Finishing and polishing procedures are important both from an esthetic point of view and a functional one. Obtaining a well-polished surface of restorative esthetic materials guarantees the esthetics of the restoration and it reduces dental plaque retention. ¹ The achievement of a smooth restoration surface is, therefore, a primary objective which reduces the amount of bacterial adhesion to the restorative material, and consequently, it reduces the secondary microinfiltration. ² , ³ Two factors affect the quality of polishing: the material used for restoration and the finishing and polishing procedures adopted. ⁴

The size and shape of the filling particles of esthetic restorative materials greatly affect the final surface of the restoration, both in terms of roughness and gloss. ⁵ The evolution of restorative materials has led to the introduction of filler nanoparticles which guarantee the possibility to obtain high-quality restoration surface. ² However, the heterogeneous composition of esthetic restorative materials influences the roughness of the restoration; resin matrix and filler particles do not wear down in the same way. This is due to different hardness: imperfectons are often formed around hard quartz particles of conventional composites, and for this reason, irregularities appear on the surface of the restoration. ⁶ , ⁷

Various techniques of finishing and polishing have been proposed and analyzed over time: many studies have shown that the smoothest surface is obtained using a polyester matrix in direct contact with the material during the curing phase. However, it is not always possible to use this method because of the anatomical complexity of the tooth. ⁸ Other finishing and polishing methods include the use of aluminum oxide finishing discs, fine diamond finishing burs, carbide finishing burs, resin polishing points, and polishing pastes. ⁹ , ¹⁰ , ¹¹ , ¹² The results of previous investigations suggest that each material behaves independently: the same finishing and polishing procedures applied to different materials lead to different smoothness results. ¹³ , ¹⁴

The aim of this in vitro study was to evaluate and compare the surface roughness of esthetic restorative materials achieved using different finishing and polishing techniques.

Specimens' preparation

In this in vitro study one microfilled composite (Gradia Direct Anterior, GC Corporation, Tokyo, Japan), one nanofilled composite (Filtek Supreme XTE, 3M ESPE, St. Paul, MN, USA), one nanoceramic composite (Ceram.X Universal, Dentsply DeTrey, Konstanz, Germany), one ultra-fine hybrid composite (Essentia enamel, GC Corporation, Tokyo, Japan), one nanohybrid Ormocer-based composite (Admira Fusion, Voco, Cuxhaven, Germany), and one supra-nano spherical hybrid composite (Estelite, Tokuyama Dental corporation, Taitou-ku, Tokyo, Japan) were evaluated Table 1.{Table 1}

All materials were polymerized according to manufacturers' instructions into silicon rings (height 2 mm; internal diameter 6 mm; external diameter 8 mm) to obtain 40 specimens identical in size of each material. Cavities of rings were slightly overfilled with material, covered with transparent polyester film strip (Mylar Strip, Henry Schein, Melville, NY, USA), pressed between glass plates, and polymerized for 40 s on each side using a curing unit (Celalux II, Voco, Cuxhaven, Germany). One light polymerization mode was used for each material standard: 1000 mW/cm ²for 40 s. The intensity of the light was verified with a radiometer (SDS Kerr, Orange, CA, USA). The light was placed perpendicular to the specimen surface, at distance of 1.5 mm. A total of forty specimens of each esthetic restorative material were prepared. After polymerization, the specimens were stored in distilled water at 37°C for 15 days.

Finishing/polishing procedures

The specimens of different esthetic restorative materials were randomly assigned to four groups (10 specimens of each composite for each group): the upper surface of each specimen was finished/polished with different finishing/polishing procedures Table 2.{Table 2}

Group 1: control group (no finishing/polishing procedures)

All finishing/polishing procedures were performed by the same investigator, to reduce variability. All these procedures were performed strictly following manufacturer's instructions. At the end of the finishing and polishing procedures, all specimens were thoroughly rinsed with water and allowed to dry for 24 h at 20°C before the average surface roughness (Ra) was measured.

Surface roughness analysis

A profilometer (Alicona Infinite Focus, Alicona Imaging GmbH, Raaba/Graz, Austria) was used to measure the surface roughness of specimens. Ra (surface roughness) was used as unit of measure in this process. Three different types of measurements (Ra) were recorded for each specimen; all the measures were taken from the middle of each specimen, giving a total evaluation length of 1 mm. The mean of all Ra measurement values was calculated for each specimen, and an overall Ra measure was calculated for the total sample group. Microscopy images Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6were taken during the measurements through the microscope of the profilometer (optical measurement: noncontact, three-dimensional, based on Focus-Variation). Figure 1

Microscopy images (×100) about surface morphology of Gradia Direct Anterior in control group (a), after polishing with REF 4312A (b), REF 4652 (c), and REF 4546 (d).

Ra values (μm) are presented in Table 3. The statistical analysis showed significant differences (P < 0.05) in Ra values for each composite when different finishing and polishing methods were applied Table 4. Group 1 (control group) had the lowest Ra values for all materials tested except for Estelite that showed the lowest values for specimens in Group 4 Table 5. Specimens of Gradia Direct Anterior, Filtek Supreme XTE and Ceram.X Universal treated with finishing and polishing technique included in Group 4 showed similar Ra values to Group 1 Table 4(P > 0.05). The finishing and polishing methods in Groups 2 and 3 provided worst results, respectively, for specimens of Gradia Direct Anterior, Admira, Estelite (Group 2) and Filtek Supreme, Ceram.X Universal and Essentia enamel (Group 3). When we considered groups as treatment in the ANOVA model, the Ra values recorded for Admira were the lowest in Group 1. Group 2 showed significantly higher roughness for all the materials Table 2and Filtek Supreme showed the highest Ra values, even for Group 3. Estelite showed the significantly higher smoothness for Groups 3 and 4.{Table 3}{Table 4}{Table 5}

The effectiveness of finishing/polishing procedures on composite surface is an important goal to be achieved in the restorative process. Esthetic restoration can be imperceptible only if its surface closely resembles to the enamel surface. ¹⁵

Two factors affect the quality of polishing: the material used for restoration and the finishing and polishing procedures adopted. As far as the material used is concerned, nano-ceramic composite resin has good mechanical properties with high esthetic features. Their structure is made by small particles clusters, and it gives the possibility to obtain surface smoother than the one obtainable with other materials. ¹⁶

Previous studies have pointed out that the smoothest obtainable surface is obtainable by curing the esthetic material in direct contact with a polyester matrix (Group 1). ⁸ , ¹⁷ , ¹⁸ The filler particle size significantly influences the final smoothness and gloss of restoration. The results of this study provide evidence of the fact that the smaller the filler particle size is, the easiest the polishing procedure is, because the surface area made by filler particles is reduced. For this reason, the only microfilled material (Gradia Direct Anterior) was not a well-performing material in terms of obtainable roughness in almost all the groups.

On the other hand, as far as the procedures are concerned, among all the finishing and polishing methods investigated in this study, carbide burs (Group 4) are the one that provides the smoother surface and previous studies have confirmed the data obtained. ¹⁹ , ²⁰ Moreover, the results given by the finishing and polishing procedure by carbide burs can be compared to the ones of the control group. Tungsten carbide burs are more cutting efficient with respect to other finishing systems. Moreover, they provide a finer finishing degree and their finishing procedure is faster than the finishing procedures of the other materials. Tungsten carbide burs quickly and easily remove the surface imperfections that are present at the end of modeling procedure and simplify the polishing phase. The use of the rubber polisher with decreasing abrasiveness (Groups 2 and 3) both for the finishing phase and the polishing one is a technique which is not able to give a surface as smoother as the one obtainable with the other techniques described in this study.

The least smooth surfaces obtained are those of Groups 2 and 3: the use of rubber polisher alone does not provide a sufficiently smooth surface with respect to the baseline. These results can be explained by the excessive rigidity of the rubber polisher and their gradual wear.

The control group (Group 1) results highlight some differences in surface roughness: these differences are attributable to the different chemical nature of the components of each material and especially to the size of the filler particles. ²¹ Although the surface of the composite materials is often sufficiently smooth after polymerization, the removal of exceeding material or the recontouring of restorations is often necessary. It is clinically important to identify the less time-consuming and material-consuming finishing technique which allows the clinician to obtain the smoothest surface. A literature review has pointed out that the surface roughness (Ra) of restorations should not be more than 0.2 μm in order to avoid an increase in bacterial adhesion to the restorative materials. ²² Although this study highlights some statistically significant differences between different finishing and polishing techniques, the majority of the materials considered in this study show Ra values lower than the previously defined benchmark (0.2 μm).

Considering all the limitations of this in vitro study, it is possible to state that tungsten carbide burs were the most effective ones among the finishing and polishing systems examined. For this reason, tungsten carbide burs are clinically recommended because they are cutting efficient and less time consuming than the other methods. Furthermore, this study highlights the effectiveness of nanofilled restorative materials to achieve the goal of a high level of smoothness. However, it is worth mentioning that further in vitro and in vivo studies on a larger sample of restorative materials are needed to obtain more reliable data.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.