Study of photopolymerization reaction of dental resins : answers of fundamental research to clinical issues

(eng) In modern dentistry, resin-based composites are about to definitely replace mercury amalgams in direct restorations, notably due to the clear aesthetic interest of tooth-like composites. However, authors have highlighted that the longevity of composite restorations is very dependent on the operator, which includes not only technical skills but also the respect of the material specificities, notably its light-responsive setting process. Therefore, the aim of the present work was to improve the understanding of the photopolymerization reaction of dimethacrylate-based dental resins and to develop strategies to improve this process. To start with, a global reflection was conducted on the theoretical aspects of photopolymerization reaction of dimethacrylate-based resins. Notably, the emphasis was put on the incomplete conversion of such resins and the associated persistence in the polymer network of unconverted monomers and trapped free radicals. The kinetics of decay of the latter was then analyzed by Electron Paramagnetic Resonance, which firstly confirmed the identification of two distinct radical species with different reactivity, and secondly highlighted the oxygen-dependance of free radicals long-term decay. Finally, the use of spin-trapping methods produced evidence of hydroxyl radical release by dental resins, which is probably due to the reaction of oxygen with the remaining methacrylate groups. As it appears that leaching of both free radicals and unconverted monomers are linked with the incomplete conversion of dimethacrylate-based resins, clinical procedures must be adapted appropriately to limit the amount of unconverted double bonds. Consequently, the second part of this thesis dealt with the two main ways to reach this goal: first, the use of a material containing a lower dimethacrylate fraction, second the adaptation of irradiation conditions in order to reach an optimal material conversion. Regarding the first method, the mechanical properties and filler fractions of different composite technologies were compared (ring-opening systems, ormocers and highly filled composites) and clinical guidelines were proposed for the use of each material. The second method focused on the evaluation of curing efficiency of new LED curing lights as well as their safety in terms of temperature rise in the pulp chamber. The results stressed the extreme importance of spectrum correspondence between light emission spectrum and absorption spectrum of the photoinitiating system. Besides, new LED lights were also shown to enable a certain reduction of curing time. Finally, the last part of this work was devoted to a frequent subject of debate in the photopolymerization of dental resin-based materials, i.e. applicability of the so-called Exposure Reciprocity Law, supporting the decrease of irradiation time by a proportional increase of irradiance. Again, the use of Electron Paramagnetic Resonance was very profitable, as it enabled to improve the understanding of the termination pathway through the measurement of trapped free radicals. Besides, the influence of photoinitiator type and filler content on the validity of this law were also investigated. These experiments revealed notably the very high curing efficiency of Lucirin-TPO, and underlined the impact of resin viscosity and filler content on the curing kinetics of Camphorquinone-based materials. These innovative approaches open the door to many other projects.