Uctasli, Arisu, Lasilla, Valittu Ltd, Fareham Hants, UK) at a crosshead speed temperature and preheated to 40°C (P=0.009) of 1 mm/min. The maximum loads were obtained and, between preheated to 45°C and 50°C (P<.001) and the flexural strength (σ) was calculated at standard curing mode. Flexural modulus values in megapascals (MPa) by using the following of Grandio were significantly higher than Z250 for formula: all the curing protocols and tested temperatures. σ = 3FL/(2BH2) No significant correlation was found between Where F is the maximum load (in newtons); L is preheating and tested mechanical properties the distance between the supports (in millimeters); [flexural strength (P=0.174) and flexural modulus B is the width of the specimen (in millimeters) and (P=0.486) of the materials. H is the height (in millimeters). dIscuIon The modulus of elasticity (GPa) was determined as: Preheating resin systems prior to photo- E = FL3/4BH3d polymerization have many potential advantages Where F is the maximum load; L is the distance like increasing the degree of conversion,16,20 between the supports; B is the width of the increasing the surface hardness.21 Previous specimen; H is the height of the specimen, and d studies have showed that preheating increased is the deflection (in millimeters) corresponding to the flow and enhance the adaptation of the resin the load F. to the prepared tooth walls and, thus, potentially The data were analyzed by one-way analysis of reduced microleakage.20 Flow of commercial resin variance and the differences between materials, composites could be increased by preheating.22 curing and preheating protocols were assessed The overall extent and rate of monomer conversion with Tukey HSD tests (P=0.05) to determine the in resin systems cured at higher temperatures are effect of material, curing and preheating protocols better than that performed at room temperature.4,16 (SPSS 15.0 version, SPSS Inc., Chicago, USA). In recent studies a significant increase in Pearson’s correlation test was used to examine conversion were observed upon preheating.15-17,23 the correlation between mechanical properties In clinical situations when the resin composite (flexural strength and flexural modulus) and is preheated, the temperature of the resin preheating. composite could affect the pulp tissue when it is placed in the cavity but with the time delay between ResuLts dispensing it from the syringe and placing it into Table 2 summarizes the mean values and the preparation a significant decrease occurs.23 standard deviations of the tested groups. While Daronch et al24 found no significant differences there were no statistically significant differences in the intrapulpal temperatures between either between flexural strengths of the tested materials room temperature or preheated resin composite for curing and preheating protocols (P=0.299), at similar restorative stages. In another study, there were statistically significant differences Daronch et al25 concluded that the resin composite between the flexural modulus of the tested groups temperature decreased rapidly upon compile (P<.001). The flexural modulus values of Z250 removal from the heating device. showed significant differences between room Flexural strength and modulus are meaningful Table 1. Materials used in this study. Material Type Grandio Nanohybrid (Voco, Cuxhaven, composite Germany) resin Filtek Z250 (3M ESPE, St. Paul, Minn, USA) Microhybrid composite resin October 2008 - Vol.2 Organic matrix TEGMA, BIS-GMA BIS-GMA, UDMA, BIS-EMA Inorganic filler Spherical nanoparticles of silicon dioxide 0.02–0.05, size-matched vitro ceramics Circonium-silicon particles 0.01–3.5 (average: 0.6) Filler Filler % wt % vol Batch Number Shade 87 78 71.4 #531919 A 3,5 61 #20050104 A 3,5 265 European Journal of Dentistry