title: Pre-heating of high-viscosity bulk-fill resin composites: effects on shrinkage force and monomer conversion.
abstract: OBJECTIVE
  To investigate the influence of pre-heating of high-viscosity bulk-fill composite materials on their degree of conversion and shrinkage force formation.
  
  
  METHODS
  Four bulk-fill composite materials (Tetric EvoCeram Bulk Fill-TECBF, x-tra fil-XF, QuixFil-QF, SonicFill-SF) and one conventional nano-hybrid resin composite (Tetric EvoCeram-TEC) were used. The test materials were either kept at room temperature or pre-heated to 68°C by means of a commercial heating device, before being photoactivated with a LED curing unit for 20s at 1170mW/cm(2). Shrinkage forces (n=5) of 1.5-mm-thick specimens were recorded in real-time for 15min inside a temperature-controlled chamber at 25°C (simulating intraoral temperature after rubber dam application) with a custom-made stress analyzer. Degree of conversion (n=5) was determined at the bottom of equally thick (1.5mm) specimens using Fourier transform infrared spectroscopy. Data were analyzed with Student's t-test, ANOVA and Tukey's HSD post-hoc test (α=0.05).
  
  
  RESULTS
  Composite pre-heating significantly increased the degree of conversion of TECBF, but had no effect on monomer conversion of the other materials investigated. For each of the test materials, pre-heated composite generated significantly lower shrinkage forces than room-temperature composite. At both temperature levels, TECBF created the significantly highest shrinkage forces, and QF caused significantly higher shrinkage forces than both XF and TEC.
  
  
  CONCLUSIONS
  Both the composite material and the pre-cure temperature affect shrinkage force formation. Pre-heating of bulk-fill and conventional restorative composites prior to photoactivation decreases polymerization-induced shrinkage forces without compromising the degree of conversion.
  
  
  CONCLUSIONS
  Composite pre-heating significantly reduces shrinkage force formation of high-viscosity bulk-fill and conventional resin composites, while maintaining or increasing the degree of monomer conversion, dependent upon the specific composite material used.
authors.affiliation: Department of Preventive Dentistry, Periodontology and Cariology, Center for Dental Medicine, University of Zurich, Plattenstrasse 11, CH-8032 Zurich, Switzerland. Electronic address: tobias.tauboeck@zzm.uzh.ch.
authors.forename: Tobias T
authors.initials: TT
authors.lastname: Tauböck
authors.affiliation: Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Gunduliceva 5, HR-10000 Zagreb, Croatia.
authors.forename: Zrinka
authors.initials: Z
authors.lastname: Tarle
authors.affiliation: Department of Endodontics and Restorative Dentistry, School of Dental Medicine, University of Zagreb, Gunduliceva 5, HR-10000 Zagreb, Croatia.
authors.forename: Danijela
authors.initials: D
authors.lastname: Marovic
authors.affiliation: Department of Preventive Dentistry, Periodontology and Cariology, Center for Dental Medicine, University of Zurich, Plattenstrasse 11, CH-8032 Zurich, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Attin
mesh_terms: D003188:Composite Resins / Q000737:chemistry*; D006358:Hot Temperature; D008422:Materials Testing; D058105:Polymerization; D014783:Viscosity
chemical_list: D003188:Composite Resins
keywords: Bulk-fill; Degree of conversion; Polymerization contraction stress; Pre-heating; Resin composite