Consuming certain beverages, such as coffee, alcoholic beverages, and cola drinks, may have an impact on the esthetic and physical characteristics of resin composites, which could lower the restoration's quality [19]. The effect of these beverages on the surface microhardness and roughness of resin composites varies depending on the intrinsic features of the composite such as their chemical composition. In addition to that, brushing can have a great effect on the degradation of surface properties of dental resin composites.

Thus, the conducted study aimed to assess the effect of different beverages, with/without brushing, on the surface mechanical properties of dental resin composite. The null hypothesis was rejected as the various beverages with/without brushing caused a change in surface microhardness and roughness of the two dental resin composites.

One factor affecting the surface quality of dental resin composites is the finishing and polishing procedure. It has been reported that the microhardness of a celluloid strip finished composite surface was lower than the composite itself. However, finishing the composite surface with a celluloid strip can produce the smoothest resin composite surface [20]. In this study, the composite samples were light-cured in contact with a celluloid strip to eliminate the influence of the variability of finishing techniques on the results.

Food and drink only briefly contact the tooth surfaces while being consumed before being rinsed away by saliva. Previous studies often involved substrates coming into prolonged contact with acidic food substances, which is unrepresentative of clinical situations and does not take the washing role of saliva into account. For this reason, a cycle of 5 min of immersion was used in this study, and it was repeated three times daily to mimic the clinical situation.

While a person may wash their teeth for two minutes, only a portion of that time is likely spent actually scrubbing each tooth surface. Approximately four seconds should be spent brushing each tooth each day [21]. The specimens in our investigation were brushed for 5 s.

Regarding the surface microhardness results, the change in surface microhardness was the highest upon immersion in Pepsi Cola and red wine, and it was the least in the control subgroup. This was in agreement with Wongkhantee et al., [22] who reported that during a short period of contact (simulating drinking a can of soft drink), Cola significantly reduced surface hardness of enamel, dentine, micro-filled composite, and resin-modified glass ionomer. Also, Nazish et al., [23] reported a decrease in surface hardness of resin composites when exposed to acidic media. Cavalcantea et al., [24] observed that the immersion in ethanol induced a higher decrease in surface hardness values in all materials used in their study and attributed this reduction in microhardness to the higher amount of [–OH] present in the ethanol, so a higher absorption occurs by the polar portion of the matrix, causing swelling of the material. This dimensional change in the matrix causes stress at the matrix–silane–filler particle interfaces, resulting in the degradation of the bond [25]. In addition to that, soft drinks with an acidic nature can cause a reduction in the surface microhardness of composites by softening the Bis-GMA-based polymers present in the organic matrix [1]. As these solvents diffuse into the network system of the polymer it causes expansion and loss of the unreacted monomers, oligomers, and ions. Consequently, these solvents occupy the porosities and act as plasticizers without forming any chemical bonds to the network system thus reducing the hardness [7]. As a probable consequence, the inorganic particles are no longer provided with a stable structure, which could predispose to filler dislodgment and elution [7].

The examination of the beverage's erosive potential requires consideration of its chemical properties, including pH, titratable acidity, and buffering capacity [26, 27]. Although all the used staining solutions have an acidic nature, Pepsi cola had the lowest initial pH and the highest titratable acidity and buffering capacity which increased its erosive ability [28].

Vouvoudi and Sideridou [29] reported that storage in water or artificial saliva at 37 ℃ for 1 or 7 days caused post-curing reactions, while storage for 30 or 90 days seems to cause plasticization effect affecting some parameters analogously. This could explain why the control subgroup showed a reduction in microhardness even if it was the least.

Brushing yielded the highest change in surface microhardness. There was clear evidence that in the presence of an abrasive, resin composites are susceptible to micro-scale abrasion depending on the polymeric matrix and inorganic fillers. This behavior is mainly due to the lower hardness of the exposed polymeric matrix when compared to inorganic particles. The same kind of wear response is expected to occur in dental applications involving these materials due to the micro-scale abrasion action promoted by the hard particles present in food or toothpaste [12].

The change in surface microhardness was less obvious in Admira fusion in most tested groups which is in agreement with Moyin et al. [9]. This could be attributed to the nature of Admira fusion, being an ormocer. Ormocers are basically organically modified ceramic with poly-condensed organic–inorganic networks. This new class of material combines the surface properties of the silicones, the toughness of the organic polymers, and the hardness and thermal stability of ceramics [2]. It was reported by Monsarrat et al., [4] that after artificial aging, better surface integrity and less change were found in mechanical parameters for pure ormocers than for conventional ormocers and composites due to the elimination of the conventional methacrylate monomers. Also, polymer networks based on Bis-GMA are highly susceptible to chemical softening [29] which is why Grandio was affected more than Admira fusion. Moreover, the degree of conversion of dental composites is a fundamental criterion in determining the surface stability directly influencing the surface microhardness [30].

Regarding the surface roughness results, the change in surface roughness was the highest upon immersion in Pepsi Cola and it was the least in the control. Yet, all the surface roughness values were below the threshold for plaque retention (0.2 µm) [8].

Soft drinks may contain several different types of acids that can contribute to the low pH value such as the presence of phosphoric acid in cola. This was demonstrated to be highly erosive compared with other organic acids. Carbonated beverages contain also carbonic acid formed by carbon dioxide in solution. Even when the carbon dioxide has been blown off and drinks have become ‘flat’, the pH remains low. Such acidic pH was thought to be responsible for the increase in surface roughness of the tested resin composite [3]. This was in agreement with Hamouda [31] who reported that all restorative materials tested in his study became rougher after they had been subjected to the lower pH-cycling regimen.

Thermocycling is an in-vitro procedure in which the tested materials are exposed to significant temperature variations to imitate the oral cavity. The resin matrix and filler particles may experience different thermal volumetric changes because of different thermal expansion coefficients or thermal conductivity coefficients. It is also important to note that water sorption during heat cycling led to the hydrolytic breakdown of the bonding between the resin matrix and filler particles. Additionally, it was stated that hygroscopic expansion in the resin matrix and filler phase would occur concurrently with water sorption, hence accelerating the degradation between the filler and matrix. All these factors could be the route that led to the dislodgement of filler particles and might in part cause an increase in the surface roughness. In the current study, a typical thermocycling model was not performed. Instead, a practically simulating model was applied to imitate the clinical situation in which the used beverages were used at their consumption temperature and then returned to the artificial saliva at 37℃.

Brushing in the current study yielded an increase in surface roughness more than in the non-brushing groups. These results were in line with da Silva et al., [32], Roselino et al., [33], and Costa et al., [21]. Also in a study Paolone et al., [16] the use of toothpaste with high RDA produced detrimental effect on the nanohybrid composite used.

The change in surface roughness was less obvious in Grandio in most tested groups. The results were in agreement with Heintze et al., [34]. This could be explained by assuming that the large fillers were trimmed flat during brushing to compensate for the loss of the polymer matrix when the mean roughness is measured [34]. O’Neill et al., [35] in their study reported that Admira Fusion X-tra samples demonstrated the roughest surfaces after a 15,000 brushing cycle. This increase in surface roughness might be due to the presence of clumps of the pre-condensed inorganic filler that remained on the surface after the resin matrix had been brushed away [35]. It is worth noting that the wear behavior of composites is affected by other factors besides filler features, including monomer conversion of the resin matrix, the filler loading, and the quality of adhesion of the fillers to the matrix.

One of the limitations of this study is the difficulty in replicating the clinical situation with an in-vitro study. Also, the short period of evaluation is another limiting factor. In addition to that, only two dental composites were evaluated, and the effect of different finishing protocols was not evaluated.