title: Effect of resin liners and photoactivation methods on the shrinkage stress of a resin composite.
abstract: OBJECTIVE
  This study was undertaken to evaluate the effect of three different photoactivation methods on the polymerization shrinkage stress of a resin composite using different resin liners.
  
  
  METHODS
  Three photoactivation methods using a quartz-tungsten-halogen light were evaluated: continuous light, soft start, and intermittent light. Three lining groups were tested: one or three coats of adhesive, and flowable composite. The stress was measured using a universal testing machine. After the stress measurement, Knoop hardness numbers (KHNs) were evaluated to verify indirectly the degree of conversion of the composite using the three photoactivation methods. The data were submitted to analysis of variance and Tukey's test (p < .05).
  
  
  RESULTS
  Intermittent light was always associated with statistically lower stress values when compared with continuous light. Statistical differences were not observed when continuous light and soft start were compared. The use of a flowable composite liner significantly reduced the stress generation when compared with the use of one adhesive coat. The three adhesive coats groups showed intermediate stress values. The hardness test showed no statistical difference between that achieved with continuous light and soft start. These two methods showed statistically higher KHNs when compared with results with intermittent light.
  
  
  CONCLUSIONS
  Photoactivation using intermittent light may lead to a significant reduction in shrinkage stress levels. This effect could be attributed to a significant reduction of the polymerization rate and/or a decrease in the degree of conversion. The use of a flowable composite liner reduced the stress levels when compared with levels when one coat of adhesive was used. This is possibly related to the higher thickness of the flowable composite layer.
authors.affiliation: Department of Restorative Dentistry, Dental Materials Area, Piracicaba Dental School--UNICAMP, São Paulo, Brazil.
authors.forename: Leonardo Gonçalves
authors.initials: LG
authors.lastname: Cunha
authors.forename: Roberta Caroline Bruschi
authors.initials: RC
authors.lastname: Alonso
authors.forename: Lourenço Correr
authors.initials: LC
authors.lastname: Sobrinho
authors.forename: Mário Alexandre Coelho
authors.initials: MA
authors.lastname: Sinhoreti
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000528:radiation effects*; D003736:Dental Cavity Lining; D018772:Dental Marginal Adaptation; D003793:Dental Restoration, Permanent / Q000379:methods*; D003799:Dental Stress Analysis; D006219:Halogens; D006245:Hardness Tests; D008027:Light; D019279:Resin Cements / Q000528:radiation effects*; D013674:Technology, Dental / Q000379:methods
chemical_list: D003188:Composite Resins; C444701:Filtek Z250; D006219:Halogens; C409388:Protect Liner F; D019279:Resin Cements; C084018:Scotchbond Multi-Purpose