Effect of Different Surface Treatments on the Surface Roughness and Gloss of Resin-Modiﬁed CAD/CAM Ceramics

: The purpose of this in vitro study is to compare the roughness and gloss of four resin-modified CAD/CAM ceramics after different surface treatments. Forty-eight specimens (1.20 × 12 mm 2 ) were prepared from Lava Ultimate (LU), Vita Enamic (VE), Cerasmart (CS), and Crystal Ultra (CU) CAD/CAM ceramics. The prepared specimens were polished with silicon carbide paper before being roughened with a 30 µ m grit diamond bur. Each material was allocated into four groups: control (no treatment), Luster Meisinger polishing (MP), Optiglaze (OG), or Meisinger polishing, followed by a ﬁnal polishing with Shofu Direct Dia Paste (MP+PP). The roughness and gloss were measured after the surface treatment using a proﬁlometer and gloss meter, respectively. Scanning electron microscopy micrographs were obtained to analyze the surface characteristics. Statistical analysis was performed using a multivariate analysis of variance (MANOVA), one-way ANOVA, and Dunnett’s post hoc test ( α = 0.05). The surface treatments signiﬁcantly affected the surface roughness and gloss of the tested materials ( p < 0.05). All the tested resin-modiﬁed CAD/CAM ceramics demonstrated a lower surface roughness and higher gloss after glazing using OG, followed by MP+PP and MP. The highest and lowest Ra were presented by the control VE (0.63 ± 0.23 µ m) and glazed LU specimens (0.04 ± 0.01 µ m), respectively. The highest and lowest GU were presented by the glazed LU (90.48 ± 4.69 GU) and the control VE specimens (23.32 ± 2.41 GU), respectively. After clinical adjustment, ﬁnishing, and polishing, the restoration is essential to obtain a restorative surface with lower roughness and the highest gloss. Surface glazing using Optiglaze exhibited the smoothest and glossiest surface of all the tested resin-modiﬁed ceramics.


Introduction
Digital technology is a significant breakthrough in the fast-paced dentistry field that has increased the quality of dental treatment and oral appliances in terms of time, effort, and cost-effectiveness [1,2].Clinicians should be familiar with the strengths and limitations of different dental restorative materials, as digital dentistry has become part of routine dental care.Ceramic is one of the four main materials used to fabricate dental restorations to replace lost or damaged tooth structures.Dental ceramics have evolved from fragile materials based on natural minerals to the highly durable synthetic ceramics available at present [3].Based on their composition, dental ceramics are classified into glass-matrix, polycrystalline, and resin-modified ceramics [3].Resin-modified ceramics are the latest addition to the group, combining the advantages of both ceramics and resin composites [3,4].
A wide range of CAD-CAM resin-modified ceramic materials have been introduced after the technological innovation and material revolution [5][6][7].The combination of an organic resin matrix and various proportions of inorganic ceramic fillers has led to materials exhibiting different physical, mechanical, and optical properties [8,9].Lava Ultimate (LU) and Cerasmart (CS) are resin composites that contain glass particles embedded in a polymer matrix.Vita Enamic (VE) is an example of a porous, interconnected, feldspathic porcelain infiltrated by a polymer [10,11].A recently developed resin-modified ceramic, Crystal Ultra (CU), comprises 30% cross-linked polymer blends and 70% ceramic-like inorganic silicate glass fillers [4].Comparatively, CU has a higher polymer content than other resinmodified ceramics, such as Vita Enamic, which only contains about 50% of CU's polymer content [4,12,13].
In clinical situations, after the try-in of the prosthetic crown, the occlusal or interproximal necessary corrections are mostly performed with a diamond bur.This affects the properties of the restoration by increasing its surface roughness [14,15], and thereby negatively affecting the optical properties of the restoration [16,17].The clinical adjustments result in micro-irregularities that promote microbial bio-film adhesion [18].The roughened surface must be re-smoothed and re-polished using the best method currently available for each material class to ensure clinical success [19,20].The manufacturer of resin-modified ceramic materials recommends using diamond-impregnated rubber points for finishing, followed by polishing with a #9 soft Robinson bristle brush and a polishing agent.Another option is to apply a light-polymerized glazing agent over the modified surfaces to regain the surface gloss [21].
It is crucial to provide the patient with a smooth restoration for the health of the tooth and the periodontium, in addition to the esthetics of the restoration [22].On the other hand, rough surfaces provide a suitable environment for plaque accumulation and discoloration, and this might lead to plaque-induced gingivitis and secondary caries, and can negatively affect translucency [23,24].Surface roughness should be minimized to achieve a healthy oral cavity with pleasant esthetics.It is recommended to have the surface roughness below 0.2 µm and a minimal plaque-retentive restoration [25].
The surface roughness of dental materials has been assessed using profilometry, scanning electron microscopy, and atomic force microscopy [20,26,27].Nevertheless, the profilometer is frequently used because it provides a quantitative measurement of surface characteristics [28].The most frequently used roughness parameter is the average arithmetic height (Ra), which is easy to define and calculate and provides a good profile of height variations [23,29].Since voids and irregularities complicate the measurements, it is well-known that studying surface roughness is challenging [30].
Gloss is defined as "a specific light intensity reflectance on a surface with the incident angle equal and opposite to the reflectance angle" [31].After the adjustment of the final restoration and re-polishing it in clinical conditions, the gloss of the crown is affected.The gloss of conventional resin composite materials deteriorates faster than ceramics [29].In a study by Lawson et al. [32], the gloss of resin-modified ceramics (VE and LU) was compared with lithium disilicate ceramic.They concluded that these materials have a gloss slightly lower than that of lithium disilicate ceramic but higher than a composite.Another study conducted by Mormann et al. [30] demonstrated that lithium disilicate maintained its gloss, while the gloss of the resin-ceramic (VE and LU) decreased after toothbrush abrasion.Surface gloss is highly dependent on surface roughness, and both depend on the material itself, its particles' size, and the polishing technique applied [32,33].In addition, using a polishing paste can also affect surface gloss [34,35].
The gloss of restorative materials is measured using a gloss meter, and the results are expressed in gloss units (GU).The maximally refractive surface has 100 GU, while the nonreflective surface has zero GU [36].The gloss of natural tooth enamel ranges between 40 and 53 GU [37].According to the American Dental Association (ADA), the gloss of dental restoration should be in the acceptable range of 40-60 [37,38].The gloss of resin-modified ceramic restorations is between 56 and 57 GU, which is within the acceptable range [30].
The surface treatment of conventional ceramics in dental practice has been studied extensively.In recent years, the use of resin-modified ceramics for the fabrication of dental restorations has increased enormously.However, there is a research gap in the literature regarding the application of the best chair-side surface treatment for these ceramics, especially with new materials being introduced to the dental market on a regular basis.Therefore, this in vitro study aims to assess the surface roughness and gloss of four resin-modified CAD/CAM ceramic materials after different surface treatment protocols.The null hypothesis states no significant difference in roughness and gloss of resin-modified CAD/CAM ceramics following different surface treatments.

Materials and Methods
Four different resin-modified CAD/CAM ceramic materials, Lava Ultimate Restorative, Vita Enamic, Cerasmart, and Crystal Ultra, were evaluated.The tested materials are detailed in Table 1.CAD/CAM blocks were milled into 12 mm cylindrical blocks using a milling device (Ceramill Motion 2, Amann Girrbach, Koblach, Austria).Next, the cylindrical blocks were sectioned (IsoMet 1000, Buehler, Bluff, IL, USA) to obtain a 1.2 mm disc shaped under water cooling.Forty-eight specimens (12 × 1.2 mm 2 ) were prepared from each material (LU, VE, CS, and CU).The specimen size was calculated using a free ware (G*Power v. 3.1.9.7 for windows, Heinrich-Heine-Universität, Düsseldorf, Germany) based on the 0.8 power, 0.032 effect size, and 0.05 significance level.The prepared specimens were polished with increasing grit sizes (300-1200 grit) of silicon carbide papers (Waterproof SiC Paper; Struers) at 300 rpm with water coolant for 30 s by a single operator (M.H.) in order to standardize and establish a baseline roughness profile for all the groups [39].A digital micrometer (Digimatic Micrometer; Mitutoyo) was used to confirm the specimen thicknesses, and any specimen that did not fit the required specifications was replaced.All the specimens underwent ultrasonic cleaning (Quantrex, L and R Manufacturing, Inc., Kearny, NJ, USA) in distilled water for 10 min, followed by 40 s of air drying [40].Afterwards, the specimens were roughened with a fine 30 µm grit diamond rotary bur (Komet, Rock Hill, SC, USA) under running water for 30 s in a single direction [41,42] to simulate the clinical adjustment of a dental prosthetic crown.Then, the specimens were randomly allocated into four subgroups (n = 12) using simple randomization (Research Randomizer, v.4; http://www.randomizer.org/,accessed on 12 June 2022).One group (n = 12) was designated as the control for each material and did not receive any surface treatment.
The experimental groups were surface treated using different protocols in accordance with the respective manufacturer's recommendations.The MP group was polished using a Luster Meisinger polishing kit (Hager and Meisinger GmbH, Neuss, Germany) comprising diamond-impregnated rubber polishers designed for resin-modified ceramics.It involves two steps: pre-polishing with a #9507U polisher at 7000-10,000 rpm to remove irregularities and smoothen the surface, followed by high-shine polishing with a #9786 polisher at 3000-10,000 rpm.For the OG group, one uniform layer of OPTIGLAZE light-polymerized glazing agent (GC America Inc., Alsip, IL, USA) was applied in a circular motion without air contamination using the micro-brush provided with the kit.The glazing agent was lightpolymerized using a hand-held curing unit (light-emitting diode, Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein) for 40 s.Finally, the specimens in the MP+PP group were treated similarly to the MP group, followed by additional polishing using a polishing paste (SHOFU Dental GmbH, Ratingen, Germany).The paste was applied on a wet surface with a #9 soft bristle brush (Abbott-Robinson, Keystone Dental Group, Bosworth, UK) at 9000 rpm to prevent excessive heat generation.The specimen distribution and the study process are presented as flowcharts in Figure 1.To ensure consistency, all the surface treatment protocols were performed by a single operator (M.H.) using a custom-made hand piece holder (Figure 2).After surface treatment, all specimens were washed with running water and air-dried before being measured for surface roughness and gloss.Surface roughness (Ra) was measured using a non-invasive 3D optical non-contact profilometer (Bruker, Tucson, AZ, USA) that employs the white light interferometry technique.The height differences on the specimen surfaces were measured using the refractive indices of the white light components [43], which provides a quantitative evaluation of the surface [44].The roughness was determined using a 5× Michelson magnification lens, a Gaussian regression filter, 1 × 1 mm 2 field of view, and 1× scan speed.The simple vision 64 application software (Bruker, Tucson, AZ, USA) converts the data into accurate high-resolution images while also controlling device movements.Before each group, the system was re-calibrated.Three parallel readings were obtained at the different specimen areas and averaged for each specimen [41].
A representative specimen from each group was observed under a scanning electron microscope (SEM, JSM-6610LV, JEOL Ltd., Tokyo, Japan) for qualitative analysis of the treated surface.The specimens were gold sputter coated (−10 nm) in a coating machine (Q150R, Quorum tech, East Sussex, UK).The specimens were then placed on the sample holder of the SEM machine, and the image was processed at a working distance of 10 mm, 20 kV power and ×100 magnification.
A gloss meter (Novo-Curve, Rhopoint Instruments Ltd., East Sussex, UK) was used to measure gloss per ISO 2813 specification [34,42].A gloss meter is a reflectometer that includes an incandescent light source, a photodetector, and a collimator.The laser beam was focused at a 60 • angle onto the surface of the specimen, and the intensity of the reflected light was measured using a photodetector positioned in the incident beam's specular direction.The reflectometer measures gloss by the proportions of the directed reflected light; as a result, high reflectometer readings indicate high gloss.The gloss measurement is expressed as gloss units (GU).
Data were analyzed using IBM SPSS software (v.25;IBM Corp, Armonk, NY, USA).Data were normally distributed, and hence parametric testing was applied.Multivariate analysis of variance (MANOVA) was used to determine the overall significance.A one-way ANOVA was used to test each dependent variable, followed by Dunnett's post hoc test for multiple comparisons between the groups (α = 0.05).

Results
Table 2 shows the outcome of the MANOVA test, which suggests that CAD/CAM material type, surface treatment, and interaction between them significantly influenced the surface roughness (p < 0.001).Table 3 presents the mean Ra of the surface-treated CAD/CAM ceramics.The highest and lowest Ra were presented by the control VE (0.63 ± 0.23 µm) and the glazed LU specimens (0.04 ± 0.01 µm), respectively.Irrespective of the CAD/CAM material type, the glazed specimens demonstrated the lowest Ra (0.05± 0.01 µm), while the control specimens had the highest Ra (0.46 ± 0.13 µm). Figure 3 presents the overall mean Ra of the CAD/CAM ceramics irrespective of the surface treatment.Among the CAD/CAM material, LU (0.16 ± 0.04 µm) and CS (0.17 ± 0.06 µm) had the smoothest surface, and the Ra values of these materials were within the threshold limit (0.2 µm).The Ra of CU and VE were 0.27 ± 0.04 µm and 0.31 ± 0.09 µm, respectively, and these values were above the threshold limit.
The Dunnett's post hoc test to identify the pairwise significance showed significant differences in Ra among the surface-treated groups within a material (p < 0.001), except between the MP-treated and control specimens of CS material (p = 0.188) (Table 4).
The SEM micrographs of the control and polished specimens are presented in Figure 4.The micrographs confirm the findings of the Ra values.The glazed OG specimens had the smoothest texture, although some small voids were present on the surface.The specimens in the control group had deep grooves, but with MP, the grooves became shallow.The VE and CS specimens polished with MP demonstrated more surface grooves and flaws than LU and CS.The surface texture of CAD/CAM ceramics polished with MP+PP showed a smooth surface compared to the OG specimens.Table 5 presents the outcome of the MANOVA of GU, which suggests that the CAD/CAM material, surface treatment, and the interaction among them significantly influenced the GU of the materials (p < 0.001).The mean GU of the surface-treated CAD/CAM ceramics is summarized in Table 6.The highest and lowest GU were presented by the glazed LU (90.48 ± 4.69 GU) and the control VE specimens (23.32 ± 2.41 GU), respectively.Irrespective of the CAD/CAM material type, the glazed specimens demonstrated the highest GU (73.16 ± 4.6 GU), while the control specimens had the lowest GU (29.29 ± 3.48 GU). Figure 5 presents the overall mean GU of the CAD/CAM ceramics irrespective of the surface treatment.Among the tested materials, LU (65.6 ± 2.9 GU) and VE (42.43 ± 2.59 GU) had the highest and lowest GU, respectively.The GU of all tested materials were within the acceptable range (40-60 GU).The Dunnett's post hoc test to identify the pairwise significance showed that GU were significantly different among the surface-treated groups within a material (p < 0.001) (Table 7).

Discussion
This in vitro investigation evaluated the effects of surface treatment protocols on the roughness and gloss of resin-modified CAD/CAM ceramics.Based on the study's outcome, the surface treatment protocols led to a significant difference (p < 0.001) in the roughness and gloss of the resin-modified ceramics, thereby rejecting the null hypothesis.Glazing with OG was the most effective protocol among the surface treatment protocols applied, with the lowest roughness and the highest gloss among all the studied materials.
Two surface treatment protocols (OG and MP+PP) resulted in surface roughness within the acceptable range (0.2 µm) [25].It is evident that applying a polishing paste after MP has a positive effect on all of the materials' surface roughness and gloss, and this is also supported by previous research [26,28].The composition of the polishing paste and the particle size of the abrasive grains affect its efficiency in polishing the surface [26].In this study, Direct Dia Paste, which is composed of a 20% diamond polishing paste and a 2-4 µm particle-sized synthetic diamond powder that acts as abrasive grains, demonstrated its effectiveness in polishing resin-modified ceramics.
On the other hand, using MP alone resulted in a borderline or slightly higher Ra.For example, the Ra after treatment with MP for VE (0.39 µm) and CU (0.34 µm) might contribute to plaque-induced gingivitis and an increased risk of dental caries [25].
The surface treatment protocol is not the only controlling factor, because the composition of the material has a great impact on surface roughness.It is challenging to choose the best surface treatment protocol for resin-modified ceramic materials because of their heterogeneous and complex nature, which includes both a soft resin matrix and very hard filler particles.According to Flury et al. [45], materials with weak micromechanical characteristics have better polishability.In particular, the organic matrix, the size, distribution, ratio, and nature of the inorganic fillers in the resin-modified ceramics play a vital role in determining the material's surface roughness [46,47].Nevertheless, comparing different materials based on their constituent parts is challenging since the organic content percentages within the tested ceramics are not explicitly defined because they are considered confidential business information [47].This study showed that LU has the lowest surface roughness value (0.16 ± 0.04).This result is consistent with a prior investigation comparing the roughness of five CAD-CAM ceramics after surface treatment, in which LU also had the lowest surface roughness [45].According to Flury et al. [45], polishability and hardness have an inverse relationship, which partially explains the surface roughness value of LU.
The study demonstrates that Optiglaze, which primarily works by sealing microcracks and surface flaws, is the most efficient method to obtain lower surface roughness.The findings of this study are corroborated by earlier research that established Optiglaze's efficacy [48].A study conducted by Tekçe et al. supported that OG is efficient in providing a smooth surface for the LU, CS, and VE materials they tested [48].In addition, they concluded that 5000 thermocycling for the glazed surfaces resulted in a negligible difference in surface roughness [48].However, there are limited studies assessing the performance of glazing materials, particularly with resin-modified ceramic materials.Halis et al. [49] found that glazing the material is superior in maintaining the surface of different nanohybrid composite resins smooth compared to the conventional polishing modalities even after 10,000 brushing cycles.Optiglaze requires less time than traditional polishing procedures when applied to the surface in a single step.However, the high viscosity of this product might cause separation from the underlying material, especially under occlusal forces [50].
For gloss, a significant difference was observed between all the surface treatment protocols within the tested resin-modified ceramics.There is no clear cutoff point for gloss, but it is recommended to keep the gloss value within the range of 40-60 GU [30,38].All surface treatment methods under investigation provide acceptable gloss levels or more.Among all the materials used, LU showed the highest gloss value.The tested CAD/CAM materials are recommended to be used as restorative materials, using the mean GU of machine-polished enamel (53 GU) as a comparison [30].The nanoparticle fillers in LU and CS ranging between 5 and 20 nm are relatively small than the wavelength of visible light (400-800 nm).This increases reflectivity and, in turn, gloss, thereby explaining the high gloss of these resin nanoceramics [51].
The current study demonstrated that gloss and surface roughness are related; a higher gloss value is related to lower surface roughness.Surface roughness and gloss have a strong correlation but have different surface characteristics.Roughness is associated with surface topography and can be described by numerous linear or three-dimensional parameters, while gloss is an optical property defined by the reflection of light intensity [33,52].The angle of the incident light, the material's refractive index, and surface characteristics are a few of the variables that affect gloss [53].In this study, a 60 • angle of incident light was applied following the ISO 2813 specification for gloss [54], which makes the gloss values dependent on the surface topography and the refractive index of the material.Rough surface topography scatters the light instead of reflecting it, reducing the gloss value.Furthermore, the difference in refractive indices between the resin matrix and fillers has a significant impact on the surface gloss [53,54].Nanoceramics would have glossier surfaces after polishing because their diffusion reflection is lessened with relatively small filler particles [54][55][56], which is consistent with our findings.
The clinician should select the proper method to perform the clinical adjustment because the grit size of a diamond bur will affect how much the material can be damaged; a larger grit size will result in more damage.This is true even though their composition greatly influences resin-modified ceramic materials' surface roughness and gloss [57].The design of the disc-shaped specimens, which do not closely mirror the anatomy of natural teeth, since cusps and fissures can have distinct effects on the results, is one of the study's shortcomings.Complicated occlusions and pH variations in the oral cavity were not replicated in our investigation.Future studies should focus on other available polishing and finishing methods.Additionally, thermocycling should be included in future studies, as it will greatly aid in evaluating the various surface treatment protocols.Furthermore, it is important to evaluate the longevity and durability of OG under occlusal stresses using dynamic loading in a chewing simulator.

Conclusions
This study assessed the most apt surface treatment protocol to regain smooth and glossier surfaces on resin-modified ceramics following simulated clinical adjustment.The outcome of this study substantially contributes to the clinicians' knowledge regarding the use of the best chair-side surface treatment protocol for dental restorations fabricated using resin-modified CAD/CAM ceramics.Within the limitations of this study, it was demonstrated that surface glazing using Optiglaze provided the smoothest, as well as a glossier, surface on all the tested resin-modified ceramics.Among the tested materials, Lava Ultimate exhibited the lowest surface roughness and the highest gloss.Furthermore, the study also showed a significant association between gloss and surface roughness; a higher gloss resulted in a lower surface roughness.

Figure 1 .
Figure 1.Flowchart for the specimen distribution and study process.

Figure 2 .
Figure 2. Customized holder used for surface treatment.

Figure 3 .
Figure 3. Overall mean Ra of the resin-modified ceramics irrespective of the surface treatment.Dashed line indicates the Ra threshold limit (0.2 µm).

Figure 4 .
Figure 4. Scanning electron microscopy micrographs of the CAD/CAM materials after surface treatment.

Figure 5 .
Figure 5. Overall mean GU of the resin-modified ceramics irrespective of the surface treatment.Area between the two dashed line indicates the acceptable GU for ceramic materials (40-60 GU).

Table 2 .
MANOVA outcome of the surface roughness.

Table 4 .
Pairwise comparison of the Ra of the surface-treated CAD/CAM ceramics.

Table 6 .
Mean gloss (GU) of the surface-treated CAD/CAM ceramics.

Table 7 .
Pairwise comparison of the GU of the surface-treated CAD/CAM ceramics.