Surface roughness (Ra) is influenced by the filler size and percentage inside the composites. In fact, a smooth surface not only determines good esthetics, but also acceptable longevity of the restoration. On the contrary, surface irregularities lead to greater food impaction, plaque retention, gingival irritation, and secondary carious lesions [19].

Flowable composites, despite having an increased surface roughness after chewing, presenting an initial lower surface roughness, can show a lower surface roughness after mastication compared to traditional packable composites.

In a recent study, the surfaces of three flowable and one packable composites were tested, with the aim of investigating the surface quality before and after the chewing cycles (approx. 4 months). The results showed that Ra increased significantly after chewing for all composites [20]. These data seem to be compliant with the findings of the present study, because all roughness values of the masticated samples appeared to be higher than the initial ones.

In another study, it emerged that Clearfil Majesty low viscosity presented a change in surface roughness comparable to that of the control packable composites [21]. Similarly, also the results of the present study showed that Clearfill Majesty ES low viscosity presents a surface roughness comparable to traditional packable composites.

Another manuscript reported G-aenial Universal Flo as the material that presented a lower surface roughness compared to other five flowable resins tested [22].

Wear is a multifactorial process that probably cannot be adequately described with a single material property. The surfaces produced by the mastication process can provide some clues about the fatigue resistance and toughness of the composites which could be decisive in determining the effects caused by wear [23]. In vitro-simulated wear processes can provide useful information for predicting the clinical performance of composite materials [24]. Wear can be measured in volume loss (mm3) or in terms of maximum depth (measured in μm) [25]. In the present manuscript, wear was considered as the maximum depth obtained, and lower values were considered better indicators of clinical performance.

From the observation carried out on the images obtained by SEM analyses, it was highlighted that the transition area between the worn and intact areas of groups 1 and 3, therefore of highly loaded flow composites, appeared to be less defined. This could indicate that the mechanical stresses to which the surfaces were subjected did not undergo significant structural modifications. Groups 2 and 4, instead, presented a more evident transition area, indicating a more important structural modification, which occurred following mechanical stimuli.

Filler amounts usually vary from one composite to another and from packabkle to flowable resins. Depending on what stated by the companies and what has been published in the literature, CM and CMf composites present a 78%wt filler (and, respectively, 66% and 64% volume), whereas GU resin contains 82%wt (and 41% volume) filler and GUf 69%wt (and 50% volume). Based on the results of the present study, the authors speculate that the lower occlusal wear of CM and CMf could be attributable, in part, to their particle size. These particles are defined as, respectively, 0.7 and 0.18–3.5 µ, and they are larger than those of the other tested composites. Improved wear resistance is often related to greater filler load [26]. This trend is confimed in the present study where the percentage of filler by volume of CM and CMf is, respectively, 66% and 64%, and is higher than GU (41%) and GUf (50%).

Several authors agree on the fact that wear is influenced by the dimensions of filler particles [28, 30]. Based on the present findings however, it could be affirmed that also the filler % volume plays a crucial role in the final material wear. In fact, the more these particles are close to each other, the less space is left for the organic matrix, that is, the part where wear mostly occurs. Probably, larger particles leave less space for the organic matrix and are less distanced.

Unlike the traditional flowables, the highly filled flowables have percentage and filler particles sizes comparable to the respective traditional composites [1, 9, 15, 27, 28]. In fact, in the results of the present research, the only two comparisons that produced statistically significant differences were the CMf10 group with mean wear values 0.01 μm lower than those of the CM10 group (p = 0.019), and the GUf10 group with mean wear values 0.03 μm greater than those of the GU10 group (p = 0.043). In both cases, the comparison highlighted was that between the groups cured for 10 s, surprisingly, in the comparison between the respective groups cured for 80 s, statistically significant differences disappeared.

These results could have important implications for dental practice. While highly filled flowables have been developed as suitable for use in all kinds of direct restorations, not all these materials could have equivalent physical and mechanical features to the conventional composite. Dental practitioners should be aware of these differences when selecting materials for their direct restorations and the results of the present manuscript underscore the importance of knowledge of the physical and mechanical properties of dental materials before choosing them for daily practice [19].

Wear of four highly loaded flow composite resins were compared to a control group made of packable composite. All the tested flow composites showed higher wear values after 40,000 simulated chewing cycles than the control group. The authors concluded by suggesting that the use of flowable composite resins for small restorations should be preferable, especially in areas with low occlusal stress [27]. The results of this study highlight the fact that the tested highly filled flowable composites always present higher wear values than packable resins. These data are in contrast with those of the present study, probably due to the fact that the authors tested Clearfill Majesty packable composite that in the present article showed to be one of the best in terms of wear resistance. This aspect has also been observed in the present manuscript.

Two years later, the same study group performed wear analysis on teeth, testing four highly loaded flowable composites and a packable composite used as the control group. Wear values of highly loaded flowable composites were comparable to those of the packable composite, showing that good wear resistance is strictly dependent of the percentage and size of the filler inside the resin, and therefore flowable composites with greater amounts of filler (respectively, 81%wt, 71%wt, 67%wt, 67%wt) had similar wear values to the control group (78%wt) [1]. This paper showed that the wear values of flowable composites are dependent on the size of the filler particles, and the results related to higly loaded flowable composites are similar to those of the present study that used the same CMf and CM resins. Similar conclusions were obtained in other studies that evaluated wear on several composite resins [26, 29].

Further, a recent study compared the wear of four flowable bulk resins with four flowable resins after chewing simulation of 400,000 cycles. In terms of both volume loss (mm3) and maximum depth (μm), G-aenial Bulk Injectable and G-aenial Universal Flow had much lower values than the other tested resins. For this reason, among the conclusions of the study, it is stated that these resins seem to be indicated for restorations of the posterior sectors in occlusal contact areas [28]. Similarly to the results of the present study, the paper by Ujiie et al. affirm that wear values of highly loaded flowables are lower or similar to those of packable composites. Highlighting the fact that GU and GUf composites present a similar wt percentage (respectively, 82 and 69%), it can be ascertained that both materials have the features to be used in occlusal areas.

A recent in vitro study aimed to comparatively assess the wear resistance of three conventional and three flowable composites containing different filler types using thermomechanical chewing simulation. Specimens were subjected to wear using a thermocycler chewing simulator against 6-mm diameter steatite balls for 240,000 cycles. The digital profiles of the treated sample surfaces were scanned using a laser scanner, and the volume loss and maximum depth of loss were calculated. The wear volume loss and loss depth of nanofilled composites were significantly higher than those of the other composite filler types, with no significant difference in either parameter between the nanohybrid and submicron-filled composite groups. With respect to apparent viscosity, wear volume loss and depth loss of conventional composites were significantly lower than those of the flowable composites. The type of composite filler and its viscosity significantly influence the in vitro wear resistance of the material [9]. Similarly to the present study, the authors affirm that the amount of wear increases when the composite filler percentage decreases.

Another recent in vitro study aimed to evaluate and compare the roughness and hardness of two bulk-fill flowable composites, two conventional flowable composites and one high-strength universal injectable composite. Flowable bulk-fill composites and high-strength injectable composite showed similar results in terms of hardness, which appeared to be statistically higher when compared with both traditional flowable composites hardness. Flowable bulk-fill composites showed significantly higher roughness values than both traditional flowable composites and high-strength injectable composite [15].

By several studies in literature, it seems evident that not only the filler properties and the polymer matrix, but also the time taken for photopolymerization affects the mechanical properties of the composite [30, 31]. Greater polymerization increases the mechanical properties of resistance to wear and fracture and makes the color of the composite more stable over time [17, 18].

In a recent study, three bulk nanohybrid composites and a control group were polymerized for 10 s or 80 s and tested for bacterial adhesion and surface roughness. Results showed that samples cured for 80 s presented lower surface roughness compared to samples cured for 10 s [32]. In agreement with literature data, the results of the present experiment showed that, regarding both the initial and final surface roughness, there were evident differences between the subgroups cured for 80 s compared to those cured for 10 s. Despite this, in terms of delta, and therefore in terms of difference between the initial and final roughness, no statistically significant differences were found, except for that in Clearfil Majesty ES Flow composite, in which the comparison between group CMf10 and CMf80 was the only one to highlight a significant difference: 0.1 μm (p = 0.038). In terms of wear, the results clearly showed that for each group, there were statistically significant differences between the subgroups cured for 80 s and those cured for 10 s. In fact, group CMf80 presented average wear values lower than 0.04 μm compared to those in group CMf10 (p = 0.000): both in the comparison between group CM80 and CM10, and in the comparison between group GUf80 and GUf10, there were differences in wear on average of 0.05 μm (p = 0.000) and also in the comparison between the GU80 and GU10 group, there was a difference in wear of 0.03 μm (p = 0.013) again in favor of the 80 s polymerized group. Thus, samples cured for 80 s had a statistically lower wear value for each of the composites tested.

Polymerization shrinkage stress of resin composite materials may have a negative impact on the clinical performance of bonded restorations. Shrinkage stress development has to be considered a multi-factorial phenomenon, and several restorative techniques aiming at stress reduction seem to have limited applicability, since their efficiency varies depending upon the materials used. Since the understanding of this matter has remarkably increased, the development of new restorative techniques and materials may help minimize this problem [33, 34].

It is mandatory to highlight that the present research has been conducted in vitro and that the interpretation of the results could present some limitations. While in vitro studies provide valuable insights, they may not fully replicate the complex environment of the oral cavity and might not perfectly replicate actual masticatory forces and conditions over extended periods. Additionally, no thermocycling was considered in the present research. Composites aging in the oral cavity and tooth brushing could modify composites’ roughness and wear.

The null hypotheses are confirmed with regard to surface roughness and wear. In fact, it has been demonstrated that there are no differences between highly filled flowable composites and traditional packable composites, when the same polymerization time was used. The last null hypothesis must be instead rejected, since differences in surface roughness and wear were found between 10 s polymerized composites and 80 s polymerized ones.

Further research, particularly in vivo and with a broader range of materials, should replicate the actual clinical condition through long-term clinical trials and would enhance the robustness and applicability of its findings. However, the value of in vitro research should not be undermined, since it enables the investigation of one or several “isolated” factors on the mechanical properties of dental materials, without the inevitable variability between subjects in a clinical study.