Investigations on a methacrylate-based flowable composite based on the SDR™ technology.

Objective: Monomer development for a reduced shrinkage of composite materials still challenges the modern research. The purpose of this study was to analyse the shrinkage behavior of an innovative composite material for dental restorations based on a resin system that is claimed to control polymerization kinetics having incorporated a photoactive group within the resin.

Methods: Shrinkage stress development within the first 300s after photoinitiation, gel point as well as micro-mechanical properties (Vickers hardness HV, modulus of elasticity E, creep Cr and elastic-plastic indentation work W(e)/W(tot)) were evaluated (n = 10). The experimental flowable resin-based composite (RBC) was measured in comparison to regular methacrylate-based micro- (Esthet X Flow) and nano-hybrid flowable RBCs (Filtek Supreme Plus Flow). Additionally, the high viscosity counterparts of the two regular flowable methacryate-based composites (Esthet X Plus and Filtek Supreme Plus) as well as a low shrinkage silorane-based micro-hybrid composite (Filtek Silorane) were considered. The curing time was 20s (LED unit Freelight2, 3M-ESPE, 1226 mW/cm(2)).

Results: The experimental material achieved the significantly lowest contraction stress (1.1 ± .01 MPa) followed by the silorane-based composite (3.6 ± .03 MPa), whereas the highest stress values were induced in the regular methacrylate-based flowable composites EsthetX Flow (5.3 ± .3 MPa) and Filtek Supreme Flow (6.5 ± .3 MPa). In view of gel point, the best values were obtained for the experimental flowable composite (3.1 ± .1s) and Filtek Silorane (3.2 ± .3s), which did not differ significant from each others, whereas EsthetX Plus and Filtek Supreme Plus did also not differ significantly, inducing the shortest gel point. The experimental flowable material achieved also the lowest shrinkage-rate (maximum at 0.1 MPa/s). For all analysed materials, no significant difference in the micro-mechanical properties between top and bottom were found when measured on 2mm thick increments 24h after polymerization. The categories of flowable materials performed in the measured micro-mechanical properties significantly inferior when compared to the hybrid-composites, showing lower HV and E and predominantly higher creep and plastic deformation. Within the flowable RBCs, the experimental material achieved the lowest Vickers hardness, the highest modulus of elasticity, the highest creep and showed the significantly lowest elastic deformation.

Significance: The experimental flowable composite revealed the lowest shrinkage stress and shrinkage-rate values in comparison to regular methacrylate composites but intermediate micro-mechanical properties. Being at the same time more rigid (higher modulus of elasticity) and more plastic (low W(e)/W(tot) and high creep values) as the regular flowable materials, its effect on interfacial stress build-up cannot be easily predicted.