Water aging reverses residual stresses in hydrophilic dental composites

Dental composites develop residual stresses during polymerization due to shrinkage. These stresses may change with time because of relaxation and water sorption in the oral environment. This phenomenon is likely dependent on the composition of the materials, specifically their hydrophilic characteristics, and could result in deleterious stresses on restorative materials and tooth structure. The purpose of this experiment was to use the thin ring-slitting method to compare the residual stress generated within composite materials of varying hydrophilicity when aged in wet and dry conditions after polymerization. Water sorption, solubility, elastic modulus, and residual stresses were measured in 6 commercial composites/cements aged in water and dry conditions. The self-adhesive resin cement showed the highest water sorption and solubility. All composites showed initial residual contraction stresses, which were maintained when aged dry. Residual stresses in 2 of the self-adhesive cements and the polyacid-modified composite aged in wet conditions resulted in a net expansion. This experiment verified that residual shrinkage stresses in dental composites can be reversed during aging in water, resulting in a net expansion, with the effect directly related to their hydrophilic properties.