Research On Depth Of Cure Of Dual-cured Flowable Resin Composites

Depth of cure was an important parameter to evaluate light-cured resin composites. Since light was attenuating within resin composites, polymerization could not achieve80%of the maximum below a critical depth which was named depth of cure. Nowadays flowable core build-up resin composites and resin cements included both light-curing and chemical-curing systems so as to obtain initial physical-mechanical properties promptly when light was sufficient, and continue to polymerize when light was blocked. Depth of cure was not born to dual-cured resin composites. It was revealed that chemical curing efficiency of most dual-cured resin composites was deficient and polymerization would still be affected by light attenuation. Experiment1Behavior of depth of cure of dual-cured flowable core build-up resin compositesWhen dual-cured flowable core build-up resin composites were used to cement prefabricated post, it was necessary to polymerize sufficiently to obtain optimal physical-chemical properties and aesthetic characteristics, so as to achieve retention and stability of post and core restorations. Clinically during post cementation light was attenuating along the canal, which would result in inhomogeneous polymerization within the resin composites, and eventually impair its physical-mechanical properties. Usually it was within0.5h to finish post cementation, core buildup and temporary crown fabrication, after that the restoration would be loaded by occlusion and mastication force. Hardness was so important to be a direct parameter of physical-mechanical properties and an indirect parameter for degree of cure, that its variation with time was significant.The study was aiming to investigate the effect of irradiation distance and post-irradiation time on microhardness and depth of cure of several dual-cured flowable core build-up resin composites by incremental surface hardness measurement, and to indirectly detect light curing and chemical curing efficiency of test materials.The results were as follows: 1. For dual-cured flowable core build-up material Luxa core, the depth of cure in0.5h,24h and120h post-irradiation was2mm. For Clearfil DC core, the depth of cure was2mm in0.5h and increased to4mm in24h. For Para core and Multi core the depth of cure in0.5h were2mm and3mm respectively, after24h the microhardness ratios exceed80%in every distance.2. With the increase of irradiation distance microhardness of dual-cured flowable core build-up resin composites decreased till certain distance, below which the microhardness was provided by chemical-curing component and did not decrease any more. The certain distance for Luxa core was3mm, for Para core and Multi core were4mm, and for Clearfil DC core was5mm.3. Microhardness increment of dual-cured flowable core build-up resin composites was great within the first0.5h after cure beginning. The microhardness increased gradually during0.5-24h and attained to the maximum after24h. Further clinical operation might be delayed to24h.4. For dual-cured flowable core build-up resin composites the efficiency of light and chemical-curing components was different. Chemical-curing efficiency of Para core and Multi core was relatively high.Experiment2Behavior of depth of cure of dual-cured self-adhesive resin cementsFor dual-cured flowable core build-up resin composites it must recur to resin adhesive systems to bond with dentin, which was full of complexity and technical sensitivity. Since dual-cured self-adhesive resin cements containing acidic functional monomers, it was no need to prime dentin to achieve optimal bond, which lowered technical sensitivity of post and core restorative procedure.The aim of this study was to discuss whether self-adhesive resin cement could be a substitute for flowable core build-up materials in post and core restoration.The results were as follows:1. For dual-cured self-adhesive resin cement RelyX Unicem, in0.5h post-irradiation the microhardness ratios were below80%in every distance while after24h the depth of cure was1mm. For BisCem and GAM-200, the depth of cure was1mm in0.5h and24h, and increased to2mm in120h. For Speedcem and Maxcem the depth of cure in0.5h were1mm and2mm respectively, and after24h the microhardness ratios exceeded80%in every distance.2. Microhardness of Maxcem was not affected by light irradiation. The certain distance, hardness in which could be influenced by light irradiation, for Speedcem was2mm, for BisCem and GAM-200was4mm, and for RelyX Unicem was5mm.3. For self-adhesive resin cements the trend of microhardness variation with post-irradiation time was the same as flowable core build-up materials, except that microhardness increment of BisCem in24-120h more than2mm distance was statistically significant.4. For dual-cured self-adhesive resin cements efficiency of light and chemical-curing components was different. Chemical-curing efficiency of GAM-200, Speedcem and Maxcem was relatively high.ConclusionsMicrohardness of dual-cured flowable core build-up resin composites and self-adhesive resin cements was affected by light irradiation and post-irradiation time. The influence distance of irradiation was different among the groups while the trend of microhardness variation with post-irradiation time was similar. Polymerization of Para core, Multi core, GAM-200, Speedcem and Maxcem was relatively homogeneous when light irradiation was inhomogeneous.