically significant relationship between the gloss and roughness values after grinding (p = 0.069). A statistically negative correlation was found between the gloss and roughness values after repolishing (r = −0.398; p = 0.011) (

Table 8. Correlation between gloss and surface roughness values.

		Control—Gloss
Control—Roughness	r	0.046
	p	0.683
		Glazing—Gloss
Glazing—Roughness	r*	0.070
	p	0.539
		Grinding—Gloss
Grinding—Roughness	r	−0.290
	p	0.069
		Repolishing—Gloss
Repolishing—Roughness	r	−0.398
	p	0.011

r: Spearman’s rho correlation coefficient; r*: Pearson correlation coefficient.

).

3.4. SEM Evaluation

Shallow scratches and grooves were clearly visible in the control images for all ceramic types. Mechanical polishing enabled a significant reduction in irregularities and scratches. The glazed specimens had a smooth surface texture with no significant grooves or scratches compared to the control readings, although some small voids were observed. Glazing reduced the surface roughness considerably. The images of the grinding groups showed that the homogeneous structure of the glaze layer was destroyed. Grinding with a diamond bur significantly altered the surface morphology. Morphological changes such as intense cracks, grooves, and scratches parallel to the direction of movement of the bur were clearly visible. By applying repolishing after grinding, the surfaces were gradually smoothed, but some deep grinding grooves could not be completely removed and some streaks were observed. Repolished surfaces were noticeably smoother than the ground surfaces (Figure 1, Figure 2, Figure 3 and Figure 4).

4. Discussion

The primary aim of this study was to evaluate the impact of various surface finishing and grinding procedures on the surface gloss and roughness of three types of monolithic lithium disilicate ceramics and one monolithic ultra-translucent zirconia ceramic. This investigation is particularly important because ceramic restorations often undergo intraoral adjustments during clinical procedures, which can compromise their surface integrity. Ensuring that these restorations maintain optimal surface properties—such as smoothness and gloss—is essential for achieving aesthetic satisfaction, minimizing plaque accumulation, and prolonging the lifespan of the restoration. However, there remains a lack of consensus in the literature regarding the most effective method to restore or enhance surface quality following such adjustments. This study contributes valuable insights by comparing the performance of mechanical polishing, glazing, and repolishing protocols across different ceramic materials. Based on the study’s outcome, glazing and mechanical polishing had a significant effect on the roughness and gloss values of the ceramic materials (p < 0.050), thereby rejecting the first null hypothesis. According to the results from the glazing, grinding, and repolishing groups, the second null hypothesis is partially rejected. After grinding the glaze layer, the mechanical polishing process was significantly able to eliminate the adverse effects of the adjustments. Since mechanical polishing and glazing have varied effects on the smoothness and appearance of dental ceramic surfaces, it was interesting to evaluate in what distinct ways the finishing processes affected surface gloss and roughness [43,44,45]. Roughness and gloss assessments allow dental ceramics to be analyzed superficially and compared in terms of surface characteristics after surface finishing. Surface roughness and gloss are two essential components that contribute to a restoration’s aesthetic appearance [46,47].

Despite advancements in CAD-CAM equipment, chairside adjustments—such as grinding, polishing, and glazing—are frequently required during prosthetic rehabilitation to refine the emergence profile and occlusal or proximal relationships of restorations [48]. Occasionally, additional surface modifications may be necessary even after the restoration has been glazed and permanently cemented to address minor interferences. For long-term clinical success, the surfaces of all ceramic restorations must be adequately smoothed [37,49]. Well-finished surfaces minimize technical and aesthetic complications by enhancing material hardness [50,51], gloss [46], translucency, and color stability [47,52]. A smooth surface also significantly improves patient comfort. Studies have shown that surface roughness (Ra) values exceeding the critical threshold of 0.2 μm are associated with increased risks of dental caries, plaque accumulation, and periodontal inflammation [53]. As a result, numerous investigations have examined the impact of various finishing and polishing techniques on the surface roughness of dental restorative materials [54,55,56]. Multiple studies consistently report that mechanical polishing not only matches but often surpasses glazing [42,57,58,59] in producing smooth ceramic surfaces. Kilinc and Turgut, for example, found that manual polishing techniques yielded outcomes comparable to those achieved with glazing [60]. According to Akar et al., using polishing kits and pastes for clinical polishing of ceramics can serve as a secure and efficient substitute for glazing procedures, particularly concerning surface roughness [61]. Mosallam et al. recommended polishing as an alternative to glazing for IPS e.max Press restorations [62]. In a study investigating the effect of polishing and glazing on surface roughness for two distinct hybrid ceramics and a lithium disilicate glass ceramic (EC), the polished surfaces demonstrated lower and clinically acceptable surface roughness values compared to the glazed surfaces across all materials [63]. Numerous studies have shown that mechanical polishing significantly reduces the surface roughness of zirconia, and that the application of polishing results in acceptable intraoral roughness values [64,65,66]. Similar to the previously mentioned studies, our research found that mechanical polishing led to lower surface roughness values than glazing for all lithium disilicate ceramic materials. Furthermore, the average roughness values of all polished ceramics were within the acceptable threshold for intraoral applications (critical value: 0.2 µm), with the following results: CT—0.09 µm, LS Block—0.08 µm, EC—0.08 µm, and KAT—0.12 µm. As a result, the study concluded that mechanical polishing can be a viable alternative to glazing. Zirconia, with its homogeneous polycrystalline structure and high hardness, presents challenges for conventional polishing systems as they may not achieve a satisfactory polishing effect [67,68,69]. Numerous studies have shown that zirconia-specific polishing systems outperform universal polishing systems when used on zirconia restorations. These specialized polishing sets are equipped with abrasives, such as diamonds, SiC, and Al₂O₃, that are highly effective in dealing with the elevated hardness of zirconia [70,71,72,73]. According to Sasahara et al., the variations in ceramic microstructures make it highly challenging to determine the best polishing technique for each ceramic [23]. A study by Al-Shammery et al. confirmed that different materials require different polishing techniques [74]. In our study, the mechanical polishing and glazing groups exhibited comparable average surface roughness values for the monolithic zirconia ceramic samples. This finding aligns with previous research by several investigators, who also reported similar surface roughness values for both polishing and glazing of monolithic zirconia materials [75,76]. Nonetheless, the outcomes for our lithium disilicate samples were distinct from those obtained for the zirconia samples. These disparities could potentially be ascribed to the different polishing sets utilized or variations in the microstructure of the materials, as indicated in the previously mentioned research [69,70]. Several studies in the literature have examined the effectiveness of mechanical polishing versus glazing after ceramic surfaces have been adjusted with a diamond bur. One study [34] reported that monolithic zirconia specimens, when repolished or overglazed after grinding, exhibited comparable surface roughness values within clinically acceptable limits, supporting the use of chairside mechanical polishing as a viable method to restore smoothness after adjustments [34]. Similarly, research on a newer generation of translucent zirconia [77] found no significant difference in surface roughness between specimens that underwent mechanical polishing and those that were glazed following diamond-bur grinding [77]. Another study [78] comparing monolithic zirconia and feldspathic ceramics demonstrated that samples polished after grinding showed roughness values similar to their glazed counterparts [78]. In a separate investigation on lithium disilicate ceramics (IPS e.max Press) [79], immediate mechanical polishing after glaze removal with a fine-grit diamond bur produced surface roughness levels comparable to those achieved through glazing [79]. Lawson et al. [80] also reported that, for both zirconia and lithium disilicate ceramics, mechanical polishing and glazing following grinding yielded similar roughness values, while the ground-only group exhibited the highest roughness 80]. In agreement with these findings, our study demonstrated that all ceramic materials tested—both lithium disilicate and monolithic zirconia—achieved clinically acceptable surface roughness values after glazing and after grinding followed by mechanical polishing. These results support the effectiveness of mechanical polishing as a chairside alternative to glazing for restoring surface quality.

Surface gloss is one of the desirable properties for restorative materials to mimic the appearance of enamel [81]. The American Dental Association considers a gloss of 40 to 60 GU to be appropriate for dental restorations [40]. Cook and Thomas reported that a poorly finished restoration is generally considered to be below 60 GU, while an acceptable finish is between 60 and 70 GU, and a finish above 80 GU is considered excellent [82]. A gloss value of more than 70 GU means that the human eye cannot distinguish between high and very high gloss [83]. In other words, a material that reaches 70 GU does not appear to be brighter than a material that reaches 90 GU. In contrast to roughness, a clinically accepted threshold for gloss in terms of GU has not yet been established. Nevertheless, some data are available for enamel surface gloss. When the visual luster of different composites was compared to and approximated natural enamel, the gloss of the latter ranged from 40 to 47 GU. In this study, after mechanical polishing, glazing, and repolishing processes, gloss values higher than 80 GU with an excellent finish were obtained in all materials [83].

Chavali et al. [72] conducted a study to evaluate the effects of different polishing systems on the surface roughness and gloss of conventional zirconia. Glazed specimens were used as the control group and were mechanically polished after being ground with a fine-grit diamond bur. The polishing process led to gloss values that were clinically acceptable and either comparable to or better than the glazed control specimens [72,84,85]. Algahtani investigated the impact of various surface finishing techniques on the surface roughness and gloss of different ceramic materials. The gloss values obtained in the glaze and mechanical polishing after grinding groups were within the clinically acceptable values for EC ceramic, as we found in our study [86]. The gloss values achieved by glazing, mechanical polishing, and repolishing for all the ceramic groups in our study were within acceptable intraoral limits, indicating that these procedures can be successfully applied in dental restorations for achieving clinically satisfactory outcomes. In a separate study [87], evaluating the gloss retention and surface roughness of contemporary aesthetic CAD-CAM dental restorative materials, the average gloss value of monolithic translucent zirconia was found to be higher than that of IPS e.max CAD (EC) following polishing. This difference was attributed to zirconia’s high refractive index and whiteness, which enhance light reflection 87]. In a similar study, Lee et al. [88] reported that the gloss values of polished monolithic zirconia ceramics were higher than those of the lithium disilicate ceramic groups [88]. In line with previous research, our study also showed that the gloss attained through mechanical polishing for monolithic zirconia was significantly higher than that achieved by EC. The different responses of lithium disilicate and zirconia ceramics to surface treatments can be attributed to their distinct microstructures and compositions. Lithium disilicate ceramics (e.g., EC, LS) contain a glassy matrix and are less hard, allowing polishing to create smoother, glossier surfaces [3]. In contrast, zirconia (e.g., KAT) is a fully crystalline, harder material with no glassy phase, making it more resistant to polishing and requiring specialized protocols to achieve similar gloss. These intrinsic differences explain the variation in surface outcomes after identical treatments [47].

Heintze et al. [46] conducted a study to investigate the relationship between surface roughness and gloss in various dental materials. The study revealed that surface roughness is a contributing factor to gloss, but it is not the sole determinant. In other words, high surface roughness does not always mean low gloss, and vice versa [46]. Monaco et al. [40] investigated the impact of prophylactic polishing pastes on leucite-reinforced glass ceramics, lithium disilicate glass ceramics, and zirconia, focusing on roughness and gloss [40]. They observed a negative relationship between surface roughness and gloss. Similar findings have been reported in other studies concerning the mechanical polishing of feldspathic and hybrid CAD-CAM ceramics; Monaco et al. [40] found that the relationship between 2D roughness and gloss was statistically significant and negatively correlated: roughness increased as surface gloss decreased. The correlation between 2D roughness and translucency was weak for e.max. Further, as a novel study we also found a negative correlation between gloss and roughness in our own study [89,90]. If the restoration was subjected to furnace glazing, the procedure would require a considerable delay of cementation or, more likely, a second appointment. This could save operator time but cannot be considered an effective chairside procedure. [85]. However, there is no “gold standard” finishing and polishing material and/or technique patterned in the literature. In order to improve the comparison of data concerning the efficacy of finishing and polishing systems, it would be useful to standardize methodologies among studies [91].

The difference in gloss values observed between mechanically polished and glazed ceramics can be attributed to both the mechanism of surface modification and inherent material properties. Mechanical polishing involves gradual abrasion using finer-grit abrasives, resulting in a uniform and microscopically smoother surface. This process effectively removes surface irregularities and flattens the microstructure, enhancing specular light reflection, which directly correlates with higher gloss levels [90]. In contrast, glazing relies on applying and firing a thin glassy layer over the ceramic. While it improves esthetics and seals porosities, the glaze layer may exhibit microcracks, uneven thickness, or surface waviness—especially after clinical adjustments—which can diffuse reflected light and reduce gloss [92,93]. Moreover, the ceramic material itself plays a crucial role. Lithium disilicate ceramics (e.g., EC, LS) consist of a fine crystalline phase embedded in a glassy matrix, allowing for more effective polishing due to their relatively homogeneous and softer microstructure). In contrast, zirconia ceramics (e.g., KAT) are fully crystalline, harder, and denser, making them more resistant to mechanical abrasion and more difficult to polish to the same degree of smoothness [37]. These differences impact the surface topography after finishing, and consequently, the gloss outcome.

This study has some limitations. The main limitation of this study is that it was conducted in vitro. While linear roughness parameters such as Ra have been widely used to characterize surfaces, it is also true that these alone are insufficient to fully understand most of the features present in the topography of a material. Therefore, 3D profilometry could be used in future investigations. The specimens were not milled using CAD-CAM but prepared with a low-speed sectioning device. In contrast to our study, the majority of surfaces on crowns or bridges are anatomically curved. For forthcoming investigations, a specialized apparatus capable of enabling standardized pressure during polishing could be employed. This study was performed purely in vitro. While the roughness values (<0.2 µm) and gloss (>60 GU) met clinically acceptable thresholds, the absence of biological testing (e.g., bacterial adhesion, wear resistance) limits direct clinical applicability [92,93]. Future work should consider these factors. The findings of this research are applicable solely to the specific zirconia and lithium disilicate brands utilized in the study and the testing conditions employed. The performance of other ceramic brands might vary due to variations in grain size, sintering temperature, or phase stability.

5. Conclusions

Both mechanical polishing and glaze application significantly affected the surface roughness and gloss of the materials. Surface roughness decreased substantially after surface finishing, reaching clinically acceptable levels (<0.2 μm), while gloss values increased notably, exceeding the clinically acceptable threshold (>60 GU). Among the materials tested, KAT achieved the highest gloss values in the mechanical polishing and repolishing groups. The highest roughness values were observed in KAT for the mechanical polishing groups and LS Block for the repolishing groups.

The mechanical polishing performed after the diamond-bur adjustment of the glaze layer effectively eliminated the negative effects of grinding. A ceramic surface roughened by grinding following clinical adjustments can be restored to its optimal condition through mechanical polishing rather than reglazing.

Clinical Considerations

Manual polishing systems enable chairside restorations in a single session, delivering results comparable to or even better than glazing. This approach allows clinicians to avoid heat treatment and streamline the workflow, which is especially beneficial with the increasing popularity of monolithic restorations. By choosing the appropriate polishing system for the specific ceramic material, clinicians can achieve outcomes as effective as glazing and dependable in clinical practice.