title: Contraction stress of flowable composite materials and their efficacy as stress-relieving layers.
abstract: BACKGROUND
  The authors compared the polymerization contraction stress produced by flowable resin-based composites with stress values produced by nonflowable composites. They also measured the stress reduction produced by placing a precured layer of flowable composite under a nonflowable composite.
  
  
  METHODS
  The authors first tested four flowable and six nonflowable composite materials for contraction stress in a tensiometer. In the second part of the study, they applied a 1.4-millimeter-thick layer of flowable composite or unfilled resin and precured it in the test apparatus to assess the stress relief produced by a low-modulus material during light curing of a subsequent layer of highly filled composite. Flexural moduli of the precured materials were determined via a three-point bending test.
  
  
  RESULTS
  The stress values ranged between 6.04 and 9.10 megapascals. The authors found no significant differences in stress between flowable and nonflowable composites. Microfilled composites produced lower contraction stress than did hybrids. The flexural modulus of the flowable composites varied between 4.1 and 8.2 GPa. Regarding the effect of a precured layer of composite on contraction stress, the authors observed significant reductions with only one of the flowable materials and with the unfilled resin.
  
  
  CONCLUSIONS
  The flowable composites produced stress levels similar to those of nonflowable materials. Most of the flowable materials tested did not produce significant stress reductions when used under a nonflowable composite.
  
  
  CONCLUSIONS
  Using a flowable resin-based composite as a restorative material is not likely to reduce the effects of polymerization stress. When used in a thin layer under a nonflowable composite, the stress reduction depended on the elastic modulus of the lining material.
authors.affiliation: Depto. de Materiais Dentarios da FOUSP, São Paulo, SP, Brazil. rrbraga@usp.br
authors.forename: Roberto R
authors.initials: RR
authors.lastname: Braga
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Hilton
authors.forename: Jack L
authors.initials: JL
authors.lastname: Ferracane
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D003799:Dental Stress Analysis; D004548:Elasticity; D008422:Materials Testing; D011108:Polymers / Q000737:chemistry*; D019279:Resin Cements / Q000737:chemistry*; D012212:Rheology; D014783:Viscosity
chemical_list: D003188:Composite Resins; D011108:Polymers; D019279:Resin Cements