title: Shrinkage stress in light-cured composite resins: influence of material and photoactivation mode.
abstract: OBJECTIVE
  The aim of this study was to record the effect of composite type and photoactivation mode on the stresses resulting from polymerization of five established composite resins: packable (Solitaire, Solitaire 2), micro-hybrid (Aelitefil, Z100) and hybrid (Clearfil AP-X).
  
  
  METHODS
  A mechanical testing machine was used to record the polymerization contraction stress (MPa) of cylindrical composite specimens (d=5mm; h=2mm; C-factor=1.25) at 0.1s intervals over a period of 400s. The samples were photopolymerized using a halogen light curing device under two types of light exposure: group 1, Standard (800mW.cm(-2)x60s); group 2, Exponential (logarithmic increase from 150 to 800mW.cm(-2) over 15s+800mW.cm(-2)x45s). The stress rate (SR: slope(MPa>0-60s)) and the maximum shrinkage stress (MSS: MPa(400s)) of each material (five replications) were statistically analysed by one-way ANOVA/Scheffe's test and Pearson's correlation procedure (alpha=0.05). Finally, Student's t-test (two matched series) enabled the assessment of the effect of the irradiation method on the results.
  
  
  RESULTS
  For group 1, in decreasing order, the MSS was 1.51+/-0.07MPa (Solitaire) statistically equivalent to 1.45+/-0.06MPa (Aelitefil), 1.29+/-0.08MPa (Solitaire 2), and 1.04+/-0.03MPa (Z100) statistically equivalent to 0.92+/-0.05MPa (Clearfil AP-X). Z100 showed the highest SR (0.045+/-6x10(-3)) and Solitaire, the lowest (0.017+/-2x10(-3)). There was no correlation between SR and MSS (r<-0.33, p<0.05). For group 2, the MSS and SR values were distributed in a similar way to those from group 1. There was a negative correlation between SR and MSS (r<-0.43 and p<0.01). The exponential ramp successfully reduced the MSS (-3.9%) and SR (-11%) values.
  
  
  CONCLUSIONS
  There is no relationship between composite resin type, stress rate and shrinkage stress levels. The slower stress rate development, resulting from ramped light intensity, helped slightly to reduce the maximum polymerization stress.
authors.affiliation: Laboratoire des Sciences du Génie Chimique, UPR 6811, INPL, ENSIC, 1, rue Grandville, BP 451 F-54.001 Nancy Cedex, France. charton.christophe@wanadoo.fr
authors.forename: Christophe
authors.initials: C
authors.lastname: Charton
authors.forename: Pierre
authors.initials: P
authors.lastname: Colon
authors.forename: Fernand
authors.initials: F
authors.lastname: Pla
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects*; D003748:Dental Equipment; D003799:Dental Stress Analysis; D006219:Halogens; D008027:Light; D008422:Materials Testing; D044367:Phase Transition; D018709:Statistics, Nonparametric; D013674:Technology, Dental
chemical_list: D003188:Composite Resins; D006219:Halogens