title: Influence of shade and storage time on the flexural strength, flexural modulus, and hardness of composites used for indirect restorations.
abstract: BACKGROUND
  Fracture resistance, elastic modulus, and hydrolytic degradation resistance are important properties of indirect composite restorations. Composite systems developed specifically for indirect application are said to have enhanced mechanical properties due to their elevated monomer conversion.
  
  
  OBJECTIVE
  This study evaluated the influence of shade and the effect of 30-day water storage on the flexural strength, flexural modulus, and hardness of 4 commercially available indirect composite systems and 1 composite used with the direct technique.
  
  
  METHODS
  A variety of commercially available indirect resin composites (Artglass, Belleglass, Sculpture, and Targis) and 1 directly placed composite (Z100, control) were used. Specimens made with either incisal or dentin shade (n = 10) were fractured with a 3-point bending test. Pre-failure loads corresponding to specific displacements of the crosshead were used for flexural modulus calculation. Knoop hardness was measured on fragments (n = 3) obtained after the flexural test. Tests were performed after 24 hours and after a 30-day water storage at 37 degrees C. Flexural strength data were analyzed with the Weibull distribution. Flexural modulus and Knoop hardness data were analyzed with 3-way ANOVA and Tukey's post-hoc test (alpha=0.05).
  
  
  RESULTS
  In general, the directly placed composite (Z100) demonstrated flexural strength similar to that of Artglass, Targis, and Sculpture. Belleglass presented the highest flexural strength (221.7 MPa for incisal shade after 24 hours storage; 95% confidence interval: 208.3-235.4). Z100 demonstrated the highest flexural modulus (range: 10.9 +/- 0.6 to 12.0 +/- 0.9 GPa) and Targis the lowest (range: 5.1 +/- 0.5 to 5.9 +/- 0.9 GPa). Sculpture was the only material that showed differences in flexural strength with respect to shade (incisal-24 hours: 149.8 MPa; incisal-30 days: 148.7 MPa; dentin-24 hours: 200.0 MPa; dentin-30 days: 177.9 MPa). The flexural modulus and hardness of the dentin shade of Sculpture were higher than those of the incisal shade after 30 days. Belleglass also showed a significant difference in flexural modulus (dentin-24 hours: 11.1 GPa; incisal-24 hours: 9.6 GPa). The effect of water storage was more evident on hardness since all composite systems softened after 30 days. Prolonged water storage decreased flexural strength only for Artglass-dentin and Z100, both incisal and dentin shades. Water aging did not affect the flexural modulus of any composite tested.
  
  
  CONCLUSIONS
  In general, indirect composites did not show enhanced mechanical properties compared to the directly placed composite. Property differences due to shade were more evident for Sculpture. Prolonged water storage had a deleterious effect on the hardness of all composites tested. However, water storage did not affect the flexural strength of most of the indirect composites or the flexural modulus of any composite tested.
authors.affiliation: Department of Dental Materials, Faculty of Dentistry, University of São Paulo, São Paulo, Brazil. p-paulo@siso.fo.usp.br
authors.forename: P F
authors.initials: PF
authors.lastname: Cesar
authors.forename: W G
authors.initials: WG
authors.lastname: Miranda
authors.forename: R R
authors.initials: RR
authors.lastname: Braga
mesh_terms: D000704:Analysis of Variance; D017609:Barium Compounds / Q000737:chemistry; D003116:Color; D003188:Composite Resins / Q000737:chemistry*; D016001:Confidence Intervals; D003793:Dental Restoration, Permanent / Q000379:methods*; D004548:Elasticity; D005899:Glass Ionomer Cements / Q000737:chemistry; D006244:Hardness; D006801:Humans; D006868:Hydrolysis; D008689:Methacrylates / Q000737:chemistry; D018583:Pliability; D011076:Polycarboxylate Cement / Q000737:chemistry; D012823:Silicate Cement / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D013223:Statistics as Topic; D013314:Stress, Mechanical; D013696:Temperature; D013997:Time Factors; D014867:Water / Q000737:chemistry; D015040:Zirconium
chemical_list: D017609:Barium Compounds; D003188:Composite Resins; D005899:Glass Ionomer Cements; D008689:Methacrylates; D011076:Polycarboxylate Cement; C083568:Z100 composite resin; C102738:barium glass filler; C110370:belleGlass; C111282:dental polyglass; D014867:Water; D012823:Silicate Cement; C005506:polycarbonate; D012822:Silicon Dioxide; D015040:Zirconium