title: Hardness and postgel shrinkage of preheated composites.
abstract: OBJECTIVE
  To evaluate the effect of composite preheating and light-curing duration on hardness and postgel shrinkage.
  
  
  METHODS
  A hybrid composite (Filtek Z250, 3M ESPE) and a nanofilled composite (Filtek Supreme Plus, 3M ESPE) were preheated in a Calset (AdDent) heating device at its highest setting (68°C) before being light cured for 5, 10, 20, or 40 seconds. Vickers hardness of the top (0 mm) and bottom (2 mm) surface of the samples was measured 5 minutes after polymerization. The development of postgel shrinkage during polymerization was measured with a strain gauge. The results were compared to those of samples cured at room temperature. The sample size was five for each composite/temperature/light-curing duration combination. The results were statistically analyzed with two-way ANOVA and least squares means.
  
  
  RESULTS
  Preheating composites before light curing did not affect the bottom hardness or postgel shrinkage (P>.05) but significantly increased the top hardness of the Supreme Plus composite at all light-curing durations (P<.05). Shorter light-curing durations than recommended by the manufacturer significantly reduced top and bottom hardness of both composites, regardless of preheating (P<.05).
  
  
  CONCLUSIONS
  Under the experimental conditions in this study, preheating of the composites only slightly increased hardness values and did not negatively affect postgel shrinkage. Reducing the light-curing durations, however, significantly reduced the hardness at both the top (0 mm) and bottom (2 mm) surfaces. Clinicians should ensure sufficient light-curing duration to obtain good mechanical properties rather than depending on composite preheating.
authors.affiliation: Department of Restorative Sciences, University of Minnesota, Minneapolis, MN, USA. dverslui@uthsc.edu
authors.forename: Daranee
authors.initials: D
authors.lastname: Tantbirojn
authors.forename: Supaporn
authors.initials: S
authors.lastname: Chongvisal
authors.forename: David G
authors.initials: DG
authors.lastname: Augustson
authors.forename: Antheunis
authors.initials: A
authors.lastname: Versluis
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry; D018772:Dental Marginal Adaptation; D003799:Dental Stress Analysis; D006244:Hardness; D006358:Hot Temperature; D016018:Least-Squares Analysis; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D053761:Nanocomposites; D058105:Polymerization; D013314:Stress, Mechanical; D013997:Time Factors
chemical_list: D003188:Composite Resins; C515951:Filtek Supreme Plus; C444701:Filtek Z250