title: Calculation of contraction stresses in dental composites by analysis of crack propagation in the matrix surrounding a cavity.
abstract: OBJECTIVE
  Polymerization contraction of dental composite produces a stress field in the bonded surrounding substrate that may be capable of propagating cracks from pre-existing flaws. The objectives of this study were to assess the extent of crack propagation from flaws in the surrounding ceramic substrate caused by composite contraction stresses, and to propose a method to calculate the contraction stress in the ceramic using indentation fracture.
  
  
  METHODS
  Initial cracks were introduced with a Vickers indenter near a cylindrical hole drilled into a glass-ceramic simulating enamel. Lengths of the radial indentation cracks were measured. Three composites having different contraction stresses were cured within the hole using one- or two-step light-activation methods and the crack lengths were measured. The contraction stress in the ceramic was calculated from the crack length and the fracture toughness of the glass-ceramic. Interfacial gaps between the composite and the ceramic were expressed as the ratio of the gap length to the hole perimeter, as well as the maximum gap width.
  
  
  RESULTS
  All groups revealed crack propagation and the formation of contraction gaps. The calculated contraction stresses ranged from 4.2 MPa to 7.0 MPa. There was no correlation between the stress values and the contraction gaps.
  
  
  CONCLUSIONS
  This method for calculating the stresses produced by composites is a relatively simple technique requiring a conventional hardness tester. The method can investigate two clinical phenomena that may occur during the placement of composite restorations, i.e. simulated enamel cracking near the margins and the formation of contraction gaps.
authors.affiliation: Department of Operative Dentistry, Tsurumi University School of Dental Medicine, 2-1-3, Tsurumi, Tsurumi-ku, Yokohama, Kanagawa 230-8501, Japan. yamamoto-tk@tsurumi-u.ac.jp
authors.forename: Takatsugu
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Jack L
authors.initials: JL
authors.lastname: Ferracane
authors.forename: Ronald L
authors.initials: RL
authors.lastname: Sakaguchi
authors.forename: Michael V
authors.initials: MV
authors.lastname: Swain
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D000465:Algorithms; D002516:Ceramics; D003188:Composite Resins / Q000737:chemistry; D003387:Cracked Tooth Syndrome / Q000209:etiology / Q000517:prevention & control*; D018772:Dental Marginal Adaptation; D003776:Dental Porcelain; D003799:Dental Stress Analysis; D055119:Elastic Modulus; D006244:Hardness; D055115:Light-Curing of Dental Adhesives / Q000009:adverse effects; D008422:Materials Testing / Q000379:methods; D044367:Phase Transition; D011140:Polyurethanes / Q000737:chemistry; D019279:Resin Cements / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D015040:Zirconium / Q000737:chemistry
chemical_list: D000180:Acrylic Resins; D003188:Composite Resins; C513708:Heliomolar; D011140:Polyurethanes; D019279:Resin Cements; C083568:Z100 composite resin; C065770:Herculite XR; D003776:Dental Porcelain; D012822:Silicon Dioxide; C023506:Glass ceramics; D015040:Zirconium