title: A comparative study between crack analysis and a mechanical test for assessing the polymerization stress of restorative composites.
abstract: OBJECTIVE
  To verify the hypothesis that crack analysis and a mechanical test would rank a series of composites in a similar order with respect to polymerization stress. Also, both tests would show similar relationships between stress and composite elastic modulus and/or shrinkage.
  
  
  METHODS
  Soda-lime glass discs (2-mm thick) with a central perforation (3.5-mm diameter) received four Vickers indentations 500 μm from the cavity margin. The indent cracks were measured (500×) prior and 10 min after the cavity was restored with one of six materials (Kalore/KL, Gradia/GR, Ice/IC, Wave/WV, Majesty Flow/MF, and Majesty Posterior/MP). Stresses at the indent site were calculated based on glass fracture toughness and increase in crack length. Stress at the bonded interface was calculated using the equation for an internally pressurized cylinder. The mechanical test used a universal testing machine and glass rods (5-mm diameter) as substrate. An extensometer monitored specimen height (2mm). Nominal stress was calculated dividing the maximum shrinkage force by the specimen cross-sectional area. Composite elastic modulus was determined by nanoindentation and post-gel shrinkage was measured using strain gages. Data were subjected to one-way ANOVA/Tukey or Kruskal-Wallis/Mann-Whitney tests (alpha: 5%).
  
  
  RESULTS
  Both tests grouped the composites in three statistical subsets, with small differences in overlapping between the intermediate subset (MF, WV) and the highest (MP, IC) or the lowest stress materials (KL, GR). Higher stresses were developed by composites with high modulus and/or high shrinkage.
  
  
  CONCLUSIONS
  Crack analysis demonstrated to be as effective as the mechanical test to rank composites regarding polymerization stress.
authors.affiliation: Department of Dental Materials, University of São Paulo School of Dentistry, São Paulo, SP, Brazil. rrbraga@usp.br
authors.forename: Roberto R
authors.initials: RR
authors.lastname: Braga
authors.forename: Takatsugu
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Kenneth
authors.initials: K
authors.lastname: Tyler
authors.forename: Leticia C
authors.initials: LC
authors.lastname: Boaro
authors.forename: Jack L
authors.initials: JL
authors.lastname: Ferracane
authors.forename: Michael V
authors.initials: MV
authors.lastname: Swain
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D001840:Dental Bonding / Q000379:methods*; D003799:Dental Stress Analysis / Q000379:methods*; D055119:Elastic Modulus; D005898:Glass / Q000737:chemistry*; D008422:Materials Testing / Q000379:methods*; D058105:Polymerization
chemical_list: D003188:Composite Resins