The Effect of Different Polishing Systems on the Surface Roughness of Nanocomposites: Contact Profilometry and SEM Analyses
The aim of this study was to evaluate the effects of different polishing systems on the surface roughness of different nanocomposite resins using various analysis methods. Three types of nanocomposite resins were investigated in this study: supra-nanohybrid (Estelite Asteria), nanohybrid (GrandioSo), and nanoceramic composite resins (Ceram-X Spheretec One). Forty-eight disc-shaped specimens (4 mm in diameter, 2 mm in thickness) were fabricated using a Teflon mold and divided into four groups according to the different polishing systems (n=12). All specimens were processed with one of the following methods: Mylar strip (control), one-step polishers (Opti1step), two-step polishers (Clearfil TwistDia), or multistep polishers (Sof-Lex XT Pop-on). The surface roughness (Ra, μm) was measured by contact profilometry (Mahr, Marsurf PS1) (n=10) and scanning electron microscopy (SEM) (Thermo Fisher Scientific, Phenom XL) at 400× magnification (n=2). The data were statistically analyzed using Kruskal-Wallis and Bonferroni correction tests (p<0.05). In addition, the surface morphology and elemental content were examined by SEM and energy dispersive x-ray spectroscopy (EDS) analyses. Under SEM evaluation, in terms of the polishing systems, there were no significant differences in the surface roughness for supra-nanohybrid composite resin (p>0.05). The multistep polishers created lower surface roughness than the one-step polishers for nanohybrid and nanoceramic composites. In terms of the composite resins, supra-nanohybrid composite exhibited lower surface roughness than nanohybrid composite for all polishing systems (p<0.05). The SEM observations confirmed the surface roughness measurements related to the surface morphology. One-step and two-step polishers created porosity on the surface of nanohybrid and nanoceramic composites. EDS analysis indicated the elemental composition of the particles in the porous zones was quite close to diamond abrasives and glass fillers.SUMMARY
INTRODUCTION
The clinical use of composite resins has widely increased because of esthetic demands and advances in adhesive dentistry. Along with the development of composite technology, the type, distribution, and size of filler particles in composite resins have been changed to improve their properties.1,2 In recent years, composite resins containing nanoparticles have been introduced and frequently used in operative dentistry. Manufacturers claim that these composite resins have superior esthetic properties and better mechanical and physical stability when exposed to the oral environment.1,3 Moreover, composite resins have increased wear resistance and better surface smoothness due to the reduced particle size and increased filler content.4
The polishing procedure used for composite resin restorations ensures natural tooth appearance and longevity.5 The surface roughness of a restorative material may cause suboptimal esthetics, adverse effects on abrasion and wear resistance, plaque accumulation, and secondary caries development.6,7 However, it is still difficult to select an adequate polishing system for different types of composite resins that produces better surface smoothness.8 Various polishing systems are used to finish and polish composite resin restorations. These systems differ in the type, composition, and hardness of the abrasive particles that may significantly affect the surface roughness of the composites.9,10 It has been reported that simplified polishing systems (one- or two-step systems) are less time-consuming than multistep polishing systems and can provide comparable surface roughness.7 In contrast, some studies have indicated that—unlike multistep systems—these simplified systems cannot sufficiently ensure surface smoothness.11,12 Thus, there is still no consensus in the literature on which type of polishing system is more efficient and less time-consuming.
The surface roughness of restorative materials can be evaluated by several methods, including profilometry (contact/noncontact), scanning electron microscopy (SEM), and atomic force microscopy.13 Contact profilometers have commonly been used for in vitro studies. However, this method can fail to accurately measure the surface roughness since only a certain part of the surface is analyzed.14 Thus, SEM evaluations are necessary for more extensive analysis and comparison with the findings from contact profilometry. In addition, SEM evaluations capture high-resolution 3D images of the surface topography, which can give some information about the micromorphological and microtopographical characteristics of restorative materials and provide more reliable evaluations to mimic the clinical conditions.15,16
Several studies have evaluated the effect of various polishing systems on different types of composite resins.9,11 However, there are few studies in the literature where different analysis methods are used to evaluate the surface roughness of various nanocomposites processed using polishing systems with varying steps.
Thus, the objectives of this study were as follows:
Compare the surface roughness values of different nanocomposite resins after the use of each polishing regimen.
Determine the effect of different polishing systems on the surface roughness values of each nanocomposite.
Evaluate the surface roughness values obtained with different analysis methods for all tested groups.
The research null hypotheses were as follows:
There would be no difference in the surface roughness values among the various nanocomposites after the use of each polishing system.
There would be no difference in the surface roughness values among the various polishing systems for each nanocomposite.
There would be no difference in the surface roughness values obtained with various analysis methods for all tested groups.
METHODS AND MATERIALS
Sample Size Calculation
The sample size was calculated based on the estimated effect size between groups according to the literature for contact profilometry.17 It was determined that 10 specimens were needed for each group to achieve a medium effect size (d=0.50), with 95% power and a 5% type 1 error rate in this study. In particular, SEM is a quite precise but expensive and time-consuming method. Thus, in the literature, this method is usually used with a few representative samples for supportive characterization purposes.18 On this basis, SEM was used to perform five surface roughness measurements and imaging at different magnifications for two representative specimens from each group.
Specimen Preparation
The components of the composite resins used in this study are shown in Table 1. Three types of nanocomposite resins were used (A2 shade): supra-nanohybrid composite resin (Estelite Asteria, Tokuyama Dental Corp, Tokyo, Japan), nanohybrid composite resin (GrandioSo, Voco GmbH, Cuxhoven, Germany), and nanoceramic composite resin (Ceram-X Spheretec One, Dentsply Sirona, York, USA). Forty-eight disc-shaped specimens (4 mm in diameter and 2 mm in thickness) of each composite resin were fabricated using a Teflon mold (N=48 for each composite). All composite resins were placed into the mold in a single increment and covered on both sides with a transparent polyethylene terephthalate matrix strip (Mylar strip, SS White Co, Philadelphia, PA, USA) and a glass slide. The mold was compressed between glass slides under finger pressure to produce a flat surface. Then, the top surfaces of all composites were polymerized with a light-emitting diode (LED) light curing unit (Valo, Ultradent, South Jordan, UT, USA) for 10 seconds (irradiance of 1000 mW/cm2) according to the manufacturer instructions (ZCO). The light intensity was assessed by a radiometer (Demetron LED Radiometer, Kerr Corp, Orange, CA, USA). The specimens were removed from the mold after light curing, and the bottom surfaces were marked with a permanent marker. The specimens were kept in distilled water at 37°C for 24 hours in a dark vial. For standardization, the top surfaces of the specimens (except for the control [Mylar strip] group) were ground with 600-grit silicon carbide abrasive paper under water for 30 seconds and then ultrasonically cleaned for five minutes to remove debris.19 Then, according to the manufacturer instructions, the top side of each specimen was subjected to a polishing regimen for 30 seconds at 10,000 rpm under water cooling and light pressure with constant linear movements using a slow handpiece.20 A new polishing disc or set of discs (for two-step and multistep polishers) was used for each specimen. After each polishing step, the surface was ultrasonically cleaned. For standardization, all polishing procedures were performed by a single operator (BO) who was unaware of the restorative material used for each specimen.13
The specimens were divided into four groups according to the polishing systems for each composite resin (n=12):
Mylar strip (ML), control: These specimens were not subjected to finishing/polishing procedures after light-curing under Mylar strips.
Opti1step (OS), one-step polishers: These specimens were polished with the flat, broad surface of the disc-shaped instrument (Opti1step Polisher, Kerr Corp).
Clearfil TwistDia (TD), two-step polishers: These specimens were prepolished with a medium-grit prepolishing wheel followed by a fine-grit polishing wheel (Clearfil TwistDia, Kuraray, Okayama, Japan).
Sof-Lex XT Pop-on (SL), multistep polishers: These specimens were polished with a series of aluminum oxide polishing discs (Sof-Lex XT Pop-on, 3M ESPE, St Paul, MN, USA) from coarse grit to superfine grit.
Surface Roughness Evaluation by Contact Profilometry
The surface roughness (Ra, mm) of 10 specimens from each group was measured by a contact profilometer (Mahr GmbH, Marsurf PS1, Göttingen, Germany) (N=40 for each composite). Three measurements were performed in different locations of the polished surface for each specimen with a stylus tip radius of 5 μm, a stylus driving speed of 0.5 mm/second, a traversing length (Lt) of 1.75 mm and five cut-off lengths of 0.250 mm, in accordance with EN ISO 4288.21 Phase corrections of the profile were automatically realized by the profilometer software in accordance with EN ISO 16610–21:2011.22 The arithmetic mean of the measurements was obtained. The tool was calibrated after testing five specimens to ensure reliable readings. A second operator (BOK), who was unaware of the type of composite resin or the type of finishing and polishing systems used for the specimens, performed all of the surface roughness measurements.
Surface Roughness Evaluation by Scanning Electron Microscope
Surface roughness of two specimens (Ra, mm) for each group was measured and evaluated for SEM analysis (Phenom XL, Thermo Fisher Scientific, Phenom XL, Waltham, MA, USA) (n=8 for each composite) (Figure 1). The specimens were sputter-coated with gold prior to imaging. SEM investigations were performed with a 5 kV accelerating voltage, which is suitable for polymer surfaces. Surface images were captured randomly using 400x magnification and analyzed using the 3D roughness measurement application (Phenom XL, Thermo Fisher Scientific). These surface roughness values from SEM were compared to the contact profilometry roughness measurements due to a higher accuracy. Five measurements from the polished surface of each specimen were performed by scanning the surface roughness along different random linear paths with a 20-nm shortest cut-off (λs) and 120-μm longest cut-off (λc) filtering selections using λ locations. An example image showing the heat map of the surface explaining the roughness changes that include random linear path selections (numbered colored arrows) to measure the Ra values, cut-off values, measured values of all paths, and their profiles in the main measurement screen of 3D Roughness Reconstruction software is presented in Figure 1. Then, the arithmetic mean of the measurements was obtained. In addition, surface roughness values and morphologies were crosschecked to conduct a precise overall polishing assessment. A second operator (BOK), who was unaware of the type of composite resin or the type of finishing and polishing systems used for the specimens, performed all of the surface roughness measurements.
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
Morphology and Content Evaluation by Scanning Electron Microscope
The detailed surface morphology and content investigation was realized by using SEM (Phenom XL, Thermo Fisher Scientific) imaging. First, the specimens were sputter-coated with a nanoscale layer of gold to achieve the electrical conductivity necessary for SEM imaging. Illumination during SEM imaging was provided by a CeB6 thermionic source using 5, 10, or 15 kV acceleration voltages, which can generate a resolution below 20 nm. The morphology evaluations were carried out using a secondary electron detector, which observed the geometrical differences on the surface in three dimensions. On the side, the surface content evaluation was performed by the back-scattered electron detector (BSD), which determines the chemical composition from the density differences of various elemental contents.
Energy Dispersive X-Ray Spectroscopy Analysis
A semiquantitative chemical microanalysis method, energy dispersive x-ray spectroscopy (EDS), was used for the chemical characterization of the unrecognized particles observed on the surface of the polished composite resins. The EDS module of the scanning electron microscope (Phenom XL, Thermo Fisher Scientific) was used because this module has the capacity to determine elements from boron (B) to americium (Am) owing to the ultrathin silicon nitride x-ray window. A thermoelectrically cooled (LN2-free) silicon drift detector was used, and an energy resolution below 137 eV at Mn Kα was achieved with a 10 eV/ch processing capability with 2048 channels and 300,000 counts/second. Thus, elemental analysis with a high certainty was achieved for the basic elements (C, Si, O, Ba, etc).
Statistical Analysis
Statistical analysis was performed using SPSS 22.0 for Windows (SPSS Inc, Chicago, IL, USA). The surface roughness data obtained from contact profilometry and SEM analysis (at 400×) were analyzed with nonparametric tests, and these data were not normally distributed. The Kruskal-Wallis test was used to compare the groups. Pairwise comparisons were made using the Dunn test and the Bonferroni correction test for the contact profilometry results and SEM roughness results, respectively. Statistical significance was determined at a confidence level of 0.05 in all the analyses.
RESULTS
Surface Roughness Evaluation by Contact Profilometry
The mean surface roughness values and standard deviations obtained with the contact profilometer for all the tested groups are shown in Table 2.
When comparing the composite resins for each polishing system, there were no significant differences in surface roughness for the ML, OS, and TD groups (p>0.05). In addition, GrandioSo showed significantly higher surface roughness than Estelite Asteria and Ceram-X for the SL group (p<0.05).
When comparing the polishing systems for each composite resin, the OS and TD groups showed significantly higher surface roughness than the ML and SL groups for Estelite Asteria (p<0.05). For GrandioSo, the OS and TD groups showed significantly higher surface roughness than the ML group (p<0.05). In addition, the SL group did not significantly differ from the OS, TD, and ML groups (p>0.05). For Ceram-X, the OS and TD groups showed significantly lower surface roughness than the SL group (p<0.05). Additionally, the ML group did not significantly differ from the OS, TD, and SL groups (p>0.05).
Surface Roughness Evaluation by Scanning Electron Microscope
The mean surface roughness values and standard deviations obtained via SEM at 400x magnification for all the tested groups are shown in Table 3. Representative images of the SEM 3D roughness measurements for all tested groups showing the linear Ra measurement paths are shown in Figure 2.
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
When comparing the composite resins for each polishing system, Estelite Asteria showed significantly lower surface roughness than Ceram-X and GrandioSo for the ML, OS, and TD groups (p<0.05). Estelite Asteria showed significantly lower surface roughness than GrandioSo for the SL group (p<0.05). In addition, there were no significant differences between Estelite Asteria and Ceram-X (p>0.05).
When comparing the polishing systems for each composite resin, there were no significant differences in surface roughness among the ML, OS, TD, and SL groups for Estelite Asteria (p>0.05). For Ceram-X and GrandioSo, the OS group showed significantly higher surface roughness than the SL and ML groups (p<0.05). In addition, the TD group did not differ significantly from the ML, OS, or SL groups (p>0.05).
Scanning Electron Microscope Observations
When comparing the composite resins with ML, Estelite Asteria presented the smoothest surface with no sign of exposed filler particles (Figure 3A, D), whereas coarse and irregular filler particles were found on the surface of GrandioSo (Figure 3B, E) and Ceram-X (Figure 3C, F).
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
When comparing the composite resins for each polishing system, the surface of GrandioSo polished with OS had some dislodged larger filler particles (pores) with slight wear scars (Figure 4A). Abrasive particles that dislodged from this polishing system attached to the deformed pores (Figure 4C). However, increased numbers and depths of wear scars were predominantly observed for the Ceram-X polished with OS (Figure 4B). Abrasive particles slightly filled the pores on the surface of this composite resin (Figure 4D).
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
Figure 5A shows moderate wear scars at the surface of GrandioSo polished with TD. In addition, some details were detected, such as large, dislodged particles (Figure 5B), filler dislodgement (Figure 5C), wear debris (Figure 5D), and wear scars (Figure 5E). Some superficial abrasion marks (black arrows) were found on the surface of the larger glass ceramic filler particle (Figure 5E). In addition, some laminar-shaped wear debris is shown in Figure 5D. Figure 5F shows that Ceram-X polished with TD showed similar surface properties to the polished surface with TD of GrandioSo. In addition, the particles and debris stuck to the surface of Ceram-X (Figure 5G).
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
Similar to the ML groups, the SL groups had a smooth surface appearance without any porosity for all tested composite resins (Figures 3 and 6). It was found that there were fewer wear scars and a greater number of flat zones. In addition, the amount and size of wear debris and attached abrasive particles were very low on the surfaces of GrandioSo (Figure 6B) and Ceram-X. The pores on the surface of Ceram-X were filled with wear debris (Figure 6C).
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
Energy Dispersive X-Ray Spectroscopy Analysis
The representative semiquantitative chemical analyses used to determine the elemental composition of the particles found on the polished surface of the composite resins are shown in Figures 7 and 8. According to the EDS results given in Figure 7, an increase in the carbon peak intensity was detected on the particles attached on the surface of Ceram-X polished with OS. In addition, a low amount of other elements originating from the filling and applied polishers were detected. The EDS results of the particles observed on the surface of GrandioSo polished with TD are given in Figure 8, which show that the elemental concentrations were close to the composite resins that contain glass fillers.
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
DISCUSSION
In the present study, the effects of different polishing systems on the surface roughness values of various nanocomposite resins were evaluated with different analysis methods. Based on the results of this study, the first null hypothesis, which proposed that there would be no difference in the surface roughness values among the various nanocomposites after the use of each polishing system, was rejected since the supra-nanohybrid and nanohybrid composite resins had significantly different surface roughness values in each polishing system. The second null hypothesis, which proposed that there would be no difference in the surface roughness values among the various polishing systems for each nanocomposite, was partially rejected since the polishing systems significantly affected the surface roughness values of the nanohybrid and nanoceramic composite resins. The third null hypothesis, which proposed that there would be no difference in the surface roughness values obtained with different analysis methods for all tested groups, was rejected since different surface roughness data were obtained with the various analysis methods.
Contact profilometry is the most common analysis method due to its easy accessibility and cost effectiveness. However, this approach has some disadvantages, such as being limited by the spatial dimensions of the stylus, which results in low resolution. In addition, limitations in the sampling rate and the calibration in the z-axis are some other notable disadvantages of contact profilometry.3 Microscale irregularities of the surfaces cannot be penetrated and detected with this method because of the size of the stylus. Therefore, this method may underestimate surface roughness and remain incapable of understanding the differences that occurred as a result of polishing procedures, as demonstrated in the schematic in Figure 9. From a different viewpoint, a contact profilometer can only measure the surface roughness in a 2D profile. However, the surface topography of restorative materials has a 3D structure in nature. Thus, the evaluation of 3D surface topography is needed. Particularly, the measurement of 3D surface roughness can give a complete description of surface topography and represent the surface characteristics of composite resins.23 It is possible to compensate the results by profile selections with or without irregularities; however, in the present study, the total surface roughness results (Sa) obtained by the reconstruction technique in SEM are in conjunction with the selected profile results (Ra) obtained. Thus, we can state that whether we pick up profiles through the irregularities or not, they do not affect the overall results in an effective manner, and we can use the Ra results in comparisons.
Citation: Operative Dentistry 46, 2; 10.2341/20-157-L
In the literature, one clinical study reported that a mean surface roughness of 0.2 μm is the critical threshold value for bacterial retention.24 Another study indicated that a surface roughness of 0.25–0.5 μm could be detected by a patient’s tongue.25 The SEM evaluation of 3D surface topography can provide more accurate and precise information about the surface characteristics of a restorative material with a higher resolution than that of a 2D profile contact profilometer.23,26 Moreover, the field of view in SEM is much larger than that of a contact profilometer, which gives us the possibility to examine a wider area with a sufficiently high resolution (17 nm in the current study) in a very short time. In addition, a better depth of field in SEM imaging is provided by the secondary electron detection and topographic detection capability of the BSD of the scanning electron microscope.27,28 Thus, SEM imaging was performed at different magnifications to obtain results that could be used as a comparison for the contact profilometer results. In this study, two analysis methods (contact profilometry and SEM) were chosen to measure the surface roughness of composite resins polished with various techniques. Both roughness analysis methods were used to numerically measure the average surface roughness values of the specimens. The SEM images at 400x magnification showed higher surface roughness values than the contact profilometer measurements. It is known that the high resolution of SEM gives us the ability to interact with points that cannot be probed with a mechanical profilometer tip. Thus, achieving higher average surface roughness values is expected. In addition, SEM images were used to analyze the surface characteristics of the specimens (Figures 3–6).
In this study, different measurements were obtained with a profilometer and SEM for some tested groups. When evaluating the different composite resins, for Mylar strip, one-step, and two-step polishers, the profilometer indicated that there were no significant differences in surface roughness, whereas SEM revealed that the supra-nanohybrid composite resin showed significantly lower surface roughness values than the other composite resins. When evaluating the polishing systems, for the supra-nanohybrid composite resin, the profilometer results showed that the multistep polishers had significantly lower surface roughness values than one-step and two-step polishers, whereas the SEM results revealed that there were no significant differences among these groups. For the nanohybrid composite resin, the profilometer results revealed that there were no significant differences among the different groups, whereas the SEM results revealed that multistep and two-step polishers had significantly lower surface roughness values than one-step polishers. These findings again highlighted that the evaluation of SEM 3D surface topography can provide more detailed and realistic surface roughness measurements than contact profilometry. In addition, this approach can help determine the most adequate polishing systems for different composite resins in clinical practice.
The surface roughness of restorative materials depends on various factors, such as filler content, size, shape, monomer type, and effective filler-organic matrix junction.29 In this study, in terms of composite resins with Mylar strips, SEM measurements reported that supra-nanohybrid composite resins had significantly lower surface roughness values than the other composite resins. This finding could be attributed to the supra-nano spherical filler technology used in this material, which can facilitate a much smoother surface than the irregular particles found in the other composite resins.
There are various polishing instruments, such as polishing discs, rubber wheels, cups, discs, and pastes, that can be used to decrease the surface roughness of restorative materials.12 In this study, one-step, two-step, and multistep polishers were used to process nanocomposite resins, and the corresponding surface roughness results were evaluated. The literature has suggested that lower surface roughness could be obtained after the use of polishing systems for composite resins that have smaller filler particle sizes.30 In terms of the composite resins used in this study, the SEM measurements indicated that the supra-nanohybrid composite resin showed significantly lower surface roughness values than the other composite resins with one-step and two-step polishers. In addition, the supra-nanohybrid composite resin exhibited lower surface roughness than the nanohybrid composite resin with multistep polishers. These findings could be explained by the fact that the supra-nanohybrid composite resin (Estelite Asteria) had the smallest filler particle size (0.2 μm).
The polishing system, polishing time, applied force, handpiece speed, abrasive particle hardness, and abrasive particle size can also influence the surface roughness of composite resins.31 Polishing systems have a wide variety of abrasives, such as aluminum oxide, diamond, and silicon dioxide. These are impregnated in rubber and aluminum oxide or diamond silica-coated abrasive discs that can be used in one-, two- or multistep applications.13 The abrasive particles of polishing systems must be harder than the fillers of composite resins for an effective finishing and polishing procedure.32 If not, the polishing systems could only remove the soft resin matrix and leave the filler particles protruding from the surface of the restorative material.7 In addition, insufficiently bonded fillers may debond and dislodge, leaving a dull surface.20 The grit size of abrasive particles must be small to prevent scratches on the composites.11 In addition, the effectiveness of the polishing systems can be related to the flexibility of this material and the shape of the instrument (cusp, discs, or cones).33
In this study, in terms of the polishing systems, for the supra-nanohybrid composite resin, the SEM measurements revealed that there were no significant differences in the surface roughness values of the different groups. This finding is consistent with Korkmaz and others,34 who reported that one-step polishers created nanocomposites with similar surface roughness to those processed with multistep polishers. This finding could be attributed to the uniform, small, and spherical fillers of this composite resin. For the nanohybrid and nanoceramic composite resins, the multistep polishers created significantly lower surface roughness values than the one-step polishers. The multistep polishers with higher flexibility (Sof-Lex) used in this study contain aluminum oxide particles that are mostly harder than the fillers of the composite resins. Therefore, this polishing system could remove equal amounts from both filler particles and the resin matrix of the resin composite, which could explain the smoother surface obtained with the multistep polishers.35 These findings are in line with many studies that reported that the smoothest surfaces were obtained with multistep aluminum oxide polishers.17,29,32 In addition, previous studies reported that one-step polishers created higher surface roughness than multistep polishers.20,36 The one-step polishers (Opti1Step) used in this study comprise diamond-impregnated polishers and silicone synthetic rubbers.37 The nanohybrid (GrandioSo) and nanoceramic (Ceram-X) composite resins used in this study contain glass filler particles. In the literature, it was indicated that diamond particles are harder than aluminum oxide particles and can be detrimental to glass particles in an inorganic matrix of composite resins, forming porous surfaces.33 This supports the results of the current study, which reported that one-step polishers caused higher surface roughness for nanohybrid and nanoceramic composite resins. In addition, the findings of this study are in line with the study that reported that one-step polishers might need a finishing step before polishing to make the surfaces smoother.37 On the other hand, the findings of the current study are in contrast with Ereifej and others,9 who concluded that one-step polishers (Opti1Step) resulted in lower surface roughness than aluminum oxide multistep polishers (OptiDisc) for nanohybrid composites. Their findings could be explained by the greater abrasive particle size (extrafine: 10 μm) of the multistep polishers used in their study.
In this study, the SEM measurements indicated that the two-step polishers caused no significant differences in surface roughness when compared to the multistep and one-step polishers for all tested composite resins. This finding is in contrast with Daud and others,38 who revealed that two-step polishers (Enhance/PoGo) resulted in lower surface roughness than polishing discs (Sof-Lex). This might be explained by the type of abrasive particles (aluminum oxide, diamond, and silicon dioxide) of the two-step polishers they used. In addition, in this study, with Mylar strips and multistep polishers, the SEM results indicated that similar surface roughness values were obtained for all tested composite resins. This finding is in line with Kemaloglu and others,31 who reported that multistep aluminum oxide polishers exhibited the smoothest surfaces, and the results were no different than those of the Mylar-finished group.
When the SEM micrographs were observed, even at low magnification (500x), the supra-nanohybrid composite resin (Figure 3A) exhibited a smoother surface appearance than the other tested composite resins (Figure 3B, C) with Mylar strips.
The one-step polishers created pores and wear scars on the surface of the nanohybrid composite resin (Figure 4A). However, the effects of the one-step polishers on the nanoceramic composite introduced a different behavior as the number and depth of wear scars increased since the scuffing mechanism of abrasive wear was dominant (Figure 4B). Abrasive particles of this polishing system attached in the porous zones, which eased the sticking of hard and sharp-edged particles after scuffing the surface (Figure 4C, D). However, it is difficult to determine the chemical structure of the particles from the BSD image given in Figure 4D due to the low contrast difference between the particles and the composite surface. Thus, EDS analysis was used to determine the elemental composition of such unrecognized particles. EDS analysis is a reliable technique to identify and quantify major components on the surfaces of restorative materials.39 The EDS analysis given in Figure 7 found a high amount of carbon (despite the gold content), indicating that this particle originated from the one-step polishers, as this is analytical evidence of diamond abrasive particles. Thus, the slight contrast difference in Figure 4D could explain the source of these particles.
The two-step polishers produced a similar surface appearance to the one-step polishers for the nanohybrid and nanoceramic composite resins (Figure 5). In Figure 5A and F, moderate wear scars, wear debris, and some particles that might be abrasive particles from this polishing system were observed for the nanohybrid composite. Figure 5A and B indicate that some particles could scratch the surface and cause wear scars (black arrows) as a sign of scuffing. When analyzing the chemical composition achieved by EDS given in Figure 8, it was found that elemental concentrations are quite close to the glass filler–reinforced composite resin. It was understood that these surface-scratching particles are glass filler reinforcements dislodged from the resin.
Figure 5C and E show that surface roughness could also originate from dislodged larger filler particles (white arrows). In addition, some superficial abrasion marks (black arrows) were observed on the surface of the large glass filler particles (Figure 5E). These submicron wear patterns could be a result of abrasion by a harder particle, which was the fine-grit diamond abrasive of the two-step polishers. In Figure 5D, some laminar-shaped wear debris was observed, which might be formed by excessive plastic deformation on the filler surface. These findings are consistent with Kemaloglu and others,31 who reported that some defects were seen on the surface of a nanohybrid composite resin polished with two-step polishers (Clearfil TwistDia). When the surface of the nanoceramic composite polished with two-step polishers was investigated in detail, the wear scar density increased (Figure 5F), and the particles and debris attached to the highly porous zones (Figure 5F). Consequently, the reason for the lower surface roughness obtained with the two-step polishers compared to the one-step polishers could be that the attachment of these deposits affected the surface less than other issues mentioned above. In addition, the surface deterioration of these composites (containing glass fillers) could be attributed to the diamond abrasive particles of the one-step and two-step polishers.
For all tested composite resins, the multistep polishers created similar surface smoothness to the Mylar strip (Figures 3 and 6). In addition, the amount and size of wear debris and attached abrasive particles were very low on the surfaces of the nanohybrid (Figure 6B) and nanoceramic composites. The pores were filled with wear debris at the surface of Ceram-X (Figure 6C). This finding supported the results of the SEM measurements, which found that lower surface roughness was obtained with the multistep polishers for the nanoceramic composites. In addition, the smooth surface for all tested composite resins can be explained by the smaller abrasive particles used for each step to remove scratches from the previous polishers until highly polished surfaces were obtained.20
Regarding the limitations of this in vitro study, although all polishing procedures were performed by a single operator under light pressure with constant linear movements, a custom-made device should be used to standardize the pressure and keep the position of the handpiece constant in future studies.
CONCLUSIONS
Within the limitations of this study, the following conclusions can be drawn:
Different surface roughness data were obtained through contact profilometry and SEM. Of these two approaches, the 3D surface topography obtained through SEM analysis could provide more detailed and realistic surface roughness measurements of composite resins.
SEM examination of all polishing systems showed that the supra-nanohybrid composite resin had lower surface roughness than the nanohybrid composite resin.
There were no significant differences among all polishing systems for the supra-nanohybrid composite, whereas the multistep polishers created lower surface roughness than the one-step polishers for the nanohybrid and nanoceramic composite resins.
One-step and two-step polishers created porosity on the surface of the nanohybrid and nanoceramic composite resins, which contained glass fillers. According to EDS analysis of the porous zones, the elemental composition of these particles was quite close to diamond abrasive particles and glass filler debris.
An example of the SEM-3D Roughness Measurement of a specimen (GrandioSo + Opti1Step) surface. The image showing the heat map of the surface explaining the roughness changes that include random linear path selections (numbered colored arrows) to measure the Ra values, cut-off values, measured values of all paths, and their profiles in the main measurement screen of 3D Roughness Reconstruction software.
Representative images of scanning electron microscope (SEM)–3D Roughness Measurement Application at 400× magnification for all tested groups. Representative images of the SEM 3D roughness measurements for all tested groups showing the linear Ra measurement paths. (A): Estelite Asteria + Mylar Strip. (B): Estelite Asteria + Opti1Step. (C): Estelite Asteria + Clearfil TwistDia. (D): Estelite Asteria + Sof-Lex XT Pop-On. (E): GrandioSo + Mylar Strip. (F): GrandioSo + Opti1Step. (G): GrandioSo + Clearfil TwistDia. (H): GrandioSo + Sof-Lex XT Pop-On. (I): Ceram-X Spheretec One + Mylar Strip. (J): Ceram-X Spheretec One + Opti1Step. (K): Ceram-X Spheretec One + Clearfil TwistDia. (L): Ceram-X Spheretec One + Sof-Lex XT Pop-On.
Scanning electron microscope (SEM) micrographs of composite resins with mylar strip. (A): Estelite Asteria + Mylar Strip. (B): GrandioSo + Mylar Strip. (C): Ceram-X Spheretec One + Mylar Strip at 500× magnification for superficial examination. (D): Estelite Asteria + Mylar Strip. (E): GrandioSo + Mylar Strip. (F): Ceram-X Spheretec One + Mylar Strip at 2500× magnification for particle size observation.
Scanning electron microscope (SEM) secondary electron detector micrographs of composite resin surfaces after being polished with Opti1Step. (A): GrandioSo + Opti1Step showing wear scars, deformed and particle attached pores (black arrows). (B): Ceram-X Spheretec One + Opti1Step showing wear scars and abrasive attached zones (black arrows), as well as BSD images to investigate the details of (C) pores on GrandioSo + Opti1Step (black arrows) and (D) attached abrasive particles on Ceram-X Spheretec One + Opti1Step (black arrows).
Scanning electron microscope (SEM) micrographs of composite resin surfaces polished with Clearfil TwistDia. (A): secondary electron detector (SED) image of GrandioSo + Clearfil TwistDia showing wear scars and debris where some details (black arrows). (B): Abrasive particles. (C): Filler dislodgement (white arrows). (D): Wear debris. (E): Wear scars (black arrows) and superficial abrasion marks on the surface of filler particle (white arrows). (F): SED image of Ceram-X Spheretec One + Clearfil TwistDia with particle and debris in detail. (G): BSD image of Ceram-X Spheretec One + Clearfil TwistDia with porosity in detail.
Scanning electron microscope (SEM) micrographs of composite resin surfaces polished with Sof-Lex XT Pop-On. (A): Estelite Asteria + Sof-Lex XT Pop-On. (B) GrandioSo + Sof-Lex XT Pop-On. (C): Ceram-X Spheretec One + Sof-Lex XT Pop-On.
Energy dispersive x-ray spectroscopy analysis of the abrasive particle lodged on the surface of Ceram-X polished with Opti1step.
Energy dispersive x-ray spectroscopy analysis of the dislodged particle observed on the surface of GrandioSo polished with Clearfil TwistDia.
Basic schematic explanation of the resolution difference between contact profilometry and scanning electron microscope (SEM) surface roughness measurements.
Contributor Notes
Clinical Relevance
For all polishing systems, a smoother surface was obtained for supra-nanohybrid composite than nanohybrid composite.
*Burcu Oglakci, DDS, assistant professor, Department of Restorative Dentistry, Bezmialem Vakif University, Fatih-Istanbul, Turkey; e-mail: burcu923@hotmail.com
Bedri Onur Kucukyildirim, PhD, associate professor, Yildiz Technical University, University of Instanbul, Department of Mechanical Engineering, Besiktas/Istanbul, Turkey
Zümrüt Ceren Özduman, DDS, assistant professor, Department of Restorative Dentistry, Bezmialem Vakif University, Fatih-Istanbul, Turkey
Evrim Eliguzeloglu Dalkilic, DDS, PhD, professor, Department of Restorative Dentistry, Bezmialem Vakif University, Fatih-Istanbul, Turkey