Effect of preheating on the mechanical properties of resin composites mechanical properties for brittle materials, although the results cannot be extrapolated to the clinical behavior without considering some aspects, namely flaw distribution26 and structural reliability of the material.27 Nonetheless, the in vitro three- point bending flexural test is recommended by the ISO 4049/200028 specification for polymer based materials and is widely used for comparative purposes.29,30 An optimal tooth restoration material should mimic structural, mechanical and physical characteristics of dentin and enamel.31 Xu et al32 measured the elastic modulus of human enamel and dentin and obtained a mean value of 19 GPa for the dentin. Young’s modulus of enamel was 94 Gpa while depending significantly on tooth orientation. The mechanical properties of all the resin composites tested in this study either at room temperature or preheated, are far from those of enamel. However, nanohybrid resin composite had similar flexural modulus (16.83- 20.12 GPa) to flexural modulus of dentin. Filler content, filler size and the distribution of the filler particles were determined to highly influence the physical and mechanical properties of the resin composites. It has been shown that the filler volume fraction and filler load level of the resin composites correlate with the material strength and elastic modulus, as well as the fracture toughness of the material.33-38 Kim et al39 found out that the mechanical properties of the resin composites are related to their filler content. Resin composites with the highest by volume exhibited the highest flexural strength and flexural modulus. In the present study, no significant difference were found between flexural strengths of the groups, but microhybrid resin composite had higher flexural strength values than nanohybrid resin composite at all tested temperatures. However, there were significant differences between the flexural modulus of two materials in all the tested temperatures and curing protocols. The nanohybrid resin composite had significantly higher flexural modulus values than microhybrid resin composite with both curing protocols and all tested temperatures. This result is in agreement with Beun et al40 who reported that the nanohybrid resin composite has higher elastic modulus while the universal hybrid resin composite has higher flexural strength. Table 2. Mean values and standard deviations of the tested groups. The same superscript letters indicate no significant difference between groups (Tukey HSD test, α= 0,05). Materials Curing Protocol Temperature Flexural Strength (MPa) Flexural modulus (GPa) Grandio (VOCO, Cuxhaven, Germany) Standard cure Filtek Z250 (3M ESPE, St. Paul, MN, USA) Standard cure Step cure Step cure 266 European Journal of Dentistry RT 40 45 50 RT 40 45 50 RT 40 45 50 RT 40 45 50 143 (±10) a 144 (±27) a 151 (±13) a 146 (±14) a 149 (±10) a 150 (±20) a 149 (±13) a 159 (±9) a 152 (±11) a 159 (±19) a 150 (±26) a 157 (±25) a 158 (±16) a 144 (±23) a 146 (±17) a 166 (±26) a 18.16 (±1.79) abcd 18.84 (±2.72) bcd 19.93 (±2.03) d 19.71 (±1.95) cd 16.83 (±2.02) abcde 19.62 (±4.23) cd 20.12 (±4.08) d 18.90 (±2.17) cd 13.71 (±1.22) ef 16.13 (±2.34) abce 14.64 (±1.57) aef 12.43 (±0.82) f 13.52 (±1.97) ef 15.24 (±3.00) abef 13.81 (±0.88) ef 13.90 (±0.95) ef