Surface Characterisation of Dental Resin Composites Related to Conditioning and Finishing

Due to the little information related to surface processing and conditioning of resin matrix ceramic materials previous glazing, the main purpose of this in vitro study was to investigate the effect of different surface treatments on the surface morphology of different resin composite materials. Five types of resin composite CAD-CAM materials: a resin composite ceramic Vita Enamic (E) and four types of nanoparticle-filled resins, like Lava Ultimate (L), Cerasmart (C), Shofu HC (S), Hyramic (H) were taken into consideration. Specimens received the following surface treatment protocols: conventional polishing [p], polishing and glazing [pg], conditioning with CoJet [c], conditioning with CoJet and glazing [cg], sandblasting [s], sandblasting and glazing [sg], etching [e], etching and glazing [eg]. Surface roughness was analyzed for all samples and nanosurface topographic characterization was made by Atomic Force Microscopy. The highest roughness was registered for sandblasted surfaces [s], followed by tribochemical silica airborne particle abrasion [c], and etching [e]. A very strong correlated conditioning behavior of resin nanoceramic materials, like L, C and S samples was found. The microroughness decreased thus [s] > [c] > [e]. These are moderate correlated with H, and are moderate negative correlated to E, where e is more efficient. Three-dimensional images indicated visible grain boundaries after conditioning, for all materials. After polishing and glazing, surfaces became smoother. For all tested conditioning and finishing methods, surface roughness values were within clinically acceptable limits. Finishing by polishing was proved to be a good choice for all materials taken into consideration, polishing and glazing likewise, excepting Hyramic. For Enamic and Shofu HC sandblasting or tribochemical conditioning and glazing and for Hyramic polishing and glazing are not the best options, related to nanoroughness values. Referring to the nanosurface topography, for Enamic, Cerasmart and Hyramic, glazing would be the method of choice, associated with the adequate conditioning method for each material.


Introduction
Resin composite materials combine the properties of glass ceramics and resin composite for dental restorations [1,2]. It is well known that ceramic materials can achieve aesthetic restorations and are characterized to have high fracture resistance, but low material wear and tend to accelerate the abrasion of opposing teeth. Good processability and ease of intraoral repair are also great advantages of these materials compared to ceramics. However, polymers have poor color stability and lose surface polish after wear [1,[3][4][5][6][7][8]. In this context, resin-composite materials are easy to mill, to process, to polish, have low abrasiveness, can be easy repaired, and different shade and translucency options are available [9]. As Five types of resin-composite CAD-CAM Vita Enamic (VITA Zahnfabrik, Bad Säckingen, Germany) (E) and four types of nanoparticle-filled resins (Lava Ultimate, 3M ESPE, St. Paul, MN, USA) (L), (Cerasmart, GC Corporation, Tokyo, Japan) (C), (Shofu HC, Shofu, Kyoto, Japan) (S), (Hyramic Upcera, Shenyang, China) (H) were taken into consideration (Table 1). Samples were obtained by slicing the blocks in rectangular-shaped plates (1 mm thick) (n = 32 per material), resulting in 64 surfaces, polished using silicon carbide papers (600-2000 grit) and the final thickness of each specimen was checked with a digital caliper. The samples were finally manually polished using a diamond polishing paste Renfert polish all-in-one (Renfert, Hilzingen, Germany) with a low-speed handpiece. They were cleaned for 60 to 180 s with 98% ethylic alcohol and dried. Specimen surfaces of each material were then divided into 8 groups (n = 8) in terms of the applied surface treatment method: conventional polishing, polishing and glazing, conditioning with CoJet, conditioning with CoJet and glazing, sandblasting, sandblasting and glazing, etching, etching and glazing. Specimens received the following surface treatment protocols: no treatment [only polished]; 9.5% hydrofluoric acid (Yellow Porcelain Etch; Cerkamed, StalowaWola, Poland) etching for 1 min airborne-particle sandblasted with 50 µm aluminum oxide particles (Ronvig Dental, Daugaard, Denmark) at a 10 mm distance for 15 s and tribochemical silica airborne particle abrasion with 30 µm particles CoJet (3M ESPE, Seefeld, Germany) at a 10 mm distance for 15 s. The sandblasting and tribochemical silica airborne particle abrasion treatments were performed by using an airborne-particle abrasion device DENTO-PREP™ Micro blaster (Ronvig Dental, Daugaard, Denmark) at 0.25 MPa. Tribochemical silica airborne particle abrasion is a chemical surface treatment. After the surface treatment protocols, the specimens were ultrasonically cleaned and degreased with 98% ethylic alcohol. Resin Glaze Primer (Shofu, Kyoto, Japan) was applied to the ceramic surfaces for 60 s and allowed to dry, and then two thin layers of glaze Resin Glaze Liquid (Shofu, Kyoto, Japan) were applied with a soft brush, in one direction to eliminate air bubbles and were polymerized for each 180 s in a light-polymerizing device SibariSr 620 (Sirio Dental, Meldola, Italy).

Surface Roughness Measurements
Specimens surface roughness was analyzed in a surface profilometer Surftest SJ-201 (Mitutoyo, Kawasaki, Japan). The arithmetic average roughness (R a ) and maximum absolute vertical roughness (R z ) [22] measurements were tested in 5 different directions and all data were registered. The mean value of the five measurements was calculated for each surface. The sampling length was 0.8 mm, and a force of 0.7 mN was applied.

Nanosurface Topographic Characterization by Atomic Force Microscopy (AFM)
Samples were examined using an atomic force microscope Nanosurf Easy Scan 2 Advanced Research (NanosurfAG, Liestal, Switzerland), in noncontact mode. Values for the average nanoroughness S a (nm) and amplitude of heights S y (nm) were registered. AFM investigation generated a three-dimensional image of the sample surface (2.2 µm × 2.2 µm). AFM is a cantilever-based technique that utilizes a sharp tip to interrogate surfaces at resolutions below the optical diffraction limit. It is also a powerful tool for nano-mechanical probing and measurements.

Statistical Analysis
Statistical analyses were performed using the Analyse-it software (Analyse-it Software, Ltd., Leeds, UK). The unpaired t-test was used to evaluate the comparisons between the means. A p-value of under 0.05 was considered statistically significant. Spearman correlation was used to assess monotonic similar or dissimilar relationships (whether linear or not) between variables. It measures the strength of association between variables and the direction of the relationship. The significance was related to: 0.00-0.19 "very weak", 0.20-0.39 "weak", 0.40-0.59 "moderate", 0.60-0.79 "strong", 0.80-1.0 "very strong". The power of the statistical test was calculated using the software IBM SPSS (IBM, New York, NY, USA).

Results
Surface roughness values R a , R z , S a , S y for conditioned surfaces are presented in Figure 1b.

Nanosurface Topographic Characterization by Atomic Force Microscopy (AFM)
Samples were examined using an atomic force microscope Nanosurf Easy Scan 2 Advanced Research (NanosurfAG, Liestal, Switzerland), in noncontact mode. Values for the average nanoroughness Sa (nm) and amplitude of heights Sy (nm) were registered. AFM investigation generated a three-dimensional image of the sample surface (2.2 µm × 2.2 µm). AFM is a cantilever-based technique that utilizes a sharp tip to interrogate surfaces at resolutions below the optical diffraction limit. It is also a powerful tool for nanomechanical probing and measurements.

Statistical Analysis
Statistical analyses were performed using the Analyse-it software (Analyse-it Software, Ltd., Leeds, UK). The unpaired t-test was used to evaluate the comparisons between the means. A p-value of under 0.05 was considered statistically significant. Spearman correlation was used to assess monotonic similar or dissimilar relationships (whether linear or not) between variables. It measures the strength of association between variables and the direction of the relationship. The significance was related to: 0.00-0.19 "very weak", 0.20-0.39 "weak", 0.40-0.59 "moderate", 0.60-0.79 "strong", 0.80-1.0 "very strong". The power of the statistical test was calculated using the software IBM SPSS (IBM, New York, NY, USA).

Results
Surface roughness values Ra, Rz, Sa, Sy for conditioned surfaces are presented in Figure  1b.  Relative to conditioning method, the results of the statistical analyses are presented in Tables 2-5. The highest roughness was registered for sandblasted surfaces, followed by tribochemical silica airborne particle abrasion, and etching. Significant differences were found between sandblasted and etched surfaces on microroughness level and for the amplitude of heights on nanolevel. On nanolevel a very strong correlation was registered between sandblasted and etched surfaces, that means that the behavior of the materials is similar to sandblasted and etched conditioning, tribochemical silica airborne particle abrasion is less efficient for E and more efficient for H.    Table 5. Correlation factor values related to material, after conditioning. E (Enamic), L (Lava Ultimate), H (Hyramic), S (Shofu), C (Cerasmart). Surface roughness values R a , R z , S a , S y for finished surfaces are presented in Figure 2. creased thus sandblasted >tribochemical silica airborne particle abrasion >etched with a significant variation between sandblasted and etched. Previous materials are moderate correlated with Hyramic, to which in addition sandblasted and tribochemical silica airborne particle abrasion are close together and moderate negative correlated to Enamic, where etching is more efficient, all the conditioning methods are close together. That means the first null hypothesis: "the surface morphology of the resin-composite materials is not influenced by the conditioning method" was rejected.  As a result of materials correlations records, a very strong correlation has been measured between Lava Ultimate, Cerasmart, and Shofu, relative to microroughness (R a and R z ). A moderate correlation was registered between the listed materials and Hyramic, and a negative moderate correlation between these and Enamic. The correlation is very strong negative between Enamic and Hyramic. On nanolevel, the behavior is different, is strong correlated for L and H. Between materials, roughness values do not vary significantly, with few exceptions, for S y , between Lava Ultimate and Shofu, and between Cerasmart and Shofu. It can be stated that all conditioning methods are effective on all materials taken into consideration. R a values are 0.745 ± 0.107 µm for tribochemical silica airborne particle abrasion, 1.006 ± 0.294 µm for sandblasted surfaces and 0.383 ± 0.287 µm for etched surfaces. S a values are 56.052 ± 16.664 nm for tribochemical silica airborne particle abrasion, 62.764 ± 27.280 nm for sandblasted and 40.651 ± 24.378 for etched surfaces.

Spearman Correlation Factor
Related to the conditioning method, Lava Ultimate, Cerasmart and Shofu samples demonstrated a very strong correlated conditioning behavior, the miroroughness decreased thus sandblasted >tribochemical silica airborne particle abrasion >etched with a significant variation between sandblasted and etched. Previous materials are moderate correlated with Hyramic, to which in addition sandblasted and tribochemical silica airborne particle abrasion are close together and moderate negative correlated to Enamic, where etching is more efficient, all the conditioning methods are close together. That means the first null hypothesis: "the surface morphology of the resin-composite materials is not influenced by the conditioning method" was rejected. Relative to finishing method, the results of the statistical analyses are presented in Tables 6-9.   After surface finishing, significant differences were found for air abraised, glazed surfaces and polished surfaces and etched, glazed and polished samples on microroughness level and for air abraised, glazed surfaces and polished surfaces and polished, glazed and simply polished for the amplitude of heights, on nanolevel. Polished surfaces recorded significantly lower values compared to these. The cg and p are strong correlated for R a , that means that between materials, the behavior related to these finishing methods is similar. R a values are higher for cg than for p, for all materials. Between materials, roughness values after finishing are not significant different, excepting R z for Enamic-Cerasmart and S a for Lava Ultimate-Shofu. For R a it is a strong correlation for Enamic-Shofu, for them the    (e) (f) (g) (h) Figure 3. 3D images of Vita Enamic specimens: (a) Enamic-tribochemical silica airborne particle abrasion, (b) Enamicsandblasted, (c) Enamic-etched, (d) Enamic-polished, (e) Enamic tribochemical silica airborne particle abrasionand glazed +, (f) Enamic-sandblasted and glazed (g) Enamic etched and glazed, (h) Enamic polished and glazed.

Discussion
The characteristics of the material surfaces and the material compositions play an important role in obtaining favorable finishing of the surface by polishing or a thin and durable sealant layer after glazing. Therefore an improvement in bond strength is expected for microretentive surfaces and well-distributed silica content [23,24]. Because resin-composite materials combine the composition of glass ceramics and composite resins, and within this class of materials are also differences, processing procedures have to be customized. Given that there are not many such materials available on the market, surface processing investigations are welcome to be reported for each type of material.
Previous studies have suggested that pretreatments like sandblasting and tribochemical silica airborne particle abrasion are efficient in improving the bond strength of resin nanoceramic and resin-based materials [25,26]. HF etching is recommended for etchable ceramics, in order to enhance surface roughness, wettability and microretention and to reveal hydroxyl groups, which favors the chemical bonding with monomers [27].
Studies reported that the effect of HF etching on the repair bond strength for resin nanoceramics was not as high as that of sandblasting and tribochemical silica airborne particle abrasion, but more similar for hybrid ceramics [28]. Other investigations show that for the resin nanoceramics the treatment with Al2O3 was statistically higher than that with CoJet, followed by HF. For the polymer infiltrated ceramics (Vita Enamic), the airborne-particle abrasion techniques (Al2O3 or CoJet) have been reported to be ineffective and the best results were obtained with HF (9%, 60 s) followed by silane application [29,30]. Another study concluded that surface blasting followed by one layer of glaze is the best method to condition Vita Enamic [31]. Another study concluded that using 5% hydrofluoric acid for aproximatly 60 s is one of the best treatments in order to increase adhesion of the characterization layer to hybrid ceramic [32].
The investigations of resin matrix ceramic materials within this study, related to the conditioning method led to a very strong correlated conditioning behavior of resin nanoceramic materials, like Lava, Cerasmart and Shofu HC samples. For these materials the microroughness decreased thus sandblasting > tricochemical conditioning > etching, with a significant variation between sandblasting and etching. Previous materials are moderate correlated with Hyramic, another resin ceramic material, to which in addition sandblasting and tribochemical conditioning are close together and are moderate negative correlated to Enamic, a polymer infiltrated ceramic material, where etching is more

Discussion
The characteristics of the material surfaces and the material compositions play an important role in obtaining favorable finishing of the surface by polishing or a thin and durable sealant layer after glazing. Therefore an improvement in bond strength is expected for microretentive surfaces and well-distributed silica content [23,24]. Because resin-composite materials combine the composition of glass ceramics and composite resins, and within this class of materials are also differences, processing procedures have to be customized. Given that there are not many such materials available on the market, surface processing investigations are welcome to be reported for each type of material.
Previous studies have suggested that pretreatments like sandblasting and tribochemical silica airborne particle abrasion are efficient in improving the bond strength of resin nanoceramic and resin-based materials [25,26]. HF etching is recommended for etchable ceramics, in order to enhance surface roughness, wettability and microretention and to reveal hydroxyl groups, which favors the chemical bonding with monomers [27].
Studies reported that the effect of HF etching on the repair bond strength for resin nanoceramics was not as high as that of sandblasting and tribochemical silica airborne particle abrasion, but more similar for hybrid ceramics [28]. Other investigations show that for the resin nanoceramics the treatment with Al 2 O 3 was statistically higher than that with CoJet, followed by HF. For the polymer infiltrated ceramics (Vita Enamic), the airborne-particle abrasion techniques (Al 2 O 3 or CoJet) have been reported to be ineffective and the best results were obtained with HF (9%, 60 s) followed by silane application [29,30]. Another study concluded that surface blasting followed by one layer of glaze is the best method to condition Vita Enamic [31]. Another study concluded that using 5% hydrofluoric acid for aproximatly 60 s is one of the best treatments in order to increase adhesion of the characterization layer to hybrid ceramic [32].
The investigations of resin matrix ceramic materials within this study, related to the conditioning method led to a very strong correlated conditioning behavior of resin nanoceramic materials, like Lava, Cerasmart and Shofu HC samples. For these materials the microroughness decreased thus sandblasting > tricochemical conditioning > etching, with a significant variation between sandblasting and etching. Previous materials are moderate correlated with Hyramic, another resin ceramic material, to which in addition sandblasting and tribochemical conditioning are close together and are moderate negative correlated to Enamic, a polymer infiltrated ceramic material, where etching is more efficient, resulting that all the three investigated conditioning methods lead to close microroughness values.
The topography of the surfaces was investigated by three-dimensional image generations of the surface samples, indicating visible grain boundaries after conditioning, for all materials. Nanoroughness values were strong correlated for Lava and Hyramic, with values as following: tribochemical conditioning > sandblasting > etching, but the values were not significantly different, valid for other materials as well. Significant higher values were recorded just for sandblasting, related to etching, for the nanoamplitude of heights.
In the literature the most frequent investigated CAD-CAM polymer infiltrated ceramic material was Vita Enamic (VITA Zahnfabrik), and the most frequent investigated resin nanoceramics were Lava Ultimate (3M ESPE) and Cerasmart (GC America). Among the types of surface-conditioning methods airborne-particle abrasion techniques with Al 2 O 3 (50 µm) and CoJet 30 µm (dry and wet), grinding with a rotary instrument or, conditioning with HF (10%) followed by silane application, adhesive systems, or combinations of these techniques were used. The studies compared different treatments in the same material and found different results [33,34].
Other studies investigated the surface conditioning for adhesive repairs, in order to obtain optimal conditioning, related to surface finishing after glazing and associated with the best optical properties. Although various repair systems based on different conditioning protocols are available on the dental market, it is challenging for clinicians to choose the most appropriate system that would provide a reliable outcome [35]. Likewise, the optimal surface processing is a challenge for these newly developed materials and for which in vivo tests do not keep up.
After surface finishing, all the studied samples registered R a values below 0.2 µm, generally accepted to be the cut-off value [36,37]. R a and R z parameters describe the surface roughness of the samples at a micro level, and S a and S y correspond to nanoroughness. In order to have a complex view of the samples' behavior, micro and nanoroughness should be evaluated.
Polished, respective polished and glazed surfaces show significant lower microroughness values for Enamic and Shofu HC materials, compared to sandblasted or tribochemical conditioned and glazed ones. Finishing by polishing was proved to be a good choice for all materials taken into consideration, polishing and glazing likewise, excepting Hyramic. It could be concluded that for Enamic and Shofu HC sandblasted or tribochemical conditioned and glazing and for Hyramic polishing and glazing are not the best options, even surface roughness values are within clinically acceptable limits. Thereby the second null hypothesis "there are no differences in surface characteristics between different finishing protocols" was also rejected.
The investigations on nanolevel using AFM analyses demonstrated higher S a and S y values for polished surfaces for Enamic, Cerasmart and Hyramic samples, which indicates the choice of glaze, associated with the adequate conditioning method. For the other finishing methods S a values are similar and between 0.52 and 3.80 nm, and for S y between 6.57 and 85.22 nm.
Previously problems related to the sealants consist of low abrasion resistance, weak retention to the applied material, poor surface quality, and microcracks [38,39]. Further studies should be performed in order to investigate if surface conditioning before glazing increases the bond strength between the resin-composite CAD-CAM and the sealant, after aging procedures because resin-composite are expected to be hydrophilic [40][41][42][43][44][45][46][47]. In connection with supposed color alterations, a subsequent challenge is to evaluate the changes in optical properties of resin-composite ceramics after aging, related to surface conditioning and finishing. The study limitations were that only one type of glaze was used and that it was held in vitro and not in vivo.