The setting stress in composite resins was studied as a function of restoration shape. The shape is described by the configuration factor, C, the ratio of the restoration's bonded to unbonded (free) surfaces. In an experimental set-up, the shape of the restoration was simulated by cylindrical forms of various dimensions. The shrinkage stress was measured continuously. It was shown that in most of the clinically relevant cavity configurations, the stress-relieving flow is not sufficient to preserve adhesion to dentin by dentin-bonding agents.

The influence of contraction stresses, developed during the polymerization of composites, on adhesion to dentin treated with a dentin adhesive was studied for a chemically- and a light-activated microfilled composite. In both linear and 3-D models. The linear model consisted of an arrangement set up in a tensilometer in which the composites could be applied to a flat dentin surface fixed to the stationary cross-head at one end, and mechanically clamped to the cross-head connected to the load cell at the other end. The increase of the bond strength was measured at different time intervals from the start of mixing and was compared with the developing contraction stress. Throughout the complete polymerization process, the adhesion survived the contraction stress, which is explained by flow relaxation, which can occur sufficiently in this configuration. In the three-dimensional model, the composites are attached to more than two dentin walls. In this situation, flow is severely limited, and contraction stress values can exceed the bond strength, leading to separation. This was demonstrated in Class V cavities. The shape of the cavity is considered to be of great importance in conservation of the composite-dentin bond.

The results of this study showed that the use of high intensity curing light units negatively affected the integrity of the restoration-cavity interface in class V restorations This is explained by the high reaction rates of light curing resin composites. The interfacial integrity was better preserved with low light intensity as it extends the visco-elastic stage of the setting materials, thereby moderating the setting stress development. The ultimate polymerization shrinkages for both conditions were equal, which suggested equal degrees of conversion and thus equal material properties. The results may alleviate the trend in using higher intensity light curing units and in particular the development of units with laser beams in an attempt to further increase conversion rates.

The hardening stresses in wall-to-wall bonded composites were measured and related to calculated values obtained from freely shrinking materials. Since the ultimate tensile strength of the materials contracting under restricted conditions was not lower than that of composites which were not hindered during the polymerization shrinkage, the conclusion was drawn that the contraction was compensated for by flow instead of internal disruptions.