Formation of gaps at the filler–resin interface induced by polymerization contraction stress: Gaps at the interface

OBJECTIVES: To investigate the movement of resin matrix with respect to the filler particles of filled composites during their photo cure without or with polymerization contraction stress (PCS). METHODS: Two types of composites were prepared. Glass beads as macroscopic fillers were placed into the center of a bis-GMA/TEGDMA resin to make single bead-embedding "composites" and a variety of fillers of different compositions, sizes, and shapes were mixed with another bis-GMA/TEGDMA resin to make lightly filled composites. They were photo cured in a cavity constructed with an acrylic or aluminum ring sitting on a polyester strip. Bonding to the ring constrained the polymerization shrinkage and thus produced a PCS. The formation of gaps between the filler and the resin was detected by optical microscopy for the glass bead-resin systems, and by light attenuation and scanning electron microscopy (SEM) for the filler-resin composites. RESULTS: In general, for composites with untreated fillers, the optical microscopy and SEM revealed gaps at the filler-resin interface only when they were cured under constrained shrinkage conditions. These composites attenuated more light when cured under constrained shrinkage conditions than when under non-constrained conditions. For the composites with silane-treated fillers, no gaps were observed. Some did not show any significant difference in light attenuation when cured under either constrained or non-constrained conditions. CONCLUSIONS: The resin tends to move away from the filler particles under the influence of PCS. Strengthening the filler-resin interaction, such as by the use of silane-treated filler, may help prevent the resin departure and thus the formation of gaps.