## 4 Discussion
This is an important factor to be considered when selecting restorative materials for clinical use as many variables that derive from their composition directly influence their wear resistance [20]. The properties of dental composites depend on various factors related to the filler particles, the polymeric matrix and the coupling between matrix and filler [27; 28]. Filtek(tm) Supreme(tm) contains bis-GMA, UDMA, bis-EMA and a low quantity of TEGDMA resins [29]. Activa(tm) results from a mixture of UDMA and other methylates with modified polyacrylic acid [30]. Ketac(tm) results from a mixture of calcium fluoro-alumino-silicate glass powder and an aqueous solution with the copolymer of polyacrylic acid-taconic acid and tartaric acid [31]. Regarding the composition of the studied materials, the results are in line with those of [PERSON] et al. [27]. The authors reported that a composite resin with a high percentage of UDMA (-70 mol%), a low percentage of TEGDMA (-30 mol%) and no Bis-GMA would result in optimum strength. Such a material would present a relatively low elastic modulus of about 8 GPa [27]. According to this investigation, Activa(tm) obtained an initial elastic modulus of 8.03 \(\pm\) 0.15 GPa, followed by Filtek Supreme(tm) (12.48 \(\pm\) 0.18 GPa) and finally Ketac(tm) (26.69 \(\pm\) 0.26 GPa). The elastic modulus provides an idea about the rigidity of the material. The higher the value of the elastic modulus, the greater the stress required for the same degree of deformation, and therefore the more rigid the material. Flexural strength (FS) is one of the key mechanical properties selected by the International Organization for Standardization (ISO) for screening of resin-based materials [5]. The fracture properties of materials are determined by their FS and are critical if the material is used for filling dental cavities [28]. Surface characteristics of restorative materials may affect FS [32]. In this investigation, Activa(tm) had the highest fracture toughness, followed by Filtek Supreme(tm) and Ketac(tm), regardless of temperature and whether it had been subjected to thermocycling. These facts corroborate the results obtained in the tribological tests, where a large number of cracks could be observed through an SEM micrograph for Ketac(tm) after thermocycling, which weakened the integrity of the specimen after testing. Some studies report that flexural strength increases when bis-GMA or TEGDMA are substituted by UDMA [27; 33], which could justify the highest FS of Activa(tm). These materials, which the manufacturer reports as containing a resilient resin matrix with energy-absorbing elastomeric components (diurethane and methylates with modified polyacrylic acid and polybutadiene modified diurethane dimethacrylate), may be more suitable in higher-stressareas that are contraindicated for conventional glass-ionomer cements [7]. According to the existing literature, FS values for conventional resin-modified glass-ionomer cement (RMGIC) materials range from 42 MPa to 66 MPa, 25 MPa to 60 MPa, and 16.9 MPa to 59 MPa [23]. In a study by [PERSON] et al. [5], the FS of RMGIC was found to be greater than the one of conventional composite resin materials. In addition, bioactive RMGIC showed a higher FS than that of conventional glass-ionomer cements (Fuji(tm), Ketac(tm)) [5]. Thus, the results from this study are in accordance with the available literature. The difference in the FS of the tested materials may be attributed to the difference in their composition, type of resin, inorganic filler, and size and content of the fillers [34]. Activa(tm) had a homogeneous wear track where no cracks were visible outside the wear track and where the track width was approximately constant. This fact may be related to the low hardness value obtained in the Vickers test (HV0.2 = 20.2 Kgf/mm\({}^{2}\)), which allows the removal of material, but at the same time a high fracture toughness as evidenced in the calculated value of work of fracture (WOF = 2767.78 J/m\({}^{2}\)) which maintains the integrity of the composite.