title: Effect of variable light intensity on composite shrinkage.
abstract: BACKGROUND
  Polymerization shrinkage is a critical limitation of dental composites and may contribute to postoperative pain, tooth fracture, microleakage, and secondary caries. Polymerization with high-intensity light sources has been related to increased depth of cure and improved mechanical properties. However, high-intensity light initiation has also been associated with greater polymerization shrinkage.
  
  
  OBJECTIVE
  The purpose of this study was to investigate the effect of sequentially increasing light intensity on the polymerization shrinkage of 2 composites, a hybrid and a microfil. A Knoop hardness test was used to evaluate effectiveness of the cure with each intensity increase.
  
  
  METHODS
  Four groups of 12 samples were measured for polymerization shrinkage by using a linometer. Light intensity curing sequences were as follows: full-intensity control (100% intensity for 40 seconds), low-intensity control (25% intensity for 40 seconds), test group 1 (25% intensity for 20 seconds, 50% for 10 seconds, 100% for 10 seconds), and test group 2 (25% intensity for 10 seconds, 50% for 10 seconds, 100% for 20 seconds). Statistical comparisons were made using a 1-factor ANOVA and a Tukey multiple comparisons test within each material.
  
  
  RESULTS
  Results showed a significant difference (P<. 05) in mean linear shrinkage between the full-intensity control group and the other 3 sequences for both composites. No difference existed within the other 3 groups for either composite. Knoop hardness was similar for the full-intensity control and test group 2. The low-intensity control group and test group 1 were also similar but significantly lower.
  
  
  CONCLUSIONS
  Curing composites for 10 seconds at 25% intensity, 10 seconds at 50%, and 20 seconds at 100% significantly reduced polymerization shrinkage while not compromising depth of cure.
authors: Department of Cariology, Restorative Sciences and Endodontics, University of Michigan School of Dentistry, Ann Arbor, MI 48109-1597, USA.
authors: J B
authors: JB
authors: Dennison
authors: P
authors: P
authors: Yaman
authors: R
authors: R
authors: Seir
authors: J C
authors: JC
authors: Hamilton
mesh_terms: D000704:Analysis of Variance; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003188:Composite Resins / Q000737:chemistry / Q000528:radiation effects*; D008027:Light; D008422:Materials Testing / Q000379:methods / Q000706:statistics & numerical data; D013997:Time Factors
chemical_list: D003188:Composite Resins