With a majority of the more than 160 million dental restorative treatments performed in the US each year involving the placement of resin-bonded composite materials, and the acknowledgement that a large portion of a dentist?s time is consumed with revising and replacing these restorations, there is a clear need for materials with improved clinical performance. The relatively short average clinical lifespan (~6 years), particularly for large restorations, is not just inefficient and expensive, but each revision escalates to more invasive treatment. The primary reasons for the failure rate of restorations are fracture or loss of the composite as well as secondary caries. These problems are all potentially linked to inadequate curing during placement of the restoration since polymer properties are correlated with degree of conversion while adhesion to the tooth and the associated reinforcement of a compromised tooth are also dependent on the extent of cure of the composite resin. Uncured monomer leaching from a restoration is known to promote more aggressive bacterial activity in the vicinity of a restoration, which further underscores the need for well cured composites. It is notable that there are significant discrepancies in the clinical service life of similar commercial composite materials, which indicates that the reliable placement of the materials is highly technique sensitive. The goal here is to design novel photoinitiating systems capable of delivering free radicals for reliable polymerization beyond the temporal and spatial footprint of a visible curing light. The intended clinically relevant application is for the direct placement of dimensionally thick, highly filled composite restorative materials in which substantial light attenuation and/or shadow patterning is encountered. These heavy-body materials cannot be accommodated in dual-cure auto-mixing syringes. The product of this proposal could alleviate all concern of incompletely photocured composite restorations by development of completely new photo-activated redox polymerization initiation systems that in one version can augment the initial cure in under-cured regions of a composite by engaging a dark curing process that continues beyond exposure to the curing light. Our early results demonstrate limited photopolymerizations reaching only 10% conversion during direct light exposure that can then progress to full conversion of colorless polymer in the ensuing dark phase. Alternatively, another version of the initiator is expected to produce full conversion following a brief activation by visible light even within regions that received no exposure from the curing light. If successful, this demonstration project will introduce a breakthrough in photocuring that would dramatically increase depth of cure and provide full assurance of complete conversion in dental composite and resin cement photopolymerizations. In addition to the obvious benefits of assured full conversion and optimized properties, other advantages such as reduced polymerization stress and improved adhesive bonding due to a more gradual polymerization process at the composite base may be realized. Success here would represent a tremendously important advance in photopolymerization that would undoubtedly extend beyond dentistry.
Dental practitioners have no rational means to verify whether their photocuring technique and their curing lights are well matched to a material and optimally producing appropriate high levels of conversion throughout a composite restoration including the placement of bulk fill restorations. The intent of this proposal is the development of a novel photo-activated initiator system that can be used generically to consistently achieve not only rapid polymerization during light exposure, but also complete through-cure by augmenting polymerization within any under-cured regions. This approach is directed towards assuring full polymer conversion, maximized dental material properties and clinical performance along with minimized leachable components with application to resin cements and dental composites.
