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

• Published in: Dental materials Vol. 18; no. 6; pp. 436 - 444
• Main Authors: Lim, B.-S, Ferracane, J.L, Sakaguchi, R.L, Condon, J.R
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.09.2002
• Subjects: Acrylic Resins - chemistry; Analysis of Variance; Composite Resins - chemistry; Composite Resins - radiation effects; Dental composite; Dentistry; Humans; Light; Materials Testing - instrumentation; Polymerization contraction stress; Polymers - chemistry; Polymers - radiation effects; Polyurethanes - chemistry; Resin Cements - chemistry; Shrinkage; Silicon Dioxide; Spectroscopy, Fourier Transform Infrared; Statistics as Topic; Stress, Mechanical; Surface Properties; Time Factors; Transducers; Two-step light-activation; Zirconium; Polymerization contraction stress; Shrinkage; Two-step light-activation; Dental composite
• ISSN: 0109-5641; 1879-0097
• DOI: 10.1016/S0109-5641(01)00066-5

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