Reduction of polymerization contraction stress for dental composites by two-step light-activation

B. S. Lim, J. L. Ferracane, R. L. Sakaguchi, J. R. Condon

Research output: Contribution to journalArticlepeer-review

237 Scopus citations


Objectives: 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 330mW/cm2. For the two-step curing method, a 5s light exposure at 60mW/cm2 was followed by 2min without light exposure, and then a second light exposure for 60s at 330mW/cm2. 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 10min 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. Significance: 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.

Original languageEnglish (US)
Pages (from-to)436-444
Number of pages9
JournalDental Materials
Issue number6
StatePublished - Sep 2002


  • Dental composite
  • Polymerization contraction stress
  • Shrinkage
  • Two-step light-activation

ASJC Scopus subject areas

  • Materials Science(all)
  • Dentistry(all)
  • Mechanics of Materials


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