title: Comparison of post-gel shrinkage strains in light-polymerized composite resins.
abstract: BACKGROUND
  Polymerization shrinkage is associated with light-polymerized and polyacid-modified composites.
  
  
  OBJECTIVE
  This in vitro study compared the polymerization shrinkage of a light-polymerized hybrid composite, a posterior composite, and a polyacid-modified composite.
  
  
  METHODS
  Five disk-shaped specimens (1 cm in diameter and 2 mm in thickness) were prepared in a mold from each of the following test materials: a light-polymerized hybrid composite (3M Valux Plus), a posterior composite (Filtek P60), and a polyacid-modified composite (Dyract AP). The hybrid composite served as the control material. A linear strain gauge was placed at the center of each specimen. After the specimens were light-polymerized for 60 seconds under 400-mW/cm(2) light intensity, microstrains were recorded with the strain indicator continuously every 4 seconds for 120 seconds. The data were evaluated with 1-way analysis of variance (P<.05) followed by a post hoc least significant difference test.
  
  
  RESULTS
  During the first 20 seconds of light polymerization, all materials exhibited tensile strains between 6 and 55 mu epsilon that quickly converted into compressive strains and continued rising after the light source was removed. The strains for all materials increased at a declining rate until they reached a constant level between 148 and 180 seconds. The polymerization shrinkage recorded for Filtek P60 was the lowest (-190.56 mu epsilon), followed by 3M Valux Plus (-303.94 mu epsilon) and Dyract AP (-345.64 mu epsilon) (P<.05). The dimensional change between Filtek P60 and Dyract AP and between Filtek P60 and 3M Valux Plus was significant (P=.001). However, the difference between Dyract AP and 3M Valux Plus specimens was not significant (P=.208).
  
  
  CONCLUSIONS
  Within the limitations of this study, the light-polymerized posterior composite tested demonstrated less polymerization shrinkage than the polyacid-modified and hybrid composites tested. Polymerization shrinkage velocity significantly affected the magnitude of strains.
authors.affiliation: Faculty of Dentistry, Hacettepe University, Gazi Mustafa Kenal Bulvari, 61/11, TR 06570 Maltepe, Ankara, Turkey. mcehreli@hotmail.com
authors.forename: Murat C
authors.initials: MC
authors.lastname: Cehreli
authors.forename: Senay
authors.initials: S
authors.lastname: Canay
mesh_terms: D000704:Analysis of Variance; D020770:Compomers / Q000737:chemistry* / Q000528:radiation effects; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects; D005782:Gels / Q000737:chemistry / Q000528:radiation effects; D006801:Humans; D008027:Light; D008422:Materials Testing; D008689:Methacrylates / Q000737:chemistry / Q000528:radiation effects; D011108:Polymers / Q000737:chemistry / Q000528:radiation effects; D011829:Radiation Dosage; D017640:Silicates / Q000737:chemistry / Q000528:radiation effects; D013223:Statistics as Topic; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors
chemical_list: D020770:Compomers; D003188:Composite Resins; C088486:Dyract; C438835:Filtek P60 Composite Resin; D005782:Gels; D008689:Methacrylates; D011108:Polymers; D017640:Silicates; C112192:valux plus