Effects of restoration and substrate on polymerization contraction stress of dental composites

The aim of this study was to treat both restoration and substrate as a combined factor (RS-factor) to complement the popular C-factor in prediction of polymerization contraction stress (PCS). A simple model consists of a uniaxial restoration with a curing composite sandwiched between two solid mountings (substrates). By using the equal stress principle and taking into account substrate deformation, a set of equations were developed and solved, resulting in a mathematical relationship between PCS and the size and stiffness of the substrate and the restoration. The strain gage method was used to experimentally assess the PCS of a light-cured composite encircled in an aluminum ring. Differently sized inserts made of cured composites and glass were placed in the center of the ring to control the thickness of the composite to be cured and created different RS factors as well as C-factors. According to the model, a restoration with a small RS-factor will produce a high PCS. The model also predicts that a restoration with a small C-factor will have a high PCS because of the compliance of substrates. These predictions were tentatively confirmed by the strain gage measurements. A higher PCS was detected when a smaller insert was used, which created a smaller RS-factor or C-factor, or when a glass insert was used instead of a less stiff composite insert, in which the former created a smaller RS-factor. The RS-factor may be a dominant factor in determining the PCS outcome in some special cases.