title: Reduction of polymerization contraction stress for dental composites by two-step light-activation.
abstract: OBJECTIVE
  The goal of this study was to assess the reduction of polymerization contraction stress of composites during a two-step light-activation process and to relate this reduction to the process of polymerization shrinkage and specimen thickness.
  
  
  METHODS
  Three test procedures were performed to compare two-step light-activation with delay with one-step continuous irradiation of composites: polymerization contraction stress using a closed-loop servohydraulic testing instrument, polymerization shrinkage by a mercury dilatometer, and degree of conversion by FTIR. For the one-step continuous curing method, the samples were light-activated for 60s at 330 mW/cm(2). For the two-step curing method, a 5s light exposure at 60 mW/cm(2) was followed by 2 min without light exposure, and then a second light exposure for 60s at 330 mW/cm(2). The same light parameters were used for measurements of stress, shrinkage, and degree of conversion. Three composites, Heliomolar, Herculite and Z100 were evaluated. The contraction stress experiments were repeated with varying thickness for Herculite using the one-step and two different two-step techniques.
  
  
  RESULTS
  Polymerization contraction stress 10 min after light-activation was significantly reduced (P<0.05) by the two-step method: 29.7% for Heliomolar, 26.5% for Herculite, and 19.0% for Z100. Total volumetric shrinkage and degree of conversion were not significantly different for composites cured by the two different techniques. Increasing the thickness of the composite sample reduced the measured contraction stress, especially for one of the two-step curing methods.
  
  
  CONCLUSIONS
  A combination of low initial energy density followed by a lag period before a final high-intensity light irradiation provides a reduction of polymerization contraction stresses in dental composites. The stress reductions cannot be attributed to reductions in degree of conversion or unrestrained volumetric shrinkage.
authors.affiliation: Department of Dental Biomaterials, College of Dentistry, Seoul National University, South Korea.
authors.forename: B-S
authors.initials: BS
authors.lastname: Lim
authors.forename: J L
authors.initials: JL
authors.lastname: Ferracane
authors.forename: R L
authors.initials: RL
authors.lastname: Sakaguchi
authors.forename: J R
authors.initials: JR
authors.lastname: Condon
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects; D006801:Humans; D008027:Light; D008422:Materials Testing / Q000295:instrumentation; D011108:Polymers / Q000737:chemistry / Q000528:radiation effects; D011140:Polyurethanes / Q000737:chemistry; D019279:Resin Cements / Q000737:chemistry; D012822:Silicon Dioxide; D017550:Spectroscopy, Fourier Transform Infrared; D013223:Statistics as Topic; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D014159:Transducers; D015040:Zirconium
chemical_list: D000180:Acrylic Resins; C029012:Composite Dental Resin; D003188:Composite Resins; C513708:Heliomolar; D011108:Polymers; D011140:Polyurethanes; D019279:Resin Cements; C083568:Z100 composite resin; C065770:Herculite XR; D012822:Silicon Dioxide; D015040:Zirconium