title: The influence of curing times and light curing methods on the polymerization shrinkage stress of a shrinkage-optimized composite with hybrid-type prepolymer fillers.
abstract: OBJECTIVE
  The aim of the study was to determine the influence of different light curing units (LCU) and regimes on the polymerization shrinkage stress (PSS) and the mechanical properties of a nano-hybrid composite.
  
  
  METHODS
  The polymerization shrinkage force (PSF) was measured continuously with compliance compensation for 300s after photo-initiating the composite, Tetric EvoCeram (Ivoclar Vivadent, Schaan, FL, Shade A3) in a Stress-Strain Analyser. Astralis 10, Bluephase and MiniL.E.D LCU with exposure times 10, 20 and 40s were used (C-factor=0.33, n=8 per group). Immediately after the PSF measurements, mechanical properties of the samples were measured at the top and the bottom using a Fischerscope H100C (Helmut Fischer GmbH, Sindelfingen, Germany). Statistical analyses were done using one-way ANOVA (p<0.05) and Tukey post hoc test.
  
  
  RESULTS
  Significant differences in the PSS for 10, 20 and 40s polymerization using Astralis 10 were found. The MiniL.E.D recorded low stress values. Modulus of elasticity is high after curing the composite with Astralis 10 at 10, 20 and 40s and for Bluephase 40s. Low moduli of elasticity were recorded for the MiniL.E.D and for the Bluephase 20 and 10s. The hardness values (HV) followed the same pattern as the modulus of elasticity. The Ramping mode of the MiniL.E.D had prolonged gel point.
  
  
  CONCLUSIONS
  High intensity LCU produce not just high HV but also high shrinkage, making it important to balance both the effects by choosing the appropriate curing time. Soft-start regimes have no paramount benefit in a LED regarding stresses in the clinical situation.
authors.affiliation: Department of Restorative Dentistry, Ludwig Maximilians University, Goethestrasse 70, D-80336 Munich, Germany. anuvish2@rediffmail.com
authors.forename: Anuradha
authors.initials: A
authors.lastname: Visvanathan
authors.forename: Nicoleta
authors.initials: N
authors.lastname: Ilie
authors.forename: Reinhard
authors.initials: R
authors.lastname: Hickel
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Kunzelmann
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry / Q000528:radiation effects*; D003748:Dental Equipment; D018772:Dental Marginal Adaptation; D003799:Dental Stress Analysis; D004548:Elasticity; D006244:Hardness; D008027:Light; D008422:Materials Testing; D053761:Nanocomposites / Q000737:chemistry / Q000528:radiation effects; D044367:Phase Transition; D013997:Time Factors
chemical_list: D003188:Composite Resins; C501891:Tetric EvoCeram