title: Polymerization stress of resin composites as a function of system compliance.
abstract: OBJECTIVE
  Evaluate the effect of testing system compliance on polymerization stress and stress distribution of composites.
  
  
  METHODS
  Composites tested were Filtek Z250 (FZ), Herculite (HL), Tetric Ceram (TC), Helio Fill-AP (HF) and Heliomolar (HM). Stress was determined in 1-mm thick specimens, inserted between two rods of either poly(methyl methacrylate), PMMA, or glass. Experimental nominal stress (sigmaexp) was calculated by dividing the maximum force recorded 5 min after photoactivation by the cross-sectional area of the rod. Composites' elastic modulus (E) was obtained by three-point bending. Data were submitted to one-way ANOVA/Tukey's test (alpha=0.05). Stress distribution on longitudinal (sigmay) and transverse (sigmax) axes of models representing the composites with the highest and lowest E (FZ and HM, respectively) were evaluated by finite element analysis (FEA).
  
  
  RESULTS
  sigmaexp ranged from 5.5 to 8.8 MPa in glass and from 2.6 to 3.4 MPa in PMMA. Composite ranking was not identical in both substrates, since FZ showed sigmaexp statistically higher than HM in glass, while in PMMA FZ showed values similar to the other composites. A strong correlation was found between stress reduction (%) from glass to PMMA and composite's E (r2=0.946). FEA revealed that system compliance was influenced by the composite (FZ led to higher compliance than HM). sigmax distribution was similar in both substrates, while sigmay distribution showed larger areas of compressive stresses in specimens built on PMMA.
  
  
  CONCLUSIONS
  sigmaexp determined in PMMA was 53-68% lower than in glass. Composite ranking varied slightly due to differences in substrates' longitudinal and transverse deformation.
authors: University of São Paulo, Department Biomaterials and Oral Biochemistry, São Paulo, Brazil.
authors: Flavia
authors: F
authors: Gonçalves
authors: Carmem S C
authors: CS
authors: Pfeifer
authors: Josete B C
authors: JB
authors: Meira
authors: Rafael Y
authors: RY
authors: Ballester
authors: Raul G
authors: RG
authors: Lima
authors: Roberto R
authors: RR
authors: Braga
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D003187:Compliance; D003188:Composite Resins / Q000737:chemistry*; D003764:Dental Materials / Q000737:chemistry*; D004548:Elasticity; D020342:Finite Element Analysis; D005898:Glass / Q000737:chemistry; D006801:Humans; D008422:Materials Testing / Q000379:methods; D008962:Models, Theoretical; D018583:Pliability; D011108:Polymers / Q000737:chemistry; D019904:Polymethyl Methacrylate / Q000737:chemistry; D011140:Polyurethanes / Q000737:chemistry; D013314:Stress, Mechanical
chemical_list: D000180:Acrylic Resins; D003188:Composite Resins; D003764:Dental Materials; C444701:Filtek Z250; C513708:Heliomolar; D011108:Polymers; D011140:Polyurethanes; C121818:Tetric ceram; C078906:Herculite Condensable; D019904:Polymethyl Methacrylate