title: Polymerization stresses in low-shrinkage dental resin composites measured by crack analysis.
abstract: OBJECTIVE
  The objective of this study was to compare several dental restoratives currently advertised as low-shrinkage composites (Clearfil Majesty Posterior, Kalore, Reflexions XLS Dentin and Venus Diamond) with a microfill composite (Heliomolar) in terms of polymerization stress, polymerization shrinkage and elastic modulus.
  
  
  METHODS
  Cracks were made at several distances from the edge of a precision cavity in a soda-lime glass disk. The composites were placed into the cavity and lengths of the cracks were measured before and after light curing. Polymerization stresses generated in the glass at 2 and 10 min after the irradiation were calculated from the crack lengths and K(c) of the glass. Polymerization shrinkage and elastic modulus of the composites also were measured at 2 and 10 min after irradiation using a video-imaging device and a nanoindenter, respectively. The data were statistically analyzed by ANOVAs and Tukey's test (p<0.05).
  
  
  RESULTS
  The stress was significantly affected by composite brand, distance and time. The stress was directly proportional to time and inversely proportional to distance from the edge of the cavity. Clearfil Majesty Posterior demonstrated the highest stress and it resulted in the fracture of the glass at 2 min. Venus Diamond and Heliomolar exhibited the greatest shrinkage at both times. The elastic moduli of Clearfil Majesty Posterior and Reflexions XLS Dentin were greatest at 2 and 10 min, respectively.
  
  
  CONCLUSIONS
  Among the four low-shrinkage composites, two demonstrated significantly reduced polymerization stress compared to Heliomolar, which has previously been shown in in vitro tests to generate low curing stress.
authors.affiliation: Department of Operative Dentistry, Tsurumi University School of Dental Medicine, Yokohama, Kanagawa, Japan. yamamoto-tk@tsurumi-u.ac.jp
authors.forename: Takatsugu
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Yu
authors.initials: Y
authors.lastname: Kubota
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Momoi
authors.forename: Jack L
authors.initials: JL
authors.lastname: Ferracane
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000528:radiation effects*; D055117:Curing Lights, Dental; D003799:Dental Stress Analysis / Q000295:instrumentation / Q000379:methods*; D055119:Elastic Modulus / Q000528:radiation effects*; D005898:Glass; D008422:Materials Testing / Q000379:methods; D058105:Polymerization / Q000528:radiation effects*
chemical_list: D003188:Composite Resins