title: Effect of resin thickness, and curing time on the micro-hardness of bulk-fill resin composites.
abstract: BACKGROUND
  Bulk-fill resin composite has been introduced, their manufacturers claimed that they can be applied in bulks of 4mm, without necessitating a prolonged curing time, or a light curing unit with increased irradiance. Thus this study was conducted to evaluate the effect of resin thickness, and curing time on the micro-hardness of two bulk -fill resin composites; Tetric Evo-Ceram [TE], and X-trafil [XF].
  
  
  METHODS
  120 cylindrical specimens were prepared, and divided into 24 groups (n=5/group), representing the two bulk-fill resin composites, three different material thicknesses (2, 3 and 4 mm) and the four curing times used in the study (10, 20, 40, and 60 seconds). The specimens were light-cured from the top surface only. Specimens were stored in light proof containers in complete darkness at 37°C for 24 hours. Micro-hardness test was conducted on both top and bottom surfaces using Vickers micro-hardness tester with 500 g load and a dwell time of 15 seconds. Data were statistically analyzed by Four-way ANOVA of Variance. The significance level was set at P ≤ 0.05. Pearson Correlation used to determine significant correlations between mean micro-hardness (top) and (bottom) surfaces.
  
  
  RESULTS
  Four way-ANOVA shows that different tested materials produce a statistically significant effect on mean micro-hardness (VHN) at p≤0.001, while thickness, curing time, and surface revealed statistically non significant effect on mean micro-hardness (VHN) at p≥0.05. [XF] (92.01±3.15 VHN) showed statistically significant higher mean micro-hardness than [TE] (54.13±4.96 VHN). Pearson Correlation revealed that there was a significant direct correlation between micro-hardness (bottom) and mean micro-hardness (top) (mm), r = 0.985, p (2-tailed) ≤0.001.
  
  
  CONCLUSIONS
  Within the limitations of this study, the bulk-fill resin composites used in this study can be placed and cured properly in the 4 mm bulk.
  
  
  BACKGROUND
  Bulk-fill resin composite, micro-hardness, thickness, curing time.
authors.affiliation: Restorative and Dental Materials Research department, National Research centre, Giza. Egypt.
authors.forename: Shaymaa M
authors.initials: SM
authors.lastname: Nagi
authors.affiliation: Restorative and Dental Materials Research department, National Research centre, Giza. Egypt.
authors.forename: Lamiaa M
authors.initials: LM
authors.lastname: Moharam
authors.affiliation: Restorative and Dental Materials Research department, National Research centre, Giza. Egypt.
authors.forename: Mohamed H
authors.initials: MH
authors.lastname: Zaazou