Effect of Polishing Systems on the Surface Roughness of Nano-Hybrid and Nano-Filling Composite Resins: A Systematic Review

Background: The polishing of surface roughness is an important characteristic of composite resins and is directly related to the longevity of the restoration and patient comfort. Different polishing systems utilize different protocols, as reported in the literature. This systematic review (SR) aimed to synthesize and analyze the available scientific evidence about the effect of polishing systems on the surface roughness of nano-hybrid and nano-filling composite resins. Methods: The study protocol of this SR was registered at the International Prospective Register of Systematic Reviews -PROSPERO- (CRD4201705653). A search was conducted in PubMed-Medline, Scopus, LILACS, EMBASE, for the period 2007–2020. Quality appraisal and a descriptive analysis of the papers that met the inclusion criteria were conducted. Results: 18 records were included. Seven polishing systems (PS) of one step were found, seven PS of two steps, eight PS of three steps, three PS of four steps, and four PS of five steps. Polishing protocols (PP) varied, with application times ranging from 10 s to 60 s with speeds between 10,000 and 30,000 RPM. Regarding composition, the aluminum oxide was one of the most important components used to achieve a smooth surface. Conclusions: Multistep polishing systems were the most effective (i.e., Astropol and Sof-Lex Discs).


Introduction
Composite resins were introduced to dentistry more than 50 years ago [1]. This kind of dental material has presented disadvantages, such as high pigmentation and accelerated wear [2]. However, these materials are universally accepted for direct restorations due to their aesthetics, adhesion capacity (including an adhesive system), longevity, and thermal insulation (especially when light cured composite resins are considered) [2][3][4]. The mechanical properties of composite resins are related to their long term success [2], and this depends on their microstructure [5]. Therefore, the amount of load, size, morphology, and distribution of the filler particles are essential for material selection [1,2]. The most important current changes consist in reducing the size of the filler particles, thus obtaining materials that are easier to use and more effective [1,6]. Surface hardness is influenced by the degree of surface roughness and predicts wear resistance and ability to grind the opposing tooth [7,8].
Polishability is an important characteristic of composite resins: a smooth surface gives the restoration better aesthetics and comfort to the patient, reduces discoloration, facilitates hygiene [9][10][11][12], decreases bacterial adhesion [10], and reduces tissue inflammation, secondary caries, biofilm retention [9,10,13,14] and the risk of fracture [11]. Similarly, the use of composites is relevant in other clinical fields, such as orthodontics, where polishing systems play an important role after the treatment to ensure aesthetics and to reduce associated dental or periodontal complications [15][16][17][18]. A polished surface reduces initial bacterial adhesion and the development of biofilm in the restoration and the adjacent dental surfaces [19]. The surface roughness threshold for bacterial retention is 0.2 µm; below this, a reduction in plaque accumulation could be expected [20]. Changes of even 0.3 µm in the surface finish can be easily detected by the tongue [8,12]. Finishing procedures remove excess material with particles larger than 25 µm, while polishing procedures remove particles smaller than 25 µm [12]. At present, it is believed that composite resins with nano-filled particles obtain a better polish and shine after being subjected to different polishing processes. However, in a systematic review carried out by Kaizera et al. [21], it was concluded that there are no statistically significant differences in the polishing and brightness of composite resins with nano-filled particles vs composite resins with hybrid or nano-hybrid particles.
To obtain an optimum polishing and shine of the composite resins, whose particles vary in hardness, shape and size, it is necessary to subject the material to abrasion processes [9,11]. Currently, an attempt has been made to determine which abrasion system provides the most polished surface for the composite resins, and several methods have been introduced without reaching a consensus that demonstrates which is the best. Some polishing methods or systems are: silicone discs, tungsten carbide burs, rubber cups, abrasive belts, and polishing pastes. These are available in one step polishing systems and multistep polishing systems [8,13,19,24,25].
Research in the field of the polishing and shine of composite resins is justified by the need to create a restored dental surface with optimal aesthetics and biological and functional properties in the patient. The amount of in vitro research that currently exists suggests the need to work on the comparison of the results obtained which will guide the improvement in behaviors and parameters to obtain better polished restorations [21].
Accordingly, this study aimed to conduct a systematic review that seeks to analyze and synthesize the available scientific evidence on the polishing systems of nano-hybrid and nano-filling composite resins to guide the clinician in obtaining an optimal polish that favors the aesthetic expectations of the patient and improves dental and periodontal health.

Study Protocol and Registration
The study was approved by the Ethical Committee of the Faculty of Dentistry at the University of Antioquia (Act 2/2017). In addition, the protocol of the systematic review was registered at the International Prospective Register of Systematic Reviews -PROSPERO-(CRD42017056536). This paper was written according to the PRISMA statement for systematic reviews and meta-analysis [26].

PICOs Question and Eligibility Criteria
The PICOs (population/participants, intervention, control, outcome and study design) strategy to formulate a focused question was used as follows: What is the scientific evidence related to in vitro studies (s) about the effect of different polishing systems (I-C) on the surface roughness (O) in nano-hybrid and nano-filling composite resins (P)? According to this question, we considered papers that accomplished these criteria:

Search Strategy and Data Extraction
Four electronic databases in health sciences were searched: PubMed-Medline, Scopus, LILACS (Latin American scientific literature in health sciences), EMBASE (The Excerpta Medica Database). In addition, grey literature (Google Scholar) searches through the reference lists of articles included, and manual consultations in specialized journals were conducted. This process was led by two reviewers (R.J.C and E.J.L.G). In the first round, the title and abstract were evaluated to define potential articles. After this, duplicate references were excluded; the full texts of remaining articles were reviewed. This process was conducted manually by R.J.C and E.J.L.G. Any discrepancies were resolved by the consensus of all four reviewers. We conducted a pilot test for one database to ensure concordance in the data extraction process, with a simple concordance index of 85% (by using ten abstracts).

Critical Appraisal and Studies' Analysis
Two of the authors (R.J.C and E.J.L.G) reviewed the quality for reporting in vitro studies. This process was checked by a third reviewer (A.A.A.S) who has expertise in epidemiology and systematic reviews. For this purpose, we applied a modified CONSORT checklist of 14 items for reporting in vitro studies of dental materials [27,28]. After a careful evaluation of papers, we selected those accomplishing at least 70% of all items for subsequent descriptive analysis. To guarantee the quality process, we conducted a pilot test with 5 articles, and we calculated a simple concordance index, with a score of 90%.

Data Analysis
We carried out a descriptive analysis of the main characteristics of the included reviews: the first author and year of publication, country, journal, study objective(s), composite resin name ® , type of composite resin, manufacturer ® .
To group the results obtained in Ra from the different studies, it was decided to average the different samples using the formula "Mean of Mean" ( In this formula, CxPx is mean in Ra, P is the mean value, and Cx is the quantity (samples). All values are represented with means and their standard deviation (±SD).
To group the results obtained in Ra from the different studies, it was decided to average the different samples using the formula "Mean of Mean" ( of ): In this formula, CxPx is mean in Ra, P is the mean value, and Cx is the quantity (samples). All values are represented with means and their standard deviation (± SD).

Discussion
The main findings of this systematic review showed the available scientific evidence about the effect of polishing systems on the surface roughness of nano-hybrid and nanofilling composite resins through the analysis of in vitro studies. The decision to include studies expressing surface roughness results in Ra units was mainly because it was the most widely used unit of measure in most of the studies that evaluated the surface roughness of the composite resins. Other units of measure for surface roughness, such as Rz, Ry and Sa, were absent in the vast majority of studies, which was why it was decided not to use these units.
The vast majority of studies reported a control group, with the application of a pressure Mylar band on the composite resin being the most popular, with a total of 328 samples and an average of Ra 0.1281. However, Alawjali and Lui in 2013 published an in vitro study in which they demonstrate that Mylar groups generate greater color change after being subjected to different substances with pigments, and mention that the increase in

Discussion
The main findings of this systematic review showed the available scientific evidence about the effect of polishing systems on the surface roughness of nano-hybrid and nanofilling composite resins through the analysis of in vitro studies. The decision to include studies expressing surface roughness results in Ra units was mainly because it was the most widely used unit of measure in most of the studies that evaluated the surface roughness of the composite resins. Other units of measure for surface roughness, such as Rz, Ry and Sa, were absent in the vast majority of studies, which was why it was decided not to use these units.
The vast majority of studies reported a control group, with the application of a pressure Mylar band on the composite resin being the most popular, with a total of 328 samples and an average of Ra 0.1281. However, Alawjali and Lui in 2013 published an in vitro study in which they demonstrate that Mylar groups generate greater color change after being subjected to different substances with pigments, and mention that the increase in discoloration can be explained by the presence of a layer rich in the composite resin matrix that is formed on the surface of the restoration [44]. It has also been shown that the polishing achieved with the Mylar strip results in surfaces with less hardness (versus abrasive polishing systems) and, consequently, greater discoloration [44]. Therefore, the removal of the composite resin matrix by finishing and polishing will allow the harder filler particles to remain in contact with the surface during polishing, producing a harder and pigment resistant surface [20].
Concerning the PP, the great heterogeneity in this regard is evidenced from the total of 11 different protocols. Besides, once the results of the studies selected were averaged, only 11 polishing systems exceeded the 0.2 µm threshold, which is considered important to reduce plaque accumulation, the possibility of secondary caries and periodontal problems [29,35,36,38,41]. The best result was obtained by the Astropol system, with an average Ra of 0.0549µm after being applied in 85 samples in the studies of Antonson et al. in 2011 andSenawongse et al. in 2007 [29,36]. Two protocols were reported, one of 20 s and another of 60 s of application, and it was appreciated that with both protocols Astropol could be below the threshold of Ra 0.2 µm [29,36]. The Super-Snap Rainbow Technique Kit system obtained the second best performance with an Ra mean of 0.0799. However, it is important to be clear that this system was only reported by Yadav et al. in 2016 [20], and this system was applied in 10 samples. The Sof-Lex Discs four step system obtained an average Ra of 0.0961 µm after being applied in 154 samples in five of the selected articles, with the use of this system six different protocols were found, all managing to exceed the limit of 0.2 µm [29,35,37,38,42].
On the other hand, the three step Sof-Lex Discs system was applied, obtaining an average of 0.3577 µm in a total of 221 samples in eight studies [3,20,31,32,36,40,41,43], of the five protocols reported with this system, it is well known that to gain optimal results, each disc must be applied for a time longer than 45 s, since whenever the application time is shorter, the result will be greater than 0.2 µm. This information is important, since according to Yadav et al. [20], the prolonged application of the Sof-Lex Discs system may have a disadvantage, that the friction heat generated by the discs causes microcracks in the polymer matrix that creates a rougher surface for hybrid composites.
It should be clarified that in this systematic review, some studies used the Sof-Lex Discs as a finishing and polishing system, fully (four steps) or partially (three steps, which means, ignoring the first disk in the system stream). Accordingly, the results were grouped separately according to whether four or three steps were used in the Sof-Lex Discs PS.
According to the results, in most systems that exceeded the 0.2 µm Ra threshold, aluminum oxide was found to be a common denominator within components which produce much smoother surfaces and this is explained by the fact that aluminum oxide has a higher hardness than most filler particles in composite resins. Otherwise, the polishing agent would only remove the soft composite resin matrix leaving filler particles protruding from the surface [29,36,40]. According to Weinstein [45], when systematically decreasing the particle size of the abrasive, a smoother surface can be achieved. Likewise, the polishing material must be smaller than the particle size of the restorative material being polished to produce better results [45]. This reveals the success of the Astropol system in most of the studies, since in its last step only abrasive particles of 0.3 µm were observed.
The number of steps of the systems that exceeded the 0.2 µm Ra threshold was between two and five. It was observed that the EXL-695 experimental disc system, with an Ra of 0.12 µm [36], and Clearfil Twist Dia, with an Ra of 0.2 µm [41], were two step systems. In the study by Antonson et al. [36], the EXL-695 system was applied only to five composite resin types with a small sample being meaningful, and in the study by Aytac et al. [41], the Clearfil Twist Dia system was applied to 40 samples with a protocol of 45 s per step. However, unlike most PS, this is not composed of aluminum oxide, but rather rubber with diamond grains, and in this case, it was shown that they exceeded the control group of the Mylar band and the three step Sof-Lex Discs, Occlubrush, Sof-Lex Spiral Finishing and Polishing Wheels systems, and obtained an Ra higher than 0.2 µm. This suggests that smooth surfaces can also be achieved with diamond abrasive particles or diamond polishing pastes using finishing and polishing systems containing these particles [41].
Considering the reduced steps, application time and elimination of the risks of cross infection, single step polishing systems have become an interesting option [36]. In this systematic review, no single step polishing system managed below the 0.2 µm Ra threshold. However, the Enhance systems with a mean Ra of 0.2167 µm after being applied in 30 samples with a 20 s PP, as is shown in the study of Nunes et al. [39], and PoGo with an Ra of 0.2218 µm after being applied in 121 samples with a PP of 10 to 30 s [3,30,31,38,43], achieved results that were close to the threshold, which means greater simplicity in the clinical protocol. When the PP was reduced to 10 s with the PoGo system, as is shown in the study by Buchgraber et al. [38], a result of an Ra of 0.560 µm was obtained. Although the one step PS mostly offer results somewhat distant from the 0.2 µm threshold, the Enhance and PoGo systems can become a moderately acceptable option.
It should be noted that some studies have reported that the PoGo one step polishing system exhibited a similar or even better Ra when compared to the three step Sof-Lex Discs system [3,31,43]. According to the authors of these studies, the superior performance of PoGo could be attributed to the fine diamond powders used instead of aluminum oxide [3]. However, whenever the PoGo system was compared to the complete Sof-Lex Discs or four step systems, it was generally surpassed [38].
The manufacturer recommends the use of the Enhance system followed by the PoGo system for more favorable results [3]. However, this combination did not achieve good results, since they obtained an average Ra of 0.4631µm after being applied in 74 samples in five studies, with protocols between 20 and 30 s [32][33][34]36,37]. This situation does not necessarily mean a bad combination, since this result was obtained after calculating a general mean of all the results found and, in some of these investigations, the Ra was significantly higher for all participants [32][33][34]. In addition, in some Enhance PoGo studies, they obtained results below the Ra threshold of 0.2 µm and reached the heights of the Astropol and Sof-Lex Discs of three steps [36], and even managed to overcome the complete Sof-Lex Discs or four step systems, as happened in the study of Berger et al. [37]. Therefore, the manufacturer's instructions must be followed and the Enhance/PoGo system should be used as one. Combining systems, as observed in this systematic review in most cases, did not seem to be very practical since it resulted in greater clinical complexity, requiring up to five steps per protocol [42], and did not always represent an improvement in the Ra, as can be seen in the study by Baseren et al. [46], where they combined the Astropol system, of proven efficiency, with Astrobrush and obtained an average Ra of 0.3450 µm, a value surpassed by the other systems evaluated in that research.
Regarding the composite resins reported in the different studies, it should be noted that although Filtek Supreme XT (transparent shade and dentin shade) and Tetric Evo-Ceram resin obtained the best results, the results obtained were averaged considering only 30 samples for each composite resin and these were obtained only from the study of Senawongse et al. [29], the results obtained after the composite resins with nanoparticles were subjected to the different polishing systems are highlighted in this study, with average values of Ra (µm) from 0.016 to 0.088 obtained. Therefore, it is evident that this study is one of the lowest reported surface roughness values, which suggests that Filtek Supreme XT and Tetric EvoCeram resins are not necessarily the best polished composite resins.
Within the limitations of this systematic review, there is great heterogeneity in the methodology of the selected studies, which makes it difficult to conduct a meta-analysis. However, it is considered a strength that all the included studies reported surface roughness in Ra units, which made it possible to average samples from different studies. For future systematic reviews related to the topic, it is recommended to evaluate the surface roughness of other materials, such as ceramics or metals, among others. Finally, it should be noted that the lacking of instruments for assessing the risk of bias or methodological quality specifically for in vitro studies is evident. For that, the research team decided to use standardized guidelines for improving quality and transparency in reporting in vitro studies in experimental dental research or dental materials [27,28]. New studies are needed, including variables such as the type and size of filling, degree of conversion, and the mechanical strength of several composite resins, to enable a complete discussion about the effectiveness of the polishing systems. Further systematic reviews in other fields, such as orthodontics, could elucidate the importance of polishing systems to ensure dental and periodontal health during and after the orthodontic treatment [15][16][17][18].

Conclusions
This systematic review reveals that the most effective polishing systems are those in which the size of the abrasive particle is systematically decreased, as is the case with the Astropol and Sof-Lex Discs system. Aluminum oxide is one of the most important components used to achieve a smooth surface. Likewise, diamond particles seem to be equally effective. Although the single step systems have evolved, they are still surpassed by multistep systems if simplicity and effectiveness are sought in the clinic. Similarly, according to the findings, it is advisable to opt for systems that have been demonstrated to be effective even with application times of a few seconds.

Institutional Review Board Statement:
This study is a systematic review. However, the protocol for this review was approved by the Ethical Committee of the Faculty of Dentistry of the University of Antioquia (Act 2/2017).

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.