Effect of light power density variations on bulk curing properties of dental composites.
OBJECTIVE
The hypothesis that low light intensity and long but sufficient curing time can produce composites with volumetric shrinkage, degree of conversion (DC%) and Young's modulus (E-modulus) comparable to those of high light intensity cured composite was tested, when the contraction strain and heat generation were lower with low light intensity curing.


METHODS
Dental composites (Z100 and Z250, 3M ESPE) were investigated. Specimens were cured with light intensities of 200, 450 and 800 mW/cm(2) for 140, 60 and 35 s from a distance of 7 mm. Strain-gages were used for contraction strain measurements. DC% was measured at the top and the bottom of 4 mm thick samples using FT-Raman spectroscopy. Volumetric polymerization shrinkage was determined using a water displacement method. E-modulus was determined in tension on composite specimens.


RESULTS
The results were analyzed using ANOVA and Duncan's multiple range tests and regular t-test. Polymerization stress level decreased significantly (p<0.05) when cured with 200 mW/cm(2) rather than with 800 mW/cm(2). Temperature rises were significantly different (p<0.05) for different composites and light intensity values. Reduction in light intensity did not decrease the DC% values significantly at the top surfaces. The most dramatic differences existed between top and bottom surfaces (p<0.05) rather than among curing groups. Measured E-modulus and volumetric shrinkage values were not significantly different (p>0.05) between different light intensity groups.


CONCLUSIONS
DC%, E-modulus and the volumetric shrinkage values in cured composites were not affected by low light intensity, however, the contraction strain and polymerization's exotherm were decreased. Thus our results support the proposed hypothesis.
Division of Polymer Engineering, Department of Applied Physics and Mechanical Engineering, Luleå University of Technology, Luleå SE-971 87, Sweden. emami@mb.luth.se
Nazanin
N
Emami
Karl-Johan M
KJ
Söderholm
Lars A
LA
Berglund
D000704:Analysis of Variance; D003188:Composite Resins / Q000528:radiation effects*; D003799:Dental Stress Analysis; D004548:Elasticity; D008027:Light; D008422:Materials Testing; D011108:Polymers / Q000737:chemistry; D012822:Silicon Dioxide / Q000528:radiation effects; D013059:Spectrum Analysis, Raman; D018709:Statistics, Nonparametric; D013674:Technology, Dental / Q000379:methods; D015040:Zirconium / Q000528:radiation effects
D003188:Composite Resins; C444701:Filtek Z250; D011108:Polymers; C083568:Z100 composite resin; D012822:Silicon Dioxide; D015040:Zirconium