title: Polymerization stress, shrinkage and elastic modulus of current low-shrinkage restorative composites.
abstract: OBJECTIVE
  To compare currently available low-shrinkage composites with others regarding polymerization stress, volumetric shrinkage (total and post-gel), shrinkage rate and elastic modulus.
  
  
  METHODS
  Seven BisGMA-based composites (Durafill/DU, Filtek Z250/FZ, Heliomolar/HM, Aelite LS Posterior/AP, Point 4/P4, Filtek Supreme/SU, ELS/EL), a silorane-based (Filtek LS, LS), a urethane-based (Venus Diamond, VD) and one based on a dimethacrylate-derivative of dimer acid (N'Durance, ND) were tested. Polymerization stress was determined in 1-mm high specimens inserted between two PMMA rods attached to a universal testing machine. Total volumetric shrinkage was measured using a mercury dilatometer. Maximum shrinkage rate was used as a parameter of the reaction speed. Post-gel shrinkage was measured using strain-gages. Elastic modulus was obtained by three-point bending. Data were submitted to one-way ANOVA/Tukey test (p=0.05), except for elastic modulus (Kruskal-Wallis).
  
  
  RESULTS
  Composites ranked differently for total and post-gel shrinkage. Among the materials considered as "low-shrinkage" by the respective manufacturers, LS, EL and VD presented low post-gel shrinkage, while AP and ND presented relatively high values. Polymerization stress showed a strong correlation with post-gel shrinkage except for LS, which presented high stress. Elastic modulus and shrinkage rate showed weak relationships with polymerization stress.
  
  
  CONCLUSIONS
  Not all low-shrinkage composites demonstrated reduced polymerization shrinkage. Also, in order to effectively reduce polymerization stress, a low post-gel shrinkage must be associated to a relatively low elastic modulus.
authors.affiliation: Department of Dental Materials, University of São Paulo, São Paulo, SP, Brazil.
authors.forename: Letícia Cristina Cidreira
authors.initials: LC
authors.lastname: Boaro
authors.forename: Flávia
authors.initials: F
authors.lastname: Gonçalves
authors.forename: Thayse Costa
authors.initials: TC
authors.lastname: Guimarães
authors.forename: Jack Liborio
authors.initials: JL
authors.lastname: Ferracane
authors.forename: Antheunis
authors.initials: A
authors.lastname: Versluis
authors.forename: Roberto Ruggiero
authors.initials: RR
authors.lastname: Braga
mesh_terms: D017438:Bisphenol A-Glycidyl Methacrylate / Q000737:chemistry; D003188:Composite Resins / Q000737:chemistry*; D018772:Dental Marginal Adaptation; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis; D055119:Elastic Modulus; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D018583:Pliability; D058105:Polymerization; D012833:Siloxanes / Q000737:chemistry; D013314:Stress, Mechanical
chemical_list: D003188:Composite Resins; D012833:Siloxanes; D017438:Bisphenol A-Glycidyl Methacrylate