Maintenance of adequate surface smoothness of dental materials is an important factor for the longevity of dental prostheses and for preserving the health of hard and soft tissues in contact with them, as well as the systemic health of their users [2, 9]. Factors such as diet, chewing, hygiene, and habits such as smoking and alcohol consumption have the potential to alter the surface roughness and color of prosthetic materials because they constantly act in the oral cavity [2, 6, 12–16, 18, 19, 39]. Considering that, this study evaluated the effect of a mechanical polishing protocol performed continuously on denture base acrylic resin and artificial teeth submitted to coffee immersion and exposure to cigarette smoke. For most experimental conditions, the polishing reduced pigmentation and increased the smoothness of the tested materials, controlling changes in surface roughness.
Coffee immersion increased surface roughness and promoted color change above the clinical perceptibility threshold (ΔE00 > 1.30) for the artificial teeth, as well as above the clinical acceptability threshold (ΔE00 > 2.25) for the base resin, in agreement with previous studies [2, 6, 10, 19, 34, 40, 41]. The changes in surface roughness caused by coffee seem to be related to the characteristic of solubility of the acrylic resin. Once in contact with the aqueous medium, the material undergoes degradation in its polymeric network through the spaces between the molecules, promoting separation and expansion of the polymer chains, causing the opening of microcracks on the surface [42]. Another important aspect is the level of polarity of the liquids; less polar dye substances, such as coffee, have an easier time penetrating the polymer matrix, promoting its oxidation and chemical instability [43]. Moreover, the tannic acid present in coffee has the potential to raise the pH of the medium, thus favoring the solubilization of the material surface [18, 33]. The pigmentation of polymers occurs due to the sorption of liquids by means of absorption and adsorption processes, in which the dyes are deposited on the surface or penetrate the internal structure and can be found free or bound to the polymer chains [10, 19, 42]. Moreover, the tannins present in coffee present more ionizable molecular groups or reactive secondary metabolites that are soluble in water and stable at high temperatures, which promote staining of the material by the sorption of these metabolites on the surface, as well as by binding and occupying the spaces between the polymer molecules [44].
Cigarette smoke caused a significant increase in the surface roughness of the artificial teeth and it was responsible for the greatest color change in both materials, generating ΔE00 values significantly higher than the clinical perceptibility (ΔE00 > 1.30) and acceptability (ΔE00 > 2.25) thresholds, in accordance with previous studies [5, 14, 21, 22]. Cigarette smoke produces the deposition of a dark and viscous layer on the surface of the specimens, which is difficult to be removed, as observed throughout the study, consisting of carbon monoxide, carbon dioxide, arsenic, ammonia, nickel, nicotine, tar, and heavy metals such as lead and cadmium [4, 5, 14]. It is attributed to the aromatic hydrocarbons present in elements contained in cigarettes, such as tar, the deleterious action to polymeric materials [45]. Tar is considered one of the main causes of pigmentation, with the power to generate great intensity stains [46], represented by reduced luminosity (lower L* values) and greater yellowish appearance (increased b* values) [14, 31]. In addition, due to cigarette combustion, these changes can be enhanced by increased temperature [31]. Thermal variations can modify the surface texture of these materials, making them more susceptible to staining [47]. Some other substances that may also be present in the composition of cigarettes can be transferred to the smoke during combustion, such as cocoa and sugars, presenting staining potential due to their dark tone and ability to adhere to the surface of composites [22]. Although saliva was not used in the present study, changes in the surface roughness of artificial teeth are also attributed to the formation of an acidic pH solution by the contact of cigarette smoke with saliva [39].
The polishing tested in groups CF + Pol, CS + Pol significantly decreased the mean values of Ra already in the first evaluation (T1) for both materials, reaching lower values than the control groups. Thus, for surface roughness under these experimental conditions, the hypothesis of the study was accepted. This reduction is due both to the change in the surface layer of the materials, because of the controlled friction promoted by the set of abrasive paste and felt wheel, and to the removal of the coffee and cigarette pigments adhered to the surface [2–4]. Also, the increased smoothness of the materials may have hindered the impregnation of these pigments, as well as facilitated the removal of the viscous nicotine film formed at each experimental cycle [2, 5, 10].
Considering the habit of associating coffee consumption with smoking, a group submitted to contact with coffee and cigarette smoke concomitantly (CFCS) was included. The aim of this group was to investigate whether the deleterious effects to materials would be potentiated by this association, as well as the capacity of polishing to interfere in their degradation. It was believed that there would be a more expressive increase in surface roughness; however, in both materials, the variation of this property in the CFCS group was comparable to that observed in the isolated groups (CF, CS), reaching final values at T2 that were statistically similar. Thus, this association seems not to potentiate the effect on surface roughness observed in the isolated groups.
At the end of the study, the polishing protocol instituted in the CFCS + Pol group reduced the roughness of the denture base specimens to values similar to those observed in the CF + Pol and CS + Pol groups, reaching values much lower than the CF, CS and CFCS groups. In the artificial teeth, the reduction in roughness in the CFCS + Pol group was less significant, generating values higher than those in the CF + Pol and CS + Pol groups and similar to their controls. Thus, for the association of pigmenting agents, the study hypothesis was partially accepted. This difference between the materials may be related to the composition of artificial teeth, represented by low molecular weight polymeric chains (of small extension) that are chemically bonded to each other by means of simple covalent cross-linkings [48], which are not found in the denture base acrylic resin [49]. This type of bonding provides the material with higher strength characteristics, which may make it less susceptible to the effects of mechanical polishing.
Unlike what was observed in surface roughness, the association coffee + cigarette generated the highest mean values of ΔE00, statistically superior to the isolated groups CF and CS in most evaluations. As observed throughout this study, the intense pigments of coffee and cigarettes seems to have a synergistic effect, causing a cumulative effect, in agreement with previous studies that found exacerbation of polymer pigmentation caused by cigarette smoke when other beverages were associated, such as coffee [50] and whiskey [22].
The tested polishing significantly reduced the color change caused by coffee and cigarette smoke in both materials, in all groups (CF + Pol, CS + Pol, CFCS + Pol), keeping the ΔE00 values below the clinical acceptability threshold (ΔE00 < 2.25) in most experimental conditions. Thus, for this property, the study hypothesis was accepted. The mechanical action of the felt wheel and the abrasive particles of the polishing paste reduces staining [3, 4] by improving the roughness pattern of the material, decreasing the chances of pigment penetration [31], as well as removing the viscous film of cigarette components deposited over the course of the experimental cycles [2, 12, 21]. It was interesting to observe that, in both materials, the ΔE00 values achieved in the CF + Pol and CS + Pol groups were similar at T1 and T2. However, in the CFCS groups, the ΔE00 values at T2 were higher than those observed at T1, demonstrating the greater staining capacity of this association, as well as greater resistance to the action of continuous polishing. Nevertheless, ΔE00 values in the CFCS + Pol group were reduced to levels similar to those in the isolated CF and CS groups.
In studies that subjected polymeric dental materials to cigarette smoke and tested the effect of polishing methods performed previously [21] or later [31, 50], or the action of sanitizing solutions [14] after the cigarette cycles, a reduction in staining was observed; however, without returning to the initial color values. The results of such studies point out that these procedures do not minimize the intrinsic pigmentation caused by the absorption of substances in the polymeric matrix. In this study, it was found that, in the denture base acrylic resin specimens, the change in color in the CS + Pol group was considerably reduced, assuming ΔE00 values similar to the groups immersed in water. This more expressive effect on the color may be attributed may be attributed to the continuous nature of the polishing method instituted concomitant to exposure to cigarette smoke, which may have exerted a greater protective effect on the material. In the artificial teeth, this reduction was significant, but it did not reach the values observed in the control group, probably due to the greater resistance of this material to the wear generated by polishing.
The present study points out the potential of the proposed mechanical polishing, when performed continuously, to minimize alterations in the acrylic resin used in dental prostheses, such as staining by coloring substances like coffee and cigarettes. These results add to previous studies that tested the same polishing protocol and observed a reduction in the deleterious effects of coffee, sodium hypochlorite, and mechanical brushing on acrylic resin properties, such as color stability, surface roughness, mass change, microhardness, as well as on biofilm adhesion [2, 36, 51]. The constant maintenance of the adequate surface smoothness makes the material less susceptible to such alterations [31]. With the results of the present study, the authors point out the possibility of creating a portable device for the execution of mechanical polishing of removable dental prostheses in a home environment, which can provide greater health to its users, reducing costs associated with the degradation of materials and dental consultations, as well as with the possibility of increasing the longevity of materials.
Among the main limitations of this study, the in vitro methodology used is highlights, as it does not simulate the real conditions of the intraoral environment. The absence of a protective film of saliva, mechanical cleaning promoted by the tongue, and chewing may have potentiated the effects observed in the materials, especially in relation to pigmentation by coffee and cigarette smoke. Another limitation was the absence of a reference in the literature for a cigarette quantity that simulates long-term use, as there is for coffee (24 days of immersion in coffee simulating two years of consumption). The studies that subject dental materials to exposure to cigarette smoke do not make this type of comparison, probably because of the difficulty in replicating the large number of units consumed in one or two years of smoking. However, significant effects were observed in these studies, even if the cigarette was used for only a few days. Because of this, there was difficulty in temporal extrapolation of the results in both the CS and CFCS groups. Nevertheless, the results obtained can be used as parameters for future studies, which may include different pigmenting substances and sanitizing solutions, evaluate prosthetic materials of different compositions, and extend the exposure time of the materials to test the limit of the polishing effectiveness.