To measure shrinkage strain, exotherm, and coefficient of thermal expansion (CTE), simultaneously for a set of representative resin-composites.
Six commercially available resin-composites with different filler loadings were selected. A modified bonded-disk instrument that includes temperature-monitoring apparatus was used to measure simultaneously: shrinkage strain, exotherm, and CTE. Shrinkage strain and temperature of disk specimens ( n = 3/materials) were monitored for 1 h after irradiation for 20 s at 1200 mW/cm 2 (energy density = 24 J/cm 2 ). Disks were irradiated for a second time 60 min after the first irradiation. Axial expansion strain and temperature were monitored for 3 min. Exotherm was obtained from differences between temperature rise during 1st and 2nd irradiations. CTE was calculated from disk axial expansion due to irradiation heat (Δ L ) and rise in temperature (Δ T ) during the second irradiation.
The final shrinkage strain values ranged from 1.7% to 2.34%, exotherm values ranged from 4.66 to 9.43 °C, and CTE ranged from 18.44 to 24.63 (10 −6 /°C). Negative correlations were found between filler loading and shrinkage strain, exotherm, and CTE. Positive correlation was apparent between shrinkage strain and CTE.
The modified bonded-disk instrument could be used to measure simultaneously shrinkage strain, exotherm, and CTE of resin-composites.
Increased desire for esthetics has led to growing demands on resin-composites. Resin-composites and their application techniques have undergone significant progress since their first introduction to dentistry by Bowen . Photo-activated resin-composites are polymerized through an addition reaction . Inevitably, their polymerization is accompanied with polymerization shrinkage. Current resin-composites including flowable types exhibit a volumetric polymerization shrinkage ranging from less than 1–6% . Shrinkage may create stress at the restoration tooth interface and may lead to restoration failure . Also, resin-composites after placement in teeth are subjected to dimensional changes due to thermal variations of the oral environment, which can be expressed by their coefficient of thermal expansion (CTE) . CTEs of enamel and dentin are 17 (10 −6 /°C) and 11 (10 −6 /°C), respectively . Therefore, it is ideal if the CTE of resin-composites are in the range of enamel and dentin to help preserve the tooth-restoration bond.
Halogen lights, plasma arc lamps, argon ion lasers, and light emitting diodes (LED) have been used for photo-activation of resin-composites . However, owing to several advantages, LED light curing units (LCUs) have gained in popularity relative to quartz tungsten halogen (QTH) units. These advantages include higher efficiency, lower consumption of energy, no need for external cooling, and remarkably long lifetime without a significant loss of intensity . Progress in LED technology has led to the availability of high power LED LCUs. However, their high light intensities of up to 2000 mW/cm 2 lead to heat generation that may harm the pulp and gingiva . This means a change in one of the important characteristics of previous LED versions, namely now a return to greater heat generation .
The polymerization of resin-composites is an exothermic reaction. Temperature rise during photo-activation of resin-composites is a result of light source energy and the polymerization reaction . Some previous studies reported temperature rises of more than 20 °C during photo-activation of resin-composites . Theoretically, transmission of heat through dentin can damage pulpal tissues. Zack and Cohen, in their animal study in 1965, demonstrated that an intra-pulpal temperature rise of 5.5 °C is damaging . Later, Lloyd et al. argued against the Zack and Cohen methodology and suggested that greater rises in temperature than 5.5 °C may be necessary before any pulpal damage occurs . Additionally, dentin could protect the pulp against any thermal shock if it is sufficiently thick .
The aims of the current study were: (a) to determine simultaneously the shrinkage strain, exotherm, and CTE of a range of representative resin-composites; (b) to explore the role of filler loading on shrinkage-strain, exotherm, and CTE. The specific objectives were to: (i) measure shrinkage-strain and temperature changes using the bonded-disk instrument, (ii) calculate the exotherm, and (iii) calculate the CTE.
The null hypotheses were: (1) there are no correlations between filler loading and shrinkage strain, exotherm, and CTE, and (2) there is no correlation between shrinkage strain and CTE.
Materials and methods
Six representative photo-activated resin-composites were selected on the basis of their matrix resin composition and filler loading ( Table 1 ).
|Code||Resin composites||Filler loading (wt.% a )||Resin matrix||Lot no.||Manufacturer|
|GRO||Grandio||87||Bis-GMA, TEGDMA||581793||Voco, Cuxhaven Germany|
|GCK||GC Kalore||82||DX-511, UDMA and dimethacrylate co monomers||0906021||GC America Inc.|
|VDD||Venus Diamond||81.2||TCD-DI-HEA UDMA||010035||Heraeus Kulzer|
|FXE||Filtek Supreme XTE||78.5||BIS-GMA, BIS-EMA (6), TEGDMA, PEGDMA and UDMA||N147105||3M ESPE Germany|
|GDP||Gradia Direct Posterior||77||UDMA and dimethacrylate co-monomers||0910191||GC America Inc.|
|GDA||Gradia Direct Anterior||73||UDMA and dimethacrylate co-monomers||0909021||GC America Inc.|