title: Relationship between contraction stress and degree of conversion in restorative composites.
abstract: OBJECTIVE
  To verify the relationship between contraction stress and degree of conversion (DC) in different composites (Filtek Z250, Filtek A110, Tetric Ceram and Heliomolar).
  
  
  METHODS
  For the contraction stress test, composite (2 mm thick) was applied between two 5-mm diameter glass rods, mounted in a tensilometer. DC was determined by Infrared Photoacoustic spectroscopy in specimens with similar dimensions and geometry, submitted to identical curing conditions. Specimens were exposed to different energy densities (4.5, 13.5, 27.0, 54.0 and 108.0 J/cm2) by varying exposure time. Contraction stress and DC were recorded 10 min after the beginning of photoactivation. Results were analyzed by ANOVA/Tukey's test and regression analysis.
  
  
  RESULTS
  For contraction stress, the interaction between composite and energy density was significant. Stress values ranged between 0.6+/-0.2 and 2.0+/-0.3 MPa at 4.5 J/cm2, 2.3+/-0.5 and 4.3+/-0.4 MPa at 13.5 J/cm2, 3.8+/-0.5 and 5.8+/-0.9 MPa at 27.0 J/cm2, 4.2+/-0.8 and 7.9+/-0.9 MPa at 54.0 J/cm2 and 6.6+/-0.8 and 8.1+/-0.9 MPa at 108.0 J/cm2. Tetric Ceram (39+/-5.8%) showed a higher average DC than the other materials. Heliomolar (28+/-5.2%) showed an average DC similar to Filtek Z250 (32+/-6.6%) and to Filtek A110 (24+/-7.5%) regardless of the energy density level. No significant increase in DC was observed above 27 J/cm2.
  
  
  CONCLUSIONS
  At high energy levels, DC had a tendency to level off earlier than contraction stress values.
  
  
  CONCLUSIONS
  Using high energy densities may cause a significant increase in stress values, without producing a significant increase in conversion.
authors.affiliation: Department of Dental Materials, School of Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, São Paulo, SP 05508-900, Brazil. fcalabro@usp.br
authors.forename: Fernanda C
authors.initials: FC
authors.lastname: Calheiros
authors.forename: Roberto R
authors.initials: RR
authors.lastname: Braga
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Kawano
authors.forename: Rafael Y
authors.initials: RY
authors.lastname: Ballester
mesh_terms: D000180:Acrylic Resins / Q000528:radiation effects; D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry / Q000528:radiation effects*; D018772:Dental Marginal Adaptation; D003793:Dental Restoration, Permanent; D003799:Dental Stress Analysis; D006244:Hardness; D008027:Light; D008422:Materials Testing; D044367:Phase Transition; D011140:Polyurethanes / Q000528:radiation effects; D012044:Regression Analysis; D013055:Spectrophotometry, Infrared / Q000379:methods; D018709:Statistics, Nonparametric; D013718:Tensile Strength
chemical_list: D000180:Acrylic Resins; C029012:Composite Dental Resin; D003188:Composite Resins; C452225:Filtek A110 composite resin; C444701:Filtek Z250; C513708:Heliomolar; D011140:Polyurethanes; C121818:Tetric ceram